d
(d) : According to the fluid mosaic model of cell membrane structure proposed by Singer and Nicolson \((1972)\) plasma membrane contains about \(50-60\%\) proteins and \(50-40\%\) lipids. Lipids form a bilayer with hydrophilic heads pointing outwards. Cell membrane allows transport of some molecules bypassive transport \(e.g.\), water, neutral solutes while some are transported actively \(e.g.\), \(Na^+/K^+\) pump.