The Plasma Membrane and the Fluid Mosaic Model

A phospholipid is glycerol joined to two fatty acids and a phosphate group (often with a small extra group such as choline).

• It is amphipathic: the phosphate head is hydrophilic (polar) and the fatty acid tails are hydrophobic (non-polar).
• In water, phospholipids form a bilayer about 7 to 10 nm thick: heads outward toward the watery cytoplasm and tissue fluid, tails inward forming an oily core.
• The core lets small non-polar molecules through ($\mathrm{O_2}$, $\mathrm{CO_2}$, steroids) and blocks ions and large polar molecules such as glucose. Water crosses slowly on its own, and fast through protein channels.
• The main job of the plasma membrane is to control exchange between the inside and the outside of the cell.

The fluid mosaic model (Singer and Nicolson, 1972):

• Fluid: phospholipids and many proteins move sideways within their layer. Flipping from one layer to the other is rare.
• Mosaic: proteins are scattered through the bilayer, not spread as a layer over it.
• The main components are phospholipids and proteins, roughly half and half by mass in a typical plasma membrane; the share of protein varies with the membrane's job.
• Integral (intrinsic) proteins sit in the bilayer; those crossing it completely are transmembrane proteins. Peripheral (extrinsic) proteins are attached to one surface.
• Carbohydrate chains are joined to proteins (glycoproteins) and lipids (glycolipids) on the outer surface only. Together they form the glycocalyx.

Fluidity is how freely the lipids and proteins of a membrane can move.

• Cold-adapted organisms (cold-water fish, plants that overwinter) raise the share of unsaturated fatty acids, whose C=C double bonds put kinks in the tails.
• Cholesterol is a fluidity buffer. At high temperature its rigid rings hold neighbouring tails still (less fluid); at low temperature it keeps tails apart so they cannot set solid (more fluid). It also makes the membrane less permeable to small water-soluble molecules.
• Cholesterol is also the raw material for steroid hormones, vitamin D and bile salts. Plant membranes use similar sterols; most bacteria have none.
• Membranes are damaged by organic solvents such as ethanol (they dissolve the lipids) and by heat or strong acid (they denature the membrane proteins). A damaged membrane leaks: beetroot pigment, for example, escapes into the water around the tissue.

In cell signalling, a signal molecule (a ligand) binds a specific receptor protein on or in the target cell.

• Water-soluble signals (insulin, adrenaline) cannot cross the bilayer. They bind receptors in the plasma membrane, which pass the message on inside, often through a second messenger such as cyclic AMP.
• Lipid-soluble signals (steroid hormones, thyroid hormone) cross the membrane and bind receptors in the cytoplasm or nucleus, changing which genes are expressed.
• Range: endocrine (hormones carried in the blood), paracrine (to nearby cells), autocrine (to the cell itself), and direct contact through junctions or surface molecules.
• The glycocalyx lets cells recognise each other, which matters in tissue formation and in the immune response.