Active transport is a type of cellular transport by which dissolved molecules move through the cell membrane, from an area where the concentration of solutes is low, to an area where their concentration is high.
What happens naturally is that the molecules move from the side where they are concentrated to the side where they are less concentrated; This process occurs spontaneously without applying any energy. In this case it is said that the molecules move along the side of the concentration gradient.
In contrast, in active transport the particles move against the concentration gradient and, as a result, consume energy from the cell. This energy usually comes from adenosine triphosphate (ATP).
Sometimes there are higher concentrations of dissolved molecules in the cell than outside, but if the organism needs them, these molecules are carried inwards by certain transport proteins that are found in the cell membrane.
What is Active Transport?
To understand what happens in active transport, it is necessary to understand what happens on both sides of the membrane through which transport takes place.
When a substance is in different concentrations on opposite sides of a membrane, it is said to have a concentration gradient. Because atoms and molecules can have an electric charge, an electric gradient can also form between the compartments on either side of the membrane.
Each time the net separation of charges in space results in a difference in electric potential. In fact, living cells often have what is called a membrane potential, which is a difference in the electric potential (voltage) across the membrane, caused by the uneven distribution of charges.
Gradients are common in biological membranes, which is why energy is often expended to move certain molecules against these gradients.
Energy is used to move these compounds through proteins that are inserted across the membrane and act as transporters.
If the protein inserts 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 chemical energy source (eg, ATP) to move molecules across a membrane against its gradient.
One of the most important examples in biology to elucidate this mechanism of primary active transport is the sodium-potassium pump, which is found in animal cells and whose function is essential to these cells.
The sodium-potassium pump is a membrane protein that transports sodium and potassium from the cell to the cell. To perform this transport, the pump needs energy from ATP.
secondary active transport
Secondary active transport is one that uses the energy stored in the cell, this energy is different from ATP and from there differentiates between the two types of transport.
The energy used by secondary active transport comes from gradients generated by primary active transport, and they can be used to transport other molecules against their concentration gradients.
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 concentration difference of this ion on either side of the membrane.
Under these conditions, sodium ions would like to move along their concentration gradient and return to the interior of the cell via cell proteins.
This energy of sodium’s electrochemical gradient can be used to transport other substances against their gradients. What happens is a shared transport and carried out by transporter proteins called co-transporters (since they transport two elements together).
An example of an important co-transporter is the sodium and glucose exchange protein, which moves sodium cations to the side of its gradient and, in turn, uses this energy to penetrate glucose molecules against its gradient. This is the mechanism by which glucose enters living cells.
In the previous example, the co-transporter protein transports two elements in the same direction (to the cellular interior). When both elements move in the same direction, the protein that moves them is called a passport.
However, co-transporters can also mobilize compounds in opposite directions; In this case the carrier protein is called an antiporter, although they are also known as exchangers or countertransporters.
An example of an antiporter is the sodium and calcium exchanger, which is one of the most important cellular processes to remove calcium from cells. It uses the energy of the electrochemical sodium gradient to mobilize calcium outside the cell: one calcium cation is released 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 waste material from the cell into the extracellular fluid. Transport is carried out by vesicles in exocytosis.
The main difference between exocytosis and active transport is that the particle being transported in exocytosis is wrapped in a structure surrounded by a membrane (vesicle), which fuses with the cell membrane to release its contents outside .
Elements carried in active transport can be moved in either direction, inward or outward. In contrast, exocytosis only carries its contents to the outside.
Finally, active transport involves proteins as a means of transport, not membranous structures as in exocytosis.