¨ 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.
Ø 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
Ø 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.
