ACTIVE TRANSPORT: PRIMARY AND SECONDARY TRANSPORT - BIOLOGY - 2025

The active transport is a transport cell type by which solute molecules move across the cell membrane, from an area with lower concentration of solutes to an area where the concentration of these is higher.

What happens naturally is that the molecules move from the side where they are more concentrated to the side where they are less concentrated; It is what occurs spontaneously without any type of energy being applied in the process. In this case, the molecules are said to move down the concentration gradient.

In contrast, in active transport the particles move against the concentration gradient and consequently consume energy from the cell. This energy normally comes from adenosine triphosphate (ATP).

Dissolved molecules sometimes have a higher concentration inside the cell than outside, but if the body needs them, these molecules are transported inside by carrier proteins found in the cell membrane.

What is active transport?

To understand what active transport consists of, it is necessary to understand what happens on both sides of the membrane through which transport occurs.

When a substance is in different concentrations on opposite sides of a membrane, it is said that there is a concentration gradient. Because atoms and molecules can be electrically charged, then electrical gradients can also form between the compartments on either side of the membrane.

Ionic movement is selective to cations or anions due to the size of the pore and its polarization. As two anions pass from the interior to the exterior of the cell, the exterior changes from +5 to +3. Source: Wikimedia commons. Author: Methylisopropylisergamide.

There is an electric potential difference every time there is a net separation of charges in space. In fact, living cells often have what is called a membrane potential, which is the difference in electrical potential (voltage) across the membrane, which is caused by an uneven distribution of charges.

Gradients are common in biological membranes, so energy expenditure is often required to move certain molecules against these gradients.

Energy is used to move these compounds through proteins that are inserted in the membrane and that function as transporters.

If proteins insert molecules against the concentration gradient, it is an active transport. If the transport of these molecules does not require energy, the transport is said to be passive. Depending on where the energy comes from, active transport can be primary or secondary.

Primary active transport

Primary active transport is one that directly uses a source of chemical energy (eg, ATP) to move molecules across a membrane against its gradient.

One of the most important examples in biology to illustrate this primary active transport mechanism is the sodium-potassium pump, which is found in animal cells and whose function is essential for these cells.

The sodium-potassium pump is a membrane protein that transports sodium out of the cell and potassium into the cell. To carry out this transport the pump requires energy from ATP.

Secondary active transport

The secondary active transport is the one that uses the energy stored in the cell, this energy is different from ATP and hence comes its distinction between the two types of transport.

The energy used by secondary active transport comes from the gradients generated by primary active transport, and can be used to transport other molecules against their concentration gradient.

For example, by increasing the concentration of sodium ions in the extracellular space, due to the operation of the sodium-potassium pump, an electrochemical gradient is generated by the difference in concentration of this ion on both sides of the membrane.

Under these conditions, the sodium ions would tend to move along their concentration gradient and would return to the interior of the cell through the transporter proteins.

Co-transporters

This energy from the electrochemical gradient of sodium can be used to transport other substances against their gradients. What happens is a shared transport and is carried out by transporter proteins called co-transporters (because they transport two elements simultaneously).

An example of an important co-transporter is the sodium-glucose exchange protein, which transports sodium cations down its gradient and, in turn, uses this energy to enter glucose molecules against its gradient. This is the mechanism by which glucose enters living cells.

In the previous example, the co-transporter protein moves the two elements in the same direction (inside the cell). When both elements move in the same direction, the protein that transports them is called a symporter.

However, co-transporters can also move compounds in opposite directions; in this case, the transporter protein is called an anti-carrier, although they are also known as exchangers or counter-transporters.

An example of an anti-carrier is the sodium-calcium exchanger, which carries out one of the most important cellular processes in removing calcium from cells. It uses the energy of the sodium electrochemical gradient to mobilize calcium out of the cell: one calcium cation leaves for every three sodium cations that enter.

Difference between exocytosis and active transport

Exocytosis is another important mechanism of cellular transport. Its function is to expel the residual material from the cell to the extracellular fluid. In exocytosis, transport is mediated by vesicles.

The main difference between exocytosis and active transport is that in exositosis the particle to be transported is wrapped in a structure surrounded by a membrane (the vesicle), which fuses with the cell membrane to release its contents to the outside.

In active transport the items to be transported can be moved in both directions, inward or outward. In contrast, exocytosis only transports its contents to the outside.

Finally, active transport involves proteins as the transport medium, not membranous structures as in exocytosis.

References

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