OCR AS Biology

Revision Notes

2.5.1 The Cell Surface Membrane

The Fluid Mosaic Model of Membranes

  • 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, and intracellular membranes form compartments within the cell such as the nucleus, mitochondria and RER
  • Membranes do not only separate different areas but also control the exchange of material across them, as well as acting as an interface for communication
    • Membranes are partially permeable
    • Substances can cross membranes by diffusion, osmosis and active transport
  • Cellular membranes are formed from a bilayer of phospholipids which is roughly 7nm wide and therefore just visible under an electron microscope at very high magnifications
  • The fluid mosaic model of the membrane 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

Phospholipids

  • Phospholipids structurally contain two distinct regions: a polar head and two nonpolar tails
  • The phosphate head of a phospholipid is polar (hydrophilic) and therefore soluble in water
  • The fatty acid tail of a phospholipid is nonpolar (hydrophobic) and therefore insoluble in water
  • If phospholipids are spread over the surface of water they form a single layer with the hydrophilic phosphate heads in the water and the hydrophobic fatty acid tails sticking up away from the water
    • This is called a phospholipid monolayer

_Phospholipid monolayer, downloadable AS & A Level Biology revision notes

A phospholipid monolayer

  • If phospholipids are mixed/shaken with water they form spheres with the hydrophilic phosphate heads facing out towards the water and the hydrophobic fatty acid tails facing in towards each other
    • This is called a micelle

Micelle, downloadable AS & A Level Biology revision notes

A micelle

  • Alternatively, two-layered structures may form in sheets
  • These are called phospholipid bilayers – this is the basic structure of the cell membrane

Phospholipid bilayer, downloadable AS & A Level Biology revision notes

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

  • Phospholipid bilayers can form compartments – the bilayer forming the cell surface membrane establishing the boundary of each cell
  • Internally, membrane-bound compartments formed from phospholipid bilayers provide the basic structure of organelles, allowing for specialisation of process within the cell
  • An example of a membrane-bound organelle is the lysosome (found in animal cells), each containing many hydrolytic enzymes that can break down many different kinds of biomolecule
    These enzymes need to be kept compartmentalised otherwise they would breakdown most of the cellular components

Membranes in the cell, downloadable AS & A Level Biology revision notes

Membranes formed from phospholipid bilayers help to compartmentalise different regions of the cell

Structure of membranes

  • The phospholipid bilayers that make up cell membranes also contain proteins
    • The proteins can either be intrinsic (or integral) or extrinsic (peripheral)
    • Intrinsic proteins are embedded in the membrane with their arrangement determined by their hydrophilic and hydrophobic regions
    • Extrinsic proteins are found on the outer or inner surface of the membrane
  • 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

1. and 2. Fluid mosaic model, downloadable AS & A Level Biology revision notes

The distribution of the proteins within the membrane gives a mosaic appearance and the structure of proteins determines their position in the membrane

Exam Tip

You must know how to draw and label the fluid mosaic model, as well as ensure that you can describe why the membrane is called the fluid mosaic model.
Fluid mosaic model exam example, downloadable AS & A Level Biology revision notes

An example of the diagram you could draw

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

  • 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
    • They also contribute to the impermeabilty of the membrane to ions
  • 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

Exam Tip

Membranes become less fluid when there is:

  • An increased proportion of saturated fatty acid chains as the chains pack together tightly and therefore there is a high number of intermolecular forces between the chains
  • A lower temperature as the molecules have less energy and therefore are not moving as freely which causes the structure to be more closely packed

Membranes become more fluid when there is:

  • An increased proportion of unsaturated fatty acid chains as these chains are bent, which means the chains are less tightly packed together and there are less intermolecular forces
  • At higher temperatures, the molecules have more energy and therefore move more freely, which increasing membrane fluidity
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