Plants have cells very similar to yours. So do the other animals. Humans are classified as animals—gasp!—because our cells look and act remarkably like all the other animal cells. Take a look at one of your cells under a microscope, then compare it with a similar one from a duck-billed platypus, if you can find one. If you switched the cells and handed them to a friend, the friend would likely not be able to tell them apart! Try it with a plant cell. But don’t bet any money on it this time. The plant cell is likely to have some green things in it that are a sure giveaway. It also has a cell wall, and you don’t need one. There are some fundamentals that go into every cell, uniting the world of living things into a oneness of the universe; there are also cellular modifications that make some cells look like they are from a different planet. We can sort it all out in this section. Perhaps you will look at your dog or a plant through new eyes after reading this section.

Cell Theory

While observing dead cork samples with a crude lens, Robert Hooke identified and named “cells.” He thought that the small, simple units looked like the bare prison cells of his time, and the name cell stuck. His work launched a new frontier in scientific exploration that led to modern cell theory:

• All living things are made of cells.
• Cells are the basic units of structure and function in all living things.
• All cells come from the reproduction of existing cells.

Size Limitations

Most plant cells are approximately 0.002 inches in diameter, whereas most bacteria are even smaller at 0.000008 inches long (10 to 50 nanometers in metric units), making them impossible to see without magnification. Cell size is limited due to the inability of very large cells to provide nutrients and water and remove wastes in an efficient manner. The size limitation is due to the ratio between their outer surface area and the internal volume, making large cubical or spherical cells too big for the surface areas to accommodate all of their cellular life functions. Cells are three-dimensional, so as the cell grows, the volume increases geometrically as the cube of the side length, but actual surface area increases arithmetically with the square of the side length. In other words, a cell’s volume increases more rapidly than the surface area. This becomes biologically important when a cell becomes too large for the available surface area to allow passage of nutrients and oxygen into, and cellular waste out of, the cell. Conversely, smaller cells can move materials in and out through the cell membrane at a faster rate because they have a more favorable surface area-to-volume ratio. Interestingly, the shape of muscle and nerve cells tend to be long and thin, which also provides a favorable surface area-to-volume ratio.