1. The significant function of plasma membrane is transport of molecules across it. Plasma membrane is selectively permeable.
2. Passive transport:
a. Many molecules move across the membrane without spending energy.
b. Some molecules move by simple diffusion along the concentration gradient i.e. from higher to lower concentration.
c. Neutral molecules may move across the membrane by the process of simple diffusion.
d. Water may also move by osmosis.
3. Active transport:
a. Few ions or molecules are transported against concentration gradient i.e. from lower to higher concentration.
b. This requires energy, hence ATP is utilized. As such a transport is an energy dependent process in which ATP is utilized, it is called Active transport e.g. Na /K pump.
c. Polar molecules cannot pass through nonpolar lipid bilayer. Therefore, they require carrier proteins to facilitate their transport across the membrane.
