Sometimes organisms require certain substances and therefore need to move dissolved substances from a region of low concentration to a region where they are at a higher concentration. This process is of course the opposite to the direction in which particles would normally move in diffusion. This is active transport. In active transport particles move against a concentration gradient and therefore require energy which must be supplied by the cell. Carrier proteins that are found in the cell membrane of cells use energy to transport molecules or ions across the membrane, against the concentration gradient. When organisms utilise active transport the energy for the process comes from respiration. Due to this the cells capable of active transport usually have more mitochondria, in which respiration takes place than other cells.
We can say thus say the definition of active transport is as follows:
The movement of particles from an area of low concentration to an area of high, against a concentration gradient.
Active Transport in Plant Cells
Plants require mineral salts such as nitrates for growth. The concentration of nitrates is higher on plant root cell than it is in the soil solution surrounding it. The plant cannot rely on diffusion as the nitrates would diffuse out of root cell into the soil. Hence the cells utilise energy to actively transport nitrates across the cell membrane into the root cell, against the concentration gradient.
Active Transport in Animals
The process of active transport takes place in humans during digestion of food in the ileum (small intestine). Once food has been absorbed by the villi after some time the concentration of food molecules inside the villi increases at this point no more food can diffuse in. As more food is still required simple sugars, amino acids, vitamins and minerals are actively transported into the villi form an area of low concentration to an area of high concentration, against the concentration gradient.