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WHEN/WHY DO CELLS DIVIDE? BIGGER IS NOT BETTER! • IMPORTANCE OF CELL MEMBRANE – Controls what goes into/out of a cell – All cells possess a cell membrane – Cells must have a membrane surface large enough for all necessary exchanges. • O2, CO2, H2O SURFACE AREA TO VOLUME RATIO • Smaller cells have LARGER SA/V ratios! • Larger cells have SMALLER SA/V ratios! SURFACE AREA TO VOLUME RATIO • As cells increase in size, the volume increases more rapidly than does surface area. • This makes it more difficult for nutrients to diffuse into the center of a cell. – The nucleus of a cell is often a central structure! CARDS READY!! Here we go… Which cell would be more efficient at delivering Oxygen to the nucleus? A) A 3 cm cell B) A 2.5 cm cell C) A 5 cm cell D) Oxygen doesn’t diffuse into cells. Which type of cells need a plasma membrane? A) Eukaryotic B) Prokaryotic C) Plant D) All of the above Which of the following would have the lowest efficiency in ridding the cell of waste? A) A cell with a 15 SA/V ratio B) A cell with a 3 SA/V ratio C) A cell with a 6 SA/V ratio D) Not enough information to answer. Which of the following explains why cell need to remain small in order to be efficient? A) Materials diffuse faster in a smaller cell B) The nucleus does not have very far to travel in a smaller cell. C) Materials have a shorter distance to travel to reach the nucleus. D) None…the cell should remain bigger. Moving on…. CELLS DIVIDE FOR: • GROWTH – As the number of cells increases, an organism grows. – Differentiation • REPAIR – Repair damaged tissue – Mending skin, bone, blood – Replacing old cells CELLS DIVIDE FOR: • REPRODUCTION – ASEXUAL • One organism produces an identical copy of itself – SEXUAL • Two organisms join genetic material to produce a unique individual