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• Components of the phospholipid bilayer

  • Phospholipids
  • Cholesterol
  • Extrinsic & intrinsic proteins
  • Glycolipids
  • Glycoproteins
• Fluid mosaic model of membranes

  • Fluid - individual phospholipids can move laterally or flip between monolayers, membrane is flexible
  • Mosaic - extrinsic and intrinsic proteins of different sizes and shapes form a pattern
• Role of phospholipids in the phospholipid bilayer

  Form basic structure. Hydrophilic heads face outwards and hydrophobic tails face inwards on either side of the bilayer. These interactions allow lipid-soluble molecules to cross but prevent water-soluble molecules from crossing.
• Role of cholesterol in membranes

  • Steroid molecule
  • Connects phospholipids, providing strength to membrane and regulating its fluidity
• Role of glycolipids in cell membranes

  Involved in cell signalling and cell recognition
• Functions of extrinsic proteins in membranes


  • Binding sites/receptors e.g. for hormones and drugs
  • Help cells adhere to each other
  • Involved in cell signalling
• Role of glycoproteins in cell membranes


  Serve as recognition sites for chemicals
• Functions of intrinsic proteins in membranes

  • Electron carriers (respiration/photosynthesis)
  • Channel proteins (facilitated diffusion)
  • Carrier proteins (facilitated diffusion/active transport)
• Arrangement of cholesterol, glycolipids and glycoproteins in the bilayer

  • Cholesterol is dispersed in the membrane alongside the phospholipids
  • Glycolipids and glycoproteins extend from either lipid or protein components within the membrane. This region is known collectively as the glycocalyx
• Functions of the cell surface membrane

  • Isolates cytoplasm from extracellular environment
  • Selectively permeable to regulate transport of substances
  • Involved in cell signalling/cell recognition
• Cell signalling


  Specific stimulus (e.g. light) → sender cell manufactures chemical to be sent → molecules (e.g. ligands) released by exocytosis and transported through bloodstream to target cell → binds to complementary receptor → effector cell stimulated
• Osmosis


  The diffusion of water across a partially permeable membrane from an area of higher water potential to an area of lower water potential. This occurs until a dynamic equilibrium is established.
• Water potential (ψ)


  • Pressure created by water molecules, measured in kPa
  • Ψ of pure water at 25°C and 100 kPa is 0
  • More solute, ψ more negative
  • Movement always from a region of high ψ to a region of low ψ
• How osmosis affects plant and animal cells

  • Osmosis INTO cell: Plant - protoplast pushes against cell wall, turgid; Animal - lysis
  • Osmosis OUT of cell: Plant - protoplast pulls away from cell wall, flaccid; Animal - crenation
• Simple diffusion


  • Net spreading out of particles from an area of higher concentration to an area of lower concentration, down their concentration gradient
  • Passive process requiring no energy from hydrolysis of ATP
• Facilitated diffusion


  The net movement of substances from a higher concentration to a lower concentration (down their concentration gradient) through transport proteins without the use of energy
• How channel proteins work

  • Form selective pores in phospholipid bilayer
  • Allow polar and charged molecules to pass through
  • Some channel proteins may be gated, opening or closing depending on the binding of a specific molecule or ion
• How carrier proteins work

  • Specific shape for the molecule they transport
  • Binds to complementary molecule, conformational change passes molecule to other side of membrane
• Active transport


  Active movement of substances from a low concentration to a higher concentration (against the concentration gradient) with the use of energy in the form of ATP
• Active transport in cell membranes

  1. Molecule binds to carrier protein with complementary shape

  2. ATP binds to separate binding site on carrier protein

  3. Carrier protein changes shape, moving molecules to the other side of the membrane

  4. Molecules released via ATP hydrolysis

  5. Carrier protein changes back to original shape
• Exocytosis


  The bulk transport of substances out of a cell via a vesicle that fuses with the plasma membrane using energy in the form of ATP
• Endocytosis


  The bulk uptake of substances into a cell by invagination of the membrane to form a vesicle trapping the substances inside the cell with the use of energy in the form of ATP
• Factors that affect the rate of diffusion

  • Temperature
  • Diffusion distance
  • Surface area
  • Size of molecule
  • Difference in concentration (how steep the concentration gradient is)
• Calculating the surface area to volume ratio of a cube

  First: Surface area = 6 × (height × width)

  Then: Volume = depth × (height × width)

  Finally: State the surface area to volume ratio and simplify. As size increases, SA:V decreases.
• How surface area to volume ratio impacts the rate of diffusion

  The greater the surface area to volume ratio, the greater the rate of diffusion