Meaning of Golgi Apparatus
Golgi complex (Golgi Apparatus, Dalton Complex, Apparato Reticulare) is a complex cytoplasmic structure made up of smooth membrane saccules or cistemae, a network of tubules with vesicles and vacuoles, which takes part in membrane transformation, secretion and production of complex biochemicals.
It is surrounded by an organelle free cytoplasm called zone of exclusion or Golgi ground substance. It was first seen by George (1867) but is named after Italian scientist Camillo Golgi, who in 1898 recognised the apparatus as reticular structure (apparato reticulare) near the nucleus.
In the nerve cells of barn owl and cat by means of metallic impregnation method. Its structure was studied under electron microscope by Dalton and Felix (1954).
Occurrence of Golgi Apparatus
Golgi apparatus or complex is absent in prokaryotic cells (PPLO, bacteria and blue-green algae). It is present in all eukaryotic cells except sieve tubes of plants, sperms of bryophytes and pteridophytes and red blood corpuscles of mammals.
Location of Golgi Apparatus
In animal cells Golgi complex or apparatus is either single or consists of a single connected complex. The two conditions are respectively called localised (most vertebrate cells) and diffused (most invertebrate cells, liver and nerve cells of vertebrates).
The localised organelle is compact. It generally occurs at one end between the nucleus and the periphery. The diffused organelle is found to form a network, e.g., around the nucleus in nerve cells.
In plant cells, Golgi apparatus is formed of a number of unconnected units called dictyosomes. Their number is highly variable— from one in certain simple algae to 25000 in rhizoidal cell of Chara. Commonly there are 10-20 dictyosomes per plant cell. A liver cell may possess up to 50 units of Golgi apparatus called Golgisomes.
Structure of Golgi Apparatus
The shape and size of Golgi complex are not fixed. They depend upon the physiological state of the cells. A typical plant dictyosome is 0.5-1.0 pm in diameter. Usually Golgi complex is made up of four parts— cisternae, tubules, vesicles and vacuoles.
Golgi complex consists of a stack of generally 4-8 (range 3-20) membrane bound saccules or cisternae. Unicistemal dictyosomes are found in fungi.
The membranes of the saccules or cisternae are smooth but of variable thickness they enclose a lumen of 60-90 A. Lumen contains a fluid substance or matrix. In a stack, the adjacent cisternae are separated by a distance of 100-300 A. The intercisternal space contains thin layer of cytoplasm having parallel fibrils.
The saccules are frequently curved to give a definite polarity to the Golgi apparatus. One face of the apparatus is convex while the other is concave. The convex side is called forming (=formative, cis-face) face while the concave side of the apparatus is known as maturing face (trans-face). The membranes of the maturing face are 7-8 nm in thickness while those of the forming face are about 4 nm in thickness.
The forming face receives (transitional) vesicles from endoplasmic reticulum. Their contents pass through various cisternae with the help of coated vesicles and intercisternal connectives. They ultimately reach the maturing face where they are budded off as secretion, coated or Golgian vesicles or vacuoles. While passing through the apparatus, bio-chemicals are variously transformed.
They form a complicated network towards the periphery and maturing face of the apparatus. Actually tubules arise due to fenestrations of the cisternae. They have a diameter of 30-50 nm. The tubules interconnect the different cisternae.
They are small sacs of 20-80 nm diameters. The vesicles are found attached to the tips of tubules at various levels in the network. They are of two types, smooth and coated. The coated vesicles have a rough surface. They elaborate membrane proteins. The smooth vesicles have a smooth surface. They contain secretory substances and are hence known as secretion vesicles.
They are expanded parts of the cisternae which have become modified to form vacuoles. The vacuoles develop from the concave or maturing face. Golgian vacuoles contain amorphous or granular substance. Some of the golgian vacuoles function as lysosomes.
Functions of Golgi Apparatus
All glandular cells depend upon Golgi complex for concentrating and packaging their products inside a soluble protein coat visible as dark staining under electron microscope. They are sent out of the cells through exocytosis or reverse pinocytosis.
Transformation of Membranes
Golgi complex brings about membrane transformation, that is, converting one type of membrane (e.g., that of ER) into other types (e.g., selectively permeable plasma membrane, differentiated membrane of lysosome). The complex also takes part in the recycling of plasma membrane.
Glycoproteins and Glycolipids
Proteins synthesised by the rough endoplasmic reticulum and lipids synthesized by smooth endoplasmic reticulum reach the cisternae of the Golgi apparatus. Here, they combine with carbohydrates to form glycoproteins and glycolipids.
Special Simple Carbohydrates
Sialic acid and galactose are made inside Golgi complex.
Most of the complex carbohydrates, other than glycogen and starch, are synthesized inside the Golgi complex, e.g., pectic compounds, mucopolysaccharides, hyaluronic acid, chondroitin sulphate, hemicelluloses, etc.
Production of hormones by endocrine glands is mediated through it.
Matrix of connective tissue is formed by Golgi complex of its cells.
Fatty acids and glycerol absorbed by intestinal epithelium are transferred as fat to lacteal through Golgi complex.
Synthesis of Pigments
In Chick embryo the retinal pigment has been observed to be synthesized by Golgi complex.
Formation of Acrosome
Acrosome is an important constituent of the tip of animal sperms which helps in digesting away the covering sheath of the egg or ovum during fertilization. It is synthesized by Golgi complex with the help of its vesicles.
In oocytes of animals, Golgi apparatus functions as the centre around which yolk is deposited. The process is called vitellogenesis.
The formation of root hair from their mother cells is believed to take place through the agency of Golgi apparatus.
Formation of Lysosomes
Some of the vesicles or vacuoles of the Golgi apparatus store digestive enzymes obtained through ER in the inactive state. They act as primary lysosomes.
Hypnotoxin of nematoblasts is formed by Golgi apparatus.
Formation of Plasma-lemma
Membranes of the vesicles produced by Golgi apparatus join in the region of cytokinesis to produce new plasma-lemma.
Formation of New Cell Wall
Pectic compounds of middle lamella and various polysaccharides of the cell wall are secreted by Golgi complex. They are brought to the area of new wall synthesis by secretion vesicles.