Transcript File

Why are Cells Small?
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cells must exchange nutrients & other molecules with
their environment
food must come in and wastes go out
the cell membrane is the only way in and out of the
cytoplasm
the cell membrane has a limited surface area
the cytoplasm has a volume to “feed” or have
wastes removed
Surface area of a cell
Distance equals on unit
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If it were a perfect square, how would you calculate
surface area?
Each side is 1 x 1=1unit squared, 6 sides therefore,
surface area = 6 units squared
Volume of a cell - cytoplasm
Distance equals one unit
Volume equals: Length x Width x Height
Thus the volume equals 1 x 1 x 1
Volume equals 1 unit cubed
Comparison of Size, Surface Area &
Volume
Cell size
Surface area
(units2)
Volume
(units3)
1
1 unit
6
2 units
24
8
3 units
54
27
Ratio of
SA/V
6/1
24/8
3/1
54/27
2/1
Conclusion
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As cell size increases ratio of surface area to
volume decreases.
Therefore, the rate of diffusion slows down….
…..thus a cell cannot get rid of wastes fast enough
or get nutrients fast enough
. . . so it will either be poisoned by its wastes or
starve.
Surface Area to Volume
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Cell surface area is important in taking in nutrients
Surface area increases as the square of cell
diameter
But… entire cell volume needs to be fed
And, cell volume increases as the cube of cell
diameter so it increases at a faster rate
Consider 2 Cells...
Surface Area to Volume
Cell Radius
(R)
5 µm
50 µm
Surface Area
(4πr2)
Volume
3
(4/3πr )
Surface Area
to Volume Ratio
314 µm2 31,400 µm2
3
524 µm3 524,000 µm
0.6
0.06
Prokaryotes vs. eukaryotes
All living cells can be divided into two
major types:
1. Prokaryotes – simple cells that lack a
nucleus, always single-celled
2. Eukaryotes – complex cells with a
distinct nucleus, can be single-celled or
multicellular
Prokaryotic cells
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lack a nuclear membrane – the DNA floats freely in
the cytoplasm
lack any membrane-bound organelles (like
mitochondria)
Plant vs. animal cells
Within eukaryotes, there are two major kinds of
cells:
1. Plant cells – stiff cell wall gives the cell
shape, large vacuoles, special organelles
called chloroplasts
2. Animal cells – lack cell walls and
chloroplasts, vacuoles are small, centrioles
are visible
What they have in common:
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both have a cell membrane
both have a nucleus with DNA inside
both have many of the common
organelles (ER, Golgi complex, etc.)
both have mitochondria for making
energy
Common structures in plant and animal
cells:
Organelles found in plant cells
Cell wall – made of cellulose,
gives cell strength and
protection
Larger vacuoles – store water
and keep cell rigid
Chloroplasts – make sugar
from CO2 and sunlight photosynthesis
Organelles found in animal cells
Centrioles – attachment
for the cytoskeleton,
organise spindle fibres
during cell division
Plant Cells
Animal cells
Have a cell wall
Do not have a cell wall
Have chloroplasts
Do not have chloroplasts
Have large vacuoles
Have small vacuoles
No visible centrioles
Centrioles visible
Some exceptions to this!
Protist cells
 single-celled eukaryotes (protists) can have organelles
typical of both plant AND animal cells
Fungus cells
 multicellular eukaryotes with a cell wall made of chitin
(the same substance in the exoskeleton of insects)
 cell walls can remain open after cell division, and the
cells are free to move back and forth