Download Bio392-Chapter 10-1

Why are cells so small?
Cells are found in every living
organism, ranging from unicellular (onecelled) organisms, such as amoebas, to
multicellular (many-celled) organisms,
such as human beings. Even though
each organism is very unique, their cells
are typically about the same small size.
This similarity then leads to the
questions:
1. Why are cells so small?
2. Why are we composed of billions of
tiny cells instead of a few large cells?
Getting Through
• Materials move through cells by diffusion. Oxygen
and food move into cells, while waste products
move out of cells. How does the size of a cell affect
how efficiently materials get to all parts of a cell?
• Work with a partner to complete this activity.
1. On a sheet of paper, make a drawing of a cell that
has the following dimensions: 5 cm x 5 cm x 5 cm.
Your partner should draw another cell about one
half the size of your cell on a separate sheet of
paper.
2. Compare your drawings. How much longer do you
think it would take to get from the cell membrane to
the center of the big cell than from the cell
membrane to the center of the smaller cell?
3. What is the advantage of cells being small?
Cell Growth
Chapter 10.1
Reasons For Cell Division
The larger a cell becomes, the more demands
the cell places on its DNA.
 In addition, the cell has more trouble moving
enough nutrients and wastes across the cell
membranes
DNA Overload
• DNA  contains the
information that controls a
cell’s functions
– Eukaryotes DNA is found in
the nucleus
– Prokaryotes DNA is found in
the cytoplasm
• As cell increases in size, DNA
does not have enough
information to maintain it and
service the needs of the cell
Exchanging Materials: Surface Area
• Rate at which exchange of wastes, nutrients, and
gases takes place depends on the surface area of
the cell
Surface area = length x width x # of sides
What is the surface area of the cube?
5
Surface area = l x w x # sides
5
5
 Surface area = 5 x 5 x 6 = 150
Exchanging Materials: Volume
• Rate at which nutrients and gases are used up
and wastes are made depends on volume of the
cell
Volume = length x width x height
What is the volume of the cube?
5
Volume = l x w x h
5
5
 Volume = 5 x 5 x 5 = 125
Exchanging Materials
Volume increases much faster than surface area
when a cell is growing
– Exchange of materials cannot keep up with the demand
–  Materials have to travel farther to get to all parts of cell
Surface Area-to-Volume ratio
Smaller cells have a
larger surface area to
volume ratio
– Means there is more
surface area for
materials to fit into
Ratio of Surface Area to
Volume in Cells
Section 10-1
Cell Size
Surface Area
(length x width x 6)
Volume
(length x width x height)
Ratio of Surface Area
to Volume