Cellular Organelles: Lysosomes, Peroxisomes, Vacuoles, and Mitochondria

Lysosomes, Peroxisomes, and Vacuoles

These are all membrane-bound organelles.

Structure and Function of Lysosomes

Lysosomes are organelles containing enzymes that degrade all types of biological polymers. They are acid hydrolases and act as a cellular digestive system, breaking down material captured from outside the cell. When a cell incorporates material via endocytosis, it forms a phagosome. A primary lysosome then fuses with this to form a secondary lysosome, or phagolysosome, where enzymes break down substances for cellular use. When material to be digested comes from within the cell, the process is called autophagy. This forms an autophagosome, which fuses with a primary lysosome for digestion.

Structure and Function of Peroxisomes

Peroxisomes are small organelles involved in processes such as fatty acid oxidation, the glyoxylate cycle, and photorespiration. Their enzymes include oxidases and catalase. They oxidize fatty acids and amino acids, providing metabolic energy for the cell and detoxifying various toxic molecules, especially in the liver and kidneys. They also perform the glyoxylate cycle, crucial for providing energy for germination and plant growth; these peroxisomes are known as glyoxysomes.

Structure and Function of Vacuoles

Vacuoles are enclosed by a membrane called the tonoplast. Their main functions include:

• Maintenance of cell turgor: Water enters vacuoles by osmosis to balance osmotic pressure, keeping the cell swollen.
• Cellular digestion: In plant cells, vacuoles are involved in intracellular digestion processes.
• Storage of various substances.

Mitochondria

Mitochondria are cellular organelles.

Ultrastructure of Mitochondria

Mitochondria are composed of two membranes that enclose two chambers:

• Outer mitochondrial membrane: This lipid bilayer contains approximately 40% lipids, with cholesterol being the most important, and 60% protein. It includes enzymes involved in lipid metabolism and integral proteins called porins. This membrane is highly permeable, allowing certain large molecules to pass freely into the outer chamber.
• Inner mitochondrial membrane: This membrane features folds called mitochondrial cristae. It contains about 20% lipids (with little cholesterol) and 80% protein, including ATP synthase, respiratory chain proteins, enzymes for beta-oxidation of fatty acids, oxidative phosphorylation enzymes, and transferases.
• Elementary Particles F: Located on the outer surface of the cristae, oriented towards the matrix. These are ATP synthase complexes, consisting of a spherical head (F₁ complex) and a stalk (F₀ factor). Similar complexes are found in chloroplast thylakoid membranes and bacterial plasma membranes.
• Internal space or mitochondrial matrix: This is a semi-fluid material containing mitochondrial DNA, mitochondrial RNA forming mitoribosomes, enzymes for DNA replication, transcription, and translation, enzymes involved in the Krebs cycle and beta-oxidation of fatty acids, and ions like calcium, phosphate, and ribonucleoproteins.