GOLGI APPARATUS, LYSOSOMES AND VACUOLES SMG

A brief account of Golgi apparatus, Lysosomes and Vacuoles

1. GOLGI APPARATUS, LYSOSOMES AND VACUOLES Dr. Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry

2. I. GOLGI APPARATUS (GOLGI COMPLEX, GOLGIBODIES) Golgi apparatus is a cell organelle present in all eukaryotic cells, except red blood corpuscles(RBCs), sperm cells of Bryophytes and Pteridophytes and sieve tubes of plants - d iscovered by Camillo Golgi (1898) in the cytoplasm of nerve cells of an owl. The localization, size and development of Golgi apparatus vary from one cell to another. Most of the Golgi apparatus appears to be surrounded by a zone which is free of ribosomes , glycogen and mitochondria – Zone of exclusion. Camillo Golgi

3. ULTRASTRUCTURE OF GOLGI APPARATUS Structural organization of Golgi apparatus varies among eukaryotes. Golgi apparatus consists of i ) Cisternae ii) Tubules iii) Vesicles

4. i ) Cisternae (Lamellae) Double membrane bound, flattened, curved, sac like structures filled with a fluid. The lumen of the Golgi cisternae are usually narrow (10nm -20nm ). Cisternae are arranged in parallel stacks. The cisternal stacks (Golgi stacks) are called dictyosomes . Within each dictyosome , the cisternae are separated from one another by a space of 20 nm to 30 nm and exhibit co- ordinated function.

5. In mammalian cells, the Golgi apparatus is located close to the nucleus and centrosome and is made up of stacked cisternae , which are interconnected by tubular elements to form a single continuous body. In plant cells, Golgi apparatus are organized as individual stacks of cisternae ( dictyosomes ) that are dispersed through out the cytoplasm.

6. Golgi apparatus

7. Number of cisternae in Golgi stacks varies from one cell type to another. In most plant cells, the number of cisternae may vary from 3 to 7. Within a single cell, cisternal number per stack and diameter are constant. Cisternae in a particular stack are individual separate entities, but there are occasional tubular connections.

8. Cisternae vary greatly depending on cell type and their position in the stack. Dictyosomes ( cisternal stack or Golgi stack) are polarized structures in that cisternae at one pole or face of the cisternal stack differ from those at the opposite pole or face. The proximal pole of each dictyosome is associated with the nuclear envelope or endoplasmic reticulum. This pole or face is referred to as the Cis face or forming face. The opposite pole(the distal pole) is known as the maturing, secreting or Trans face. Golgi cisternae are fenestrated(having perforations). These fenestrae are necessary for movement of secretory granules.

9. Golgi apparatus https://www.cleariitmedical.com

10. Golgi apparatus - Transmission Electron Micrograph credit: modification of work by Louisa Howard https://opentextbc.ca

11. ii) Tubules From the peripheral area of cisternae arise a complex network of tubules. The tubules are 300 - 500 Å in diameter and may serve to connect the cisternae of adjacent dictyosomes . iii) Vesicles The secretions of cisternae are packed in the vesicles. These vesicles may be either free or attached to the tubule ends.

12. FUNCTIONS OF GOLGI APPARATUS 1. Golgi apparatus plays an important role in endocytic and secretory pathways. Golgi apparatus receives newly synthesized molecules (proteins, carbohydrates, lipids etc.) from endoplasmic reticulum. The Golgi apparatus modifies, sorts and packages proteins and lipids. Some will remain in the Golgi apparatus while others, the newly synthesized lysosomal enzymes will be transported to the endocytic pathway and some others to the plasma membrane ( secretory pathway).

13. 2. The Golgi apparatus synthesize certain polysaccharides from simple sugars. The plant Golgi apparatus has an important role in synthesis of large quantities of cell wall polysaccharides. 3. Golgi apparatus plays an important role in the glycosidation of lipids ( glycolipids ) and proteins ( glycoproteins ).

14. 4. Secretory granules originating from Golgi apparatus will fuse with the cell membrane (plasma membrane) during exocytosis . The membrane of the granules get incorporated into the cell membrane and thus contributes to the renewal of the membrane constituents . 5. In plant cells, the Golgi apparatus synthesizes pectin and some other carbohydrates ( hemicellulose ) necessary for the formation of cell wall. 6. The Golgi apparatus gives rise to primary lysosomes by budding from the maturing face of cisternae .

15. II. LYSOSOMES https://outlet.historicimages.com First reported by Christian Rene de Duve (1955)

16. Lysosomes (Gr. Lysis = dissolution, soma = body) Tiny single membrane bound organelle. Lysosomes are commonly found in animal cells. Not common in plant cells. But they have been reported in certain plant cells- meristematic cells of plant roots (e.g. root tip cells of maize) , yeast etc.

17. The number of lysosomes in a cell varies with the cell type. Lysosomes may be spherical, ovoid or rarely tubular in shape. However, meristematic cells of plant roots have irregular lysosomes . Lysosome is a tiny vacuolar structure bound by a single lipoprotein membrane. The size of the lysosome varies from 0.2 μ m to 0.8 μ m . https://microbenotes.com

18. Lysosome contains a dense , finely granular fluid which contain a number of hydrolytic enzymes. At present more than fifty lysosomal hydrolases are known which are able to digest most of the biological substances. These enzymes generally act at acid pH (approximately at pH 4.6 - acid hydrolases ). e.g. acid phosphatase , cathepsin , beta- glucuronidase ( β- glucuronidase ), neuraminidase etc. Lysosomal membrane contains a variety of highly glycosylated integral proteins whose carbohydrate chains are thought to form a lining that protects the membrane from the enclosed hydrolases .

19. Polymorphism in Lysosomes (Types of Lysosomes ) Lysosomes exhibit polymorphism with regard to the size of the particle and the irregularities of their internal structure. Polymorphism is the result of the association of the primary lysosomes with the different materials that are phagocytosed by the cell. Four types of lysosomes are recognized, of which only the first is the primary lysosome . The other three are the secondary lysosomes that are formed by the fusion of a primary lysosome and a phagosome , which could be either a heterophagosome or an autophagosome .

20. Primary Lysosomes (Storage granule) Dense particles of about 0.4 μ m surrounded by a single membrane. The enzymatic content is synthesized by ribosomes of the rough endoplasmic reticulum (RER) and transferred to Golgi apparatus where it is packaged to form the primary Lysosome . https://pathresidents.com

21. ii) Heterophagosomes (Digestive vacuole) These are secondary lysosomes resulting from the association of primary lysosomes with vacuoles containing phagocytosed material. The phagosome fuses with a primary lysosome and form the heterophagosome and is digested by the hydrolytic enzymes. iii) Residual bodies Secondary lysosomes containing indigestible material are known as residual bodies. In some cells , they remain for a long time in the cell as pigment inclusions and may be related to the ageing process.

22. iv) The autophagic vacuole ( Cytolysosome or Autophagosome ) The autophagic vacuole is a special case in which parts of the cell are digested. This process is stimulated during starvation and pathological conditions. Under such conditions, the lysosomes bring about the digestion of cellular organelles like mitochondria and endoplasmic reticulum. This process is known as autophagy (autolysis). Because of this self destructive nature, lysosomes are called as ‘suicidal bags’.

23. Polymorphism in Lysosomes - Primary lysosomes , Heterophagosomes , Residual bodies and Autophagic vacuoles( Autophagosomes )

24. Functions of Lysosomes Some of the important functions of lysosomes include Intracellular digestion Lysosomes enclose a number of hydrolytic enzymes that are involved in the mechanism of cellular digestion. The digestion of the phagocytosed material takes place with the help of the hydrolytic enzymes of lysosomes . Many unicellular organisms ingest food particles which are enzymatically disassembled in a lysosome . The lysosomal enzymes digest protein into dipeptides and carbohydrates into monosaccharides . The resulting nutrients pass through the lysosomal membrane into the cytosol .

25. 2. Autophagy Regulated destruction of cell’s own organelles and their replacement. Through the process of autophagy , lysosomes are involved in the renovation and turnover of cellular components. 3. Degradation of extracellular material Lysosomes are involved in specific processes outside the cell. This involves the release of primary lysosomes by exocytosis for degradation of extracellular moieties. This occurs when the target to be degraded can not be internalized by endocytosis . e.g. Lysosomes are involved in the bone-degrading capacity of osteoclasts - in bone remodelling ( osteoclast is a type of bone cell that breaks down bone tissue ). 4. Crinophagy A process by which excess secretory granules are removed.

26. III. VACUOLES Vacuoles are bubble like sacs which are bounded by a single semipermeable membrane called tonoplast . They are found in most cells. Young plant cells have no vacuoles or if present, are very small in size. But as the cell matures, the small vacuoles fuse and form a large central vacuole pushing the cytoplasm and nucleus to the periphery.

27. The vacuoles are filled with a fluid called cell sap. The cell sap consists of free water and a variety of compounds such as mineral ions, sugars, oxygen, carbon dioxide, pigments, organic acids, waste products etc., either in the form of solution or as crystals. The semipermeable membrane tonoplast helps to maintain concentration of materials different from that found in the cytoplasmic matrix. They help to maintain the turgor of plant cells. Vacuoles may act as storage organs.

28. Electron micrograph of vacuoles https://encyclopedia2.thefreedictionary.com/vacuole

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