Active transport is a physical process in which molecules and ions move from a region of lower concentration to a region of higher concentration. This means that they move against a concentration gradient unlike in diffusion which is an equally physical process, where molecules or ions move from a region of higher concentration to a region of lower concentration. A vivid example of active transport can be seen in seaweeds which take up iodine in such a vigorous manner that it is more than two million times as concentrated inside the cells as in the surrounding water environment. Therefore any movement against a concentration gradient is called active transport.
Active transport can also occur in biological systems that is actively producing energy by respiration. Physical conditions such as oxygen concentration and temperature influence the rate of active transport and there are further evidences from biochemical studies to show that active transport is linked with energy production. Anything that inhibits the formation or hydrolysis of ATP(Adenosine triphosphate), or inhibits it from being used also stops active transport from taking place or proceeding. Cyanide a a good example of a compound that prevents active transport because it prevents ATP from being synthesized. The involvement of active transport in energy production can also be explained by the fact that certain cells that are known to undertake active transport on a very large scale have an unusually large amount of mitochondria, which is the energy storehouse of the cell.
The explanation of active transport can be found in several hypothesis but it is generally assumed by biologists that there are certain carriers in the plasma membrane which can attach themselves to the molecules or ions at the outer surface of the membrane and then convey them to the inner surface where they are deposited in the cytoplasm. The carrier then returns to the outer surface and repeats this process which definitely involves the resupply of energy from ATP. There is another school of thought that suggest that the carrier molecule is a protein which transverses the entire membrane. The explanation goes further to explain that the substance to be transported becomes attached to the carrier on one side of the membrane such that the configuration of the carrier molecule then changes in such a way that the substance is moved through it to the other side of the membrane. This hypothesis is consistent with the view by biologists on how proteins are arranged in the plasma membrane.
Active transport gives further explanation that cells are selectively permeable, taking in or expelling certain ions while excluding some others. Movement of ions across the cell membrane is at different rates because it can be seen that most cells absorb potassium ion more readily compared to other cations. Interestingly, research on the uptake of ions by the roots of plants have shown that certain ions which show identical charges and similar properties compete with each other for uptake by the root and the one which is taken up is the ion that is most common or abundant in the plant surroundings. What this means is the ions are transported by one and the same carrier and that this carrier cannot distinguish between the ions. It must be noted that once a substance has been actively transported into the cell, it cannot leak out again down the concentration gradient and so the carrier acts as a one way valve.
Active transport controls what goes in and out of the cell. Many animal and plant cells take in potassium ions but expel sodium ions. These two ions are being moved in opposite directions within one and the same cell membrane. In investigating the passage of some radioactive ions across the cell membrane of certain algal cells, it was discovered that the concentration of both potassium and sodium ions on the two sides of the membrane are interdependent, suggesting that the same carrier is used for transporting both ions.