transport across cell membranes

• Cell membrane components


  • Proteins
  • Cholesterol
  • Glycolipids
  • Glycoproteins
• Cell surface membrane


  • Barrier between cell and environment
  • Partially permeable
• Membrane within cells


  • Divide cell into compartments
  • Partially permeable and control what enters and exits
• Fluid mosaic model


  Phospholipid bilayer with proteins scattered throughout
• Channel proteins and carrier proteins

  Allow large molecules to pass through membrane
• Receptor proteins


  Detect chemicals released from other cells and signal cell to respond
• Phospholipids


  • Form a barrier to water-soluble substances
  • Have hydrophilic 'head' and hydrophobic 'tail'
  • Automatically arrange into bilayer
• Cholesterol


  • Gives membrane stability
  • Fits between phospholipids
  • Restricts phospholipid movement
• Temperature and membranes


  Affects phospholipid movement and membrane permeability
• Diffusion


  • Net movement of particles from higher to lower concentration
  • Passive process requiring no energy
• Factors affecting rate of diffusion

  • Concentration gradient
  • Thickness of exchange surface
  • Surface area
• Facilitated diffusion


  • Large or charged particles diffuse through carrier or channel proteins
  • Still a passive process down concentration gradient
• Carrier proteins


  Move large molecules across membrane down concentration gradient
• Channel proteins


  Form pores for charged particles to diffuse through
• Factors affecting rate of facilitated diffusion


  • Concentration gradient
  • Number of channel or carrier proteins
• Facilitated diffusion


  1. Carrier protein

  2. Molecule attaches to carrier protein

  3. Protein changes shape

  4. Molecule released on opposite side of membrane
• Carrier protein


  • Move large molecules across the membrane, down their concentration gradient
  • Different carrier proteins facilitate the diffusion of different molecules
• Facilitated diffusion


  1. Channel protein

  2. Form pores in the membrane for charged particles to diffuse through

  3. Different channel proteins facilitate the diffusion of different charged particles
• Factors affecting the rate of facilitated diffusion


  • Concentration gradient
  • Number of channel or carrier proteins
• Calculating the rate of diffusion

  1. For a straight line graph, find the gradient of the line

  2. For a curved graph, draw a tangent and find the gradient of the tangent
• Osmosis


  Diffusion of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential
• Water potential


  The potential (likelihood) of water molecules to diffuse out of or into a solution
• Pure water has water potential of 0
• Adding solutes to pure water lowers its water potential
• The water potential of any solution is always negative
• Isotonic


  Two solutions that have the same water potential
• Hypotonic


  Solutions with higher water potentials compared to the inside of the cell
• Hypertonic


  Solutions with lower water potential compared to the inside of the cell
• Factors affecting the rate of osmosis

  • Water potential gradient
  • Thickness of exchange surface
  • Surface area of exchange surface
• Active transport


  Uses energy to move molecules and ions across plasma membranes, usually against the concentration gradient
• Active transport


  1. Carrier proteins involved

  2. Molecule attaches to carrier protein

  3. Protein changes shape

  4. Molecule moved across membrane and released
• Differences between active transport and facilitated diffusion

  • Active transport usually moves solutes from low to high concentration
  • Active transport requires energy
• ATP


  • Common source of energy in the cell, important for active transport
  • Undergoes hydrolysis reaction, splitting into ADP and Pi, releasing energy
• Co-transport of glucose absorption

  1. Na+ actively transported out of epithelial cells

  2. Na+ diffuses into epithelial cells down concentration gradient

  3. Na+-glucose cotransporter carries glucose into cell

  4. Glucose diffuses into blood down concentration gradient
• Factors affecting the rate of active transport

  • Decreasing concentration gradient doesn't affect rate
  • Speed of individual carrier proteins
  • Number of carrier proteins
  • Rate of respiration and availability of ATP