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Active transport
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Active transport is the mediated process of moving particles across a biological membrane against a concentration gradient. If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient.
ialized trans-membrane proteins recognize the substance to be transported and allow it (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 lipid bilayer of the membrane is impermeable or because it is moved against the concentration gradient.
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Encyclopedia
Active transport is the mediated process of moving particles across a biological membrane against a concentration gradient. If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient.
Details
Specialized trans-membrane proteins recognize the substance to be transported and allow it (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 lipid bilayer of the membrane is impermeable or because it is moved against the concentration gradient. The last case, known as primary active transport, and the proteins involved in it as pumps, uses the chemical energy of, usually, 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 which form channels through the cell membrane.
Sometimes—but not always—one substance is transported in one direction at the same time as another substance is being cotransported in the other direction. Often, the mechanism is named for both substances, as in the sodium-potassium pump.
When particles are being moved from areas of low concentration to areas of high concentration (i.e. against the concentration gradient) then specific carrier proteins in the membrane are required to move these particles. The carrier proteins bind to specific molecules (e.g. oxygen) and transport them into the cell where they are released. Because energy is required for this process, it is known as active transport. Examples of active transport include when sodium is transported 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 are in very dilute solution. Active transport enables these cells to take up salts from this dilute solution against the concentration gradient.
ABC pumps
ABC class pumps transport small molecules across membranes. They are also called the ABC superfamily.. They consist of two transmembrane domains, and two ATP binding domains. ABC pumps are involved in the transport of small molecules, phospholipids, and lipophilic drugs in mammalian cells. In bacteria they transport amino acids, sugars, and peptides.
Examples
Endocytosis
Endocytosis is the proccess by which cells ingest materials. The cellular membrane folds around the desired materials outside the cell. The ingested particle is trapped within a pouch, vacuole or inside the cytoplasm. Often enzymes from lysosomes are then used to digest the molecules absorbed by this process.
Endocyctosis can be split up into two main types: pinocytosis 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.
Exocytosis
Exocytosis is the process by which cells excrete waste and other large molecules from the protoplasm
See also
External Links
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