McDougal Littell Science Cells and Heredity 

Table of Contents
Page 62

Cell size affects transport.

Most cells are very small. In fact, most cells are too small to be seen without a microscope. The average cell in your body is about 50 micrometers (0.05 mm) in diameter. Most of the cells on this planet are bacteria, which are only 3 to 5 micrometers in diameter. How can something as important as a cell be so tiny? Actually, if cells were not so small, they could never do their jobs.

Everything the cell needs or has to get rid of has to go through the cell membrane. The amount of cell membrane limits the ability of cells to either get substances from the outside or transport waste and other materials to the outside. This ability is related to surface area. The relationship between surface area and volume controls cell size. As a cell gets larger, its volume increases faster than its surface area if the cell maintains the same shape. Why does this matter?

INVESTIGATE Cells

How does cell size affect transport?

Demonstrate how small size helps make it possible for cells to get resources.

PROCEDURE

  1. 1 Cut a large piece of egg white from the egg.
  2. 2 Use a knife to trim the egg white into one small cube, about 1 cm square, and one large cube, about 2 cm square.
  3. 3 Pour 100 mL of water into the beaker. Add 10 drops of blue food coloring and stir. Place both cubes into the solution. Let both stand in the colored water overnight.
  4. 4 Remove each gently from the water with a spoon. Place both on a paper towel. With the knife, cut each in half. Use the ruler to measure how far the blue water penetrated into the surface of each one.

WHAT DO YOU THINK?

  • Record your observations. Which piece of egg was penetrated more, compared to its total diameter, by the blue water?
  • Why was there a difference in water penetration?

CHALLENGE What do you predict would happen to an egg left in its shell?

cellsize
Page 63
Surface Area and Volumes of Cubes
  Number of Cubes Side Length Surface Area Volume
Cubes
1 4 cm 96 cm2 64 cm3
Cubes
8 2 cm 192 cm2 64 cm3
Cubes
64 1 cm 384 cm2 64 cm3

As the cell gets bigger, there comes a time when its surface area is not large enough to allow resources to travel to all parts of the cell. So the cell stops growing. Bird eggs and frog eggs are much larger than typical cells, but they have a storehouse of food and also rapidly divide to give rise to multicellular embryos. In fact, this multicellular embryo is a good illustration of another way cells get around the surface- area-to-volume problem: they divide. The ratio of surface area to volume in newly divided cells is much higher, giving more surface area for exchanging materials with the outside of cells.

A cell's shape also affects its surface area. For example, some single- celled organisms are thin and flat, providing increased surface area. Other cells, such as nerve cells and muscle cells, are long and skinny, which also gives them a higher ratio of surface area to volume.