s7 - 2 it should be pretty obvious that an increase in size...

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Animal Physiology Physiology is the study of how organisms work--how they have evolved strategies and structures for such things as nutrition, gas exchange, excretion and reproduction, and how these strategies and structures are under the control of the central nervous system and the endocrine system. In single cells, or single-celled organisms, the plasma membrane and intracellular diffusion provide for the intake of needed food and oxygen and excretion of unwanted CO 2 and the nitrogen waste (NH 3 ) that is produced when amino acids are broken down. But the rates of this diffusion are limited by the surface to volume ratio and the diffusion distances from the surface to internal organelles. For example: given a single cuboidal cell whose single side length is the following: What this tells us is that as a cell gets bigger, it's surface area relative to the volume gets smaller. If the surface is the only "window" to the outside's nutrients or O
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Unformatted text preview: 2 , it should be pretty obvious that an increase in size presents a problem. Thus, provision of necessary food and oxygen to a single cell is limited by its size. Since food and oxygen must diffuse through the cell's cytoplasm to the site of metabolism (the mitochondria), there is also a diffusion limitation . This means that natural selection has limited the size of single cells and single-celled organisms. Natural selection has also favored the evolution of organ-systems in multicellular animals that provide for the needs of the cells: Digestive System, Gas Exchange System, Circulatory System and Heart, Excretory System (kidney), Nervous System, Muscle System, Endocriine System, Reproductive System, etc. Length (L) Surface Area: L 2 x 6 Volume: L 3 Surface/Volume Ratio 1 6 1 6 2 24 8 3 5 150 125 1.2 10 600 1000 0.6...
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