Cells keep various molecules at specific concentrations. For example, Potassium ions (K+) are kept high inside cells but Sodium ions (Na+) are kept low. Whereas outside of cells, concentration of K+ is low while that of Na+ is high. There is thus a concentration gradient created across the cell membrane. How does the cell maintain this concentration difference? Through active transport, that’s how.
Active transport moves substances against their concentration gradients and thus requires the expenditure of metabolic energy, usually ATP. The transporters are called pumps so there is a Na+/K+ pump for example. This pump transports Na+ out of the cell and transports K+ back into the cell. This way, the concentration difference across the cell membrane is maintained.
What is the importance of active transport in biological systems? Well, for one, it is responsible for enabling us to absorb more food from our intestines. If there is no active transport, then most of the food that we eat will be wasted. If absorption only depends on diffusion, then once the concentration of food in and out of the intestinal cells becomes the same, then absorption will stop. But as we have experienced, we can have 2nd helpings or even 3rds of some of our favorite food. So we have active transport to thank (blame?)for that.
Second, active transport is responsible for selective reabsorption of substances in the kidneys. There is a Na pump in kidney cells that pumps back Na into the intercellular space. Through osmosis, water then naturally follows the solutes. This is how we are able to reabsorb some water.
Third, cells have proton pumps that pump hydrogen ions during energy generation. These pumps are also used to maintain intracellular pH.
Active transport is also used by plants in absorbing minerals from the soil. While animal cells use Calcium pumps to maintain intracellular Calcium (Ca++) levels. After every muscle contraction for example, Ca ions (Ca++) are pumped back into storage so that muscles can relax. When a person dies, the cells lack ATP and active transport of Ca++ cannot occur. Thus, the muscle cells remain contracted and the dead person exhibits rigor mortis.