1.3

• Phospholipids


  Molecules with a hydrophilic polar head and hydrophobic fatty acid tails
• Phospholipid bilayer formation


  1. Polar heads face outwards interacting with water outside the cell

  2. Fatty acid tails face inwards interacting with water in the cytoplasm
• Cell membrane


  • Phospholipids arranged in a bilayer
  • Proteins scattered throughout the bilayer, some extrinsic, some intrinsic
• Proteins in cell membrane


  • Receptors for hormones
  • Recognition sites
  • Channels
  • Carrier proteins
  • Enzymes
• Fluid mosaic model


  Proposed by Singer and Nicolson in 1972 to describe the structure of the animal cell membrane
• Fluid mosaic model


  • Membrane contains cholesterol which stabilises it
  • Glycoproteins act as antigens
  • Glycolipids act as receptor sites for molecules like hormones
• Diffusion


  Passive transport where molecules move from high to low concentration until equally distributed
• Factors affecting diffusion rate


  • Concentration gradient
  • Diffusion distance
  • Surface area of membrane
  • Thickness of exchange surface
• Increase in temperature


  Increases kinetic energy of molecules, increasing diffusion rate
• Facilitated diffusion


  Passive process using transport proteins to assist movement of polar molecules across membrane
• Transport proteins


  • Channel proteins with hydrophilic pores
  • Carrier proteins that span the membrane
• Active transport


  Requires energy in the form of ATP to transport molecules against concentration gradient
• Respiratory inhibitors


  Prevent aerobic respiration and ATP production, inhibiting active transport
• Co-transport


  Transporting two different molecules together, e.g. glucose and sodium ions
• Sodium-glucose co-transport


  1. Sodium ions actively transported out of epithelial cells, creating low concentration

  2. Sodium ions diffuse into cells via co-transport protein, carrying glucose with them

  3. Glucose then passes into blood via facilitated diffusion, sodium ions by active transport
• Water potential


  Measure of the tendency for water to move into or out of a solution
• Water potential values


  • Pure water 0kPa
  • Typical cell -200kPa
  • Strong glucose solution -1000kPa
• Osmosis


  Movement of water across a semi-permeable membrane from a region of higher water potential to a region of lower water potential
• Osmosis in plant cells


  1. Water enters cell, expanding vacuole and pushing cytoplasm against cell wall

  2. Creates pressure potential, making cell turgid
• Plasmolysis


  Process where plant cell loses water in a hypertonic solution, cytoplasm draws away from cell wall
• Osmosis in animal cells


  1. No pressure potential, water potential = solute potential

  2. Hypotonic solution causes water to enter and cell to burst (haemolysis)

  3. Hypertonic solution causes water to leave and cell to crenate
• Permeability of cell membranes can be investigated using beetroot discs
• Endocytosis


  1. Cell engulfs material by infolding of plasma membrane, forming vesicle

  2. Phagocytosis for solid materials

  3. Pinocytosis for liquids
• Exocytosis


  Substances leave the cell in vesicles, e.g. digestive enzymes