It is the absorption of nutrient from the higher concentration to the lower concentration without the expenditure of energy. This process is slow and occurs by simple diffusion, osmosis and facilitated diffusion. During simple diffusion, the nutrients are required to be in higher concentration in the intestine lumen than its cells. The various amino acids and monosaccharides (glucose), some of the electrolytes are absorbed by simple diffusion but it does not result in complete absorption of nutrients. The water and some water-soluble salts are absorbed by osmosis. The absorption of fructose, mannose and some amino acids occurs by facilitated diffusion with the help of Na+ where carrier molecules are required to pass the nutrients across the plasma membrane of intestinal cells but this does not need any energy.
It occurs more rapidly than diffusion. In this, the nutrient molecules move against the concentration gradient i.e. though the concentration of nutrients is less in the intestinal lumen and much higher in the blood, the nutrients are still absorbed from the lumen to blood. The nutrients are absorbed by spending energy. The various nutrients like glucose, amino acids, galactose and Na+ etc. are absorbed actively from the intestine. The active transport is always unidirectional i.e. from the lumen to the epithelial cell to the blood capillary and never in the opposite direction. The molecules are transported with the help of specific membrane proteins that act as carriers. The energy required to pull the nutrients into the cell is derived from ATP molecules. (ATP breakdown to release ADP and energy). The cells show active transport by the sodium-potassium pump.
Difference Between Passive and Active Absorption:
Diffusion (Passive Absorption)
It is physical process.
It is a vital process.
It takes place from a region of higher concentration to the lower concentration i.e. along a concentration gradient.
It takes place from the region of lower concentration to the higher concentration i.e. against the concentration gradient.
It is a very slow process.
It is an active and rapid process.
It does not end in complete absorption of nutrients.
It ends in complete absorption of nutrients.
No expenditure of energy in this process.
The energy in the form of ATP is utilised in this process.
It does not involve any carrier molecule.
It involves the participation of a carrier molecule.