Active transport refers to the process through which substances are moved across the cell membrane and against a concentration gradient.
Diffusion and osmosis alone do not account for movement of substances in and out of the cells. In particular, there are some mineral salts that occur at low concentrations in the soil water than in the cell sap. Some of these mineral salts cannot be absorbed by the plants through diffusion. A mechanism that would move them into the cells against the concentration gradient will be useful.
Active transport requires energy. This is unlike diffusion and osmosis that only depend on concentration gradient for them to take place.
It is postulated that there are protein carrier molecules on the cell membrane that aid in moving these substances across the membrane. These carrier molecules combine with the substances being transported across the membrane and then move them from one side of the membrane to the other side.
Cellular intake of solutes is largely through active transport.
Role of active transport in living organisms
Active transport is important in living things in that:
•It helps in re-absorption of sugars and some salts by the kidney to the bloodstream.
•It helps in absorption of some mineral salts from the soil by roots.
•Absorption of digested food from alimentary canal of animals into the bloodstream.
•It leads to accumulation of substances into the body to offset osmotic imbalance in arid and saline environments.
•It plays a role in excretion of waste products from body cells.
Factors affecting the rate of Active Transport
Most factors that affect active transport are those factors that would affect the energy production process in living cells.
a) Oxygen concentration
Oxygen is required in respiration process that yields energy for active transport. Under low oxygen concentration, the rate of respiration will be low hence there will be production of little energy leading to low rate of active transport. Increase in oxygen concentration translates into a higher energy production leading to high rate of active transport.
b) Change in pH
Change in pH affects the respiratory process which is enzyme controlled. Respiratory enzymes require optimum pH for their efficient activity. Extreme pH conditions will lower the rate of active transport since the enzymes controlling respiration will be denatured.
c) Glucose concentration
Glucose is the chief respiratory substrate. At low glucose concentration, there will be less production of energy leading to decreased rate of active transport. Rate of active transport increases with increase in glucose concentration due to increase in the rate of energy production.
Temperature affects the enzyme controlled respiration process. At low temperatures, the enzymes are inactive hence the rate of respiration will be low resulting into low rate of active transport since there will be less production of energy. An increase in temperature increases the rate of respiration since the enzymes become more activated. At temperatures beyond 40 degrees celcius, the enzymes become denatured, respiration stops and so does active transport.
e) Presence of metabolic inhibitors e.g. cyanide.
These are substances which act as metabolic poisons. They stop the rate of respiration leading to production of no energy. Active transport is, thus, stopped.