Membrane structure

Phospholipid structure:

• Phosphate head — polar / hydrophilic (water-loving).
• Two fatty acid tails — non-polar / hydrophobic (water-hating).

In aqueous environments, phospholipids spontaneously self-assemble into a bilayer: tails point inward (away from water), heads point outward (toward water). The bilayer is the foundation of every biological membrane.

Hydrophobic core. The interior of the bilayer is non-polar — small non-polar molecules (oxygen, CO₂) cross easily, while polar molecules (water itself crosses slowly; ions, glucose) need protein channels.

Fluid mosaic model (Singer & Nicolson, 1972):

• Fluid: phospholipids move sideways within their layer.
• Mosaic: various proteins float within the bilayer like tiles.

Integral (intrinsic) membrane proteins — embedded in or spanning the bilayer. Hydrophobic regions interact with the tails.

Peripheral (extrinsic) membrane proteins — attached to the surface; hydrophilic; easily removed.

Functions of membrane proteins (mnemonic: TREASR):

• Transport — channels (e.g. potassium channel) and pumps (e.g. Na⁺/K⁺ ATPase).
• Receptors — bind hormones / neurotransmitters; initiate signalling.
• Enzymes — catalyse reactions at the membrane surface.
• Adhesion — link cells together (e.g. cadherins).
• Structural anchoring — to the cytoskeleton (e.g. integrins).
• Recognition — glycoproteins identify "self" cells (e.g. MHC, ABO).

Cholesterol (animal cells only):

• Sits between phospholipids in the membrane.
• At high temperature: limits movement, reduces fluidity (prevents membrane becoming too fluid).
• At low temperature: prevents close packing of tails, prevents membrane from solidifying (maintains fluidity).
• Net effect: a 'buffer' that keeps fluidity steady across the body's normal range.

Glycoproteins and glycolipids. Carbohydrate chains attached to outward-facing proteins/lipids form the glycocalyx — used for cell-cell recognition. Antigens of the ABO blood group are glycoproteins/glycolipids.