Peroxisomes are small membrane-bound organisms found within the cytoplasm of eukaryotic cells. The primary function of peroxisomes is to oxidize certain biomolecules, although peroxisomes have other functions as well, such as enabling the synthesis of plasmalogens (a type of membrane lipid). Peroxisomes also have an additional set of functions in the cells of plants. Peroxisomes in plant cells are involved with photorespiration, recycling carbon out of phosphoglycolate.
That’s a quick rundown of the functions of peroxisomes, but it is important to understand how peroxisomes interact with the other organelles in the cell. Let’s take a closer look at the functions peroxisomes serve and how these functions support the cell as a whole.
The Structure Of Peroxisomes
Peroxisomes can come in a number of different sizes and shapes, with these different forms reflecting the energy needs of different cells. For instance, yeast cells have their peroxisomes increase in both size and number if the presence of toxins increases or if the cell has a lipid-rich diet. Conversely, peroxisomes can shrink in size if the cell has a carbohydrate-rich diet.
Peroxisomes are composed of a phospholipid bilayer, much like the membrane of the cell itself. As is the case with other membrane-bound organelles, the peroxisomes have many membrane-bound proteins, such as translocators and protein transporters. The enzymes which are involved in lipid metabolism and detoxification are produced in ribosomes found floating in the cytoplasm. These enzymes are then incorporated into the peroxisomes, which means peroxisomes could be considered more similar in nature to chloroplasts and mitochondria when compared to the lysosomes (which are associated with the endoplasmic reticulum).
The proteins and enzymes that will fuse with the peroxisomes are designated with one of two possible signaling sequences. Amino acid sequences will determine where the proteins end up being located in the peroxisomes. The most common signaling sequence is referred to as Peroxisome Targeting Sequence 1, but a signaling sequence made out of 9 amino acids and referred to as the N-terminal signal sequence also exists.
Peroxisomes may have very high levels of enzymes within them, giving them a crystalloid core. The influx of lipids and proteins causes the peroxisomes to grow, and it is capable of dividing into 2 organelles when it becomes large enough. The phospholipids found in the peroxisomes are mainly created in the smooth portion of the endoplasmic reticulum.
The Function Of Peroxisomes
The functions of peroxisomes include digesting fatty acids, digesting alcohol, digesting amino acids, and breaking down hydrogen peroxide.
Peroxisomes function in extremely specific ways, with the enzymes in the organelle breaking down complex molecules into their smaller, constituent parts. When the digestion of things like fatty acids and alcohol occurs, hydrogen peroxide is produced. Peroxisomes are able to then hold that hydrogen peroxide within them and further degrade them down into harmless oxygen and water. The water is a harmless byproduct while the oxygen can be used to fuel more digestive reactions.
The name of peroxisomes comes from the fact that they utilize molecular oxygen to drive their metabolic functions. Peroxisomes are tied with the metabolism of lipids and reactive oxygen processing. The peroxisomes found within lipid metabolisms deal mainly with the mobilization of lipid seed stores, steroid hormone synthesis, cholesterol biosynthesis, and β–oxidation of fatty acids.
Fats have high-energy density because they have relatively low proportions of oxygen per molecule. As an example, palmitic acid possesses only two oxygen atoms even though it has 16 carbon atoms and its molar mass is around 250 g per mole. This means that although lipids are excellent storage molecules, glycolysis cannot easily catabolize them, nor can they be easily burned as fuel. For these reasons, fats have to be processed before the mitochondria can completely oxidize them through the process of oxidative phosphorylation and the citric acid cycle.
The process of breaking down fats so that they can be oxidized by the mitochondria happens within the peroxisomes. The peroxisomes have the ability to take the long fatty acid chains and degrade them, producing acetyl-CoA through a process known as beta-oxidation. Acetyl-CoA then forms citrate when combined with oxaloacetate. The majority of carbohydrates will start off the citric acid cycle as pyruvate, a three-carbon molecule which will be decarboxylated to create acetyl-CoA, but the process of peroxisomal oxidation lets fatty acids proceed directly to the citric acid cycle. Because hydrogen peroxide can be harmful to the cell, the membrane of the peroxisome must contain the hydrogen peroxide within them until it can be broken down by the enzymes within the organelle.
Within the cells of animals, peroxisomes serve as sites for lipid biogenesis, especially for plasmalogens, which are special types of phospholipids. These plasmalogens are used to create the myelin sheath that surround nerve fibers. The synthesis of bile salts is also completed with the assistance of peroxisomes. Approximately 25% of alcohol consumed by people becomes acetaldehyde, thanks to the operations carried out by peroxisomes. Because peroxisomes oxidize and detoxify metabolic byproducts, toxins, and other substances, they are prominently found within the cells of the liver and kidneys.
Peroxisomes In Plants
Peroxisomes play a slightly different role in the cells of plants than they do in animals. In plant cells, peroxisomes are involved in the processes of photosynthesis and seed germination. Seed germination in plant cells has fat stores utilized to drive anabolic reactions, with the end result of these reactions being carbohydrates. The generation of acetylcholine and β–oxidation is referred to as the glyoxylate cycle, which has an important role in plant cells by preventing the loss of energy when photosynthetic carbon fixation takes place. This is accomplished through the recycling of photo-respirational products.
Consequences Of Protein Deficiencies
If peroxisomes have protein deficiencies, abnormalities can occur. Protein deficiencies can lead to a condition known as Zellweger syndrome, a congenital disorder that leads to developmental abnormalities in both the face and head. A different form of Zellweger syndrome is caused by a deficiency of the protein known as Peroxin Pex 2.
Comparing Other Organelles To Peroxisomes
Peroxisomes are similar in nature to lysosomes, and early microbiologists had difficulty even distinguishing peroxisomes from lysosomes with a microscope alone. Yet the process of differential centrifugation revealed that lysosomes and peroxisomes, while outwardly similar, had different compositions. The enzymes that the two organelles contain are very different from one another, as are the lipid and protein components of the two organelles. Peroxisomes have catalase within them, which lets them detoxify the hydrogen peroxide that comes from the process of beta-oxidizing fats. Lysosomal proteins are also different from peroxisomal proteins in that they are synthesized in the rough ER, budding off after the vesicles that contain the correct lysosomes are made.
Peroxisomes also have similarities with chloroplasts and mitochondria, in addition to having similarities with lysosomes. Ribosomes in the cytoplasm translate the proteins needed for these organelles. Yet unlike chloroplasts/mitochondria, peroxisomes lack genetic translation machinery and they also contain no genetic material to speak of. For this reason, the proteome of the peroxisomes comes entirely from substances pulled from the cytoplasm. Peroxisomes also only have a single lipid bilayer instead of the double membranous structures that chloroplasts and mitochondria have.
Functions Of Lysosomes And Mitochondria
Let’s contrast the function of peroxisomes with the functions of lysosomes and mitochondria. Lysosomes are spherical organelles filled with acidic hydrolase enzymes capable of degrading macromolecules. The membrane of the lysosome functions to separate the digestive enzymes from the other organelles within the cell, and help keep these internal enzymes in nature. The enzymes of the lysosomes are made of proteins that originate in the endoplasmic reticulum, and these proteins are enclosed by vesicles coming from the Golgi apparatus. The lysosomes can be thought of as the garbage disposal facility of the cell, breaking down macromolecules and enabling the parts which make up the macromolecules to be reused to create new cellular structures. The number of lysosomes within cells vary depending on the types of cells, with cells like white blood cells having many more lysosomes compared to other types of cells.
The mitochondria are the organelles in the cell responsible for creating the energy that the cell needs to function. Mitochondria absorb nutrients and break the molecules down, transforming them into useful energy for the cell. This process of chemical transformation is referred to as cellular respiration, and it takes place within the mitochondria. Mitochondria can be for found floating freely throughout the cell body, suspended in the cytoplasm of the cell. The number of mitochondria in the cell will vary depending on the needs of the cell, with cells like muscle cells have many mitochondria because they require so much energy.
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