Cell%20Size

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Cell Size
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Why are Cells Small?

• 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

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Surface area of a cell

• If it were a perfect square, how would you
  calculate surface area?
• Each side is 1 x 11unit squared, 6 sides
  therefore, surface area 6 units squared

Distance equals on unit
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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

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Comparison of Size, Surface Area Volume
Cell size Surface area (units2) Volume (units3) Ratio of SA/V
1 unit 6 1 6/1
2 units 24 8 24/8 3/1
3 units 54 27 54/27 2/1
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Conclusion

• 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.

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Surface Area to Volume

• 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

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Consider 2 Cells...

100 µm diameter
10 µm diameter
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Surface Area to Volume
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Cell types
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Prokaryotes vs. eukaryotes

• All living cells can be divided into two major
  types
• Prokaryotes simple cells that lack a nucleus,
  always single-celled
• Eukaryotes complex cells with a distinct
  nucleus, can be single-celled or multicellular

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Prokaryotic cells

• lack a nuclear membrane the DNA floats freely
  in the cytoplasm
• lack any membrane-bound organelles (like
  mitochondria)

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Plant vs. animal cells

• Within eukaryotes, there are two major kinds of
  cells
• Plant cells stiff cell wall gives the cell
  shape, large vacuoles, special organelles called
  chloroplasts
• Animal cells lack cell walls and chloroplasts,
  vacuoles are small, centrioles are visible

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What they have in common

1. both have a cell membrane
2. both have a nucleus with DNA inside
3. both have many of the common organelles (ER,
   Golgi complex, etc.)
4. both have mitochondria for making energy

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Common structures in plant and animal cells
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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

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Organelles found in animal cells

• Centrioles attachment for the cytoskeleton,
  organise spindle fibres during cell division

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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
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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