The
cell is the basic structural and functional unit of all known living
organismsIn biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s. It is also common to describe small molecules such as
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s as "
molecular building blocks". Organisms can be classified as unicellular (consisting of a single cell; including most
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
) or multicellular (including plants and animals). Humans contain about 100 trillion cells; a typical cell size is 10
µmA micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
and a typical cell mass is 1 nanogram. The human cell extrema are: largest - anterior horn in the spinal cord (135 μ vs. 120 μ for the ovum), longest - pseudounipolar cells which reach from extremities, including the toes to the lower
brain stemIn vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...
, and smallest - granule cells in the
cerebellumThe cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
, at 4 µ. The largest known cells are unfertilised
ostrichThe Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...
egg cellsAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
, which weigh 3.3 pounds.
The cell was discovered by
Robert HookeRobert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...
in 1665. In 1835, before the final cell theory was developed,
Jan Evangelista PurkyněJan Evangelista Purkyně was a Czech anatomist and physiologist. He was one of the best known scientists of his time. His son was the painter Karel Purkyně...
observed small "granules" while looking at the plant tissue through a microscope. The
cell theoryCell theory refers to the idea that cells are the basic unit of structure in every living thing. Development of this theory during the mid 17th century was made possible by advances in microscopy. This theory is one of the foundations of biology...
, first developed in 1839 by
Matthias Jakob SchleidenMatthias Jakob Schleiden was a German botanist and co-founder of the cell theory, along with Theodor Schwann and Rudolf Virchow....
and
Theodor SchwannTheodor Schwann was a German physiologist. His many contributions to biology include the development of cell theory, the discovery of Schwann cells in the peripheral nervous system, the discovery and study of pepsin, the discovery of the organic nature of yeast, and the invention of the term...
, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the
hereditary informationGenetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
necessary for regulating cell functions and for transmitting information to the next generation of cells.
The word
cell comes from the
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
cellula, meaning "a small room". The descriptive term for the smallest living biological structure was coined by
Robert HookeRobert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...
in a book he published in 1665 when he compared the
corkCork is an impermeable, buoyant material, a prime-subset of bark tissue that is harvested for commercial use primarily from Quercus suber , which is endemic to southwest Europe and northwest Africa...
cells he saw through his microscope to the small rooms monks lived in.
Anatomy
There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.
Table 1: Comparison of features of prokaryotic and eukaryotic cells
| |
Prokaryotes |
Eukaryotes |
| Typical organisms |
bacteria, archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
|
protistProtists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s, fungiA fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
, plantPlants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s, animalAnimals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s |
| Typical size |
~ 1–10 µm |
~ 10–100 µm (sperm cellsA spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...
, apart from the tail, are smaller) |
Type of nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
|
nucleoid region; no real nucleus |
real nucleus with double membrane |
| DNA |
circular (usually) |
linear molecules (chromosomeA chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s) with histoneIn biology, histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They are the chief protein components of chromatin, acting as spools around which DNA winds, and play a role in gene regulation...
proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s |
| RNA-/protein-synthesis |
coupled in cytoplasm The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
|
RNA-synthesis inside the nucleus
protein synthesis in cytoplasm |
RibosomeA ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s |
50S+30S |
60S+40S |
| Cytoplasmatic structure |
very few structures |
highly structured by endomembranes and a cytoskeleton The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
|
Cell movementChemotaxis is the phenomenon in which somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules,...
|
flagellaA flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
made of flagellinFlagellin is a protein that arranges itself in a hollow cylinder to form the filament in bacterial flagellum. It has a mass of about 30,000 to 60,000 daltons...
|
flagella and ciliaA cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....
containing microtubuleMicrotubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s; lamellipodiaThe lamellipodium is a cytoskeletal protein actin projection on the mobile edge of the cell. It contains a quasi-two-dimensional actin mesh; the whole structure propels the cell across a substrate...
and filopodiaFilopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the...
containing actinActin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
|
| Mitochondria |
none |
one to several thousand (though some lack mitochondria) |
| Chloroplast Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s |
none |
in algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
and plantPlants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s |
| Organization |
usually single cells |
single cells, colonies, higher multicellular organisms with specialized cells |
Cell divisionCell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
|
Binary fission (simple division) |
MitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
(fission or budding)
MeiosisMeiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
|
Prokaryotic cells
The
prokaryoteThe prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
cell is simpler, and therefore smaller, than a eukaryote cell, lacking a
nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
and most of the other
organelleIn cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s of eukaryotes. There are two kinds of prokaryotes:
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
; these share a similar structure.
Nuclear material of prokaryotic cell consist of a single chromosome that is in direct contact with cytoplasm. Here, the undefined nuclear region in the cytoplasm is called
nucleoidThe nucleoid is an irregularly-shaped region within the cell of a prokaryote that contains all or most of the genetic material. In contrast to the nucleus of a eukaryotic cell, it is not surrounded by a nuclear membrane. The genome of prokaryotic organisms generally is a circular, double-stranded...
.
A prokaryotic cell has three architectural regions:
- On the outside, flagella and pili
right|thumb|350px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell...
project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells;
- Enclosing the cell is the cell envelope
The cell envelope is the cell membrane and cell wall plus an outer membrane, if one is present.Most bacterial cell envelopes fall into two major categories: Gram positive and Gram negative. These are differentiated by their Gram staining characteristics....
– generally consisting of a cell wallThe cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
covering a plasma membrane though some bacteria also have a further covering layer called a capsule. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as MycoplasmaMycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
(bacteria) and ThermoplasmaIn taxonomy, Thermoplasma is a genus of the Thermoplasmataceae.Thermoplasma is a genus of archaea. It belongs to the Thermoplasmata, which thrive in acidic and high-temperature environments. Thermoplasma are facultative anaerobes and respire using sulfur and organic carbon...
(archaea). The cell wall consists of peptidoglycanPeptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and finally bursting (cytolysisCytolysis, or osmotic lysis, occurs when a cell bursts due to an osmotic imbalance that has caused excess water to move into the cell. It occurs in a hypotonic environment, where water diffuses into the cell and causes its volume to increase. If the volume of water exceeds the cell membrane's...
) from osmotic pressureOsmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....
against a hypotonic environment. Some eukaryote cells (plant cellPlant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...
s and fungi cells) also have a cell wall;
- Inside the cell is the cytoplasmic region
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
that contains the cell genomeIn modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
(DNA) and ribosomes and various sorts of inclusions. A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferiBorrelia burgdorferi is a species of Gram negative bacteria of the spirochete class of the genus Borrelia. B. burgdorferi is predominant in North America, but also exists in Europe, and is the agent of Lyme disease....
, which causes Lyme disease). Though not forming a nucleus, the DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNAExtrachromosomal DNA is DNA located or maintained in a cell apart from the chromosomes....
elements called plasmidIn microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
s, which are usually circular. Plasmids enable additional functions, such as antibiotic resistanceAntibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria in a...
.
Eukaryotic cells
Plants, animals, fungi, slime moulds, protozoa, & algae are all
EukaryoticA eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
. These cells are about 15 times wider than a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is a
cell nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
, a membrane-delineated compartment that houses the eukaryotic cell's DNA. This nucleus gives the eukaryote its name, which means "true nucleus." Other differences include:
- The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
- The eukaryotic DNA is organized in one or more linear molecules, called chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s, which are associated with histoneIn biology, histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They are the chief protein components of chromatin, acting as spools around which DNA winds, and play a role in gene regulation...
proteins. All chromosomal DNA is stored in the cell nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
, separated from the cytoplasm by a membrane. Some eukaryotic organelleIn cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s such as mitochondria also contain some DNA.
- Many eukaryotic cells are ciliated
A cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....
with primary cilia. Primary cilia play important roles in chemosensation, mechanosensationMechanosensation is a response mechanism to mechanical stimuli. The physiological foundation for the senses of touch, hearing and balance, and pain is the conversion of mechanical stimuli into neuronal signals: mechanosensation. Mechanoreceptors of the skin, called cutaneous mechanoreceptors, are...
, and thermosensation. Cilia may thus be "viewed as sensory cellular antennaeAntennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."
- Eukaryotes can move using motile cilia or flagella. The flagella are more complex than those of prokaryotes.
Table 2: Comparison of structures between animal and plant cells
|
Typical animal cell |
Typical plant cell |
| Organelles |
- Nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
- Nucleolus
The nucleolus is a non-membrane bound structure composed of proteins and nucleic acids found within the nucleus. Ribosomal RNA is transcribed and assembled within the nucleolus...
(within nucleus)
- Rough endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
(ER)
- Smooth ER
- Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s
- Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
- Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
- Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
- Mitochondria
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
- Vesicles
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
- Lysosome
thumb|350px|Schematic of typical animal cell, showing subcellular components. [[Organelle]]s: [[nucleoli]] [[cell nucleus|nucleus]] [[ribosomes]] [[vesicle |vesicle]] rough [[endoplasmic reticulum]]...
s
- Centrosome
In cell biology, the centrosome is an organelle that serves as the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression. It was discovered by Edouard Van Beneden in 1883...
- Centriole
A Centriole is a barrel-shaped cell structure found in most animal eukaryotic cells, though it is absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules...
s |
NucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
- Nucleolus
The nucleolus is a non-membrane bound structure composed of proteins and nucleic acids found within the nucleus. Ribosomal RNA is transcribed and assembled within the nucleolus...
(within nucleus)
Rough ER
Smooth ER
Ribosomes
CytoskeletonThe cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
Golgi apparatusThe Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
(dictiosomes)
CytoplasmThe cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
MitochondriaIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
PlastidPlastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
s and its derivatives
VacuoleA vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
(s)
Cell wallThe cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
|
Subcellular components
All cells, whether prokaryotic or eukaryotic, have a
membraneThe cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
that envelops the cell, separates its interior from its environment, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, a
saltIn chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
y
cytoplasmThe cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
takes up most of the cell volume. All cells possess
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
, the hereditary material of
geneA gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s, and
RNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, containing the information necessary to
buildGene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
various
proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s such as
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s, the cell's primary machinery. There are also other kinds of
biomoleculeA biomolecule is any molecule that is produced by a living organism, including large polymeric molecules such as proteins, polysaccharides, lipids, and nucleic acids as well as small molecules such as primary metabolites, secondary metabolites, and natural products...
s in cells. This article lists these primary components of the cell, then briefly describe their function.
Membrane
The cytoplasm of a cell is surrounded by a cell membrane or
plasma membrane. The plasma membrane in plants and prokaryotes is usually covered by a
cell wallThe cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a
double layer of lipidsThe lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
(hydrophobic fat-like molecules) and hydrophilic
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
molecules. Hence, the layer is called a phospholipid bilayer. It may also be called a fluid mosaic membrane. Embedded within this membrane is a variety of
proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be 'semi-permeable', in that it can either let a substance (
moleculeA molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
or
ionAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as
hormoneA hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s.
Cytoskeleton
The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during
endocytosisEndocytosis is a process by which cells absorb molecules by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane...
, the uptake of external materials by a cell, and
cytokinesisCytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
, the separation of daughter cells after
cell divisionCell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of
microfilamentMicrofilaments are the thinnest filaments of the cytoskeleton, a structure found in the cytoplasm of all eukaryotic cells. These linear polymers of actin subunits are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton...
s,
intermediate filamentIntermediate filaments are a family of related proteins that share common structural and sequence features. Intermediate filaments have an average diameter of 10 nanometers, which is between that of 7 nm actin , and that of 25 nm microtubules, although they were initially designated...
s and
microtubuleMicrotubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s. There is a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis.
Genetic material
Two different kinds of genetic material exist:
deoxyribonucleic acidDeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
(DNA) and
ribonucleic acidRibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
(RNA). Most organisms use DNA for their long-term information storage, but
some virusesAn RNA virus is a virus that has RNA as its genetic material. This nucleic acid is usually single-stranded RNA but may be double-stranded RNA...
(e.g.,
retrovirusA retrovirus is an RNA virus that is duplicated in a host cell using the reverse transcriptase enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an integrase enzyme. The virus thereafter replicates as part of the host cell's DNA...
es) have RNA as their genetic material. The biological information contained in an organism is
encodedThe genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells....
in its DNA or RNA sequence. RNA is also used for information transport (e.g., mRNA) and
enzymaticEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
functions (e.g.,
ribosomalA ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
RNA) in organisms that use
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
for the genetic code itself.
Transfer RNATransfer RNA is an adaptor molecule composed of RNA, typically 73 to 93 nucleotides in length, that is used in biology to bridge the three-letter genetic code in messenger RNA with the twenty-letter code of amino acids in proteins. The role of tRNA as an adaptor is best understood by...
(tRNA) molecules are used to add amino acids during protein
translationTranslation is the communication of the meaning of a source-language text by means of an equivalent target-language text. Whereas interpreting undoubtedly antedates writing, translation began only after the appearance of written literature; there exist partial translations of the Sumerian Epic of...
.
Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial
chromosomeA chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
) in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called
chromosomeA chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see
endosymbiotic theoryThe endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...
).
A human cell has genetic material contained in the
cell nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
(the
nuclear genomeIn modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
) and in the mitochondria (the mitochondrial genome). In humans the nuclear genome is divided into 23 pairs of linear DNA molecules called
chromosomeA chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the
mitochondrial DNAMitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.
Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called
transfectionTransfection is the process of deliberately introducing nucleic acids into cells. The term is used notably for non-viral methods in eukaryotic cells...
. This can be transient, if the DNA is not inserted into the cell's
genomeIn modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
, or stable, if it is. Certain
virusA virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es also insert their genetic material into the genome.
Organelles
The human body contains many different
organsIn biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs," called
organelleIn cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s, that are adapted and/or specialized for carrying out one or more vital functions. Both eukaryotic and prokaryotic cells have organelles but organelles in eukaryotes are generally more complex and may be membrane bound.
There are several types of organelles in a cell. Some (such as the
nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
and
golgi apparatusThe Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
) are typically solitary, while others (such as mitochondria, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The
cytosolThe cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
is the gelatinous fluid that fills the cell and surrounds the organelles.
Cell nucleus – eukaryotes only - a cell's information center : The cell nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
replication and RNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
synthesis (transcriptionTranscription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelopeA nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol...
. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
is transcribedTranscription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
, or copied into a special RNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, called messenger RNAMessenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...
(mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolusThe nucleolus is a non-membrane bound structure composed of proteins and nucleic acids found within the nucleus. Ribosomal RNA is transcribed and assembled within the nucleolus...
is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasmThe cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
. |
|
Mitochondria and Chloroplasts – eukaryotes only - the power generators : MitochondriaIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria play a critical role in generating energy in the eukaryotic cell. Mitochondria generate the cell's energy by oxidative phosphorylationOxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP,...
, using oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
to release energy stored in cellular nutrients (typically pertaining to glucoseGlucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
) to generate ATPAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
. Mitochondria multiply by splitting in two. Respiration occurs in the cell mitochondria. |
Endoplasmic reticulum – eukaryotes only : The endoplasmic reticulumThe endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
(ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface and secretes proteins into the cytoplasm, and the smooth ER, which lacks them. Smooth ER plays a role in calcium sequestration and release. |
Golgi apparatus – eukaryotes only : The primary function of the Golgi apparatus is to process and package the macromoleculeA macromolecule is a very large molecule commonly created by some form of polymerization. In biochemistry, the term is applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles...
s such as proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s and lipidLipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s that are synthesized by the cell. |
|
Ribosomes : The ribosomeA ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
is a large complex of RNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
and proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
molecules. They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes). |
Lysosomes and Peroxisomes – eukaryotes only : Lysosomethumb|350px|Schematic of typical animal cell, showing subcellular components. [[Organelle]]s: [[nucleoli]] [[cell nucleus|nucleus]] [[ribosomes]] [[vesicle |vesicle]] rough [[endoplasmic reticulum]]...
s contain digestive enzyme'Digestive enzymes' are enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body. Digestive enzymes are found in the digestive tract of animals where they aid in the digestion of food as well as inside the cells,...
s (acid hydrolaseIn biochemistry, a hydrolase is an enzyme that catalyzes the hydrolysis of a chemical bond. For example, an enzyme that catalyzed the following reaction is a hydrolase:-Nomenclature:...
s). They digest excess or worn-out organelleIn cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s, food particles, and engulfed virusA virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es or bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. PeroxisomePeroxisomes are organelles found in virtually all eukaryotic cells. They are involved in the catabolism of very long chain fatty acids, branched chain fatty acids, D-amino acids, polyamines, and biosynthesis of plasmalogens, etherphospholipids critical for the normal function of mammalian brains...
s have enzymes that rid the cell of toxic peroxideA peroxide is a compound containing an oxygen–oxygen single bond or the peroxide anion .The O−O group is called the peroxide group or peroxo group. In contrast to oxide ions, the oxygen atoms in the peroxide ion have an oxidation state of −1.The simplest stable peroxide is hydrogen peroxide...
s. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system. |
Centrosome – the cytoskeleton organiser : The centrosomeIn cell biology, the centrosome is an organelle that serves as the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression. It was discovered by Edouard Van Beneden in 1883...
produces the microtubules of a cell – a key component of the cytoskeletonThe cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
. It directs the transport through the ERThe endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
and the Golgi apparatusThe Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
. Centrosomes are composed of two centrioles, which separate during cell divisionCell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
and help in the formation of the mitotic spindleIn cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...
. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells. |
Vacuoles : VacuoleA vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
s store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably AmoebaAmoeba is a genus of Protozoa.History=The amoeba was first discovered by August Johann Rösel von Rosenhof in 1757. Early naturalists referred to Amoeba as the Proteus animalcule after the Greek god Proteus, who could change his shape...
, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of eukaryotic cells are usually larger in those of plants than animals. |
Capsule
A gelatinous capsule is present in some bacteria outside the cell wall. The capsule may be
polysaccharidePolysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
as in pneumococci, meningococci or polypeptide as
Bacillus anthracisBacillus anthracis is the pathogen of the Anthrax acute disease. It is a Gram-positive, spore-forming, rod-shaped bacterium, with a width of 1-1.2µm and a length of 3-5µm. It can be grown in an ordinary nutrient medium under aerobic or anaerobic conditions.It is one of few bacteria known to...
or hyaluronic acid as in streptococci.
Capsules are not marked by ordinary stain and can be detected by special stain.
Flagella
Flagella are the organelles of cellular mobility. They arise from cytoplasm and extrude through the cell wall. They are long and thick thread-like appendages, protein in nature. Are most commonly found in bacteria cells but are found in animal cells as well.
Fimbriae (pili)
They are short and thin hair like filaments, formed of protein called pilin (antigenic). Fimbriae are responsible for attachment of bacteria to specific receptors of human cell (adherence). There are special types of pili called (sex pili) involved in conjunction.
Growth and metabolism
Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions:
catabolismCatabolism is the set of metabolic pathways that break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides, and amino...
, in which the cell breaks down complex molecules to produce energy and reducing power, and
anabolismAnabolism is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy. One way of categorizing metabolic processes, whether at the cellular, organ or organism level is as 'anabolic' or as 'catabolic', which is the opposite...
, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions.
Complex sugars consumed by the organism can be broken down into a less chemically complex sugar molecule called
glucoseGlucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
. Once inside the cell, glucose is broken down to make adenosine triphosphate (
ATPAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
), a form of energy, through two different pathways.
The first pathway,
glycolysisGlycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
, requires no oxygen and is referred to as
anaerobic metabolismFermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy.
The second pathway, called the Krebs cycle, or
citric acid cycleThe citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...
, occurs inside the mitochondria and can generate enough ATP to run all the cell functions.
Creation
Cell division involves a single cell (called a
mother cell) dividing into two daughter cells. This leads to growth in
multicellular organismMulticellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...
s (the growth of
tissueTissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
) and to procreation (
vegetative reproductionVegetative reproduction is a form of asexual reproduction in plants. It is a process by which new individuals arise without production of seeds or spores...
) in
unicellular organismA unicellular organism, also known as a single-celled organism is an organism that consists of only one cell, in contrast to a multicellular organism that consists of multiple cells. Historically simple single celled organisms have sometimes been referred to as monads Prokaryotes, most protists,...
s.
ProkaryoticThe prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
cells divide by binary fission.
EukaryoticA eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
cells usually undergo a process of nuclear division, called
mitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
, followed by division of the cell, called
cytokinesisCytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
. A diploid cell may also undergo
meiosisMeiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
to produce haploid cells, usually four. Haploid cells serve as
gameteA gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s in multicellular organisms, fusing to form new diploid cells.
DNA replicationDNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.
Protein synthesis
Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps:
transcriptionTranscription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
and
translationIn molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...
.
Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give
messenger RNAMessenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...
(mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called
ribosomeA ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s located in the
cytosolThe cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to
transfer RNATransfer RNA is an adaptor molecule composed of RNA, typically 73 to 93 nucleotides in length, that is used in biology to bridge the three-letter genetic code in messenger RNA with the twenty-letter code of amino acids in proteins. The role of tRNA as an adaptor is best understood by...
(tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.
Movement or motility
Cells can move during many processes: such as wound healing, the immune response and cancer metastasis. For wound healing to occur, white blood cells and cells that ingest bacteria move to the wound site to kill the microorganisms that cause infection.
At the same time fibroblasts (connective tissue cells) move there to remodel damaged structures. In the case of tumor development, cells from a primary tumor move away and spread to other parts of the body. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. The process is divided into three steps – protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton.
Evolution
The origin of cells has to do with the origin of life, which began the
history of lifeThis timeline of evolution of life outlines the major events in the development of life on planet Earth since it first originated until the present day. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations...
on Earth.
Origin of the first cell
There are several theories about the origin of small molecules that could lead to life in an early Earth. One is that they came from meteorites (
see Murchison meteoriteThe Murchison meteorite is named after Murchison, Victoria, in Australia. It is one of the most studied meteorites due to its large mass , the fact that it was an observed fall, and it belongs to a group of meteorites rich in organic compounds....
). Another is that they were created at
deep-sea ventsA hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots. Hydrothermal vents exist because the earth is both...
. A third is that they were synthesized by lightning in a reducing atmosphere (
see Miller–Urey experiment); although it is not clear if Earth had such an atmosphere. There are essentially no experimental data defining what the first self-replicating forms were.
RNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
is generally assumed the earliest self-replicating molecule, as it is capable of both storing genetic information and catalyzing chemical reactions (
see RNA world hypothesisThe RNA world hypothesis proposes that life based on ribonucleic acid pre-dates the current world of life based on deoxyribonucleic acid , RNA and proteins. RNA is able both to store genetic information, like DNA, and to catalyze chemical reactions, like an enzyme protein...
). But some other entity with the potential to self-replicate could have preceded RNA, like clay or peptide nucleic acid.
Cells emerged at least 4.0–4.3 billion years ago. The current belief is that these cells were
heterotrophA heterotroph is an organism that cannot fix carbon and uses organic carbon for growth. This contrasts with autotrophs, such as plants and algae, which can use energy from sunlight or inorganic compounds to produce organic compounds such as carbohydrates, fats, and proteins from inorganic carbon...
s. An important characteristic of cells is the
cell membraneThe cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
, composed of a bilayer of
lipidLipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s. The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. Lipids are known to spontaneously form bilayered
vesicleA vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
s in water, and could have preceded RNA, but the first cell membranes could also have been produced by catalytic RNA, or even have required structural proteins before they could form.
Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a
symbiotic communitySymbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
of prokaryotic cells. DNA-bearing organelles like the mitochondria and the chloroplasts are almost certainly what remains of ancient symbiotic oxygen-breathing
proteobacteriaThe Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....
and
cyanobacteria, respectively, where the rest of the cell appears derived from an ancestral
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
n prokaryote cell—an idea called the
endosymbiotic theoryThe endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...
.
There is still considerable debate about whether organelles like the
hydrogenosomeA hydrogenosome is a membrane-enclosed organelle of some anaerobic ciliates, trichomonads and fungi. The hydrogenosomes of trichomonads produce molecular hydrogen, acetate, carbon dioxide and ATP by the combined actions of pyruvate:ferredoxin oxido-reductase, hydrogenase, acetate:succinate CoA...
predated the origin of mitochondria, or viceversa: see the
hydrogen hypothesisThe hydrogen hypothesis is a model proposed by William F. Martin and Miklós Müller in 1998 that describes a possible way in which the mitochondrion arose as an endosymbiont within a prokaryote , giving rise to a symbiotic association of two cells from which the first eukaryotic cell could have...
for the origin of eukaryotic cells.
Sex, as the stereotyped choreography of meiosis and syngamy that persists in nearly all extant eukaryotes, may have played a role in the transition from prokaryotes to eukaryotes. An 'origin of sex as vaccination' theory suggests that the eukaryote genome accreted from prokaryan parasite genomes in numerous rounds of lateral gene transfer. Sex-as-syngamy (fusion sex) arose when infected hosts began swapping nuclearized genomes containing co-evolved, vertically transmitted symbionts that conveyed protection against horizontal infection by more virulent symbionts.
History
- 1632–1723: Antonie van Leeuwenhoek teaches himself to grind lenses
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
, builds a microscopeA microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
and draws protozoaProtozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
, such as VorticellaVorticella is a genus of protozoa, with over 16 known species. They are stalked inverted bell-shaped ciliates, placed among the peritrichs. Each cell has a separate stalk anchored onto the substrate, which contains a contractile fibril called a myoneme. When stimulated this shortens, causing the...
from rain water, and bacteria from his own mouth.
- 1665: Robert Hooke
Robert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...
discovers cells in cork, then in living plant tissue using an early microscope.
- 1839: Theodor Schwann
Theodor Schwann was a German physiologist. His many contributions to biology include the development of cell theory, the discovery of Schwann cells in the peripheral nervous system, the discovery and study of pepsin, the discovery of the organic nature of yeast, and the invention of the term...
and Matthias Jakob SchleidenMatthias Jakob Schleiden was a German botanist and co-founder of the cell theory, along with Theodor Schwann and Rudolf Virchow....
elucidate the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.
- The belief that life forms can occur spontaneously (generatio spontanea
Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...
) is contradicted by Louis PasteurLouis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...
(1822–1895) (although Francesco RediFrancesco Redi was an Italian physician, naturalist, and poet.-Biography:The son of Gregorio Redi and Cecilia de Ghinci was born in Arezzo on February 18, 1626. After schooling with the Jesuits, he attended the University of Pisa...
had performed an experiment in 1668 that suggested the same conclusion).
- 1855: Rudolf Virchow
Rudolph Carl Virchow was a German doctor, anthropologist, pathologist, prehistorian, biologist and politician, known for his advancement of public health...
states that cells always emerge from cell divisionCell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
s (omnis cellula ex cellula).
- 1931: Ernst Ruska
Ernst August Friedrich Ruska was a German physicist who won the Nobel Prize in Physics in 1986 for his work in electron optics, including the design of the first electron microscope.Ruska was born in Heidelberg...
builds first transmission electron microscope (TEM) at the University of Berlin. By 1935, he has built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
- 1953: Watson
James Dewey Watson is an American molecular biologist, geneticist, and zoologist, best known as one of the co-discoverers of the structure of DNA in 1953 with Francis Crick...
and CrickFrancis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...
made their first announcement on the double-helixA helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...
structure for DNA on February 28.
- 1981: Lynn Margulis
Lynn Margulis was an American biologist and University Professor in the Department of Geosciences at the University of Massachusetts Amherst. She is best known for her theory on the origin of eukaryotic organelles, and her contributions to the endosymbiotic theory, which is now generally accepted...
published Symbiosis in Cell Evolution detailing the endosymbiotic theoryThe endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...
.
See also
- Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
- Cell culture
Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...
- Cell type
A cell type is a distinct morphological or functional form of cell. When a cell switches state from one cell type to another, it undergoes cellular differentiation. A list of distinct cell types in the adult human body may include several hundred distinct types.-References:...
- Cellular component
Cellular component refers to the unique, highly organized substances and substances of which cells, and thus living organisms, are composed. Examples include membranes, organelles, proteins, and nucleic acids...
- Cytorrhysis
Cytorrhysis is the complete collapse of a plant cell's cell wall within plants due to the loss of water through osmosis. This always follows plasmolysis...
- Cytotoxicity
Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are a chemical substance, an immune cell or some types of venom .-Cell physiology:...
- Plasmolysis
Plasmolysis is the process in plant cells where the cytoplasm pulls away from the cell wall due to the loss of water through osmosis. The reverse process, cytolysis, can occur if the cell is in a hypotonic solution resulting in a higher external osmotic pressure and a net flow of water into the cell...
- Stem cell
This article is about the cell type. For the medical therapy, see Stem Cell TreatmentsStem cells are biological cells found in all multicellular organisms, that can divide and differentiate into diverse specialized cell types and can self-renew to produce more stem cells...
- Syncytium
In biology, a syncytium is a large cell-like structure; filled with cytoplasm and containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms.The term may also refer to cells that are connected by specialized membrane...
External links
- Inside the Cell
- Virtual Cell's Educational Animations
- The Inner Life of A Cell, a flash video showing what happens inside of a cell. Daniel Reda of Singularity University
Singularity University is an academic institution in Silicon Valley whose stated aim is to "assemble, educate and inspire a cadre of leaders who strive to understand and facilitate the development of exponentially advancing technologies and apply, focus and guide these tools to address humanity’s...
narrates (beginning at 22:24) http://www.youtube.com/watch?v=It83JKAxejM
- The Virtual Cell
- Cells Alive!
- Journal of Cell Biology
- The Biology Project > Cell Biology
- Centre of the Cell online
- The Image & Video Library of The American Society for Cell Biology, a collection of peer-reviewed still images, video clips and digital books that illustrate the structure, function and biology of the cell.
- HighMag Blog, still images of cells from recent research articles.
- New Microscope Produces Dazzling 3D Movies of Live Cells, March 4, 2011 - Howard Hughes Medical Institute
Howard Hughes Medical Institute is a United States non-profit medical research organization based in Chevy Chase, Maryland. It was founded by the American businessman Howard Hughes in 1953. It is one of the largest private funding organizations for biological and medical research in the United...
.
- WormWeb.org: Interactive Visualization of the C. elegans Cell lineage - Visualize the entire cell lineage tree of the nematode C. elegans