Re: why doesn't an animal have a cell wall?

Hi, good question.

I'm assuming that you are comparing animal cells with plant cells that do have a rigid cell wall. The cell wall structure provides a lot of support and stabilizes the overall structure of the plant - cells can't just move around. In animals, multiple interactions between cells and molecules around them (referred to as the extracellular matrix) help to hold everything in place, but there is generally no real equivalent to the cell wall. The plant cell wall is important for resisting the fluid pressure that builds up in the plant cells, which is part of the mechanism by which they can stand upright. Animals need bones or an exoskeleton to achieve a similar result.

There are a couple of consequences of having a cell wall, and one of them is probably a decrease in mobility of the individual organism - animals move around much more than plants. This in turn affects the different ways that plants and animals respond to environmental changes. Because the cell wall tends to limit the directions of cell growth, plant embryos tend to develop by having specialized groups of cells that divide, whereas animal embryos tend to grow in all directions and in all places at once. Thus, animals appear to have traded the structural stability of the cell wall for the increased mobility and somewhat more flexibility in overall structure. However, there are many similarities between plant and animal cells. Good places to start some further reading are:

http://www.purchon.co.uk/science/cells.html
http://www.cybernet.net/aedmills/biorefweb/biohome.html

Good luck!