Re: why do plants have larger vacuoles?

Dear LAKEISHA,

I like your question! Cells are cells, right? True, but not all cells are 
created equal and some of the differences explain why plant cells have 
larger vacuoles than animal cells.

There are a couple of functions that plant vacuoles perform that require a 
large vacuole.  We can look at several.

1. STORAGE OF INORGANIC IONS. Plants are important parts of the food chain 
because they change INORGANIC or non-living material into ORGANIC, living 
material. Of course we usually think about the use of light energy by 
plants to "feed" food chains, but plants also take non-living compounds 
in and make them into living tissue. Some of these are gases like carbon 
dioxide. Others are IONS, or salt-like substances like nitrate, phosphate 
and even iron.  Plants store these ions for later use. Remember that a 
plant is different from an animal because it cannot move. So it is very 
handy for a plant to be able to store extra nutrients when they are around 
for use later when nutrients may not be available. Having a large 
vacuole means the plant can store more ions, and they are less 
concentrated, which makes storage easier. Animal cells store far fewer 
inorganic salts.

2. EXPANSION OF CELLS: Plant cells grow by swelling. Unlike animal cells, 
plant cells are surrounded by a RIGID WALL. To make this wall expand, plant 
cells fill their vacuoles (mostly with water) to swell the cell and cell 
wall. If a plant had to use CYTOPLASM, the liquid that contains all the 
proteins, enzymes and other working chemicals of the cell, it would be much 
harder because the cell would have to make more and more protein. Making 
protein is much harder to do than take in water.

3. SPREADING OF CYTOPLASM: That working cell sap inside the plant cell gets 
its carbon dioxide and nutrients from outside the cell and must move them 
inside, so the more cell sap that is close to the outside, the better. 
Plants must also collect light. Both nutrients and light are easier to 
collect if the plant spreads-out its cell sap and CHLOROPLASTS (the little 
factories that use light to make sugar) around the inside walls of the 
cell. It is somewhat like spreading out a net to collect fish compared to 
using a fishing pole. The more spread out, the better chance to catch 
something. So for most plant cells, there is a thin layer of cell sap 
inside the cell wall with a large storage vacuole inside to push the cell 
sap to the edges.

These are the main reasons that plants have larger vacuoles than animal 
cells. Some purposes are unique to plants like spreading out to collest 
light -- animals don't need to collect light. They also don't have to swell 
up to expand a cell wall because animals cells don't have rigid cell walls. 
The part about collecting material from outside does affect both plant and 
animal cells which must get nourishment from outside the cell. The 
difference is that animals don't have to collect inorganic materials which 
are often very dilute. Animal "food" is usually much more concentrated  and 
so animal cells don't need to spread out as much to collect nutrients.

Hope this helps!