Cell Structure & Function Chapter #3 Pages 45-63

Cell Theory 1. All organisms are composed of one or more cells 2. Cells are the basic living unit of structure and function in organisms 3. All cells come only from other cells

Cell Size Multicellular organisms are easily seen because we are composed of trillions of cells. Each of these cells contain tiny specialized structures called organelles which carry out specific cellular functions. Individual cells are microscopic ranging typically from um. Because they are limited in their size, in order for an organism to grow it must increase the number of cells.

Why is it that cells can’t get bigger? A basic input/output ratio dictates that as cells grow they will require more nutrients and create more wastes. The problem is that their transport mechanism, the cell membrane, does not grow proportionally with the body of the cell, the cytoplasm. Although both grow, the cytoplasm increases at a greater rate than the cell membrane Therefore, the surface area:volume ratio dictates when a cell can no longer support itself and therefore must divide

Imagine the cell was shaped as a cube. The surface area of a 1cm cube would be: L x W x # of sides 1cm x 1 cm x 6 = 6 cm 2 The volume of the same 1 cm cube would be: L x W x H 1cm x 1 cm x 1 cm =1cm 3

This cell then grew and doubled in size The surface area of a 2cm cube would be: L x W x # of sides 2cm x 2 cm x 6 = 24 cm 2 The volume of the same 1 cm cube would be: L x W x H 2cm x 2 cm x 2 cm =8cm 3

Ratios The original surface area:volume ratio was 6cm 2 :1cm 3 for the 1cm cube The new surface area:volume ratio became 24 cm 2 :8cm 3 for the 2cm cube Reduced (divide both by 3) the new ratio became 3cm 2 :1cm 3 What this means is that in the larger cell there is now only half the surface area available to service each 1cm 3

Summary As a cell increases in size, the surface area does not increase proportionally to the volume. As a result, the cell membrane will not be able keep up with the increased demands for nutrients/wastes that the larger volume demands. At this point the cell must divide in order to survive.