Introduction

 The cell is the basic unit of life  All living things made up of cells  ALL CELLS COME FROM PRE-EXISTING CELLS…… So how does this happen? Think about some ways we have discussed that cells replicate themselves.

 Binary Fission  Budding

 Your cells do not duplicate by binary fission or budding, but by following an orderly sequence called The Cell Cycle.

 Do you think your cells are dividing now??  YES!! How do you know??  You are still growing!  Why do cells divide?  GROWTH  REPAIR  REPLACE OLDER CELLS

 The life expectancy for skin cells is about 35 days, so those cells must be replicated to replace the skin cells that slough off. By the time you are 20 yrs, you will have been through ~200 sets of skin!  The life expectancy for cells that line your gut is about 24 hours, so your entire lining is replaced every day!!

 The cell cycle is very orderly and has checkpoints that determine when a cell is ready to divide.  Today we will look at two factors that prompt a cell to divide –  Surface Area  Volume

 The surface of a cell, its membrane, is the site of exchange between its interior and its external environment. This surface must allow sufficient exchange to support the contents of the cell.  The volume of the cell increases much faster than the surface area of the membrane.  This ratio limits how large cells can be.