Download Cellular biology

Cell Structure
&
Function
Chapter #3
Pages 45-63
Cell Theory
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
1.
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 10-100um.
 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
 The volume of the same
1cm cube would be:
 L x W x # of sides
1 cm cube would be:
 LxWxH
 1cm x 1 cm x 6 = 6 cm2
 1cm x 1 cm x 1 cm =1cm3
This cell then grew and doubled in size
 The surface area of a
 The volume of the same
2cm cube would be:
 L x W x # of sides
1 cm cube would be:
 LxWxH
 2cm x 2 cm x 6 = 24 cm2
 2cm x 2 cm x 2 cm =8cm3
Ratios
 The original surface area:volume ratio was
6cm2:1cm3 for the 1cm cube
 The new surface area:volume ratio became
24 cm2:8cm3 for the 2cm cube


Reduced (divide both by 3) the new ratio
became 3cm2:1cm3
What this means is that in the larger cell there
is now only half the surface area available to
service each 1cm3
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.
http://www.cellsalive.com/cells/cell_model.
htm