The human body is made up of cells, intercellular matrix and bodily fluids. The cell is the body’s functional unit and is the only part that is truly alive. One human body contains trillions of cells. Different types of human tissue have different types of cells (there are more than 200 types), but the cell is always basically the same, and is composed of an outer membrane, the cytoplasm or inner cellular living materialand a nucleus. Nutrients, proteins and minerals are in constant motion throughout the body and are forever traveling back and forth from the outer, extracellular, fluid to the inner cell, and vice versa. Biochemical processes that enable the body to function are occurring at every moment as these particles go into and back out of the cell.
There are a few different mechanisms by which these nutrients and particles travel in and out of cells. Some of the mechanisms are passive. These include simple diffusion, facilitated diffusion and osmosis. Diffusion is the passive movement of particles (atoms, ions, or molecules) from a region of high concentration to a region of lower concentration. An example of this is gas exchange for respiration, in which oxygen from the blood goes to tissue cells, and carbon dioxide comes from the tissue cells and enters the blood. Facilitated diffusion is the movement of molecules down a concentration gradient, passing through the membrane with the help of a specific carrier protein. Osmosis is a type of diffusion in which water travels through a semi-permeable membrane (one that can only allow some types of particles through it) from a more dilute solution to a more concentrated solution (down the water potential gradient). An example of osmosis is absorption of water by the stomach and colon.
However, some of the mechanisms by which substances pass in and out of cells require the input of energy, and are therefore called “active.” Active transport is the pumping of molecules across the cell membrane, against the concentration gradient. In other words, suppose there were more sodium ions within the cell than outside of it. In active transport, energy will be used to pump still more sodium ions through the membrane and into the cell, making the concentration of sodium ions within the cell even greater. It is pumping against the concentration gradient.
There are two types of active transport, which are primary active transport and secondary active transport. In primary active transport, chemical energy is used to power the process in the form of ATP, or adenosine triphosphate. ATP is the body’s energy currency, which is manufactured in the cell’s mitochondria, an organelle within the cell that uses food to produce energy. Secondary active transport works by using an electrochemical gradient. It does not use ATP directly, but takes advantage of a previously existing concentration gradient.
One might ask the question, why is active transport necessary for the body to function, and where does it happen? Some of the body’s cells, in order to function properly, require high concentrations of certain nutrients. These concentrations do not happen with simple diffusion, and this is where active transport comes into play. For example, the epithelial cells that line the intestines need to bring glucose that has been made available from digestion into the body, and must prevent the reverse flow of glucose from the body back to the intestines. The glucose is needed in the bloodstream, not in the gut. If the intestinal cells only functioned by diffusion, then after eating, glucose would diffuse from the high concentration in the gut to the low concentration in the bloodstream. But later, once the concentration in the bloodstream became high enough, the glucose would begin to diffuse back into the gut. This would dangerously lower the level of glucose in the blood. Active transport, however, uses the energy input of ATP to prevent this from happening and to keep the blood glucose levels regulated.
Active transport also plays an important role in many of the body’s other systems, perhaps most notably the nervous system. When a nerve cell, or neuron, is at rest, a charge difference is maintained between the inside and outside of the cell. This charge difference is produced and maintained by active transport using sodium-potassium pumps. The difference in charge between the interior and exterior of the neuron is called its resting membrane potential. As nerve impulses disturb the cell membrane, another change in relative voltage occurs and an action potential is created, causing the impulse to travel along the cell.
Active transport also occurs in muscle cells, pumping calcium ions to create muscle contraction. Active transport is also important in the kidneys, where it is used to exchange substances between the tubules and the capillaries surrounding the nephron, or kidney cell.
The order and complexity of the human body and all of its physiological processes is truly one of the greatest wonders of nature. Active transport is just one small part of that enormous interaction. It is not, however, unique to the human body. It also occurs in other living organisms. In plants, mineral uptake through plant roots is a form of active transport. In fact, the cells of all living organisms have features in common, and active transport is just one way that these cells can ensure that the organism receives adequate nutrition and maintains its homeostatic balance.