BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.10. Cellular Structures

5.10.5. The Golgi Apparatus

The Structure now known as the Golgi apparatus was first discovered in Cells in 1898 by Camillo Golgi, who used a special staining technique for his observations. However, a detailed investigation of it became possible only with the advent of the Electron microscope. The Golgi apparatus is found in almost all Eukaryotic cells and consists of a stack of flattened membranous sacs, known as cisternae, and an associated system of vesicles called Golgi vesicles. The three-dimensional STRUCTURE OF THE Golgi apparatus is difficult to discern in ultrathin sections, but it is believed that a complex system of interconnected tubules forms around the central stack (Fig. 5.28).

At one pole of the stack, new cisternae are continually formed by the fusion of vesicles that bud off from the smooth ER. This "outer" or forming face of the stack is convex, whereas the opposite, "inner" face—where maturation is completed and cisternae break down into vesicles once again—has a concave shape. The stack consists of numerous cisternae that gradually migrate from the outer face to the inner face.

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Fig. 5.28. A. Three-dimensional structure of the Golgi apparatus. B. Transmission electron micrograph showing two Golgi apparatuses: left — a dictyosome in vertical section, right — the topmost cisterna viewed from above, x50,000.

The function of the Golgi apparatus involves the Transport of substances and the chemical modification of cellular products entering it. This function is particularly vital in secretory cells, for which the acinar Cells of the Pancreas serve as an excellent example. These cells secrete digestive Enzymes of pancreatic juice into the pancreatic duct, through which they reach the duodenum. Fig. 5.29, A shows an electron micrograph of such a Cell, and Fig. 5.29, B illustrates a diagram of the aforementioned secretory pathway.

Fig. 5.29. A. Electron micrograph of an acinus — a cluster of pancreatic acinar cells secreting enzymes, x10,400. 1 — Nucleus; 2 — mitochondrion; 3 — Golgi apparatus; 4 — secretory granules; 5 — rough Endoplasmic reticulum. B. Diagram of protein Synthesis and Secretion (one of the enzymes) in a pancreatic acinar cell.

The individual stages of this pathway are revealed using radioactively labeled Amino Acids. Proteins are built from amino acids within The Cell. By utilizing labeled amino acids, one can track their incorporation into proteins and their movement across various cellular Organelles. To achieve this, tissue samples are homogenized at various time intervals following the administration of the amino acids, cellular organelles are separated by centrifugation (Fig. 5.9), and the highest radioactivity is measured to determine which organelles harbor it. After concentration in the Golgi apparatus, the protein is transported in Golgi vesicles to The Plasma Membrane. The final step is the secretion of the inactive enzyme via a process that is the reverse of pinocytosis. The digestive enzymes secreted by the pancreas are synthesized in an inactive form to prevent them from destroying the cells in which they are produced. An example is trypsinogen, which is converted into active Trypsin in the duodenum.

Proteins entering the Golgi apparatus from the ER typically possess short oligosaccharide chains, meaning they are Glycoproteins (similar to the Membrane Proteins depicted in Fig. 5.16). Such carbohydrate "antennae" may undergo modification within the Golgi apparatus, transforming them into molecular tags that direct the protein precisely to its destination. However, the exact mechanism by which the Golgi apparatus sorts and distributes molecules remains incompletely understood.

The Golgi apparatus is sometimes involved in carbohydrate secretion, such as in the synthesis of plant Cell wall material. Fig. 5.20 demonstrates its enhanced activity in the region of the "cell plate," i.e., the region where a new cell wall is laid down between two newly formed daughter nuclei following nuclear division.

Golgi vesicles are directed to the appropriate site on the cell plate with the aid of microtubules (Section 5.10.7). The membranes of these vesicles become part of the Plasma Membranes of the daughter cells, while their contents are utilized to construct the middle lamella and new cell walls. Cellulose is supplied separately, bypassing the Golgi apparatus and instead being delivered via microtubules.

Fig. 5.30. Electron micrograph of a dividing plant cell plate (extending diagonally across the micrograph). Each of the two new cells is forming a cell wall, with raw Materials supplied from the center of the plate toward the periphery. The growing cell wall is referred to as the cell plate because in three-dimensional view it resembles a plate. Note the close association of the Golgi apparatus and microtubules with the growing end of the cell plate, x15,000.

The two Examples we have examined — the secretion of enzymes by pancreatic cells and The formation of new cell walls in dividing plant cells — demonstrate how multiple cellular organelles can cooperate to perform a single unified function.

The Golgi apparatus secretes the glycoprotein mucin, which forms mucus in solution. It is produced by goblet cells located within the epithelium of the mucous membrane of the intestine and respiratory tract. In the leaf glands of certain Insectivorous Plants, such as the sundew, the Golgi apparatus secretes a sticky mucus and enzymes that these plants use to capture and digest prey. In many cells, the Golgi apparatus participates in the secretion of mucus, Waxes, Gums, and plant mucilages.

Aside from the secretion of various substances, the Golgi apparatus performs another crucial function — it is the site where Lysosomes are formed, which we will discuss next.