Camillo Golgi discovered certain peculiar ultramicroscopic membranous structures in animal cells which showed complex organization. And there were named after him as Golgi complex or Golgi bodies or Golgi membranes. But biologists call such structures found in plant cells as dictyosomes.
Golgi bodies look like a stack of flat membranous sacs of various sizes and dimensions and form a complex. The organelles measure 2 mm to 5 mm in size and consists of 6-20 double membranes. They are found in large numbers particularly in secretory tissues like salivary glands, stigmatic surface cells, tapetal cells and glandular tissues of Drosera and other insectivorous plants. But other cells contain 2 -5 golgi bodies.
Golgi bodies are specialized membranes designed to perform some specific functions. They are made up of a stack of flat circular or perforated membranous sacs containing lumen of 100 to 200 A thickness. The number of membranous sacs vary from to 20 with an inter spaces of 200 to 300 A.
Such membrane complexes are associated with SER membranes at the formation face which is slightly convex in its topology called cis Golgi face. The other face is called maturation face Tran Golgi face. At the formation face smaller membranous sacs are is association with numerous vesicles derived from SER from various directions. In fact one can observe a transition from RER to SER and from SER to Golgi vesicles at the formation face. The vesicles thus derived from various SER membranes fuse with one another. So one finds the proteins that are synthesized on RER ultimately find their way into Golgi membranes at the proximal face. Even the lipids that are synthesized in SER and mono or disaccharides accumulated in SER find their way into Golgi membranes. Formation of vesicles from SER and fusion of them into a flat sac is a continuous process, thus many such sacs are stacked one above the other. However, various components like proteins, lipids and carbohydrates are further processed and packed at the peripheral region of membranous sacs which develop into enlarged bulbous structures. Once such packaging process is completed, the membranes pinch off vesicles and the membranes disappear at the maturation face. Thus one each observes various stages of processing and packaging of different components loaded into vesicles in the complex. The membrane flow from SERR to Golgi, fgolgi to SER and among the golgi membranes and from transgolgi to tother region is performed by a variety of transport proteins such as COPs , Clathrins and Adaptor-adaptin proteins.
Time lapse microphotography clearly shows that the total time required from the assembly of vesicles at the formation face and release of vesicles at the maturation face, is about 20-40 minutes. The formation of golgi membranes at one face and disappearance of such membranes at the other is a continuous process, but the activity of golgi bodies is very high in certain specialized tissues where they are involved in secretion.
Techniques like equilibrium density gradient centrifugation have come handy in isolating fairly pure Golgi membranes. The chemical analysis of such membrane components reveal that the Golgi membranes are made up of 60 per cent proteins and 40 per cent lipids. Some of the proteins found in Golgi bodies are same as that of endoplasmic reticulum. Nonetheless they differ from ER in other protein components, because the processed and packed proteins vary. But with respect to lipid components, particularly in plant Golgi membranes they have more of phosphotidic acids and phosphotidyl glycerol. Such Golgi membranes are specifically lacking in sialic acids which are predominant components in animal liver Golgi membranes. Plant Golgi membranes also contain carbohydrates like glucosamines, galactose, glucose, mannose, fucose, xylose, arabinose and other sugars. Furthermore NAD dependent enzymes, Cyt. C oxidase and Cyt B reductases are found in the membranes. C oxidase,NAD dependent enzymes and Cyt.B are found in golgi membranes. The most characteristic rather marked enzymatic components are glycosyl transferase and thiamine pyrophosphotase. Acid phosphotase enzymes are also found because primary lysosomal structures are still in the process of formation and release.
Various cellular components are loaded into SER, then transported and processed packed into Golgi membranes before they are released into cytosol as vesicles. The problem that still puzzles biologists is that, how different components are sorted out and pack them into asserted groups. One of the possible explanations is that as the incoming products are synthesized on a particular set of polysomal complex on RER the proteins should be same. Then they are processed and packed into a single vesicle. Different components, coming from different SER membranes in from different directions, are separate into different groups at the formation face of the Golgi complex itself; later these are processed and packed into different vesicles. The golgi vesicles do contain receptors for specific epitopes, thus each and every transit proteins are recognized and sorted out.
In plant cells, Golgi bodies process and pack enzymes and carbohydrates into the same vesicles or different vesicles for the synthesis of cell walls. Later such vesicles are transported towards plasma membranes, microtubules direct the transport. Lysosomal enzymes are also packed into different vesicles. Some of the components of micro bodies or glyoxysomes or peroxisomes, mitochondria, nucleus and plastids receive proteins from cytoplasm directly. It is also known that some of the mucilaginous products in plants are produced by Golgi complex.
Golgi membranes are also involved in the aggregation of many enzyme precursors into zymogen granules which get enclosed in the vesicles. Later they are transported to different regions where the precursor proteins are activated. But proteins like insulin in animal cells are processed into active molecules and packed into vesicles which are then released into outer spaces.
Furthermore the membranes of Golgi complexes are involved in lipid accumulation. Such lipid components are either glycosylated or and proteinated to produce glycolipids and lipoproteins respectively. In animal cells, during spermatid maturation, the required acrosomal components are produced by Golgi membranes. They are also responsible for packing a group of hydrolyzing enzymes into primary lysosomal structures.
An important feature of processing, sorting and packing of different components into different vesicles is the requirement of metabolic energy like ATP. Thus Golgi membranes perform functions like receiving, sorting, modifying, packaging and releasing various components into their individual functional but targeted vesicles, which ultimately reach specific destinations to perform their functions. Remarkable organelle indeed!