# Bioilogy Essay

2958 words - 12 pages

AQA Biology RevisionChapter 10: Exchange and Transport SystemsSize and Surface AreaOrganisms need to exchange substances with their environmentEvery organism, whatever its size, needs toe exchange things with the environment.Cells need to take in oxygen (for aerobic respiration) and nutrients.They also need to excrete waste products like carbon dioxide and urea.Most organisms need to stay at roughly the same temperature, so heat needs to be exchanged too.How easy the exchange of substances is depends on the organism's surface area to volume ratio.Smaller animals have higher surface area : volume rationsA mouse has a bigger surface area to relative to its volume than a hippo. This can be hard to imagine, but you can prove it mathematically. Imagine these animals as cubes:The hippo could be represented as a block measuring 2cm x 4cm x 4cm.It's volume is 2 x 4 x 4 = 32cm cubed.It's surface area is 2 x 4 x 4 = 32cm squared (top and bottom surfaces of cube) + 4 x 2 x 4 = 32cm squared (four sides of the cube)Total surface area = 64cm squared.So the hippo has a surface area : volume ration of 64 : 32 or 2 : 1Compare this to a cube mouse measuring 1cm x 1cm x 1cm.It's volume is 1 x 1 x 1 = 1cm cubed.It's surface area is 6 x 1 x 1 = 6cm squared.So the mouse has a surface area : volume ration of 6 : 1Multicellular organisms need exchange organs and mass transport systemsAn organism needs to supply everyone of ne of its cells with substances like glucose and oxygen (for respiration). It also needs to remove waste products from every cell to avoid damaging itself.In single-celled organisms, these substances can diffuse directly into (or out of) the cell across the cell surface membrane. The diffusion rate is quick because of the small distances the substances have to travel.In multicellular animals, diffusion across the outer membrane is too slow, for two reasonsSome cells are deep within the body - there's a big distance between them and the outside environment.Larger animals have a low surface area to volume ratio - it's difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface.So rather than using straightforward diffusion to absorb and excrete substances, multicellular animals need specialised exchanged organs (like lungs).They also need an efficient system to carry substances to and from their individual cells - this is mass transport. In mammals, "mass transport" normally refers to the circulatory system, which uses blood to carry glucose and oxygen around the body. IT also carries hormones, antibodies and waste like CO2.Body size and shape affect heat exchangeAs well as creating waste products that need to be transported away, the metabolic activity inside cells creates heat. Staying at the right temperature is difficult, and it's pretty heavily influenced by your size and shape…SizeThe rate of heat loss from an organisms depends on its surface area. As on the previous page, if an...

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