The Cell Membrane
The cell surface membrane (plasma membrane) is a vital structure that controls the passage of substances into and out of cells. At A-Level, you must understand its structure according to the fluid mosaic model, the roles of its individual components, and the factors that affect membrane permeability. This lesson covers AQA specification section 3.2.3.
Key Definition: The fluid mosaic model (proposed by Singer and Nicolson, 1972) describes the cell membrane as a fluid phospholipid bilayer (the molecules can move laterally within the layer) with a mosaic of proteins embedded in or attached to it.
The Phospholipid Bilayer
Structure of a Phospholipid
A phospholipid consists of:
- A glycerol molecule.
- Two fatty acid chains (the hydrophobic 'tails'). These can be saturated (no C=C double bonds, straight chains) or unsaturated (one or more C=C double bonds, causing kinks in the chains).
- A phosphate group (the hydrophilic 'head'), which may have additional groups attached (e.g., choline in phosphatidylcholine).
This gives the phospholipid an amphipathic nature — one end is hydrophilic (water-loving) and the other is hydrophobic (water-repelling).
Bilayer Arrangement
In an aqueous environment, phospholipids spontaneously arrange into a bilayer:
- The hydrophilic phosphate heads face outward towards the aqueous environment on both sides (extracellular fluid and cytoplasm).
- The hydrophobic fatty acid tails face inward, away from water, forming a hydrophobic core.
- This arrangement is thermodynamically stable and self-assembling — it forms spontaneously when phospholipids are placed in water.
Properties of the Bilayer
- Fluid: individual phospholipid molecules can move laterally within their layer (lateral diffusion), rotate on their axes, and flex their fatty acid tails. This gives the membrane flexibility. However, flip-flop (movement from one layer to the other) is rare without the help of enzymes called flippases.
- Selectively permeable: the hydrophobic core allows small, non-polar molecules (e.g., O₂, CO₂, steroid hormones) to pass through by diffusion. It is impermeable to large molecules, polar molecules, and ions, which require transport proteins.
Membrane Proteins
Proteins make up approximately 25–75% of membrane mass (depending on the cell type) and perform a wide range of functions.
Intrinsic (Integral) Proteins
- Span the entire membrane (transmembrane proteins) or are deeply embedded in one leaflet.
- Held in place by hydrophobic interactions between their non-polar amino acid R-groups and the hydrophobic tails of the phospholipids.
- Examples include:
- Channel proteins — provide hydrophilic pores that allow specific ions or polar molecules to pass through by facilitated diffusion. Many are gated (opened or closed by specific signals, e.g., voltage-gated sodium channels in neurones).
- Carrier proteins — bind to specific molecules and undergo a conformational change to transport them across the membrane. Involved in both facilitated diffusion and active transport.
- Receptor proteins — have a specific binding site on the extracellular surface for signalling molecules (ligands) such as hormones or neurotransmitters. Binding triggers a response inside the cell (signal transduction).
Extrinsic (Peripheral) Proteins
- Found on the surface of the membrane (either inner or outer face).
- Not embedded in the hydrophobic core; attached by ionic bonds or hydrogen bonds to intrinsic proteins or to the phospholipid heads.
- Functions include acting as enzymes, anchoring proteins (connecting the membrane to the cytoskeleton), and components of cell signalling pathways.
Glycoproteins and Glycolipids
- Glycoproteins — proteins with short carbohydrate (oligosaccharide) chains attached, found on the extracellular surface of the membrane.
- Glycolipids — lipids with short carbohydrate chains attached, also on the extracellular surface.
Together, glycoproteins and glycolipids form the glycocalyx (cell coat), which:
- Acts as cell recognition markers (antigens) — allows cells to identify self from non-self (essential for the immune system).
- Provides cell-cell adhesion — helps cells stick together to form tissues.
- Plays a role in cell signalling — acts as receptor sites for hormones and other chemical signals.
- Provides protection — a physical barrier against mechanical damage and chemical attack.
Exam Tip: The ABO blood group system is an example of glycolipid/glycoprotein variation. Different glycolipid antigens (A, B, or neither) on the surface of red blood cells determine blood type.
Cholesterol