Movement through the cell membrane by active transport
When a solute moves in accordance with osmotic, concentration or electric gradients, it is said to be moving ' downhill' because it is a passive process and no energy is required. This movement along the concentration gradient may be compared to a car rolling down a hill with its engine cut off. On the other hand solutes may move in the opposite direction, against the gradients of osmotic pressure, concentration or electrical attraction. Thus the solute is said to be moving 'uphill'. It may be compared to a car going uphill with running engine at the expense of the fuel energy. This movement of ions or molecules against a particular gradient is known as the active transport. It is described as 'active' because the cell must expend energy to accomplish the transport unlike the passive force of diffusion. Adenosine Triphosphate(ATP) which is produced by oxidative phosphorylation in mitochondria, is generally used as the source of energy.
Examples of active transport
- Absorption of Glucose and Amino Acids: The cells on the inner surface of the rat intestine can transport glucose and amino acids actively from a lower concentration in the intestine to a higher concentration in the blood.
- Propagation of Nerve Impulse: The transmission of nerve impulse depends upon the active transport of sodium ions out of the nerve cell and potassium ions into the cell. This mechanism is often described as sodium pump although it should be appropriately called the sodium potassium exchange pump because it transports sodium out of the cell and at the same time also moves potassium back into the cell against concentration gradient by expenditure of energy derived from cellular metabolism.
- Urine formation: This mechanism is also involved in the transport of solutes between kidney tubules and the blood during the formation of urine.
- Osmoregulation( salt secretion) in fish.