Why are cells so small?

1. Cells HAVE to be small to be efficient

I.) What limits the size of a cell?• Most living cells are between 2 to 200 um in

diameter –Ex- if the cell size doubles it would need 8x

more nutrients to survive and it would create 8x the waste to excrete!!

• WHY? Let’s do the math…

Surface Area to Volume Ratio

• The volume increases faster than the surface area of the cell membrane!!

Need a proper Surface Area: Volume ratio to support the needs of the cell

2. Rate of Diffusion

• Remember the function of the cell membrane?– allows nutrients (ex. Glucose, oxygen,

etc.) in and wastes out (diffusion)

• Diffusion works better over short distances (agar lab)

3. Need enough DNA to support protein needs of cell

DNA codes for making___________– there needs to be enough DNA to support

the protein needs of the cell

ex. Think about how much protein is in the cell membrane

-if the cell size increases, you will need more protein!

Solving the problem

• Bigger cells have a greater metabolism than smaller cells

• BUT bigger cells have a proportionally less surface for exchange

• How to increase surface area with increasing size?

© 2007 Paul Billiet ODWS

Throw out extensions

Microvilli of small intestine epithelium

© copyright 2001 Gwen V. Childs, Ph.D., University of Arkansas for Medical Sciences

Flatten into a thin film

Chick blastodisc© Developmental Biology Online © Developmental Biology Online

Divide the cytoplasm into smaller volumes

Early human embryos

Zygote

8-cell Embryo

© Reproductive Medicine and Fertility Center

Multicellular organisms show the same adaptations

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

© Text 2007 Paul Billiet ODWS

Multicellular organisms show the same adaptations

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

lithograph plate 20th U.S. edition of Gray's Anatomy of the Human Body, originally published in 1918. This image is in the public domain because its copyright has expired

© Text 2007 Paul Billiet ODWS