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Summary OCR A/AS-Level Biology 3.1. Exchange Surfaces and Circulatory Systems
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Revision Summary of key knowledge from OCR A/AS-Level Biology 3.1. Exchange Surfaces and Circulatory Systems.
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The Need for SpecialisedGas Exchange Surfaces
Size
Larger animals = greater demand for materials and greater amounts of waste produced
Heat is also produced which needs to be dissipated to prevent overheating and
denaturing of enzymes
Central cells are far from surface – greater diffusion distance and materials cannot be
exchanged fast enough by diffusion to sustain life
Surface Area to Volume Ratio (SA:V)
Single-celled organisms (such as amoeba and bacteria) have a large SA:V ratio so can
rely on diffusion to get gases and nutrients in and waste products out
Their diffusion pathways are short enough for substances to diffuse efficiently
Multi-cellular organisms have a large SA:V ratio so diffusion is no longer effective to
transport materials
Level of Activity (Metabolism)
Some organisms are more active than others
Metabolic activity uses energy from food and requires oxygen to release this energy in
aerobic respiration
Mammals have an especially high metabolism since they need to keep warm
Features of a Good Exchange System
Large SA:V ratio
Thin permeable barrier to exchange
Good blood supply – maintain high diffusion gradient
Moist surface – allows gases to dissolve
Ventilation – maintain high diffusion gradient
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