The transport of molecules across the plasma membrane can occur by the two process- active transport and passive transport. No amount of energy is required during the active transport.
Passive transport includes the diffusion and osmosis. Both the diffusion and osmosis do not require energy and the molecules move down from the region of higher concentration towards the region of lower concentration. Small molecules, ions and proteins can go through the passive transport.
As a result of their surface area to volume ratio, smaller living cells are more active metabolically as transport through the cell organelles occur faster
