The Structure of Cell Membranes (AQA A Level Biology): Revision Note

The fluid mosaic model of membranes

• The basic structure of all cell membranes is the same

• This includes the cell surface membrane and the membranes surrounding eukaryotic organelles (e.g. nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, chloroplasts, lysosomes)

  • These membranes:

    • Are composed of a phospholipid bilayer

    • Contain intrinsic and extrinsic proteins

    • May include cholesterol (in animal cells), glycoproteins, and glycolipids

• The fluid mosaic model describes how the molecules are arranged within cell membranes

  • The term "fluid" refers to the lateral movement of phospholipids and some proteins, giving the membrane flexibility

  • The term "mosaic" reflects the scattered arrangement of proteins within the bilayer

• This model explains how membranes are:

  • Partially permeable

  • Sites for cell signalling, recognition, and communication

  • Responsible for controlling the exchange of substances across compartments

Structural components of cell membranes

Phospholipid bilayer

• Cell membranes are primarily made of a phospholipid bilayer with two layers of phospholipid molecules

• Each phospholipid has two regions:

  • A phosphate head that is polar (hydrophilic) and therefore soluble in water

  • Two fatty acid tails that are non-polar (hydrophobic) and insoluble in water

• The bilayer arranges so that the hydrophobic tails face inward, forming a hydrophobic core, while the hydrophilic heads face outward towards aqueous environments

• This structure forms a selectively permeable barrier, preventing most polar or water-soluble substances (e.g. ions, glucose, amino acids) from freely crossing the membrane

• Phospholipids can be chemically modified to act as signalling molecules by:

  • Moving within the bilayer to activate other molecules (eg. enzymes)

  • Being hydrolysed which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm

Cholesterol

• Cholesterol regulates the fluidity of the membrane

• Cholesterol molecules sit in between the phospholipids, preventing them from packing too closely together when temperatures are low; this prevents membranes from freezing and fracturing.

• Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid

  • Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together

• It also makes the membrane less permeable to small charged particles (like ions) and strengthens the membrane so that the cell doesn't burst

Glycolipids & glycoproteins

• Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface (the periphery/extrinsically), which enables them to act as receptor molecules

• This allows glycolipids and glycoproteins to bind with certain substances at the cell’s surface

• There are three main receptor types:

  • Signalling receptors for hormones and neurotransmitters

  • Receptors involved in endocytosis

  • Receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell

• Some act as cell markers or antigens, for cell-to-cell recognition (eg. the ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains)

Proteins

• Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane. There are two types:

  • channel (pore) proteins

  • carrier proteins

• Each transport protein is specific to a particular ion or molecule

• Transport proteins allow the cell to control which substances enter or leave