Cell membrane

• Fluid-mosaic model of membrane structure

  Molecules within membrane can move laterally (fluid) e.g. phospholipids

  • Mixture of phospholipids, proteins, glycoproteins and glycolipids
• The structure of a cell membrane
• - Phospholipid bilayer
• - Phosphate heads are hydrophilic so attracted to water – orientate to the aqueous environment either side of the membrane
• - Fatty acid tails are hydrophobic so repelled by water – orientate to the inside/interior of the membrane
• - Embedded proteins (intrinsic or extrinsic)
• - Channel and carrier proteins (intrinsic)
• - Glycolipids (lipids and attached polysaccharide chain) and glycoproteins (proteins with polysaccharide chain attached)
• - Cholesterol (binds to phospholipid hydrophobic fatty acid tails)
• The fluid mosaic model of membrane structure can explain how molecules can enter/leave a cell
• Phospholipid bilayer
• - Allows movement of non-polar small/lipid-soluble molecules e.g. oxygen or water, down a concentration gradient (simple diffusion)
• - Restricts the movement of larger/polar molecules
• Channel proteins (some are gated) and carrier proteins
• - Allows movement of water-soluble/polar molecules / ions, down a concentration gradient (facilitated diffusion)
• Carrier proteins
• - Allows the movement of molecules against a concentration gradient using ATP (active transport)
• Features of the plasma membrane adapt it for its other functions
• - Phospholipid bilayer
• - Maintains a different environment on each side of the cell or compartmentalisation of cell
• - Phospholipid bilayer is fluid
• - Can bend to take up different shapes for phagocytosis / to form vesicles
• - Surface proteins / extrinsic / glycoproteins / glycolipids
• - Cell recognition / act as antigens / receptors
• - Cholesterol
• - Regulates fluidity / increases stability
• The role of cholesterol
• - Makes the membrane more rigid / stable / less flexible, by restricting lateral movement of molecules making up membrane e.g. phospholipids (binds to fatty acid tails causing them to pack more closely together)
• - Note: not present in bacterial cell membranes
• Movement across membranes by simple diffusion and factors affecting rate - Net movement of small, non-polar molecules e.g. oxygen or carbon dioxide, across a selectively permeable membrane, down a concentration gradient

  Passive / no ATP / energy required

  • Factors affecting rate – surface area, concentration gradient, thickness of surface / diffusion distance
• The role of cholesterol

  Makes the membrane more rigid / stable / less flexible, by restricting lateral movement of molecules making up membrane e.g. phospholipids (binds to fatty acid tails causing them to pack more closely together)

  • Note: not present in bacterial cell membranes
• How might cells be adapted for transport across their internal or external membranes

  By an increase in surface area

  • Increase in number of protein channels / carriers
• Movement across membranes by active transport and factors affecting rate

  Net movement of molecules/ions against a concentration gradient

  Using carrier proteins

  Using energy from the hydrolysis of ATP to change the shape of the tertiary structure and push the substances though

  • Factors affecting rate – pH/temp (tertiary structure of carrier protein), speed of carrier protein, number of carrier proteins, rate of respiration (ATP production)
• Movement across membranes by osmosis and factors affecting rate

  Net movement of water molecules across a selectively permeable membrane down a water potential gradient

  Water potential is the likelihood (potential) of water molecules to diffuse out of or into a solution; pure water has the highest water potential and adding solutes to a solution lowers the water potential (more negative)

  Passive

  • Factors affecting rate – surface area, water potential gradient, thickness of exchange surface / diffusion distance