CBSE Class 9 Answered

Movement of substances through the plasma membrane happens in essentially two ways: 1. passively and 2. activelyIn Passive processes substances penetrate the membrane without any energy input from the cell.In active processes the cell provides the metabolic energy (ATP) that drives the transport process to move the substance across the membrane.

Passive Processes: Diffusion     Diffusion is the tendency of molecules or ions to scatter evenly throughout the environment. Molecules  are said to diffuse  along or down their concentration gradient.Because the driving force is the kinetic energy of the molecules themselves, the speed of diffusion is influenced by the size of the molecules and the temperature Simple diffusion of water is known as osmosis.

Active Processes Whenever a substance is unable to cross the membrane  because it may be to large to pass through the pores, unable to dissolve in the bilipid membrane core, or required to move against rather than with a concentration gradient.