transport across membranes

• Diffusion

  • the net movement of particles from a region of higher concentration to a region of lower concentration
  • passive process - down the concentration gradient
  • continues until there is equilibrium
• Factors affecting the rate of diffusion

  • Temperature
  • Concentration Gradient
  • Stirring/moving
  • Surface area
  • Distance/thickness
  • Size of molecule (small diffuse fastest)
• Facilitated diffusion

  • passive process
  • Diffusion across a membrane through a protein
  • channel proteins - mainly for ions e.g. Na+, different ions have specific changes, many are gated (can be opened/closed)
  • carrier proteins - mainly for larger molecules e.g. glucose, change shape to let molecules through
• Active transport =Movement of molecules or ions in/out of a cell from a region of lower concentration to a region of higher

  concentration, requires energy and carrier proteins
• General process of active transport

  1. Molecule/ion binds to receptors in the channel of carrier protein
  2. On inside of cell ATP binds to carrier protein and is hydrolysed into ADP and a phosphate
  3. Binding of phosphate causes carrier protein to change shape, opens up to inside of cell
  4. Molecule/ion is released into cell
  5. Phosphate molecule is released and recombines with ADP to form ATP
  6. Carrier protein returns to original shape
• Bulk transport = Type of active transport- movement of large molecules (e.g. enzymes, hormones, bacteria cells)
• Endocytosis

  • type of bulk transport INTO cells
  • Cell membrane first invaginates (encloses) when it comes into contact with material to be transported
  • The membrane enfolds the material to form a vesicle
  • The vesicle pinches and moves into cytoplasm to transfer material
  • phagocytosis - solids
  • pinocytosis - liquids
• exocytosis

  • bulk transport OUT of cells
  • Reverse endocytosis
  • Vesicles (usually formed by golgi) move towards and fuse with plasmamembrane
  • Contents of vesicle are then releasedout of cell
• Osmosis (in terms of water potential) = There is a net movement of water from the solution with a higher Ψ to solution with a lower Ψ across a partially permeable membrane and this will continue until equilibrium is reached
• effect of osmosis on animal cells

  • If animal cell put into a solution with a higher Ψ than cell

  (hypotonic), water moves into cell by osmosis, increasing hydrostatic pressure- as there is no cell wall, it cannot stand increased pressure so will burst (cytolysis)

  • If animal cell put into a solution with a lower Ψ than cell

  (hypertonic), water leaves cell by osmosis, decreasing the volume of cell and the plasma membrane ‘puckers’ (crenation)

  • To prevent cytolysis or crenation- multicellular organisms have mechanisms to ensure cells are surrounded by aqueous solution of same Ψ (isotonic)
• effect of osmosis on plant cells

  • When plant cells put into a solution with a higher Ψ than cell, increased hydrostatic pressure pushes membrane against cell wall (turgor), as turgor pressure increases it resists further entry of water, cell is turgid
  • When plant cells put into solution with a lower Ψ than cell water leaves cell by osmosis, this decreases volume of cell and eventually the plasma membrane is pulled away from cell wall (cell is plasmolysed)
• hypertonic solution = lower water potential than the cell, water moves out of the cell
• hypotonic solution = higher water potential than the cell, water moves into the cell
• isotonic solution = same water potential as inside the cell, water moves at constant rate
• cytolysis = cell bursting
• crenation = cell shrinking , forming an abnormal surface
• turgor = cell membrane presses against cell wall
• plasmolysed = cell membrane is pulled away from cell wall
• water potential (Ψ)

  • pressure exerted by water molecules as they collide with a membrane/container
  • measured in kPa
  • pure water = 0kPa
  • all other solutions have a negative water potential
  • more negative kPa value = more concentrated solution