Golgi apparatus: Definition, Function, structure.

Golgi apparatus is also known as the Golgi complex, Golgi bodies, and Golgiosome, in plant cell and lower invertebrates is usually referred to as Golgi body or dictyosome.  The Golgi apparatus act as a factory and plays an important function in transporting, modifying, and packaging proteins and lipids into vesicles for delivery to a target destination.

In addition, most glycolipids and sphingomyelin are synthesized within the Golgi bodies. In-plant cells, the Golgi bodies further serve as the site at which the complex polysaccharides of the cell wall are synthesized. Thus, the Golgi apparatus is involved in processing the broad range of cellular constituents that travel along the secretory pathway.

Discovery

Golgi apparatus was discovered by Italian physician (i.e. Neurologist) Camillo Golgi in 1898, during the investigation of Purkinje cells (that is nerve cells of the cerebral cortex of the brain) of the barn owl. It contained an internal reticular network that stains black with silver stain.

After the fist microscopic observation, he called this structure apparato reticolare interno (internal reticular apparatus). By reporting the existence of such an organelle inside a cell, he inadvertently raised a storm of controversy in the scientific world, which is commonly known as the Golgi controversy.

Occurrence and distribution

The Golgi apparatus occurs in all cells except the prokaryotic cells and eukaryotic cells of certain Fungi, sperm cells of bryophytes, and pteridophytes, cells of mature sieve tube of plants, and mature sperm and red blood cells of animals.

Their number per plant cell can vary from several hundred as in tissues of corn root and algal rhizoids, to a single organelle in some algae. Certain algal cells such as Pinularia and Microsterias, contain the largest and most complicated Golgi apparatuses.

In higher plants, Golgi apparatuses are particularly common in secretory cells and in young rapidly growing cells. In animal cells, there usually occurs a single Golgi apparatus, however, its number may vary from animal to animal and from cell to cell.

Thus, Paramoeba species has two Golgi apparatuses and nerve cells, liver cells and chordate oocytes have multiple Golgi apparatuses, there being about 50 of them in the liver cells.

Distribution

In the cells of higher plants, the Golgi bodies or dictyosomes are usually found scattered throughout the cytoplasm and their distribution does not seem to be ordered or localized in any particular manner. However, in animal cells, the Golgi apparatus is a localized organelle.

For example, in the cells of ectodermal or endodermal origin, the Golgi apparatus remains polar and occurs in between the nucleus and the periphery (e.g., thyroid cells, exocrine pancreatic cells and mucus-producing goblet cells of intestinal epithelium) and in the nerve cells it occupies a circum-nuclear position.

Structure of Golgi apparatus

Golgi apparatus is also known as the Golgi complex, Golgi bodies, and Golgiosome,
Structure of Golgi apparatus

The Golgi apparatus is morphologically very similar in both plant and animal cells. However, it is extremely pleomorphic: in some cell types it appears compact and limited, in others spread out and reticular (net-like).

Its shape and form may vary depending on cell type. Typically, however, Golgi apparatus appears as a complex array of interconnecting tubules, vesicles and cisternae. There has been much debate concerning the terminology of the Golgi’s parts.

The classification given by D.J. Morre (1977) is most widely used. In this scheme, the simplest unit of the Golgi apparatus is the cisterna. This is a membrane bound space in which various materials and secretions may accumulate.

Numerous cisternae are associated with each other and appear in a stack-like (lamellar) aggregation. A group of these cisternae is called the dictyosome, and a group of dictyosomes makes up the cell’s Golgi apparatus. All dictyosomes of a cell have a common function. The detailed structure of three basic components of the Golgi apparatus can be studied as follows:

1. Flattened Sac or Cisternae

Cisternae (about 1 μm in diameter) are central, flattened, plate-like, or saucer-like closed compartments that are held in parallel bundles or stacks one above the other. In each stack, cisternae are separated by a space of 20 to 30 nm which may contain rod-like elements or fibers.

Each stack of cisternae forms a dictyosome which may contain 5 to 6 Golgi cisternae in animal cells or 20 or more cisternae in plant cells. Each cisterna is bounded by a smooth unit membrane (7.5 nm thick), having a lumen varying in width from about 500 to 1000 nm.

Polarity. The margins of each cisterna are gently curved so that the entire dictyosome of the Golgi apparatus takes on a bow-like appearance. The cisternae at the convex end of the dictyosome comprise proximal, forming, or cis-face and cisternae at the concave end of the dictyosome comprise the distal, maturing, or trans-face.

The forming or cis face of Golgi is located next to either the nucleus or a specialized portion of the rough endoplasmic reticulum that lacks bound ribosomes and is called “transitional” endoplasmic reticulum. The Trans face of Golgi is located near the plasma membrane. This polarization is called the cis-trans axis of the Golgi apparatus.

2. Tubules

A complex array of associated vesicles and anastomosing tubules (30 to 50 nm diameter) surround the dictyosome and radiate from it. In fact, the peripheral area of dictyosome is fenestrated (lace-like) in structure.

3. Vesicles

The vesicles (60 nm in diameter) are of three types:

(i)  Transitional vesicles: are small membrane limited vesicles which are thought to form as blebs from the transitional endoplasmic reticulum to migrate and converge to the cis face of Golgi, where they coalesce to form new cisternae.

ii)  Secretory vesicles: are varied-sized membrane-limited vesicles that discharge from margins of cisternae of Golgi. They, often, occur between the maturing face of Golgi bodies and the plasma membrane.

(iii)  Clathrin-coated vesicles: are spherical protuberances, about 50 μm in diameter, and with a rough surface. They are found at the periphery of the organelle, usually at the ends of single tubules, and are morphologically quite distinct from the secretory vesicles.

The clathrin-coated vesicles are known to play a role in intracellular traffic of membranes and of secretory products, i.e., between the endoplasmic reticulum and Golgi bodies, as well as, between the GELR region and the endosomal and lysosomal compartments.

Origin of Golgi apparatus

Origin of Golgi apparatus involves the formation of new cisternae and there is great variation in shape, number and size of cisternae in each stack (dictyosome). The process of formation of new cisternae may be performed by any of the following methods:

  1. Individual stacks of cisternae may arise from the pre-existing stacks by division or fragmentation.
  2. The alternative method of origin of Golgi is based on de novo formation.

In fact, various cytological and biochemical evidences have established that the membranes of the Golgi apparatus are originated from the membranes of the smooth ER which in turn have originated from the rough ER.

The proximal Golgi saccules are formed by the fusion of endoplasmic reticulum derived vesicles, while distal saccules “give their all” to vesicle formation and disappear. Thus, Golgi saccules are constantly and rapidly renewed. The cells of dormant seeds of higher plants generally lack Golgi apparatuses but they do display zone of exclusion having aggregation of small transition vesicles.

Photomicrographs of cells in early stages of germination suggest progressive development of Golgi bodies in these zones of exclusion; and the development of Golgi apparatuses coincides with the disappearance of the aggregation of vesicles

Function of Golgi apparatus

Golgi vesicles are often, referred to as the “traffic police” of the cell. They play a key role in sorting many of cell’s proteins and membrane constituents, and in directing them to their proper destinations.

To perform this function, the Golgi vesicles contain different sets of enzymes in different types of vesicles— cis, middle and trans cisternae—that react with and modify secretory proteins passing through the Golgi lumen or membrane proteins and glycoproteins that are transiently in the Golgi membranes as they are en route to their final destinations.

For example, a Golgi enzyme may add a “signal” or “tag” such as a carbohydrate or phosphate residues to certain proteins to direct them to their proper sites in the cell. Or, a proteolytic Golgi enzyme may cut a secretory or membrane protein into two or more specific segments (e.g., molecular processing involved in the formation of the pancreatic hormone insulin: preproinsulin→ proinsulin→ insulin).

Recently, in the function of Golgi apparatus, sub compartmentalization with a division of labor has been proposed between the cis region (in which proteins of rough endoplasmic reticulum are sorted and some of them are returned back possibly by coated vesicles), and the trans region in which the most refined proteins are further separated for their delivery to the various cell compartments (e.g., plasma membrane, secretory granules, and lysosomes).

Thus, Golgi apparatus is a centre of reception, finishing, packaging, and dispatch for a variety of materials in animal and plant cells:

1. Golgi apparatus Functions in Plants In plants,

Golgi apparatus is mainly involved in the secretion of materials of primary and secondary cell walls (e.g., formation and export of glycoproteins, lipids, pectin, and monomers for hemicellulose, cellulose, lignin, etc.).

During cytokinesis of mitosis or meiosis, the vesicles originating from the periphery of the Golgi apparatus, coalesce in the phragmoplast area to form a semisolid layer, called a cell plate. The unit membrane of Golgi vesicles fuses during cell plate formation and becomes part of the plasma membrane of daughter cells

2. Golgi Functions in Animals

In animals, Golgi apparatus is involved in the packaging and exocytosis of the following materials:

  • Zymogen of exocrine pancreatic cells;
  • Mucus (=a glycoprotein) secretion by goblet cells of the intestine;
  • Lactoprotein (casein) secretion by mammary gland cells (Merocrine secretion);
  • Secretion of compounds (thyroglobulins) of thyroxine hormone by thyroid cells;
  • Secretion of tropocollagen and collagen;
  • Formation of melanin granules and other pigments; and
  • Formation of yolk and vitelline membrane of growing primary oocytes. It is also involved in the formation of certain cellular organelles such as plasma membrane, lysosomes, acrosome of spermatozoa, and cortical granules of a variety of oocytes.

comment