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Cell Sizes &
Growth
Limitations
Volume
• Volume determines the amount
metabolism in the cytoplasm
• Metabolism will require import of
precursors
• Metabolism will result in the
export of secretions
• And the export of excretory
products
© 2010 Paul Billiet ODWS
Surface area
• Surface area will determine the
exchange of materials between the cell
and its environment
• Bigger cells will metabolise more
• But they will need more surface to
support that metabolism
• And the sites of metabolism in bigger
cells will be further from the surface of
the cell
© 2010 Paul Billiet ODWS
Growing cells
1µm
2µm
3µm
4µm
5µm
6µm
© 2010 Paul Billiet ODWS
How are surface area and
volume affected by growth?
Size
/ µm
1
2
3
4
5
6
Surface
area
/ µm2
6
24
54
96
150
216
Volume
/ µm3
1
8
27
64
125
216
SA/V
6
3
2
1.5
1.2
1
© 2010 Paul Billiet ODWS
Comparing Surface Area and
Volume changes
1000
900
800
700
600
Surface area
Volume
500
400
300
200
100
0
0
1
2
3
4
5
6
Size / µm
© 2010 Paul Billiet ODWS
7
8
9
10
Surface Area:Volume Ratio
6
5
SA/V
4
3
2
1
0
0
1
2
3
4
5
6
Size / µm
© 2010 Paul Billiet ODWS
7
8
9
10
Solving the problem
• Bigger cells have a greater
metabolism than smaller cells
• BUT bigger cells have a
proportionally less surface for
exchange
• How to increase surface area with
increasing size?
© 2010 Paul Billiet ODWS
EQ: What factors controls or limit
the Cell Size?
1. Diffusion limits size
2. DNA content limits size
3. Surface area to volume
ratio limits size
How does Diffusion limit cell
size?
An oxygen molecule follows a path to
the mitochondria
(Oxygen)…
plasma membrane  cytoplasm 
mitochondria
Smaller size allows the cell
to be more efficient
•
EQ: How does DNA content limits
size?
• Cells usually contain only one nucleus
•DNA
•DNA
•RNA
•RNA
•DNA
is found in the nucleus
makes RNA
will travel through the cytoplasm to the ribosomes
assists the ribosomes in making proteins
is responsible for the whole above process
• If the cell was very large, there would not be enough
DNA to support the cell.
EQ: How does Surface area to
volume ratio limits cell size
• As a cell increases, it volume increases much faster
than its surface area
• If a cell doubled, the cell would require 8X more
nutrients and have 8X more waste to get rid of
FYI – If E.coli were left unreglated, it could
engulf the Earth in one day because it doubles in
volume every 30 minutes!!
EQ: When do cells divide?
• All organisms grow and change.
• Cells are constantly growing, dividing, and dying
• Some cells divide more than others –
•Heart & nerve system cells rarely (if ever)
divide
•Skin cells, on the other hand, are constantly
dividing
•Growth is related to cell specificity and activity
•If cell growth is uncontrolled – leads to cancer
DATA TABLE: Cell Size Comparison
Cell
1
2
3
Dimensions
(cm)
Surface Area
(cm2)
Volume
(cm3)
Ratio
Surface area
to Volume