• What does the city do with wastes?What does the city do with wastes?• How does a city get new nutrients?How does a city get new nutrients?

How Big is Too Big?

• All living organisms are made up of cells.

• Cells are really small. Why?

• Why can’t a human be made of just one big cell instead?

• All cells diffuse nutrients and wastes in and out at a constant rate.

• Basically, the more surface area (aka cell membrane), the more nutrients and wastes a cell can exchange.

• Using the formulas for volume and surface area, we can make a ratio to compare the two.

Surface Area to Volume Assignment

• Follow along for two examples then complete the rest of the assignment using the formulas provided.

• Ratio: SA / V

• If we were to watch nutrients being absorbed at a constant rate in 3 different size cubes, the nutrients could reach the entire cytoplasm of smaller cells because, for their volume, they had a much larger surface area.

Dead: nutrients can’t get to all parts of the cell.

Better, but still dead.

Alive! Nutrients make it to all parts of the cell.

4cm3 1 cm32 cm3

Surface Area = (base) (height) added up for all sides

S.A.= (4cm)(4cm)(6 sides)

= 96 cm2 x 1 cube = 96 cm2

S.A.= (2cm)(2cm)(6 sides)

= 24 cm2 x 8 cubes = 192 cm2

S.A.= (1 cm)(1cm)(6sides)

= 6 cm2 x 64 cubes = 384 cm2

Volume = (base) (width)

(height) V = (4cm)(4cm)(4cm)

= 64cm3 x 1 cube = 64cm3

V = (2cm)(2cm)(2cm)

= 8cm3 x 8 cubes = 64 cm3

Surface Area to Volume Ratio

V = (1cm)(1cm)(1cm)

= 1cm3 x 64 cubes = 64 cm3

Volume

Area Surface

€

S.A.

V=

96

64=1.5 3

64

192..

V

AS 664

384..

V

AS

The smaller the cell,the larger the SA / V ratio, and theeasier it is for diffusion to reach

all parts of the cell

• Shape also affects surface area to volume ratio.

16

7.11

• Skinnier, flatter cells large SA / V ratios.

• Cells with projections, like microvilli, have large SA / V ratios.

• In order for an organism to be large, it must be multi cellular.• Which of the equal volumes below has the greatest SA / V Ratio and is therefore most likely to survive?

• Distribution systems, such as your circulatory system deliver nutrients to and remove waste from each cell.

• Diffusion is sufficient because of each cell’s large SA / V ratio.