Cell Membrane — Fluid Mosaic Model
The fluid mosaic model describes the plasma membrane as a dynamic, two-dimensional fluid in which a mosaic of proteins, lipids, and carbohydrates can move laterally. The 13 structures shown in the diagram are central to AP Biology Unit 2 (Cell Structure and Function).
Phospholipid Bilayer
Two back-to-back sheets of phospholipids with hydrophilic heads facing the aqueous environments on each side and hydrophobic tails forming the waterproof interior core. This arrangement is the basis of selective permeability.
Hydrophilic Head
The polar, charged head group of each phospholipid — a glycerol-phosphate unit that is attracted to water and faces outward on both membrane surfaces.
Hydrophobic Tail
The two nonpolar fatty-acid chains that extend inward. Saturation affects their packing: saturated tails pack tightly (less fluid); unsaturated tails with kinks stay more loosely packed (more fluid).
Integral Protein
Permanently embedded in the bilayer, spanning one or both leaflets. The portions within the hydrophobic core have nonpolar side chains; the portions exposed to aqueous environments have polar or charged side chains.
Peripheral Protein
Loosely associated with the membrane surface — attached to head groups or integral proteins but not embedded in the hydrophobic core. Removable without detergents. Functions include signal relay and cytoskeleton anchoring.
Channel Protein
A transmembrane protein with a hydrophilic pore allowing specific ions or small polar molecules to cross by facilitated diffusion — no ATP required, movement is down the concentration gradient. Aquaporins are a key example.
Carrier Protein
A transmembrane protein that binds a solute, changes conformation, and releases it on the other side. Works by facilitated diffusion (passive) or active transport (requires ATP). Unlike channels, it does not form an open pore.
Receptor Protein
A transmembrane protein with an extracellular binding domain that recognizes a specific ligand (hormone, neurotransmitter, etc.). Ligand binding triggers an intracellular signal transduction cascade — the signal does not enter the cell.
Cholesterol
A steroid lipid wedged between phospholipids in animal cell membranes. Acts as a fluidity buffer — reducing excess fluidity at high temperatures and preventing solidification at low temperatures.
Glycoprotein
An integral protein with branching oligosaccharide chains on the extracellular face. Part of the glycocalyx; critical for cell-cell recognition and immune response. ABO blood group antigens are glycoproteins.
Glycolipid
A phospholipid in the outer leaflet with a short sugar chain on its extracellular head. Contributes to the glycocalyx and cell surface identity.
Extracellular Space
The aqueous environment outside the cell, where signaling molecules arrive and where extracellular concentration gradients drive diffusion and osmosis across the membrane.
Intracellular Space
The cytoplasm side of the membrane, where signal transduction cascades are initiated, where active transport pumps hydrolyze ATP, and where peripheral proteins anchor the cytoskeleton to the bilayer.