Mitochondria, Plastids, and Peroxisomes

Mitochondria, complex bodies about the size of bacteria and bounded on the outside by a double membrane, are present in all eukaryotic cells that use oxygen for respiration. The numbers per cell appear to vary from the one for certain tiny trypanosomes to as many as 3 x 105 in some oocytes. Liver cells often contain more than 1000 mitochondria
apiece. Study of ultrathin serial sections of a single yeast cell by electron microscopy has shown that under some growth conditions all of the yeast mitochondria are interconnected. More recent evidence from new imaging procedures, e.g. using the green fluorescent protein also supports the idea that mitochondria are interconnected in a reticulum that can become fragmented under some conditions. The inner membrane of a mitochondrion is often highly folded to form the cristae (crests). The outer membrane is porous to small molecules but the passage of substances into and out of the inner space of the mitochondrion, known as the matrix, is tightly controlled by the inner membrane. Although some of the oxidative chemical activites of the cells are located in the ER and in peroxisomes, the major energy-yielding reactions for aerobic organisms are found in the mitochondria, which are also the principal site of utilization of oxygen. Within each mitochondrion is a small circular molecule of DNA whose genes encode only a few of the many proteins needed in this organelle. Also present within mitochondria are ribosomes of a size similar to those of bacteria and smaller than those lining the rough ER. Plastids are organelles of plant cells that serve a variety of purposes. Most important are the chloroplasts, the chlorophyll-containing sites of photosynthesis. Like mitochondria they contain folded internal membranes and several small molecules of DNA. Fragile organelles, the peroxisomes or microbodies, occur in many cells. In green leaves they may occur in numbers up to one-third those of mitochondria. Peroxisomes are often about the size of mitochondria
but have only a single membrane and do not contain DNA. They often contain an apparently crystalline “core.” The single membrane of peroxisomes is porous to small molecules such as sucrose. This permits these organelles to be separated from itochondria by cen trifugation in a sucrose gradient where the microbodies assume a density of about 1.25 g/cm3 compared to 1.19 for the impervious mitochondria. Peroxisomes are rich in enzymes that produce and decompose hydrogen peroxide. They often make a major contribution to the oxidative metabolism of cells. In germinating oilseeds glyoxysomes, a type of peroxisome, contain enzymes that catalyze reactions of
the biosynthetic “glyoxylate pathway” of metabolism. Organelles that resemble eroxisomes in appearance Endoplasmic reticulum Microsomes15 but which are functionally more closely related to mitochondria are the hydrogenosomes of anaerobic protozoa. As the name suggests, these organelles are the site of formation of molecular hydrogen, a ommon product of anaerobic metabolism.