The present lecture details few other cell organelles like Peroxisomes, chloroplast and vacuoles.

Peroxisomes:

All animal cells (except erythrocytes) and most plant cells contain peroxisomes . They are present in all photosynthetic cells of higher plants in etiolated leaf tissue, in coleoptiles and hypocotyls, in tobacco stem and callus, in ripening pear fruits and also in Euglenophyta, Protozoa, brown algae, fungi, liverworts, mosses and ferns. Peroxisomes contain several oxidases.

Structure:

Peroxisomes are variable in size and shape, but usually appear circular in cross section having diameter between 0.2 and 1.5μm. They have a single limiting unit membrane of lipid and protein molecules, which encloses their granular matrix. Like mitochondria and chloroplasts, they acquire their proteins by selective import from the cytosol. Peroxisomes resemble the Endoplasmic reticulum by being self-replicating, membrane-enclosed organelle that exists without a genome of its own.

Peroxisomes are unusually diverse organelles, and even in the various cell types of a single organism they may contain different sets of enzymes. They can also adapt remarkably to changing conditions. Yeast cells grown on sugar, for example, have small peroxisomes. But when some yeasts are grown on methanol, they develop large peroxisomes that oxidize methanol; and when grown on fatty acids, they develop large peroxisomes that break down fatty acids to acetyl CoA by β oxidation. Peroxisomes are also important in plants. Two different types have been studied extensively. One type is present in leaves, where it catalyzes the oxidation of a side product of the crucial reaction that fixes CO₂ in carbohydrate. This process is called photorespiration because it uses up O₂ and liberates CO₂. The other type of peroxisome is present in germinating seeds, where it has an essential role in converting the fatty acids stored in seed lipids into the sugars needed for the growth of the young plant. Because this conversion of fats to sugars is accomplished by a series of reactions known as the glyoxylate cycle, these peroxisomes are also called glyoxysomes. In the glyoxylate cycle, two molecules of acetyl CoA produced by fatty acid breakdown in the peroxisome are used to make succinic acid, which then leaves the peroxisome and is converted into glucose. The glyoxylate cycle does not occur in animal cells, and animals are therefore unable to convert the fatty acids in fats into carbohydrates. Glyoxysomes occur in the cells of yeast, Neurospora, and oil rich seeds of many higher plants. They resemble with peroxisomes in morphological details, except that, their crystalloid core consists of dense rods of 6.0 μm diameter.

Chemical composition:
Internally peroxisomes contain several oxidases like catalase and urate oxidase—enzymes that use molecular oxygen to oxidize organic substances, in the process forming hydrogen peroxide (H₂O₂), a corrosive substance. Catalase is present in large amounts and degrades hydrogen peroxide to yield water and oxygen.

A specific sequence of three amino acids located at the C- terminus of many peroxisomal proteins functions as an import signal. Other peroxisomal proteins contain a signal sequence near the N terminus. If either of these sequences is experimentally attached to a cytosolic protein, the protein is imported into peroxisomes. The import process is yet to be understood completely, although it is known to involve soluble receptor proteins in the cytosol that recognize the targeting signals, as well as docking proteins on the cytosolic surface of the peroxisome. At least 23 distinct proteins, called peroxins, participate as components in the process, which is driven by ATP hydrolysis. Oligomeric proteins do not have to unfold to be imported into peroxisomes, indicating that the mechanism is distinct from that used by mitochondria and chloroplasts and at least one soluble import receptor, the peroxin Pex5, accompanies its cargo all the way into peroxisomes and, after cargo release, cycles back out into the cytosol. These aspects of peroxisomal protein import resemble protein transport into the nucleus.