2.  Objectives  By the end of this lesson you should be able to  Describe a property of cell membranes which allows substances to pass across it  Describe the process of diffusion  List factors which affect the movement of substances across a membrane by diffusion

3. Pores or channels

4.  Oxygen for respiring cells  Water  Carbon dioxide for photosynthesising plant cells  Simple sugars, amino acids

5.  One way is by diffusion

6.  Diffusion is the net or overall movement of molecules from an area of high concentration to an area of low concentration. It is a passive process it does not use up energy

7.  If there is a big difference between two areas diffusion will take place quickly  The difference between two areas of concentration is called the concentration gradient

8.  Temperature  If the temperature is higher the particles in a gas or solution move more quickly.  This means diffusion takes place more rapidly as the random movement of particles speeds up

9.  The thickness of the membrane Why?  The surface area of the membrane. Look at this photo of the lining of the small intestine Villi in the small intestine increase the surface area How does surface area Increase rate?

10.  List 2 things which must be able to pass into cells.  What is present in a cell membrane to allow them to pass across?  What is the name of the process which lets substances pass from high to low concentrations?  Does this process use up energy?  What factors affect the rate of this process?

11. Boiling tube containing iodine solution and water  Set up this model cell and leave it for 30 minutes. Iodine turns black when it mixes with starch. Hazard –iodine wear goggles Starch solution in the visking bag. This is your model cell visking bag has tiny holes in it

12.  Can you explain the change in colours? Boiling tube containing iodine solution and water Starch solution in the visking bag visking bag

13.  What happens when starch and iodine solution mix?  What is present in the visking tubing to allow small iodine molecules through?  Why has the inside of the visking tubing model cell gone black?  Which particles have moved?  Why have the starch molecules stayed in the bag?

14.  The iodine molecules are very _______ and can move through the small ----- in visking tubing. The iodine molecules have moved from an area of high ________to an area of ______concentration by _________  The starch molecules are too ____ to pass through the small holes in the visking tubing so they stay in the model ____