Transport of molecules across the cell membrane can occur through passive or active transport. Passive transport includes diffusion, osmosis, and facilitated diffusion, and does not require energy. Diffusion is the movement of molecules from high to low concentration down a gradient. Osmosis is the diffusion of water across a membrane, and facilitated diffusion uses transport proteins to assist molecule movement. Active transport transports molecules against a concentration gradient and uses energy in the form of ATP. The sodium-potassium pump is an example that maintains ion gradients through active transport.

1. Transport Across Cell Membrane
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2. Transport across the cell membrane
 Passive transport does NOT require energy
 Diffusion – small uncharged molecules
 Osmosis - water
 Facilitated diffusion - glucose
 Active transport REQUIRES ENERGY
 Ion pumps

3. Passive Transport
3 types
 Diffusion- simple movement from regions of
high concentration to low concentration.
 Osmosis- specifically the diffusion of water
across a semi-permeable membrane.
 Facilitated diffusion - protein transporters
which assist in diffusion.

4. DEFINITIONS
 DIFFUSION: the net movement of molecules from a region of their
higher concentration to a region of their lower concentration down a
concentration gradient, as a result of their random movement.
 OSMOSIS: the diffusion of water molecules from a region of their
higher concentration (dilute solution) to a region of their lower
concentration (concentrated solution), through a partially permeable
membrane.
 ACTIVE TRANSPORT: the movement of ions in or out of a cell
through the cell membrane, from a region of their lower
concentration to a region of their higher concentration against a
concentration gradient, using energy released during respiration.
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5. •Diffusion is the net movement of molecules (or
ions) from a region of their high concentration to
a region of their lower concentration.
The molecules move down a concentration gradient.
Molecules have kinetic energy, which makes them
move about randomly.
As a result of diffusion molecules reach an
equilibrium where they are evenly spread out.
This is when there is no net movement of molecules
from either side.
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6. Diffusion through a membrane
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Cell membrane
Inside cell Outside cell

7. Diffusion through a membrane
Biology, Cell membranes and Transport 7
Cell membrane
Inside cell Outside cell
diffusion

8. Diffusion through a membrane
8
Cell membrane
Inside cell Outside cell
EQUILIBRIUM

9. What determines the rate of diffusion?
There 4 factors:
1. The steepness of the concentration gradient. The bigger
the difference between the two sides of the membrane
the quicker the rate of diffusion.
2. Temperature. Higher temperatures give molecules or ions
more kinetic energy. Molecules move around faster, so
diffusion is faster.
3. The surface area. The greater the surface area the faster
the diffusion can take place. This is because the more
molecules or ions can cross the membrane at any moment.
4. The type of molecule or ion diffusing. Large molecules
need more energy to get them to move so they tend to
diffuse more slowly. Non-polar molecules diffuse more
easily than polar molecules because they are soluble in the
non polar phospholipid tails.
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10. Molecules that diffuse through cell membranes
1. Oxygen – Non-polar
so diffuses very
quickly.
1. Carbon dioxide –
Polar but very small
so diffuses quickly.
2. Water – Polar but
also very small so
diffuses quickly.
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11. Osmosis
‘The diffusion of water from an area of high
concentration of water molecules (high water
potential) to an area of low concentration of
water (low water potential) across a partially
permeable membrane.’
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12. Osmosis
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Cell membrane
partially
permeable.
Inside cell Outside cellVERY High conc.
of water
molecules. High
water potential.
VERY Low conc.
of water
molecules. Low
water potential.
Sugar molecule
DILUTE SOLUTION
CONCENTRATED SOLUTION

13. Osmosis
13
Cell membrane
partially
permeable.
Inside cell Outside cell
High conc. of
water molecules.
High water
potential.
Low conc. of
water molecules.
Low water
potential.OSMOSIS

14. Osmosis
14
Cell membrane
partially
permeable.
Inside cell Outside cell
OSMOSIS
EQUILIBRIUM. Equal water concentration on each side.
Equal water potential has been reached. There is no net
movement of water

15. Osmosis


16. Its about concentration
The concentration of the solution,
with respect to other solutions is
important
Isotonic --- when both solutions have the same
concentration of dissolved substances
Hypertonic --- a solution with a higher
concentration of dissolved substances
Hypotonic --- a solution with a lower
concentration of dissolved substances

17. Animal cells
Plant cells

18. Osmosis and Animal Cells
haemolysis
Hypotonic
solution
Hypertonic
solution

19. Osmosis & Plant Cells
Turgid Cell Plasmolysed Cell
Hypotonic
solution Hypertonic
solution

20. 3-Facilitated Diffusion
 Transport proteins carry specific
molecules across the cell membrane
 Movement is along a concentration
gradient (i.e. From higher to lower)
 Each type of transport protein will carry
only one type of molecule.
 This is how glucose is moved.

21. Facilitated diffusion
 This process does not require ATP but does require cell membrane proteins
which are called carrier proteins to carry the molecules across the cell
membrane from an area of higher concentration to an area of lower
concentration

22. Facilitated Diffusion
GlucoseTransport protein
Concentration
gradient

23. Glucose
Cell membrane
Transport protein
Glucose binds to the
transport protein
The transport protein turns over and releases glucose
onto the inside of the cell, along the concentration gradient
Concentration
gradient

25. Active Transport
 Active transport uses energy to send
substances against the direction they
would travel by simple diffusion: that is
from a region of low concentration to a
region of high concentration.

26. Salt ion
Cell membrane
Transport protein
Ion binds to the
transport protein
The transport protein turns over and releases the ion
onto the inside of the cell, against the concentration gradient
Concentration
gradient
Energy is used

27. Ion Channels
 Work fast: No conformational changes
needed
 Not simple pores in membrane:
 specific to different ions (Na, K, Ca...)
 gates control opening
 Toxins, drugs may affect channels

28. Sodium-Potassium Pump

29. WHAT IS ACTIVE TRANSPORT?
 Transport proteins within the membrane must
use energy (ATP) to move substances either to
the inside or outside of the membrane.
 Active transport requires the cell to spend
energy, usually in the form of ATP.
 Examples include transport of large molecules
(non-lipid soluble) and the sodium-potassium
pump.

30.  In the case of active transport, the proteins are
having to move against the concentration
gradient.
 For example the sodium-potassium pump in
nerve cells. Na+ is maintained at low
concentrations inside the cell and K+ is at higher
concentrations.
 The reverse is the case on the outside of the
cell.
 When a nerve message is propagated, the ions
pass across the membrane, thus sending the
message.

31. Comparison table
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