2. Diffusion and facilitated diffusion
• Diffusion – the net movement of molecules (or ions) from a
region of their higher concentration to a region of their lower
concentration (molecules move down a concentration
gradient)
• Molecules tend to reach an equilibrium where evenly spread
• The rate at which substance diffuses across a membrane
depends on:
– The ‘steepness’ of the concentration gradient. The greater
the difference in concentration, the greater the difference in
the number of molecules passing in the 2 directions and
hence the faster the net rate of diffusion
– Temperature. Diffusion takes place faster in high
temperatures due to higher kinetic energy
– The surface area. The greater the surface area, the more
molecules or ions can cross it at any one moment (faster
diffusion)
– The nature of the molecules or ions. Substances with large
molecules tend to diffuse more slowly. Non-polar molecules
diffuse more easily through cell membranes as they are
soluble in non-polar phospholipid tails
ALBIO9700/2006JK
4. • Facilitated diffusion – diffusion of a
substance through protein channels in a
cell membrane
• The proteins provide hydrophilic areas
that allow the molecules or ions to pass
through a membrane that would otherwise
be less permeable to them
• Rate depends on how many appropriate
channels there are in the membrane and
on whether they are open or not
ALBIO9700/2006JK
6. Active transport
• The energy-consuming transport of molecules or
ions across a membrane against a concentration
gradient made possible by transferring energy
from respiration
• Like facilitated diffusion, active transport is
achieved by special transport proteins, each of
which is specific for a particular type of molecule
or ion
• However, it requires energy (molecule ATP)
because movement occurs up a concentration
gradient
• Energy used to make the transport protein
(carrier protein) change its 3D shape,
transferring the molecules or ions across the
membrane in the process
ALBIO9700/2006JK
8. Bulk transport
• Transport of large quantities of materials
into cells (endocytosis) or out of cells
(exocytosis)
• Exocytosis – process by which materials
are removed from cells
• Endocytosis – engulfing of the material
by the plasma membrane to form a small
sac or ‘endocytotic vacuole’
– Phagocytosis: bulk uptake of solid material
– Pinocytosis: bulk uptake of liquid
ALBIO9700/2006JK
9. Osmosis
• Solution = solute + solvent
• Partially permeable membrane – membrane
which allows only certain molecules through
• If solution B has higher concentration of solute
molecules than solution A – solution B is more
concentrated than solution A/solution A is more
dilute than solution B
• Osmosis involves net movement of water
molecules only
ALBIO9700/2006JK
11. • Water potential and solute potential
– Water potential: the tendency of water molecules
to move from one place to another (psi, ψ)
– Water always moves from a region of higher water
potential to a region of lower water potential (down
concentration gradient)
– Osmosis: the movement of water molecules
from a region of higher water potential to a
region of lower water potential through a
partially permeable membrane
– Pure water has highest water potential (solutes lower
the water potential)
– Solute potential: the amount that the solute
molecules lower the water potential of a solution.
Always –ve (ψs).
ALBIO9700/2006JK
12. • Osmosis in animals
– If the water potential of the solution surrounding the
cell is too high, the cell swells and bursts
– If it is too low, the cell shrinks
– In animal cells ψ = ψs (water potential is equal to
solute potential)
• Pressure potential
– The greater the pressure applied, the greater the
tendency for water molecules to be forced back
– Increasing the pressure increases the water potential
– Pressure potential: contribution made by pressure
to water potential (ψp)
– The pressure potential makes the water potential less
negative and is therefore positive
ALBIO9700/2006JK
14. • Osmosis in plant cells
– Different from animal cells because of rigid
and strong cell wall
– Cell wall prevents the cell from bursting (lysis)
– When fully inflated with water: turgid
– Water potential is a combination of solute
potential and pressure potential (ψ = ψs + ψp)
– When protoplast shrinks and pulls away from
the cell wall: plasmolysis
– Incipient plasmolysis: the point at which
pressure potential has just reached zero and
plasmolysis is about to occur
ALBIO9700/2006JK
16. Exchange surfaces
• Gaseous exchange in
mammalian lungs
– Gaseous exchange
surface: where
oxygen from the
external environment
can diffuse into the
body, and carbon
dioxide can diffuse out
– In humans: alveoli in
the lungs
ALBIO9700/2006JK
17. • Uptake of mineral ions in a plant root
– Specialised exchange surface: root hairs (very thin
extensions of the cells that make up the outer layer
or epidermis of a root)
– Root hairs make contact with thin layer of water
coating each soil particle and absorbs it by osmosis
– Lower concentration of solutes in the water in the soil
than there is inside the root hair cell
– Water potential higher outside the root hair and
water moves passively down the water potential
gradient into the cells
– Mineral ions also absorbed by facilitated diffusion if
concentration is higher outside root hair
– By active transport (carrier proteins and energy)
when concentration is relatively low in soil
ALBIO9700/2006JK
