LIMITS TO SIZE: Why are Cells Small Lab?

Why are cells so small?

The answer to this question has as much to do with mathematics as biology. Imagine a cell that is shaped like a cube. As the cell increases, its surface area to volume ratio changes.

Surface area Surface area is the total area of the

outside surface. For cubes it is expressed in squared units (such as cm2 or m2).

The S.A. of a cell will determine the amount of materials that can be exchanged between the cell and its environment.

Equation: SA = length x width x # of sides

© 2010 Paul Billiet ODWS

Volume Volume is the amount of space

something occupies. It is expressed in cubic units (such as cm3 or m3).

The volume of a cell will determine the amount of chemical reactions or the metabolism of the cell.

Equation: V= length x width x height

© 2010 Paul Billiet ODWS

Surface Area to Volume Ratio

Surface Area to Volume Ratio is the comparison of the amount of surface area (what is on the outside) to the volume (what is on the inside)

Cells need to have a large surface area to its volume (hence a high S.A. to V ratio) because wastes need to be able to exit, and nutrients need to be able to enter the cell quickly.

Growing cells

1cm

2cm

3cm

4cm

5cm

6cm© 2010 Paul Billiet ODWS

How are surface area and volume affected by growth?

1:11.2:11.5:12:13:16:1

216125642781

2161509654246

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SA/V

Volume (cm3)

Surface area (cm2)

Length of side (cm)

© 2010 Paul Billiet ODWS

Comparing Surface Area and Volume changes

0

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0 1 2 3 4 5 6 7 8 9 10

Size / µm

Surface areaVolume

© 2010 Paul Billiet ODWS

(cm)

Surface Area:Volume Ratio

0

1

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0 1 2 3 4 5 6 7 8 9 10

Size / µm

SA

/V

© 2010 Paul Billiet ODWS

(cm)

As cells get larger, the S.A.:V gets __________________.smaller

Since transport of materials in and out of the cell can only happen at the cell’s surface, what problem does this pose?

Bigger cells have a greater volume (meaning a greater metabolism) than smaller cells

BUT bigger cells have a proportionally less surface for exchange.

Diffusion can’t take place as quickly!© 2010 Paul Billiet ODWS

1. Throw out extensions

Microvilli of small intestine epithelium

Image Credit www.cytochemistry.net/

What are ways to increase surface area while increasing size.

2. Flatten into a thin film

Chick blastodisc

Image Credits http://anatomia.uw.hu/

3. Or just divide the cytoplasm into smaller volumes

Early human embryos

Zygote

8-cell Embryo

Image Credit http://www.advancedfertility.com/ Image Credit http://www.rmfcfertility.com/

Do larger organisms have larger cells or more cells than smaller organisms?

More Cells!!!!

Multicellular organisms show the same adaptations

Flowering plants have an extensive, branched rooting system to absorb water and minerals

Image Credit http://ecomerge.blogspot.com

Multicellular organisms show the same adaptations

Mammals have a long small intestine with internal folding to absorb digested food

Image Credit C. Candalh, INSERM