The Cell Surface Membrane (OCR A Level Biology)

• 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.