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

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