The Cell Surface Membrane (AQA A Level Biology)

Cell Surface Membranes

Phospholipids

• Form the basic structure of the membrane (phospholipid bilayer)

• The tails form a hydrophobic core comprising the innermost part of both the outer and inner layer of the membrane

• Act as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane)

• This ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in

• 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

• Cholesterol also contributes to the impermeabilty of the membrane to ions and increases mechanical strength and stability of membranes; without it membranes would break down and cells 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