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Cell Size As the cell grows larger the cells DNA will no longer be able to serve the increasing needs of the growing cell. The larger the cell the more trouble it has moving nutrients such as food, oxygen and water and wastes across the cell membrane. As the cell increases in size it does not make an extra copy of DNA so the DNA that it has will no longer be able to meet the needs of the growing cell. The rate at which the nutrients and wastes are exchanged depends on the surface area of a cell. Surface Area (SA)= 6 x heigth x width The rate at which nutrients are used up and waste products are produced depends on the cells volume. Volume (V)= length x width x height There is a relationship between a cells volume and its surface area and why a cell must divide as they grow As a cell increases in size, its volume increases faster than its surface area Ideally, a cell wants to have a large surface area with a small volume or a large SA/V ratio Find the SA/V ratio of a 4cm x 4 cm x 4 cm cube. SA = 6 x 4 x 4 = 96 V = 4 x 4 x 4 = 64 SA/V = 96/64 SA/V=48/32 SA/V=24/16 SA/V=12/8 SA/V=6/4 SA/V= 3/2 Find the surface area/volume ratio of a 1 cm x 1cm x 1 cm cube. SA for a cube = 6 x height x width V = height x width x length SA/V = 6 x 1 x 1/1 x 1 x 1 = 6/1 Now find the SA/V ratio of a 2x2x2 cm cube. SA= 6x2x2 V=2x2x2 SA/V=24/8 SA/V=12/4 SA/V=6/2 SA/V=3/1 Now find the SA/V ratio of a 3x3x3 cm cube. SA= 6x3x3=54 V= 3x3x3=27 SA/V=54/27 SA/V=18/9 SA/V=6/3 SA/V=2/1 6/1 3/1 2/1 Cell division-the splitting of a single cell into 2 daughter cells A cell must divide when: 1. A cell has grown so that the nucleus can no longer control all of the cytoplasm 2. The SA/V ratio is too low Growth- Increase in size Repair- Accomplished by cell regeneration at the site of injury Reproduction- Either asexually (requires one parent) or sexually (requires two parents) http://mrsmaineswiki.wikispaces.com/Ia nW