1. Homework for next week P.176 green P.176 red 2nd lesson
Construct fish gill
Colour and label
P181 red

2. Exchange between organisms and their environment
p.176

3. Objectives
How does the size of an organism and its structure relate to its surface area to volume ratio?
How can we show in an experiment larger organisms need a specialised gas exchange system?
How do larger organisms increase their surface area to volume ratio?
How are surfaces specially adapted to facilitate exchange?

4. Make some cubes (these are fake organisms give them eyes)
Make a 1x1x1 cm cube
Make a 2x2x2 cm cube
Make a 6x6x6 cm cube
Which one has the largest surface area?
Which one has the largest surface area: volume ratio?

5. Calculate the surface area to volume ratio of the cubes;
Side length cm
Surface area
cm2
Volume
cm3
Surface area to volume ratio
As one number 1 2 3 4 5 6

6. Aqa book p

7. Surface area and volume
Teacher notes
This activity can be used by students to compare the relationship between surface area and volume between different types of shape, e.g. flat rectangles vs. cubes. Students could plot a graph to follow the trends.

8. Calculate the surface are at volume ratio of the cubes; (calcs on p176)
Side length cm
Surface area
cm2
Volume
cm3
Surface area to volume ratio
As one number 1 6
6:1 2 24 8
24:8
3:1 3 54 27
54:27
2:1 4 96 64
96:64
1.5:1
1.5 5
150
125
150:125
1.2:1
1.2
216
216:216
1:1
1.0

9. So which of your 3 cubes has the largest surface area to volume area?

10. How does the size of an organism relate to its surface area to volume ratio?
Draw a graph to show the effect of size on surface area to volume ratio
Onto the graph annotate where a small organism would be
Add that it has a large enough surface area to volume ratio for efficient exchange by diffusion across its body surface and a short diffusion distance to the centre of the organism
Add where a large organism would be on the graph
Add why this organism needs specialised exchange surfaces

11. How does the size of an organism relate to its surface area to volume ratio?
Why would my graph lose a mark?
Length of one side /cm
Small organism
it has a large enough surface area to volume ratio for efficient exchange by diffusion across its body surface (no gas exchange system)
short diffusion distance to the centre of the organism so no circulatory system needed
Large organism
Only has a small surface area to volume ratio so needs a specialised gas exchange surface for efficient diffusion
Long diffusion distance so needs circulatory system

12. How can we show in an experiment larger organisms need a specialised gas exchange system?

13. Make the cubes with jelly Put them into liquid
How can we show in an experiment larger organisms need a specialised gas exchange system?
Make the cubes with jelly
Put them into liquid
Diffusion to the centre of the organism will occur fastest in the smallest cube
Fluid
Jelly

14. Why would we using the same ‘block of jelly’?
How can we show in an experiment larger organisms need a specialised gas exchange system?
Design…
Why would we using the same ‘block of jelly’?
How much fluid is needed?
What is the independent and dependent variable in the investigation?
What things should be constant and why?
Fluid
Jelly

15. How can we show in an experiment larger organisms need a specialised gas exchange system?
Why are we using the same ‘block of jelly’? So the cubes can be compared as they will contain the same proportion of constituents
How much fluid is needed? Excess so it is not a limiting factor in diffusion
Independent = surface area to volume ratio or size of cube dependent = time taken to change colour/rate of diffusion
Temperature – an increase would speed diffusion Fluid concentration – greater concentration would speed diffusion
Fluid
Jelly

16. Practical results (alkali cubes in acid)
Explain the results Use the key words diffusion and neutralisation ‘from high to low concentration’ or concentration gradient
What is the conclusion? Include key words surface area to volume ratio, diffusion rate
Centre still alkaline
Indicator blue
Acid diffused into the jelly
Turns indicator red

17. Practical results
The acid has diffused into the cube from high to low concentration. As it moved through the cube it neutralised the alkali and the indicator went green, then red as the jelly became acidic.
The larger the surface area to volume ratio the faster the acid diffuses into the centre of the cube
Centre still alkaline
Indicator blue
Acid diffused into the jelly
Turns indicator red

18. How do larger organisms increase their surface area to volume ratio?

19. How do larger organisms increase their surface area to volume ratio?
The leaves on trees for gas exchange (and photosynthesis) spongy mesophyll layer has air spaces
Elongated or flattened shape e.g. a worm
Many alveoli in mammal lungs
Many filaments and gill plates in fish
Many tracheae in insects

20. Is the use of jelly cubes to realistic?
Yes points Diffusion occurs through the jelly It is easy to calculate surface area to volume ratio from a cube
No points No organism is cubed in shape No organism has constant cell composition

21. How are surfaces specially adapted to facilitate exchange?
List the features of a specialised exchange surface
From p. 176 list the things which need to be exchanged (add water to the list)
What ways can these pass into/out of an organism
What organ systems does this involve in humans?
What has Fick’s law got to do with this?