Diffusion

1
Diffusion

• Net movement of particles down a diffusion
  (concentration) gradient
• Stops when equilibrium is reached
• Speeded up by increased surface area, increased
  concentration gradient and decreased distance

Rate of diffusion a SA x conc. gradient
distance
2
Simple diffusion across membranes

• Small, lipid soluble (non-polar) molecules will
  diffuse across the phospholipid bilayer, e.g.
  oxygen, steroid hormones
• Large, polar (water soluble) molecules will not
  cross membranes without the help of proteins
• There are some exceptions (e.g. water)
• Normal diffusion rules apply

3
Facilitated Diffusion 1

• Large or polar molecules will not dissolve in
  phospholipid membranes
• They must diffuse through channels created by
  proteins embedded in the membrane, e.g.
  K-channel in neurone membrane
• Rate of diffusion depends on number of channels
  available

4
Facilitated Diffusion 2

• Many channels are always open
• However some can open and close in response to
  chemicals or membrane potential (voltage)
• These are called gated channels

• E.g. liver cells have glucose channels that open
  when insulin binds to a receptor site beside the
  channel
• E.g. Na-channels in neurone membranes open when
  membrane potential reaches -55mV (threshold)

5
Active transport

• Uses carrier proteins in membrane
• Moves substances against a concentration gradient
• Requires energy in the form of ATP
• Stops when respiration stops
• E.g. Na/K-pump
• E.g. minerals pumped into root xylem to maintain
  ? gradient from root to xylem

6
Water potential (?)

• ? is the tendency of a solution to lose water, it
  is decreased by the addition of a solute, it is
  increased by external pressure
• ? is measured in kPa
• ? is always negative, i.e. ? of water is 0, ? of
  sugar solution may be -2000 kPa
• Water always moves to the lowest (most negative)
  ?
• Hypotonic higher ? than cell
• Hypertonic lower ? than cell
• Isotonic same ? as cell

7
Osmosis

• Water molecules can diffuse easily across
  membranes
• Many solute molecules, e.g. Na, cannot diffuse
  across membranes (without help)
• Osmosis is the diffusion of water, down water
  potential (?) gradient, across a selectively
  permeable membrane

8
Bulk Transport

• Endocytosis takes substances into a cell
• Phagocytosis involves insoluble material (e.g.
  bacteria)
• Pinocytosis involves soluble material (liquid)
• Exocytosis removes substances (e.g. enzyme
  secretion from pancreas cells)

• Transports large quantities of material
• Involves invagination of membrane to form
  vesicles (vacuoles)
• It is sometimes triggered by the substance
  binding to a receptor on the cell surface