Homework for next week

• P.176 green • P.176 red• 2nd lesson• Construct fish gill• Colour and label• P181 red

Exchange between organisms and their environment

p.176

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?

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?

Calculate the surface area to volume ratio of the cubes;

Side lengthcm

Surface areacm2

Volumecm3

Surface area to volume ratio

As one number

1

2

3

4

5

6

Aqa book p176-177

Surface area and volume

Calculate the surface are at volume ratio of the cubes; (calcs on p176)

Side lengthcm

Surface areacm2

Volumecm3

Surface area to volume ratio

As one number

1 6 1 6:1 6

2 24 8 24:83:1

3

3 54 27 54:272:1

2

4 96 64 96:641.5:1

1.5

5 150 125 150:1251.2:1

1.2

6 216 216 216:2161:1

1.0

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

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

How does the size of an organism relate to its surface area to volume ratio?

0 1 2 3 4 5 6 70

1

2

3

4

5

6

7

Length of one side /cm

Surface area to volume ratio

Small organismit 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 organismOnly has a small surface area to volume ratio so needs a specialised gas exchange surface for efficient diffusionLong diffusion distance so needs circulatory system

Why would my graph lose a mark?

How can we show in an experiment larger organisms need a specialised gas exchange

system?

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

1. Make the cubes with jelly2. Put them into liquid3. Diffusion to the centre of

the organism will occur fastest in the smallest cube

Fluid Jelly

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

Design…1. Why would we using the

same ‘block of jelly’?2. How much fluid is needed?3. What is the independent

and dependent variable in the investigation?

4. What things should be constant and why?

Fluid Jelly

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

1. Why are we using the same ‘block of jelly’? So the cubes can be compared as they will contain the same proportion of constituents

2. How much fluid is needed? Excess so it is not a limiting factor in diffusion

3. Independent = surface area to volume ratio or size of cube dependent = time taken to change colour/rate of diffusion

4. Temperature – an increase would speed diffusion Fluid concentration – greater concentration would speed diffusion

Fluid Jelly

Practical results(alkali cubes in acid)

Acid diffused into the jellyTurns indicator red

Centre still alkalineIndicator blue

1. Explain the results. Use the key words diffusion and neutralisation ‘from high to low concentration’ or concentration gradient

2. What is the conclusion? Include key words surface area to volume ratio, diffusion rate

Practical results

Acid diffused into the jellyTurns indicator red

Centre still alkalineIndicator blue

1. 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.

2. The larger the surface area to volume ratio the faster the acid diffuses into the centre of the cube

How do larger organisms increase their surface area

to volume ratio?

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

Is the use of jelly cubes to realistic?

Yes pointsDiffusion occurs through the jellyIt is easy to calculate surface area to volume ratio from a cube

No pointsNo organism is cubed in shapeNo organism has constant cell composition

How are surfaces specially adapted to facilitate exchange?

1. List the features of a specialised exchange surface

2. From p. 176 list the things which need to be exchanged (add water to the list)

3. What ways can these pass into/out of an organism

4. What organ systems does this involve in humans?

5. What has Fick’s law got to do with this?