This document discusses various mechanisms of transport across plasma membranes, including: 1) Diffusion and facilitated diffusion, which involve the passive movement of substances down their concentration gradients, or through membrane protein channels. 2) Active transport, which uses energy to transport substances against their gradients via carrier proteins. 3) Bulk transport processes like endocytosis and exocytosis that move large quantities of material into and out of cells. 4) Osmosis, the passive movement of water through semi-permeable membranes down its concentration gradient, influenced by solute concentrations and pressure potentials.

1. Transport across the plasma
membrane
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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
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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
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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
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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
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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
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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).
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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
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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
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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
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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
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