Written by tutor Diane G.
To accomplish all the tasks a cell must do to stay alive and serve its specific purpose, it must allow a wide variety of substances to pass through the plasma membrane. There are two basic ways that this happens. If substances cross the membrane with no energy being expended, the process is called passive transport. Passive in this case means it does not require outside energy to occur. The other process is referred to as active transport because it requires the cell to expend chemical energy, typically adenosine triphosphate, commonly called ATP.
Active transport occurs when a solute must pass through the cell membrane against its concentration gradient. A good analogy is that this is like pushing the water that fell over a waterfall, back up the river to the falls above. It takes a lot of energy for the cell to do this. We’ve learned in the study of osmosis that solutes move from the higher concentration to the lower concentration. But how do you get a solute to move to a higher concentration? This requires energy to “push” the substance across the membrane. This allows the cell to have substances within it that differ chemically rom the interior environment of the cell. For example if you put salt water on one side of a permeable membrane (a barrier that allows certain molecules to pass through it) and fresh water on the other side, the salt water will move into the fresh water (higher sodium concentration to lower sodium concentration) and the result will be that all water will have exactly the same sodium concentration.
When the concentration of a substance is different inside a cell than outside the cell there is a concentration gradient, meaning there is a varying amount (i.e. gradient) of a substance in and out of the cell.
A common example of active transport, or moving a substance against its gradient, is the maintaining of a balance of sodium and potassium inside and outside a cell. Obviously too much sodium inside the cell would cause cell death. So, the cell must actively regulate how much sodium is allowed in through the membrane. The cell pumps sodium ions (Na+) out of the cell which causes a lower level of sodium inside the cell than in the surrounding environment. To constantly remove sodium from inside, the cell requires energy to push it out. The cell simultaneously provides an internal environment with a higher level of potassium than the external environment; again anytime a substance operates against the laws of diffusion it takes energy to maintain that unnatural balance.
Let’s look at a specific example of active transport in a cell.
As mentioned earlier, it is the ATP molecule that supplies the energy for active transport. A carrier protein molecule embedded in the plasma membrane binds three sodium (Na+) molecules to it from within the cell. This binding causes ATP to attach a phosphate (called phosphorylation) which results in a change in the shape of the protein. The new shape causes the protein to become less attracted to sodium and it releases the three molecules to the outside of the cell. The shape now is more receptive to potassium ions (K+) outside the cell and two of them bind to the phosphorylated carrier protein. This action causes the protein to release the phosphate group and subsequently the potassium is released inside the cell. The movement of three positively charged molecules out of the cell and two positively charged molecules into the cell results in a net negative charge, relative to the outside environment.
The result of this alternating double shape of the carrier protein is that the sodium leaves the cell and goes against its concentration gradient to a higher concentration outside the cell, and the potassium flows into the cell, again from the lower concentration outside the cell to the higher concentration inside the cell. This process of using energy, provided by phosphorylation, to move sodium and potassium ions from lower concentrations to higher concentration (against the gradient) is active transport.
There are many examples of active transport in biology. The one described above is one of the most common. When carrier proteins are used to transport substances against their gradient they are referred to as pumps. Thus, the name of this process is a sodium potassium pump.