|MadSci Network: Cell Biology|
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!
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