Protein – Lecithin Theory:

This theory was suggested by Bennet Clarke (1956). According to his theory –

• Carrier molecule composed of protein with the phospholipids called lecithin.
• There are different phospholipid groups present in the membrane that correspond with the number of competitive groups of cations and anions.
• The phosphate group in the phosphatide is regarded as an active center of carrier.
• The ions are liberated on the inner surface of the membrane by decomposition of the lecithin by the enzyme lecithinase.
• The regeneration of the carrier lecithin from phosphatide acid and choline takes place in the presence of enzymes viz. choline acetylase choline esterase and ATP. vi. ATP acts as the energy source for active transport.