Plasma membranes are selectively permeable (or semi-permeable), meaning that only certain molecules can pass through them.
Water, oxygen, and carbon dioxide can easily travel through the membrane. Generally, ions (e.g. sodium, potassium) and polar molecules cannot pass through the membrane; they must go through specific channels or pores in the membrane instead of freely diffusing through. This way, the membrane can control the rate at which certain molecules can enter and exit the cell.
Cell membranes separate the internal and external environments of a cell or organelle, preventing the intrusion of harmful substances, the dispersion of macromolecules, and the dilution of enzymes and substrates. This selective permeability maintains homeostasis. Homeostatic mechanisms attributable to cell membranes maintain optimal intracellular concentrations of ions, water, enzymes, and substrates.
Three mechanisms allow selected molecules to cross membranes.
1. Passive diffusion. Some substances (e.g., water and lipids) can cross the membrane in either direction following a concentration gradient, without the cell expending energy.
2. Facilitated diffusion. Some molecules (e.g., glucose) are helped across the membrane by a membrane component. This facilitated diffusion is often unidirectional, but it follows a concentration gradient and requires no energy.
3. Active transport. Some molecules enter or exit a cell against a gradient. This requires energy, usually asadenosine triphosphate (ATP). One active transport mechanism is the sodium pump (Na+/K+-ATPase), which expels sodium ions from a cell even when the sodium concentration is higher outside than inside