Vacuoles, vesicles, lysosomes, and perixosomes [take a quiz on this topic]

Chapter Three: Cell Structures The cell membrane and the cell wall Movement through the cell membrane Vacuoles, vesicles, lysosomes, and perixosomes The nucleus, nucleolus, and ribosomes The endoplasmic reticulum and Golgi bodies The mitochondria The cytoskeleton Cilia, flagella, and pseudopodia Plastids

Vacuoles are sacs within the cell made out of the same material as the cell membrane. That's because vacuoles are usually formed by budding off of the membrane. Vacuoles may contain large food particles, enzymes, water, or many other substances. Autotrophic cells, which require a great deal of water, often have one large vacuole filled with water. On the other hand, animal cells do not usually have very large vacuoles.

Vesicles is a term given for very small vacuoles. Often vesicles are formed at an organelle known as a Golgi body in order to carry protein molecules either to other organelles or to the cell membrane.

Contractile vacuole (image courtesy of BIODIDAC)

A special kind of vacuole is called a contractile vacuole. As mentioned in Chapter Two, water tends to flow into the cell since it is usually more concentrated in the external environment. Contractile vacuoles pump water out to prevent the cell from bursting. Not all cells have contractile vacuoles; those which do not must live in environments in which the surrounding water is less pure (so that it will have less of a tendency to diffuse into the cell).

Both lysosomes and peroxisomes are sacs similar to vacuoles which contain special enzymes, but each performs a different function.

Lysosomes contain powerful enzymes which can be used to break down large food particles into smaller molecules or destroy organelles that have been damaged. If the cell lacks an adequate amount of food, the lysosomes may actually digest healthy organelles to provide the cell with energy. Should the lysosome break and release its powerful enzymes, the entire cell can be destroyed, which is why the lysosome is sometimes called the "suicide sac." However, a pH difference between the lysosome and the cell's cytoplasm helps to prevent the enzymes from doing significant damage.

Peroxisomes are very similar to lysosomes, but they contain oxidizing enzymes (enzymes which add oxygen to their substrates) rather than digestive enzymes. Adding oxygen can neutralize many substances which would otherwise be toxic. One such poison which the peroxisomes can prevent from doing harm to the cell is hydrogen peroxide (H₂O₂).