Re: Why do differences between animal and plant cells exist?

That is a very good question.  It's also a very big question and I will 
just highlight a few points in my answer.  As you may know, structurally, 
plant and animal cells are very similar because they are both eukaryotic 
cells. Differences have eveolved based on their various needs in their 
environment.  Immediately, you will notice that plant cells also have some 
additional structures that are not present in an animal cell. These 
structures include: chloroplasts, the cell wall, and vacuoles. These 
structures perform important distinct processes necessary for plant cell.

Plant cells are surrounded by a cell wall made of cellulose.  The only 
thing which the cell wall does is to allow very high pressures to build up 
inside them because of a process called osmosis. Animal cells have semi-
permeable cell membranes, this means that water can enter or leave by 
osmosis. When plant cells are put in distilled water they start to swell 
up, but they do NOT burst.  Animal cells are different, they do not have 
cell walls. If one of your cells is placed in distilled water it will 
swell up and eventually burst. Animals have to excrete excess water: if 
your kidney stopped working your blood would get very dilute when you 
drank some water; water would start to go into your cells by osmosis and 
they might burst due to the change in pressure. This would be very bad for 
your kidney and for you.

Chloroplasts are tiny structures containing the green pigment chlorophyll. 
This complicated molecules, in association with other pigments, absorbs 
the sun's energy and enables the plant to use the energy to transform 
carbon dioxide gas and water from the air and soil into simple 
carbohydrates (sugars) and oxygen gas. The sugar molecules produced in 
this photosynthesis are linked together in chains to make larger 
carbohydrate molecules, mainly starches and cellulose. The oxygen gas is 
released into the atmosphere.

Animals cannot make their food in this way, but must depend upon plants or 
other animals for food. Plants take the sun's energy and store it in high-
energy food substances that animals can use. 

If you are interested in learning more about the differences between plant 
and animal cells, I suggest you look at a basic botany or biology textbook.

Have fun with science!
Elsa