• Membranes are?vital?structures found in all cells
• The?cell surface membrane?creates an enclosed space separating the internal cell environment from the external environment
• Intracellular?membranes?(internal membranes) form?compartments?within the cell, such as?organelles?(including the nucleus, mitochondria and RER) and?vacuoles
• Membranes not only?separate?different areas but also?control the exchange of materials?passing through them; they are?partially permeable
• Membranes form partially permeable?barriers?between the cell and its environment, between cytoplasm and organelles and also within organelles
• Substances can cross membranes by?diffusion, facilitated diffusion,?osmosis?and?active transport
• Membranes play a role in?cell signalling?by acting as an?interface?for?communication between cells

Membranes formed from phospholipid bilayers help to compartmentalise different regions within the cell, as well as forming the cell surface membrane

• The?fluid mosaic model?of membranes was first outlined in 1972 and it explains how biological molecules are arranged to form cell membranes
• The fluid mosaic model also helps to explain:
  • Passive and active movement between cells and their surroundings
  • Cell-to-cell interactions
  • Cell signalling

   

• The fluid mosaic model describes cell membranes as?‘fluid’?because:
  • The?phospholipids?and?proteins?can?move?around?via diffusion
  • The phospholipids mainly move sideways, within their own layers
  • The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although?some may be fixed?in position

   

• The fluid mosaic model describes cell membranes as?‘mosaics’?because:
  • The?scattered pattern?produced by the?proteins?within the phospholipid bilayer looks somewhat like a mosaic when viewed from above

   

• The?fluid mosaic model?of membranes includes four main components:
  • Phospholipids
  • Cholesterol
  • Glycoproteins and glycolipids
  • Transport proteins

   

Phospholipids

• Phospholipids form the basic structure of the membrane (the phospholipid bilayer)
• The tails form a hydrophobic core comprising the innermost part of both the outer and inner layer of the membrane
• Phospholipids bilayers 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
• 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

   

A phospholipid bilayer is composed of two layers of phospholipids; their hydrophobic tails facing inwards and hydrophilic heads outwards

Cholesterol

• Cholesterol?increases the fluidity of the membrane, stopping it from becoming too rigid at?low?temperatures (allowing cells to survive at lower temperatures)
• This occurs because cholesterol?stops the phospholipid tails packing too closely together
• 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
  • The impermeability of the membrane to ions is also affected by cholesterol

   

• Cholesterol?increases the mechanical strength and stability of membranes?(without it membranes would break down and cells burst)

Glycolipids and glycoproteins

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

Transport 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
    • Carrier proteins?change shape?to transport a substance across the membrane

     

   

• Each transport protein is?specific to a particular ion or molecule
• Transport proteins allow the cell to?control?which substances enter or leave

The main components of cell membranes. The distribution of the proteins within the membrane gives a mosaic appearance and the structure of the proteins determines their position in the membrane.