Active transport

What is active transport?

The Active transport It is the movement of substances from one side of the cell membranes to the other against their concentration gradient, that is, from where they are less concentrated to where they are more concentrated. Since it does not occur spontaneously, it is a process that usually requires energy.

All cells that exist in nature are delimited by a lipid membrane that behaves as a semi-permeable barrier, that is, it allows the passage of some substances and prevents the passage of others from the inside to the outside and vice versa.

A large number of molecules move by passive transport from one side of the cells to the other, but an important part of cellular mechanisms and, therefore, of life per se depend on the active transport of ions and molecules such as glucose, sodium, potassium, calcium, among many others.

Since active transport is not an energetically favorable process (it is “uphill), it is usually coupled, directly or indirectly, to another process that is, such as an oxidation reaction, of ATP hydrolysis, to the flow of chemical species in favor its gradient, to the absorption of sunlight, etc..

How do molecules move in active transport?

The movement of molecules or substances from one side of the cell membranes to the other can occur in two ways:

• Pso: when molecules spontaneously cross membranes by simple diffusion -or facilitated by pores and protein channels-. In this case, the chemical equilibrium between the compartments is being sought, that is, following their electrochemical or concentration gradient (from a place of higher concentration to one of lower concentration).
• TOctively: when the molecules are transported from one side of the cell membranes to the other against their concentration or charge gradient. This results in their uneven accumulation or in the displacement of the chemical equilibrium between the compartments; needs energy (it is thermodynamically unfavorable, that is to say endergonic) and the participation of special protein transporters.

Primary active transport

Primary active transport is one where the transport of a molecule against its chemical gradient (resulting in its accumulation on one side of the membrane) is directly coupled to an exergonic chemical reaction, that is, to a reaction where it is released Energy.

The most common examples of primary active transport are represented mainly by those that use the energy released during the hydrolysis of adenosine triphosphate (ATP), a molecule considered to be the most important cellular energy currency..

Animal cells, for example, actively move or transport (against their gradient) sodium (Na +) and potassium (K +) ions, using a very special transporter protein structure known as the sodium-potassium pump. This is responsible for expelling sodium ions and introducing potassium ions into the cell, while hydrolyzing ATP.

It is important to note that many of the proteins that participate in this type of transport are called "pumps".

How does the Na + / K transporter work+?

Sodium and potassium concentrations are different in animal cells: potassium is found in higher concentration at the intracellular level, with respect to the external environment, and sodium is less concentrated inside the cell than outside. Its active transport thanks to the sodium / potassium pump is as follows:

1. The pump "opens" in the cytosolic space and binds to 3 sodium ions (Na +), which triggers the hydrolysis of an ATP molecule (the pump is phosphorylated).
2. With the hydrolysis of ATP, the pump changes its structural shape and is oriented as "open" towards the extracellular space, where it lets the sodium ions go due to a phenomenon of decreased affinity..
3. In this position, the pump is now able to bind 2 potassium ions (K +), resulting in dephosphorylation of the pump and its change of shape to the initial form, open towards the cytosol. This opening releases the potassium ions into the cell, and it is ready for another transport cycle..

Generally, primary active transport achieves the establishment of electrochemical gradients important from multiple points of view for cellular activity..

Secondary active transport

Secondary active transport is the transport of a molecule or solute against its electrical or concentration gradient (endergonic process, which requires energy) that is coupled with the transport of another molecule in favor of its gradient (exergonic process, which releases energy ).

The particularity of this type of active transport has to do with the fact that the gradient of the molecule that apparently moves by passive transport was previously established by a primary active transport process, that is, it also used energy.

How does it work?

The primary active transport of positively or negatively charged ions establishes an electrochemical gradient within the cell; this type of transport is generally regarded as an "energy storage" mechanism..

The reason for the previous statement is due to the fact that when the same ions that were actively transported are mobilized by passive transport, or what is the same, in favor of their concentration gradient, energy is released, since it is an exergonic process.

Secondary active transport is called this way because it uses “stored” energy in the form of an ionic concentration gradient (which was established by primary active transport), to move other molecules against its concentration gradient at the same time as it is produced. passive transport of those that were first introduced by primary transport.

Usually the proteins that participate in this type of active transport are cotransporters that use the energy contained in electrochemical gradients. These cotransporters can move molecules in the same direction (symporters) or in opposite directions (anti-carriers).

A good example of the secondary active "cotransport" of the "symport" type is that carried out by the sodium / glucose cotransporter in the cell membrane of cells present in the intestinal mucosa of animals..

This transporter moves sodium ions down its concentration gradient into the cell, at the same time that it transports glucose molecules into the cell, against its concentration gradient..

Examples of active transport

Active transport is a process of fundamental importance for cellular life, for which a large number of examples can be cited, including:

• The pumps (primary active transport) that are responsible for the active transport of ions, small hydrophilic molecules, lipids, etc..
• Transporters (cotransporters, secondary active transport) that are responsible for the movement of molecules such as glucose, amino acids, some ions and other sugars, among others.

ATP-powered pumps for primary active transport

Active transport, in general, is an extremely important transport mechanism for all cells, both prokaryotes (bacteria and archaea) and eukaryotes (animals, plants and fungi)..

Primary active transport is usually mediated by a type of protein or protein complex known as "pumps", of which the pumps "moved" or "propelled" by the energy derived from ATP are the most relevant.

These proteins are essentially responsible for the movement of ions against their concentration gradient, using the energy released by the hydrolysis of ATP..

All these pumps usually have in their structure different sites for binding to ATP, generally on the side of the membrane where they are found facing the cytosol and according to these binding sites and the identity of the subunits that make them up, there are different types from bombs conveyors:

• The “P” class pumps, among which are the proton pumps of the plasma membrane of bacteria, plants and fungi; the Na + / K + and Ca + 2 pumps of the plasma membrane of all eukaryotic cells, etc..
• Class “V” pumps, such as those of the vacuolar membrane of plants, fungi and yeasts; pumps in the lysosomes of animal cells and pumps in the plasma membrane of some bone and kidney cells.
• Class “F” pumps, including those of the bacterial plasma membrane, the inner mitochondrial membrane, and the thylakoid membrane of chloroplasts in plant cells.
• The “ABC” transporter superfamily pumps, which include transporters for amino acids, sugars, peptides, phospholipids, lipophilic drugs, and other molecules in some animal and bacterial cells.

References

1. Alberts, B., Bray, D., Hopkin, K., Johnson, A. D., Lewis, J., Raff, M.,… & Walter, P. (2013). Essential cell biology. Garland science.
2. Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., & Keith Roberts, P. W. (2018). Molecular biology of the cell.
3. Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A.,… & Matsudaira, P. (2008). Molecular cell biology. Macmillan.
4. Murray, K., Rodwell, V., Bender, D., Botham, K. M., Weil, P. A., & Kennelly, P. J. (2009). Harper's illustrated biochemistry. 28 (p. 588). New York: McGraw-Hill.
5. Nelson, D. L., Lehninger, A. L., & Cox, M. M. (2008). Lehninger Principles of Biochemistry. Macmillan.