1. Cell Transport Cell transport occurs across the cell membrane.
Materials such as ions, water, molecules and nutrients are transported within cells as well as in and out of cells.
This occurs via the processes of diffusion, osmosis, active transport, phagocytosis and pinocytosis.

2. Diffusion
Diffusion is the movement of particles from an area of high concentration to an area of low concentration.
The particles can be in liquid or gas, and are said to be moving at random until both sides of the semi-permeable membrane are of equal concentration.
It is known as moving DOWN the ‘concentration’ or ‘diffusion’ gradient.

3. Diffusion

4. Diffusion
The process does not require any energy so is known as “passive transport”.
Simple diffusion allows small particles to pass through the phospholipids
Diffusion rates depend on:
Concentration gradient: the greater the gradient the faster the rate of diffusion.
Particle size: small particles diffuse faster then large particles.
State: gases diffuse faster then liquids.
Temperature: warm particles move faster then cool particles.

5. Diffusion
Food and oxygen diffuse into cells, where waste products diffuse out.
Cell membranes are semi-permeable. Only some molecules can diffuse directly through the lipid bilayer.
Molecules such as Oxygen, fatty acids, Carbon dioxide and urea can do this.
Lungs versus tissues

6. Facilitated Diffusion
A process by which substances are transported across cell membranes by means of protein channel molecules
This can be faster then diffusion alone
Proteins are within the cell membrane and specialise to diffuse certain particles or molecules that cannot cross the hydrophobic part of the lipid bilayer.
e.g. polar molecules or ions
This is known as “facilitated diffusion”.

8. Questions Describe the process of diffusion.
Explain some similarities and differences between simple diffusion and facilitated diffusion.
What 4 variables effect the rate of diffusion and how? (increase or decrease)
The plasma membrane is semi-permeable. What does this mean?

9. Investigating the Effect of Temperature on Diffusion rate
An experiment was set up to test the hypothesis that increasing temperature increases diffusion rate. The time for a purple dye to diffuse set distances in water at different temperatures was recorded

10. Results Water Temp. 2 cm 4 cm 6 cm 8 cm 10 cm 10˚C 60 s 110 s 210 s
Plot the three sets of data on the same graph. Use a key.
Do these results support the hypothesis? Explain your answer

11. Osmosis
The Diffusion of water molecules across a semi-permeable membrane from the side with the highest water concentration of to the side with the lowest water concentration.
Osmosis refers to water concentration.
A high water concentration is therefore the same as a low solute concentration (and vice versa).

12. Osmosis

13. Osmosis

14. Osmosis Osmosis requires no energy input (facilitated diffusion)
When a cell is surrounded by pure water, water moves into the cell by osmosis.
The cell then becomes turgid (swollen).
In animal cells if it becomes to turgid it can burst the cell (lysed).
In plant cells the cell wall is strong so the cell remains very turgid without bursting.
The vacuole then absorbs water, maintaining osmoregularity.

15. Osmosis

16. Osmosis
If the water concentration outside the cell is less than inside the cell, they lose water and become flaccid (limp).
This causes the animal cell to shrivel
Plant cells will plasmolyse if too much water is lost
The cell membrane will detach from the cell wall.
The vacuole then expels water to maintain osmoregularity.

17. Osmosis

18. Tonicity/osmoregulation
Tonicity relates the osmotic pressure of a cell to its surroundings/environment.
Osmoregulation is the way the cell maintains an optimal constant osmotic pressure.
This requires the cell to deal with changes in tonicity of its environment

19. Tonicity and Osmoregulation
Tonicity can be grouped into 3 categories.
Isotonic
Hypertonic
Hypotonic

20. Tonicity
Tonicity
Cells in an ‘isotonic solution’ have the SAME concentration of impermeable solutes ‘outside’ the cell as inside.
‘Hypertonic’ cells have a ‘greater’ concentration of impermeable solutes ‘outside’ the cell.
‘Hypotonic cells’ have a ‘lower’ concentration of impermeable solutes ‘outside’ the cell.

21. Hypertonic environment

22. Hypotonic environment

23. Isotonic Environment