Active transport is the movement of molecules from a lower concentration to a higher concentration. This process is vital for living organisms and is important for the following reasons:
(1) Absorption of most nutrients from the intestine, (2)Rapid and selective absorption of nutrients by cells, (3)Maintaining a membrane potential, and (4)Maintaining water and ionic balance between cells and extracellular fluids.
Basic Types of Active Transport
Primary Active Transport
Primary active transport uses a direct source of chemical energy – for example, ATP – in order to move the molecules across their gradient and across a membrane. Because the energy source of the transport process comes from ATP, it is considered primary active transport.
The sodium-potassium pump is a very important pump in animal cells, and it moves NA+ out of the cells and K+ into the cells. This sodium-potassium pump maintains the right concentrations of NA+ and K+ in living cells; however, it plays a very important role in generating the right amount of voltage across the cell membrane in animal cells as well.
These types of pumps are involved in maintaining and establishing membrane voltages and they are also known as electrogenic pumps. A plant’s primary electrogenic pump pumps hydrogen ions (H+) rather than sodium and potassium.
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The Cycle of the Sodium-Potassium Pump
A pump that transports sodium out of the cell and potassium into the cell, it consists of a cycle that repeats itself and involves conformational, or shape, changes. With each of the cycles, three sodium ions leave the cell and two potassium ions enter the cell. This is a multi-step process that can be explained in the following six steps.
- For the first step, the pump is open all the way to the inside of the cell because in this form, the pump likes to bind, i.e., has an affinity for, sodium ions and in fact, will take up three of them.
- The sodium ions will bond and trigger the pump to break down, or hydrolyze, the ATP. One of the phosphate groups from ATP then attaches itself to the pump and ADP is released as a by-product.
- The pump changes shape because of the phosphorylation process and it then re-orients itself in order to open up towards the extracellular space. During the conformation, the pump doesn’t like to bind anymore to sodium ions, having a low affinity for them, which results in the three sodium ions being released outside of the cell.
- The pump in its outward-facing form switches its allegiances and it now likes binding to or has a high affinity for, the potassium ions. In fact, it binds two of them, which starts the process of removing the phosphate group that is attached to the pump in step two above.
- The phosphate group is now gone and therefore the pump changes back to its original form. It now opens towards the cell’s interior.
- The pump is now facing inwards and has lost its interest in, or has a low affinity for, the potassium irons. Because of this, the two potassium ions get released into the cytoplasm. The pump is then back to where it was in step one above, which means that the cycle now repeats itself.
This cycle may seem complex but it merely consists of protein going back and forth between two different forms: first, an inward-facing form that has a low affinity for potassium and a high affinity for sodium; and second, an outward-facing form with a low affinity for sodium and a high affinity for potassium.
The protein can go back and forth from the different forms simply by adding or removing a phosphate group. This, in turn, is controlled by the ions that need to be transported being bound.
Generation of a Membrane Potential from the Sodium-Potassium Pump
By now, most people wonder how this pump establishes the voltage across the membrane. You can make an argument based on stoichiometry; in other words, every time that three ions of sodium move out, only two potassium ions move in, which results in a cell interior that is more on the negative side.
This type of charge ratio makes the interior of the cell a little more negative but in reality, it only accounts for a tiny part of the pump’s effect on membrane potential.
In fact, a sodium-potassium pump acts mainly to build up a high concentration of potassium ions on the inside of the cell. This makes the concentration gradient of the potassium very steep. The gradient is so steep that the potassium ions move out of the cell via channels even with the interior’s growing negative charge.
The process continues until the voltage found across the membrane is big enough to counterbalance the concentration gradient of the potassium. When this balance occurs, the membrane’s inside is negative as related to the outside.
As long as the K+ concentration in the cell remains high, the voltage is maintained. However, if K+ stops being imported, it will remain high.
Secondary Active Transport
Also called cotransport, secondary active transport uses an electrochemical gradient as the energy source in order for the molecules to move against their gradient. Generated by active transport, this type of transport does not directly require an energy source such as ATP.
It stores energy through its electrochemical gradients that are set up by the primary active transport process itself and this can be released as the ions start to back down their gradients. This type of active transport also uses the gradients’ stored energy in order to move other materials against their own gradients.
Here is another way to look at it. Let’s say that we have sodium ions at a high concentration located in the extracellular space because of the sodium-potassium pump’s hard work. If a certain route, for instance, a carrier protein or a channel, open up, these sodium ions move down their concentration gradient and then return to the cell’s interior.
With secondary active transport, the sodium ions move down their gradient and are coupled with the other substances’ uphill transport, facilitated by a shared carrier protein. This carrier protein at this point is called a cotransporter.
In the figure below, the carrier protein allows the sodium ions to move down their gradient; however, they also simultaneously bring a glucose molecule up their gradient and into the cell. The carrier protein utilizes the sodium gradient’s energy to drive the transport of glucose molecules.
In secondary active transport, two molecules are transported and move either in the same direction. For example, they both move into the cell or in totally opposite directions, such as if one of them goes into the cell and one of them goes out of the cell.
If they move in the same direction, the symporter is the protein that transports them. The protein is called the antiporter when they move in opposite directions.