Cell Organelles: Vacuoles and Peroxisomes

Cell Organelles

• Vacuoles play a role in homeostasis, conserving pH by taking in hydrogen ions, and establish a hydrostatic pressure called "turgor pressure", providing mechanical strength to the cell.
• Vacuoles are present in many types of animal cells and in unicellular protists like protozoa.
• Most protozoa have food vacuoles that fuse with lysosomes to digest food, and contractile vacuoles to remove excess water from the cell.

Peroxisomes

• Peroxisomes are simple, membrane-bound, multi-functional organelles of the cytoplasm that carry out diverse metabolic reactions, including substrate oxidation leading to the formation of hydrogen peroxide.
• Peroxisomes are involved in the oxidation of very-long-chain fatty acids, the oxidation of uric acid, and the synthesis of plasmalogens.
• Plant glyoxysomes, which carry out the glyoxylate cycle, are a type of peroxisome.
• Peroxisomes produce hydrogen peroxide during oxidation reactions, which is then broken down by catalase into harmless water and oxygen.
• Peroxisomes are found in cells synthesizing, storing, and degrading lipids, and also synthesize phospholipids of the nerve system.
• Mutations causing abnormal peroxisome synthesis are likely to cause mental retardation.

Mitochondria and Chloroplasts

• Energy is in the form of light or molecules (glucose).

Lysosomes

• Lysosomes are small sacs of digestive enzymes dispersed in the cytoplasm of most animal cells.
• They contain about 40 enzymes and have a pH of around 5.
• Lysosomes degrade complex molecules from bacteria and debris ingested by scavenger cells (phagocytes).
• Primary lysosomes are formed by budding from the Golgi, and their hydrolytic enzymes are synthesized in the RER.
• Secondary lysosomes are formed by the fusion of primary lysosomes with vesicles containing ingested materials.
• Lysosomal storage diseases occur when one of the digestive enzymes is absent, leading to the accumulation of undigested materials in cells.

Vacuoles (continued)

• Vacuoles are larger than vesicles and can occupy up to 90% of the cell volume in plant cells.
• They store salts, pigments, food, metabolic waste, inorganic compounds, and sometimes proteins.
• Vacuoles contain a lot of dissolved materials, which is why they take in water.
• They can be minor or extended in amounts, such as in liver cells, where they serve in synthesizing and processing cholesterol and detoxifying carcinogens and drugs.

Rough ER

• The rough ER is composed of a network of flattened sacs (cisternae) with ribosomes bound to its cytosolic surface.
• The functions of the rough ER include the synthesis of proteins and most of the lipids of a cell's membranes.
• The addition of sugars to the asparagine residues of proteins begins in the rough ER and continues in the Golgi complex.

Golgi Complex

• The Golgi complex consists of flattened sacs called cisternae, with a lumen in each sac.
• The cis face (entry surface) receives vesicles from the ER, while the trans face (exit surface) packages molecules in vesicles and transports them out of the Golgi.
• The Golgi complex is developed in cells producing a lot of glycoproteins, and in animal cells, it manufactures lysosomes.
• Two hypotheses for Golgi function are the vesicular transport model and the cisternal maturation model.