Active transport

Active transport

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Active transport is the movement of a substance against its concentration gradient (from low to high concentration). In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine triphosphate
Adenosine triphosphate
Adenosine-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...

 (ATP), it is termed primary active transport
Primary active transport
Primary active transport, also called direct active transport, directly uses energy to transport molecules across a membrane.Most of the enzymes that perform this type of active transport are transmembrane ATPases. A primary ATPase universal to all cellular life is the sodium-potassium pump, which...

. Secondary active transport
Secondary active transport
In secondary active transport or co-transport, uses energy to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of the cell is used...

 involves the use of an electrochemical gradient
Electrochemical gradient
An electrochemical gradient is a spatial variation of both electrical potential and chemical concentration across a membrane; that is, a combination of the membrane potential and the pH gradient...

. Active transport uses energy, unlike passive transport
Passive transport
Passive transport means moving biochemicals and other atomic or molecular substances across membranes. Unlike active transport, this process does not involve chemical energy, because, unlike in an active transport, the transport across membrane is always coupled with the growth of entropy of the...

, which does not use any type of energy. Active transport is a good example of a process for which cells require energy. Examples of active tranport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants.

Details


Specialized trans-membrane proteins recognize the substance and allows it access (or, in the case of secondary transport, expend energy on forcing it) to cross the membrane when it otherwise would not, either because it is one to which the phospholipid bilayer of the membrane is impermeable or because it is moved in the direction of the concentration gradient. The last case, known as primary active transport, and the proteins involved in it as pumps, normally uses the chemical energy of ATP. The other cases, which usually derive their energy through exploitation of an electrochemical gradient, are known as secondary active transport and involve pore-forming proteins that form channels through the cell membrane.

Sometimes the system transports one substance in one direction at the same time as cotransporting another substance in the other direction. This is called antiport. Symport is the name if two substrates are being transported in the same direction across the membrane. Antiport and symport are associated with secondary active transport
Secondary active transport
In secondary active transport or co-transport, uses energy to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of the cell is used...

, meaning that one of the two substances are transported in the direction of their concentration gradient utilizing the energy derived from the transport of the second substance (mostly Na+, K+ or H+) down its concentration gradient.

Particles moving from areas of low concentration to areas of high concentration (i.e., in the opposite direction as the concentration gradient) require specific trans-membrane carrier proteins. These proteins have receptors that bind to specific molecules (e.g., glucose
Sodium-glucose transport proteins
Sodium-dependent glucose cotransporters are a family of glucose transporter found in the intestinal mucosa of the small intestine and the proximal tubule of the nephron . They contribute to renal glucose reabsorption...

) and thus transport them into the cell. Because energy is required for this process, it is known as 'active' transport. Examples of active transport include the transportation of sodium out of the cell and potassium into the cell by the sodium-potassium pump. Active transport often takes place in the internal lining of the small intestine.

Plants need to absorb mineral salts from the soil, but these salts exist in very dilute solution. Active transport enables these cells to take up salts from this dilute solution against the direction of the concentration gradient.

Primary active transport



Primary active transport, also called direct active transport, directly uses energy to transport molecules across a membrane.

Most of the enzymes that perform this type of transport are transmembrane ATPases. A primary ATPase universal to all life is the sodium-potassium pump, which helps to maintain the cell potential. Other sources of energy for Primary active transport are redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 energy and photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

 energy (light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

). An example of primary active transport using Redox energy is the mitochondrial electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

 that uses the reduction energy of NADH to move protons across the inner mitochondrial membrane against their concentration gradient. An example of primary active transport using light energy are the proteins involved in photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 that use the energy of photons to create a proton gradient across the thylakoid membrane and also to create reduction power in the form of NADPH.

ATP utilizing Primary active transport types


(1) P-type ATPase
P-type ATPase
The P-type ATPases, also known as E1-E2 ATPases, are a large group of evolutionarily related ion pumps that are found in bacteria, archaea and eukaryotes. They are α-helical bundle primary transporters referred to as P-type ATPases because they catalyze auto- phosphorylation of a key conserved...

 : Sodium potassium pump, Calcium pump
Calcium pump
There is a very large transmembrane electrochemical gradient of Ca2+ driving the entry of the ion into cells, yet it is very important for cells to maintain low concentrations of Ca2+ for proper cell signalling; thus it is necessary for the cell to employ ion pumps to remove the Ca2+.The PMCA and...

, Proton pump
Proton pump
A proton pump is an integral membrane protein that is capable of moving protons across a cell membrane, mitochondrion, or other organelle. Mechanisms are based on conformational changes of the protein structure or on the Q cycle.-Function:...



(2) F-ATPase
F-ATPase
F-ATPase, also known as F-Type ATPase , is an ATPase found in bacterial plasma membranes, in mitochondrial inner membranes, and in chloroplast thylakoid membranes...

 : mitochondrial ATP synthase, Chloroplast ATP synthase

(3) V-ATPase
V-ATPase
Vacuolar-type H+-ATPase is a highly conserved evolutionarily ancient enzyme with remarkably diverse functions in eukaryotic organisms. V-ATPases acidify a wide array of intracellular organelles and pump protons across the plasma membranes of numerous cell types...

 : vacuolar ATPase

(4) ABC (ATP Binding Cassette) transporter : MDR, CFTR, etc.

Secondary active transport



In secondary active transport or co-transport
Co-transport
Co-transport, also known as coupled transport or secondary active transport, refers to the simultaneous or sequential passive transfer of molecules or ions across biological membranes in a fixed ratio...

, uses energy to transport molecules across a membrane; however, in contrast to primary active transport
Primary active transport
Primary active transport, also called direct active transport, directly uses energy to transport molecules across a membrane.Most of the enzymes that perform this type of active transport are transmembrane ATPases. A primary ATPase universal to all cellular life is the sodium-potassium pump, which...

, there is no direct coupling of ATP
Adenosine triphosphate
Adenosine-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...

; instead, the electrochemical potential difference created by pumping ions out of the cell is used.

The two main forms of this are antiport and symport.

Antiport


In antiport two species of ion or other solutes are pumped in opposite directions across a membrane. One of these species is allowed to flow from high to low concentration which yields the entropic
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...

 energy to drive the transport of the other solute from a low concentration region to a high one. An example is the sodium-calcium exchanger
Sodium-calcium exchanger
The sodium-calcium exchanger is an antiporter membrane protein that removes calcium from cells. It uses the energy that is stored in the electrochemical gradient of sodium by allowing Na+ to flow down its gradient across the plasma membrane in exchange for the countertransport of calcium ions...

 or antiporter, which allows three sodium ions into the cell to transport one calcium out.

Many cells also possess a calcium ATPase
Calcium ATPase
Calcium ATPase is a form of P-ATPase that transfers calcium after a muscle has contracted. The calcium ATPase are:*Plasma membrane Ca2+ ATPase *Sarcoplasmic reticulum Ca2+ ATPase - Plasma membrane Ca2+ ATPase :...

, which can operate at lower intracellular concentrations of calcium and sets the normal or resting concentration of this important second messenger. But the ATPase exports calcium ions more slowly: only 30 per second versus 2000 per second by the exchanger. The exchanger comes into service when the calcium concentration rises steeply or "spikes" and enables rapid recovery. This shows that a single type of ion can be transported by several enzymes, which need not be active all the time (constitutively), but may exist to meet specific, intermittent needs.

Symport


Symport uses the downhill movement of one solute species
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...

 from high to low concentration to move another molecule uphill from low concentration to high concentration (against its electrochemical gradient
Electrochemical gradient
An electrochemical gradient is a spatial variation of both electrical potential and chemical concentration across a membrane; that is, a combination of the membrane potential and the pH gradient...

).

An example is the glucose symporter SGLT1
Sodium-glucose transport proteins
Sodium-dependent glucose cotransporters are a family of glucose transporter found in the intestinal mucosa of the small intestine and the proximal tubule of the nephron . They contribute to renal glucose reabsorption...

, which co-transport
Co-transport
Co-transport, also known as coupled transport or secondary active transport, refers to the simultaneous or sequential passive transfer of molecules or ions across biological membranes in a fixed ratio...

s one glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 (or galactose
Galactose
Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....

) molecule into the cell for every two sodium ions it imports into the cell. This symporter
Symporter
A cotransporter is an integral membrane protein that is involved in secondary active transport. It works by binding to two molecules or ions at a time and using the gradient of one solute's concentration to force the other molecule or ion against its gradient....

 is located in the small intestines, trachea, heart, brain, testis, and prostate. It is also located in the S3 segment of the proximal tubule
Proximal tubule
The proximal tubule is the portion of the duct system of the nephron of the kidney which leads from Bowman's capsule to the loop of Henle.-Structure and appearance:...

 in each nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....

 in the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

s . Its mechanism is exploited in glucose rehydration therapy
Oral rehydration therapy
Oral rehydration therapy is a simple treatment for dehydration associated with diarrhoea, particularly gastroenteritis or gastroenteropathy, such as that caused by cholera or rotavirus. ORT consists of a solution of salts and sugars which is taken by mouth...

 and defects in SGLT1 prevent effective reabsorption of glucose, causing familial renal glucosuria.

Examples

  • Water
    Water
    Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

    , ethanol
    Ethanol
    Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

    , and chloroform
    Chloroform
    Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...

     exemplify simple molecules that do not require active transport to cross a membrane.
  • Metal ions, such as Na
    Sodium
    Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

    +, K
    Potassium
    Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

    +, Mg
    Magnesium
    Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

    2+, or Ca
    Calcium
    Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

    2+, require ion pumps or ion channel
    Ion channel
    Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...

    s to cross membranes and distribute through the body
  • The pump for sodium and potassium is called sodium-potassium pump or Na +/K+-ATPase
  • In the epithelial cells of the stomach, gastric acid
    Gastric acid
    Gastric acid is a digestive fluid, formed in the stomach. It has a pH of 1 to 2 and is composed of hydrochloric acid , and large quantities of potassium chloride and sodium chloride...

     is produced by hydrogen potassium ATPase
    Hydrogen potassium ATPase
    Gastric hydrogen potassium ATPase is also known as H+/K+ ATPase- Function and location :The gastric hydrogen potassium ATPase or H+/K+ ATPase is the proton pump of the stomach and, as such, is the enzyme primarily responsible for the acidification of the stomach contents...

    , an electrogenic pump

Endocytosis



Endocytosis is the process by which cells take in materials. The cellular membrane folds around the desired materials outside the cell. The ingested particle becomes trapped within a pouch, vacuole
Vacuole
A 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...

 or inside the cytoplasm
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...

. Often enzymes from lysosomes are then used to digest the molecules absorbed by this process.

Biologists distinguish two main types of endocyctosis: pinocytosis
Pinocytosis
In cellular biology, pinocytosis is a form of endocytosis in which small particles are brought into the cell—forming an invagination, and then suspended within small vesicles that subsequently fuse with lysosomes to hydrolyze, or to break down, the particles...

 and phagocytosis.
  • In pinocytosis, cells engulf liquid particles (in humans this process occurs in the small intestine, cells there engulf fat droplets).
  • In phagocytosis, cells engulf solid particles.

See also

  • ATP-binding cassette transporter
  • Ion channel
    Ion channel
    Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...

  • Protein targeting
    Protein targeting
    Protein targeting or protein sorting is the mechanism by which a cell transports proteins to the appropriate positions in the cell or outside of it. Sorting targets can be the inner space of an organelle, any of several interior membranes, the cell's outer membrane, or its exterior via secretion...

  • Translocon
    Translocon
    The translocon is the complex of proteins associated with the translocation of nascent polypeptides across membranes. In eukaryotes the polypeptides are transported into the interior space of the endoplasmic reticulum from the cytosol...

  • Countercurrent exchange
    Countercurrent exchange
    Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or...


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