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 / cm 2 Volume of cube / cm 3 Ratio of SA to Volume Clearing Time (seconds) 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 organismsIt would take too could be supportedlong 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