What is permeability of cell membrane?

Permeability of the cell membrane describes how easily different substances can pass through the membrane's phospholipid bilayer and its protein channels. The plasma membrane is “selectively permeable”, meaning it allows some molecules (like water, oxygen, small lipids) to cross freely, while others (ions, large polar molecules) need transport proteins or cannot cross at all.

Factors affecting permeability

• Lipid composition: More unsaturated fatty acids keep the membrane fluid, easing passage of small nonpolar molecules.
• Cholesterol: Moderates fluidity, tightening gaps at high temperature and preventing stiffness at low temperature.
• Temperature: Heat increases membrane fluidity and thus permeability.
• Presence of transport proteins: Aquaporins speed water flow; ion channels let Na⁺, K⁺, Ca²⁺ move selectively.

Types of transport

Passive diffusion moves substances down their concentration gradient without energy. Facilitated diffusion uses carrier proteins. Active transport pumps substances against the gradient using ATP, like the sodium-potassium pump.

Biological significance

Permeability maintains homeostasis. Cells must import nutrients, export waste, and keep ion balances. Abnormal permeability can lead to diseases such as cystic fibrosis (faulty chloride channel) or edema (capillary leakage).