Biology 2014-2015

Section 10-1 Cell Growth
SWS Biology
SWS Life Science

       Living things grow by producing more cells.


Limits to Cell Growth

       There are two reasons why cells divide rather than continuing to grow indefinitely:

1.       The larger a cell becomes, the more demands the cell places on its DNA.

2.       The cell has more trouble moving enough nutrients and wastes across the cell membrane.


DNA “Overload”

       The information that controls a cell’s function is stored in a molecule known as DNA.

       DNA is found in the nucleus.

       When the cell is small, the information stored in DNA is able to meet all of the cell’s needs.

       As a cell increases, it does not make extra copies of DNA.

       If a cell grows without a limit, an “information crisis” would occur.

       A larger cell makes greater demands on the available genetic “library”.

       In time, the cell’s DNA would no longer be able to serve the increasing needs of the growing cell.


Exchanging Material

       Food, oxygen, and water enter a cell through its cell membrane and wastes leave the same way.

       The rate of exchange depends on the surface area of the cell, which is the total area of its cell membrane.

       The rate at which food and oxygen are used up and waste is produced depends on the cell’s volume and its surface area is the key to understanding why cells must divide as they grow.


Ratio of Surface Area to Volume

       Imagine a cell that is shaped like a cube.

q       Length of 1 cm 

q       Surface area = length x width x number of sides

q       Surface area = 1 cm x 1 cm x 6 = 6 cm2


v      Volume = length x width x height

v      Volume = 1 cm x 1 cm x 1 cm = 1 cm3


       To get the ratio of surface area to volume, you need to divide the surface area by the volume.

       The ratio of surface area to volume is 6/1, or 6:1.


       What if the length was doubled?

q       Length of 2 cm

q       Surface area = 2 cm x 2 cm x 6 = 24 cm2

v      Volume = 2 cm x 2 cm x 2 cm = 8 cm3

       The ratio of surface area to volume is 24/8 or 3:1.


       The volume increases much more rapidly than the surface area, causing the ratio of surface to volume to decrease.

       If the cell gets too large, it is more difficult to get sufficient amounts of oxygen and nutrients in and waste products out.


Division of the Cell

       A growing cell divides forming two “daughter” cells.

       Cell division is the process by which a cell divides into two new daughter cells.

       Before cell division occurs, the cell replicates, or copies, all of its DNA.

       Each daughter cell receives one complete set of genetic information.

       Each daughter cell has an increased ratio of surface area to volume allowing efficient exchange of materials.