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 vol. 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)

Diffusion 1. Diffusion: movement of particles from high to low concentration across the membrane Very fast & efficient over short distances Becomes slow & inefficient as the distances become larger Organisms would die before nutrients could reach the organelles if the cell was too big

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!

DNA 1. There’s a limit as to how fast DNA can code for proteins in nucleus and then made into proteins in the ER 2. Cell cannot survive unless there is enough DNA to support the protein needs of the cell

Surface area – to – volume ratio (Fig 8.9) 1. As cell size increases, its volume increases much faster than surface area 2. Example: If cell size doubles, cell would require 8x more nutrients, but the membrane would only increase by 4x. This would mean that cell doesn’t have enough room to diffuse

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 © Developmental Biology Online © Developmental Biology Online Chick blastodisc

Divide the cytoplasm into smaller volumes 8-cell Embryo Early human embryos Zygote © Reproductive Medicine and Fertility Center

Multicellular organisms show the same adaptations Flowering plants have an extensive, branched rooting system to absorb water and minerals © Illinois Enviromental Protection Agency © 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