Microbodies and the Cytoskeleton

Microbodies are a further category of small, membrane-bound organelles found scattered through the cytoplasm of both plant and animal cells. What unites them as a group is less their overall shape or size, which is fairly nondescript and roughly spherical in most cases, and more the fact that each type carries a characteristic set of enzymes that carries out a particular, specialised metabolic function within its own enclosed compartment, kept separate from the general cytoplasm.

Peroxisomes are perhaps the most widely distributed type of microbody, occurring in both plant and animal cells. They characteristically contain enzymes such as catalase, along with various oxidative enzymes that oxidise particular substrates and, in the process, generate hydrogen peroxide as a by-product -- a potentially toxic compound that catalase then safely breaks down into water and oxygen within the same organelle. In plant cells, peroxisomes play an important role in a process called photorespiration, a metabolic pathway closely linked to photosynthesis. Glyoxysomes are a related but distinct type of microbody found specifically in the cells of germinating oil-rich seeds; they contain the enzymes of the glyoxylate cycle, a pathway that allows the plant to convert the fats stored in the seed into carbohydrates, providing the energy and building blocks the young seedling needs before it can begin photosynthesising on its own.

The cytoskeleton is not itself a discrete membrane-bound organelle in the way the structures described elsewhere in this chapter are, but rather an extensive, interconnected network of filamentous protein structures that extends throughout the cytoplasm of a eukaryotic cell, giving the cell much of its mechanical strength, its characteristic shape, and its capacity for internal and external movement. The cytoskeleton is generally described as being built from three main types of protein filament, differing in thickness and in the specific protein subunits from which they are assembled. …