Active transport

Active transport is the movement of molecules across a cell membrane in the direction against their concentration gradient, i.e. moving from an area of lower concentration to an area of higher concentration. Active transport is usually associated 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 (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. Active transport uses cellular energy, unlike passive transport, which does not use cellular energy. Active transport is a good example of a process for which cells require energy. Examples of active transport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants.

Details

Specialized transmembrane proteins recognize the substance and allow it access 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 against the direction of the concentration gradient. In the case of secondary transport, such proteins expend energy upon forcing the substance to cross. In this case, known as primary active transport, the proteins involved in it are pumps which normally use the chemical energy of ATP. The other cases are known as secondary active transport, and energy is usually derived through exploitation of an electrochemical gradient. This case involves pore-forming proteins that form channels through the cell membrane. The difference between passive transport and active transport is active transport uses energy (ATP) and goes from low to high concentration and passive transport uses no energy (ATP)and goes from high to low concentration.

In an antiporter one substrate is transported in one direction while a second is cotransported in the opposite direction. In a symporter, two substrates are transported in the same direction across the membrane. Antiport and symport processes are associated with secondary active transport, meaning that one of the two substances is transported in the direction of its concentration gradient, utilizing the energy derived from the transport of second substance (mostly Na+, K+ or H+) down its concentration gradient.

If substrate particles are moving from areas of lower concentration to areas of higher concentration (i.e., in the opposite direction as, or against the concentration gradient), specific trans-membrane carrier proteins are required. These proteins have receptors that bind to specific molecules (e.g., glucose) 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 or other sources, 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 metabolic 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 animal life is the sodium-potassium pump, which helps to maintain the cell potential. Other sources of energy for Primary active transport are redox energy and photon energy (light). An example of primary active transport using Redox energy is the mitochondrial electron transport chain 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 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.

Model of active transport

ATP hydrolysis is used to transport hydrogen ions against the electrochemical gradient (from low to high hydrogen ion concentration). Phosphorylation of the carrier protein and the binding of a hydrogen ion induce a conformational (shape) change that drives the hydrogen ions to transport against the electrochemical gradient. Hydrolysis of the bound phosphate group and release of hydrogen ion then restores the carrier to its original conformation.

ATP using primary active transport types

1. P-type ATPase: sodium potassium pump, calcium pump, proton pump
2. F-ATPase: mitochondrial ATP synthase, chloroplast ATP synthase
3. V-ATPase: vacuolar ATPase
4. ABC (ATP binding cassette) transporter: MDR, CFTR, etc.

Secondary active transport

In secondary active transport, also known as coupled transport or co-transport, energy is used to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead it relies upon the electrochemical potential difference created by pumping ions in/out of the cell. Permitting one ion or molecule to move down an electrochemical gradient, but possibly against the concentration gradient where it is more concentrated to that where it is less concentrated increases entropy and can serve as a source of energy for metabolism (e.g. in ATP synthase).

In August 1960, in Prague, Robert K. Crane presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption. Crane's discovery of cotransport was the first ever proposal of flux coupling in biology.

Cotransporters can be classified as symporters and antiporters depending on whether the substances move in the same or opposite directions.

Antiport

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

Many cells also possess a calcium 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 from high to low concentration to move another molecule uphill from low concentration to high concentration (against its concentration gradient). Both molecules are transported in the same direction.

An example is the glucose symporter SGLT1, which co-transports one glucose (or galactose) molecule into the cell for every two sodium ions it imports into the cell. This symporter is located in the small intestines, trachea, heart, brain, testis, and prostate. It is also located in the S3 segment of the proximal tubule in each nephron in the kidneys. Its mechanism is exploited in glucose rehydration therapy and defects in SGLT1 prevent effective reabsorption of glucose, causing familial renal glucosuria.

Examples

• Metal ions, such as Na⁺, K⁺, Mg²⁺, or Ca²⁺, require ion pumps or ion channels 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 is produced by hydrogen potassium ATPase, an electrogenic pump
• Water, ethanol, and chloroform exemplify simple molecules that do NOT require active transport to cross a membrane.

Endocytosis and Exocytosis

For further information, see Endocytosis and Exocytosis

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 or inside 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 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
• Countercurrent exchange
• Protein targeting
• Translocation

References

External links

• Secondary Active Transport

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