1. Starter Write down an exam perfect definition for diffusion
List as many factors as you can that affect the rate of diffusion

2. answers
The net movement of molecules in a gas or liquid from an area of high concentration to an area where they are less concentrated
2. Temperature, concentration gradient, surface area, distance, size of molecule

3. Objectives
To recall the definitions of diffusion, facilitated diffusion, active transport and endocytosis
To explain the role of membrane proteins in transport
Apply the principles of membrane transport in unfamiliar situations

4. Crossing membranes
Each molecule of ion in a gas or liquid will diffuse down its own concentration gradient until equilibrium is reached.
This means one molecule can be diffusing one way and another can diffuse in the opposite direction at the same time. E.g. plant cell (oxygen and carbon dioxide)

5. Diffusion Is a passive process Q1. What does passive mean?
Small molecules with no electrical charge are able to pass though the phospholipid bilayer by simple diffusion
Q2. Name two small molecules without a charge.

6. answers It does NOT use energy
2. Small, lipid soluble (non-polar) molecules which will diffuse across the phospholipid bilayer include oxygen, carbon dioxide and steroid hormones

7. Large molecules or molecules with a charge
E.g. Glucose or sodium ions
These cant pass through the bilayer and need the help from a protein. Because they are helped it is called facilitated diffusion.
Two types of protein are involved in facilitated diffusion, these are channel proteins and carrier proteins

8. Channel protein
These form pores in the membrane, often shaped to allow only one type of ion through. Many are gated to allow it to be opened or closed.
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)

9. Carrier protein
Shaped so that a specific molecule can fit into the membrane surface. When the molecule fits it changes the shape of the protein so the molecule gets carried to the other side of the membrane.

10. So what is the difference?
Compare and contrast channel and carrier proteins which are involved in facilitated diffusion.

11. Facilitated diffusion
Remember this is still diffusion, it relies on particles bumping into the protein and it does NOT use energy.

12. Task 1
Look at the images showing movement across membranes and complete the table below.
Image no. Type Description
Carrier protein molecule fits the protein and it changes the shape of the protein so the molecule gets carried to the other side of the membrane

13. Answers Diffusion (just plain old diffusion) Channel protein
Gated channel protein
Carrier protein

14. 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 Ψ gradient from root to xylem

15. 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

16. Task 2
Complete exam questions 2, 3 and 5

17. Plenary
SAQ 2 page 29