THE GOLGI APPARATUS

The Golgi apparatus, as seen under digital compound microscopes, consists of a system of membrane-bound vesicles arranged ap¬proximately parallel to each other. The smooth membranes of the Golgi apparatus often have connections (probably transient) with the membranes of the endoplasmic reticulum and therefore constitute another portion of the complex cellular mem¬brane system.

Many years ago. it was noticed that the Golgi apparatus was particu¬larly prominent in cells thought to be involved in the secretion of various chemical products, and that as the level of secretory activity of these cells changed, corresponding changes occurred in the structure of the organelle. The inference could be drawn that the Golgi played some part in the secretory process. The role of the Golgi apparatus in secretion has been confirmed by more recent studies; its functions include the storage modification (removal of water or breakup of lipids into small droplets), and packaging in vesicles of secretory products. In some cases, polysaccharides may be synthesized from simple sugars in the Golgi, and these polysaccharides may then be attached to proteins and lipids, some of which, in animal cells, will later be incorporated into the glycocalyx.

There is some reason to believe that secretory vesicles produced by the Golgi apparatus may play an important role in adding surface area to the plasma membrane. When a secretory vesicle moves from the Golgi apparatus to the cell surface, it becomes attached to the plasma membrane and then ruptures, releasing its contents to the exterior in the process of exocytosis. The membrane of the ruptured vesicle may remain as a permanent addition to the plasma membrane. Biochemi¬cal and structural studies of the membranes show that the inner por¬tion of the Golgi apparatus resembles the membranes of the nuclear envelope and endoplasmic reticulum, and that there is a progressive change within the Golgi apparatus until the outer portion, where se¬cretory vesicles are produced, resembles the plasma membrane. It has been suggested, therefore, that there is a movement and transforma¬tion of the cellular membranes from the nuclear membrane to the plasma membrane, in the following sequence: nuclear envelope, rough ER, smooth ER, Golgi apparatus, secretory vesicles and plasma membrane.

MITOCHONDRIA

Prominent among the cytoplasmic structures that can be detected under the light microscope , such as a digital compound microscope, are organelles called mitochondria, which occur in almost all types of eucaryotic cells. Electron microscopy has revealed that each mitochondrion is a double-walled vessel; the outer wall is a smooth membrane, and the inner wall is a membrane with many inwardly directed folds. These folds, called cristae, extend into a semifluid amorphous matrix.

Mitochondria are the sites of chemical reactions that extract energy from foodstuffs and make it available to the cell for its innumerable energy-demanding activities, as seen under a microscope.

LYSOSOMES

When viewed under digital compound microscopes, lysosomes are slightly smaller than the mitochondria with which they were often con¬fused. In the past, lysosomes are membrane-enclosed bodies that ap¬parently function as storage vesicles for many powerful digestive (hydrolytic) enzymes. The lysosome membrane, which is single (unlike the double-membrane envelope of mitochondria), is both impermeable to the outward movement of these enzymes and capable of resisting their digestive action. Packaged in the lysosomes, the enzymes are prevented from digesting the material of the cell itself.

It is now thought that lysosomes act, in a sense, as the digestive system of the cell, enabling it to process some of the bulk materials taken in by phagocytosis or pinocytosis.

The Golgi apparatus probably produces lysosomes. First dis¬covered in rat liver cells using a microscope, they have since been found in many kinds of animal cells, and may occur in all of them. They have also been found in some fungi, but it is not yet clear how widely they occur in plant cells.