All living organisms must exchange substances with their environment to sustain life, taking in essential molecules like oxygen and glucose while removing waste products such as carbon dioxide and urea. This topic explores the relationship between the size of an organism and its surface area to volume ratio, explaining why multicellular organisms require specialized exchange surfaces and transport systems. It covers the fundamental physical processes of diffusion, osmosis, and active transport, and applies these principles to biological systems including the human respiratory and digestive systems, as well as plant root and shoot systems.
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