biological membranes

• What are the roles of membranes?

  The 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 and vacuoles
  • 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
• How does the fluid mosaic model describe cell membranes as fluid?

  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
• How does the fluid mosaic model describe cell membranes as mosaics?


  The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
• What are the four main components of the fluid mosaic model?

  Phospholipids

  • Cholesterol
  • Glycoproteins and glycolipids
  • Transport proteins
• What are the function of phospholipids?

  To 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
• A phospholipid bilayer is composed of two layers of phospholipids; their hydrophobic tails facing inwards and hydrophilic heads outwards
• What is the role of cholesterol in membranes?

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

  • This is as cholesterol stops the phospholipid tails packing too closely together
  • Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
  • 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
• What are the 3 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
• What is the role of 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
• What is the permeability of cell membranes affected by?

  temperature

  solvent concentration
• How does temperature affect the permeability of cell membranes?

  As temperature increases, lipids become more fluid, reducing the effectiveness of the cell membrane as a barrier to polar molecules, so polar molecules can pass through.

  Changes in membrane fluidity are reversible If temps decrease, the lipids will return to their normal levels of fluidity.
• How does temperature above the optimum affect membrane permeability?

  at around 40 degrees, many proteins (including those in cell membranes) begin to denature

  • This disrupts the membrane structure, meaning it no longer forms an effective barrier
  • As a result, substances can pass freely through the disrupted membrane
  • This process is irreversible
• • You can investigate how different factors affect membrane structure and permeability using beetroot
  • Beetroot cells contain a dark purple-red pigment
  • The higher the permeability of the beetroot cell membrane, the more of this pigment leaks out of the cell
• Using a scalpel, cut 5 equal-sized cubes of beetroot

  • They must have the same dimensions so that they all have equal surface areas and volumes, as these factors may affect the rate at which the pigment leaks out
  • Rinse the beetroot pieces to remove any pigment released during cutting, pat dry
  • Add the pieces to five different test tubes, each containing the same volume of water ( 5cm3)
  • Put each test tube in a water bath at a different temperatures for the same length of time
  • The time should be long enough to allow the pigment to diffuse into the water (e.g. around 30 minutes)
• p2 of Investigating the effect of temperature on membrane permeability:

  Remove the beetroot pieces, leaving just the coloured liquid in the five test tubes

  • Use a colorimeter to measure how much light is absorbed as it passes through each of the five samples of coloured liquid
  • The higher the absorbance, the more pigment must have been released, due to a greater membrane permeability
• Investigating the effect of temperature on membrane permeability:

  As temperature increases, the phospholipids within the cell membrane move more because they have more energy

  • Increased movement means the phospholipids are not as tightly packed together, increasing the permeability of the membrane
  • At high temperatures, the phospholipid bilayer may even start to melt and breakdown, further increasing the permeability of the membrane
• Investigating the effect of temperature on membrane permeability:

  In addition, the volume of water inside the cells expands, putting pressure on the membrane, causing channel and carrier proteins to deform so they can no longer control what enters and leaves the cell. These factors also increase the permeability of the membrane

  • Temperature also affects the conformation (3D shape) of proteins as at high temperatures the intermolecular forces between amino acids are broken which affects the protein’s specificity and function
• Limitations:

  • The beetroot pieces may not be identical in size and shape, meaning some test tubes could contain slightly more beetroot tissue than others
  • Solution: cut the discs as accurately as possible using a scalpel and ruler, and repeat each investigation several times to find a mean
  • Some parts of beetroot tissue have more pigment in their cells than others
  • Solution: conduct several repeats, using different parts of the beetroot and find a mean
• What is 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.
• • Certain substances cannot diffuse through the phospholipid bilayer of cell membranes. These include:
  • Large polar molecules such as glucose and amino acids
  • Ions such as sodium ions (Na+) and chloride ions (Cl-)
  • These substances can only cross the phospholipid bilayer with the help of certain proteins
  • This form of diffusion is known as facilitated diffusion
• What are two types of proteins that enable facilitated diffusion?

  Channel proteins

  • Carrier proteins
• What are channel proteins?

  Channel proteins are water-filled pores

  • They allow charged substances (eg. 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
  • This allows the channel protein to control the exchange of ions
• What are carrier proteins?

  Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes

  • This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane 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