Exchange between Organisms and their Environment

• What needs to be exchanged between an organism and its environment?

• Respiratory Gases e.g. ?• Nutrients e.g. ?• Excretory Products e.g. ?• Heat

• How are these substances exchanged between an organism and its environment?

• Passively by ?• Actively by ?

Learning Outcomes:• Candidates should be able to explain the

significance of the relationship between size and surface area to volume ratio for the exchange of substances.

• Candidates should be able to explain why multicellular organisms need specialised exchange surfaces and single celled organisms do not.

Surface area to volume ratio

Surface Area to Volume Ratio(SA:V)

Length of edge of

cube / mm

SA of whole cube / cm2

Volume of cube / cm3

Ratio of SA to Volume

Clearing Time (seconds)

10

5

2.5

20 X 10 X 5

40 X 5 X 5

Sketch a graph of the data:

Length of edge of a cube / cm

Clearing time (seconds)

Questions

• Some of the cubes have the same volume but different surface areas. What is the effect of increasing the surface area on the clearing time?

• Why is it advantageous for a small animal to be long and thin rather than cube shaped?

Conclusions:• As an organism becomes larger, their volume

increases at a faster rate than their surface area. Problems?

Only inactive organisms It would take too could be supported long to reach the middle of the

organism

• Solutions? Flattened shape • OR Specialised exchange surfaces

Features of specialised exchange surfaces:

• Large SA:Vol – WHY?• Thin barrier– WHY?• Partially permeable – WHY?• Movement of the environmental medium

e.g. air – WHY?• Movement of the internal environmental

medium e.g. blood – WHY?

• Explain in your own words why unicellular organisms do not need specialised exchange surfaces but multicellular organisms do?

• How is a plant root adapted for taking in water and mineral ions?

Plenary