Lysosomes And Peroxisomes And Glyoxysomes 4u1e12

Lysosome: Lysosomes are single membrane bound vesicular cell organelles containing hydrolytic enzymes. The cytoplasm of both plant and animal cells contain many tiny irregular shaped membrane bound vesicles known as lysosomes. They contain various hydrolytic enzymes. De Duve in 1955 observed these organelles in animal cells and named them as lysosomes. They are now found also in plant cells. They originate from ER or Golgi bodies. The number and shape of lysosomes differ in different cells. Origin: Lysosomes are formed from ER or Golgi bodies. Distribution: They are scattered in the cytoplasm and number increases in secretary cells. Size: The size varies from 0.2 – 0.8µm. Number: Varies according to the cell type. Structure: Lysosomes look like membrane bound vesicles or vacuoles without any characteristic shape or size. Under the electron microscope lysosomes appear dense and finely granular and bounded by unit membrane. The unit membrane is trilaminar. Lysosomes contain globules of proteins and lipids and carry at least 50 types of hydrolytic enzymes. Lysosomes of the injured or dead cells probably rupture together and release enzymes which lyse the useless cells. Lysosomes are termed as ‘suicidal bags’ because they act and digest their own organelles. On the basis of function four different lysosomal forms have been identified within individual cell – Primary lysosomes, heterophagosomes, autophagosomes and residual bodies. The last three types are collectively termed as secondary lysosomes. Primary lysosomes or protolysosomes: These are newly produced lysosomes bounded by a single membrane and varying greatly in size. The primary lysosome is a virgin particle in that its digestive enzymes have not taken part in digestion. Heterophagosome: This is a type of secondary lysosome variously termed heterophagic vacuoles, digestive vacuoles, or phagolysosomes. These are formed by the fusion of one or more primary lysosomes with phagosomes (cytoplasmic vacuoles containing extracellular substances formed by endocytosis). The primary lysosomes empty their digestive enzymes into the newly formed particles called heterophagosomes. Lysogenous digestion of endocytosed material is termed heterophagy.

Autophagosomes: This type is also variously termed as autophagic vacuoles and cytolysosomes. These are formed by the fusion of primary lysosomes with partially degraded parts of mitochondria, ER, microbodies, glycogen particles or other cytoplasmic structures of the cell. The process of digestion of portions of a cell’s own cytoplasmic constituents by its lysosomes is termed autophagy.

Residual bodies: Heterophagosomes and autophagosomes after digestion and absorption are left with only residues and denatured enzymes within the vacuole, which are now termed residual bodies or teleolysosomes or dense bodies. Functions: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Helps in digestion Lysis of organelles during cellular differentiation Scavenging of worn out cell parts and denatured proteins Destruction of aged RBCs and dead cells Defence against invading bacteria and viruses Dissolution of blood clots Keratinisation of skin Secretion of hydrolyases by sperm for egg penetration during fertilisation Yolk digestion during embryonic development

Microbodies: Another class of membrane bound organelles found in both plants and animal cells are called microbodies. These bodies appear to be bounded by a single unit membrane and are characterised by the presence within the of specific enzymes. The function of microbodies in general is to compartmentalize specific enzymes and therefore specific biochemical reactions in various regions of cell. Microbodies are believed to originate from invaginations of the ER. Two main types of microbodies have described based on which reactions are catalysed by the enzymes they contain. They are peroxisomes and glyoxysomes. Peroxisome: Peroxisomes were first described in liver and kidney cells of the rat, where they carry out various oxidative reactions involving the formation and subsequent destruction of hydrogen peroxide. Plant peroxisomes are localised within the cell in close association with chloroplasts and one major role of peroxisomes in leaf cells is in the oxidation of certain intermediates formed in the dark reactions of photosynthesis. The metabolic activities of peroxisomes appear to berelated to phoyorespiration. Glyoxysome: This microbody was isolated from the cells of oil rich endosperm tissue. Glyoxysomes are similar in appearance to peroxisomes but differ markedly in their enzyme composition. The enzymes present within it are involved in the conversion of the stored fats of the seeds to carbohydrates. In the germinating seedlings where the storage fat is utilized the glyoxysomes are no longer found in the cell.