OCR Yr12 Biology A 3.1.1 Exchange surfaces
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| What is surface area to volume ratio? | The surface area of an organism divided by its volume, expressed as a ratio.
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| Which 3 main factors affect the need for an exchange system? | 1. Size
2. SA:Vol ratio
3. Level of activity
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| Why do many multicellular organisms require specialised exchange surfaces. | They have several layers of cells creating a longer diffusion pathway and therefore diffusion is too slow and inefficient to supply the innermost cells.
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| Describe the SA:Vol ratio of small organisms | Large SA:Vol ratio
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| Describe the SA:Vol ratio of large organisms | Small SA:Vol ratio
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| Some organisms increase their surface area without increasing their volume excessively, eg. flatworms have thin, flat bodies. What are the limitations of this? | It limits the overall size of the organism. Most large organisms need a range of tissues for support and strength this increases the thickness and therefore the volume of their bodies.
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| How is the surface area of a sphere calculated? | 4 pie r squared
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| How is the volume of a sphere calculated? | 4/3 pie r cubed
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| Why do mammals and organisms with high metabolic rates need specialised exchange systems? | More oxygen and nutrients are required for energy release in aerobic respiration.
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| Root hairs in a plant are an example of which feature of a good exchange system? | Increased surface area with many additional folds in the membranes.
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| Describe the features of barriers found in good exchange systems. Give an example. | They are thin and permeable providing a short diffusion pathway for efficient exchange. Alveoli walls are 1 cell thick providing a barrier of only 0.3 micrometers between blood and air.
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| How does a good blood supply aid an efficient exchange surface? Give an example. | It maintains a steep concentration gradient so that diffusion occurs rapidly. Eg. gills in fish.
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