1. Recap - You have 5 minutes!
What two things must a haemoglobin be efficient at?
What animals typically have haemoglobin with a high affinity for oxygen?
Why might an animal have haemoglobin with a low affinity for oxygen?
What effect would a increase in CO2 have on the association of oxygen with a haemoglobin? Sketch a graph to illustrate this.
Sketch a graph comparing a lugworms and a humans oxygen dissociation curve.

2. Lesson Objectives E – Needed a lot of help to write own method
C – wrote a workable method, and got a good set of results. Identified any trends in results.
A – Independently planned and carried out the investigation. Scientifically commented on results

3. Task: To investigate the effect of surface : volume ratio on diffusion rate

4. Objectives; To calculate surface-area-to-volume ratio (SA:V)
To show the effect of surface-area-to-volume ratio on the diffusion rate of hydrochloric acid

5. Surface area to volume ratio
Hint (NB units not given)
Surface area to volume ratio
Length of edge of a cube
Surface area of whole cube
Volume of cube (length x width x height)
Ratio of surface to volume (surface area / volume)

6. You need to: Plan an experiment to answer the question
Identify factors to control
You will be given
1M hydrochloric acid
Agar stained with universal indicator
Ask for additional equipment!
Make a prediction, and justify it
Record your results appropriately
Analyse and display your results appropriately
Interpret your results, making reference to any data or graph.

7. You will be given; 1M hydrochloric acid
Agar stained with universal indicator
Ask for additional equipment!

8. Questions
What prediction did you make about the rate of diffusion and the effect of surface-area-to-volume ratio?
Identify at least three key factors you controlled in this experiment.
Explain the effect of surface-area-to-volume-ratio on the rate of diffusion and how this is important in living organisms using your graph and your scientific knowledge.
What are the limitations of this experiment?
Should this procedure have a control? If so, what would it be?
The volume of a living organism is proportional to the number of cells in its body. Each cell needs oxygen and nutrients and needs to get rid of metabolic waste products such as carbon dioxide. The smallest organisms can absorb nutrients and get rid of waste by diffusion through their outer membranes. Do you think larger organisms can do the same? Why?

9. Teachers answers
Prediction should link the diffusion rate and the increased or reduced surface-area-to-volume ratio.
Key factors controlled should include: temperature, shape of the block, size of block, immersion method, volume of acid used, the depth of acid used and the type of agar.
A decrease in surface-area-to-volume ratio causes a reduction in the rate of diffusion. In living organisms this is important since larger animals have a smaller SA:V ratio and will be unable to obtain enough nutrients and oxygen by diffusion alone.
Difficulty in cutting and measuring accurately. Difficulty in maintaining a constant shape. Eye used to judge exact end point. Blocks may float, effecting immersion.
Any sensible suggestion – leave a cube in water, or on a piece of filter paper so you can be sure the change in indicator is due to diffusion of the surrounding solution inwards, not the effect of time on the cubes.
Larger organisms would not be able to get all the nutrients they need, or get rid of all their waste products by diffusion as there is not enough surface area for each cell. The rate of diffusion cannot be increased so this will limit the size of the organism unless it has specialised gas exchange surfaces or other mechanisms.

10. Rate of diffusion effected by?
Concentration gradient
Area over which diffusion takes place
Thickness of exchange surface
= (surface area x (difference in concentration)) / length of diffusion path

11. In order to ensure maximum diffusion, what can we do?

12. A typical human erythrocyte has a disk diameter of 6–8 µm and a thickness of 2 µm, being much smaller than most other human cells. These cells have a volume of about 90 fL with a surface of about 136 μm2, and can swell up to a sphere shape containing 150 fL, without membrane distension.
Adult humans have roughly 2–3 × 1013 (20-30 trillion) red blood cells at any given time, comprising approximately one quarter of the total human body cell number (women have about 4 to 5 million erythrocytes per microliter (cubic millimeter) of blood and men about 5 to 6 million; people living at high altitudes with low oxygen tension will have more).