The fluid-mosaic model and movement across membranes: diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.
The cell-surface membrane controls what enters and leaves the cell. Its structure is described by the fluid-mosaic model.
The fluid-mosaic model
A phospholipid bilayer with hydrophilic heads facing the watery solutions and hydrophobic tails inside. It is "fluid" (phospholipids move) and a "mosaic" of proteins:
Channel proteins — water-filled pores for specific ions/molecules (facilitated diffusion).
Carrier proteins — change shape to move substances (facilitated diffusion and active transport).
Cholesterol — sits between phospholipids, regulating fluidity and stability.
Glycoproteins/glycolipids — cell recognition and receptors.
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