Understand that the cell is the smallest independent unit of life.
All independent life on this planet is cellular. The living cell is the smallest unit capable of showing all the "signs and symptoms" that define the phenomenon we call "life".
Source: http://www.brooklyn.cuny.edu/bc/ahp/LAD/C9/C9_reproduction.html
Very interesting site with a lot of detail about cells. Go here or die (just joking of course).
This site from the University of Utah shows the size of cells compared to other common items. Take a look,, why don't you?
A conventional definition of Life
Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life exhibits the following phenomena:
Source: http://en.wikipedia.org/wiki/Life
Here is basically the same information but in pictures!
Explain the significance of surface area to volume ratio.
Why are single-celled organisms so small? Tiny organisms have to take in food, water, gases, and chemicals, and they have to excrete waste products. They are full of organic molecules that have to be moved around and processed. In small cells, all parts of the cell are near the cell membrane, and chemical exchanges with the environment are simplified. As single cells grow larger, the contents increase in volume, and making everything work together becomes more complex. The cell membrane, the interface between the living cell and the world, does not increase in area as quickly as the volume of the cell contents. At some point, the structures and processes in the cell will become inefficient if the cell becomes too large.
A sphere is a strong, efficient shape. Cells that are shaped like tiny bubbles contain the greatest volume for their surface area. However, cells come in a great variety of shapes. We have seen many cells that are shaped like tiny rods. Rod-shaped cells will have more surface area compared to their contents than spherical ones. However, as cells evolve to increase their surface area, they risk becoming increasingly fragile.
Source: curriculum.calstatela.edu
Go to this site for a more detailed explanation of this important idea.
Go here to see an animation calculating surface area and volume.
This interactivity will change the three dimensions of a rectangle and show you the surface area and volume calculations. Try starting with 1 x 1 x 1 cm cube and then adding one to each dimension - what happens to the SA/V ratio?