biological membranes

• Membranes are vital structures found in all cells
• Cell surface membrane


  Creates an enclosed space separating the internal cell environment from the external environment
• Intracellular 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
  • 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
• 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
• Fluid mosaic model of membranes

  Explains how biological molecules are arranged to form cell membranes
• Fluid mosaic model


  • Helps to explain passive and active movement between cells and their surroundings
  • Cell-to-cell interactions
  • Cell signalling
• Fluid


  • (in the fluid mosaic model) 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
• Mosaic

  (in the fluid mosaic model) The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
• Components of the fluid mosaic model of membranes

  • Phospholipids
  • Cholesterol
  • Glycoproteins and glycolipids
  • Transport proteins
• 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
• Cholesterol


  • Increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures (allowing cells to survive at lower temperatures)
  • 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


  • 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, and receptors involved in cell adhesion and stabilisation
  • Some glycolipids and glycoproteins act as cell markers or antigens, for cell-to-cell recognition
• Transport proteins


  • Create hydrophilic channels to allow ions and polar molecules to travel through the membrane
  • There are two types: channel (pore) proteins and 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 permeability of cell membranes is affected by different factors or conditions, such as temperature and solvent concentration
• Temperature


  • As temperature increases, lipids become more fluid
  • This increased fluidity reduces the effectiveness of the cell membrane as a barrier to polar molecules, meaning polar molecules can pass through
  • At higher temperatures, any diffusion taking place through the cell membrane will also occur at a higher speed (due to increased kinetic energy)
  • Changes in membrane fluidity are reversible
  • At a certain temperature (often around 40°C) many proteins (including those in cell membranes) begin to denature, disrupting the membrane structure and meaning it no longer forms an effective barrier
• Solvent concentration


  Organic solvents can increase cell membrane permeability as they dissolve the lipids in the membrane, causing the membrane to lose its structure
• Investigating the effect of temperature on membrane permeability

  1. Cut equal-sized beetroot pieces

  2. Rinse the beetroot pieces

  3. Add the beetroot pieces to test tubes with water at different temperatures

  4. Leave for 30 minutes

  5. Remove the beetroot pieces, leaving just the coloured liquid

  6. Use a colorimeter to measure how much light is absorbed as it passes through each sample
• The general pattern is that as temperature increases, membrane permeability also increases
• If experimenting with temperatures below 0°C, membrane permeability may also be increased (once the cells have thawed again)
• Diffusion is the net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration
• Factors affecting the rate of diffusion across a membrane

  • Concentration gradient
  • Molecular size
  • Membrane thickness
  • Temperature
• Facilitated diffusion


  A type of diffusion that occurs across the cell membrane, where transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane
• Diffusion


  A type of transportation that occurs across the cell membrane
• Diffusion


  The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration
• The molecules or ions move down a concentration gradient
• The random movement is caused by the natural kinetic energy of the molecules or ions
• As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are evenly spread within a given volume of space
• Factors affecting the rate of diffusion

  • Provided in Diffusion Factors Table
• Facilitated diffusion


  Certain substances cannot diffuse through the phospholipid bilayer of cell membranes, so they can only cross the phospholipid bilayer with the help of certain proteins
• Substances that require facilitated diffusion

  • Large polar molecules such as glucose and amino acids
  • Ions such as sodium ions (Na+) and chloride ions (Cl-)
• Channel proteins


  Water-filled pores that allow charged substances (e.g. ions) to diffuse through the cell membrane
• The diffusion of these ions does not occur freely, most channel proteins are 'gated', meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
• Carrier proteins


  Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes, causing the binding site to be open to one side of the membrane first, and then open to the other side when the carrier protein switches shape
• The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
• Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
• Practical: Investigating the rate of diffusion using Visking tubing

  Fill Visking tubing with starch and glucose solutions, suspend in water, test water for presence of starch and glucose