### Agar cubes - MrFranta.org

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Cells are limited in size – you can’t see them
without a microscope for good reason!
As a cell gets larger and larger (as it eats more
and more), it takes longer and longer to get
food/waste in and out
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Cell becomes “bloated”, can’t compete as well
with smaller, quicker cells
Needs to divide to become competitive again
Mitosis!
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Surface area – the total area on the outside of
an object
Volume – the amount of space taken up by an
object
The ratio between the two limits the sizes of cells!
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Surface Area (S.A.) =
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Area of all sides, added together
3cm cube
3 x 3 = 9cm2
6 sides to a cube
Total surface area = 54cm2
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Length x height x width
Same cube
3x3x3
Total volume = 27cm3
Volume
Ratio S.A.:Volume:
Surface Area / volume
 54:27 or 2:1
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2cm cube
Surface area:
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Volume:
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2x2x6
24cm2
2x2x2
8cm3
Ratio S.A.:Volume:
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24:8
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3:1
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Surface Area:
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Volume:
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1x1x6
6cm2
1x1x1
1cm3
Ratio S.A.:Volume:
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6:1
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0.1cm cube
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Surface area = 0.06cm2
Volume = 0.001cm3
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SA/V = 60:1
0.01cm cube
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Surface area = 0.0006cm2
Volume = 0.000001cm3
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SA/V = 600:1
The smaller the cell, the higher the
ratio is! This is why cells divide to stay
small!
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Cutting cubes of
agar into 1cm, 2cm,
and 3cm cubes
Phenolphthalein in
the agar cubes is an
indicator
Turns pink in base
 Turns clear in acid
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We will be soaking the cubes in vinegar (acetic
acid)
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Look to see how far it penetrates into larger vs.
smaller cubes
 Which do you think will diffuse faster?
 Why?
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This is similar to how long it takes food/waste to
diffuse in/out of larger vs. smaller cells
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Wear goggles AND aprons today
Cut your agar block down the middle, and split
between two groups
Carefully cut out your 1cm, 2cm, and 3cm
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Remember – cubes have the same height, length,
and width!
Be sure to accurately measure the length of
the sides, so you can do proper calculations
Place the three cubes in the 250ml beakers
Cover with vinegar and wait until
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After 10 minutes has expired, pull cubes out of
vinegar and gently rinse off
Measure the remaining Nuclear Pink Agar
Calculate the rate of diffusion
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Final Volume of Nuclear Pink Agar – Then Trash
Clean up & answer analysis questions
Return the following to Prep Table:
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Vinegar
Ruler
Plastic Knife,
Milk Carton (Clean, Dry)
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