Golgi apparatus

The Golgi apparatus, named after its discoverer Camillo Golgi, has a similar structure to the smooth endoplasmic reticulum but is more compact. It is composed of stacks of sacs made of membranes. The sacs are fluid-filled. There is normally only one Golgi apparatus in each animal cell. The position and size of the Golgi apparatus varies from cell to cell but it is well developed in secretory cells and neurones and is small in muscle cells. Proteins and glycoproteins that reach the Golgi apparatus are synthesized in the endoplasmic reticulum. Transport vesicles then deliver these products to the Golgi apparatus. Inside the Golgi apparatus, enzymes modify the arriving proteins and glycoproteins. For example, the enzymes may change the carbohydrate structure of a glycoprotein, or they may attach a phosphate group, sugar, or fatty acid to a protein.

One of the unique things about this organelle is that compounds which enter the cisternae are constantly in motion, traveling up the stack from the ER toward the cell membrane. Small vesicles move material from one cisterna to the next. There are three types of vesicles that carry materials away from the Golgi apparatus:

1. Secretory Vesicles. Secretory vesicles contain secretions that will be discharged from the cell. These vesicles fuse with the cell membrane and empty their contents into the extracellular environment. This process is known as exocytosis.

2. Membrane Renewal Vesicles. When vesicles produced at the Golgi apparatus fuse with the surface of the cell, they are adding new lipids and proteins to the cell membrane. At the same time, other areas of the cell membrane are being removed and recycled. The Golgi apparatus can thus change the properties of the cell membrane over time. For example, new glycoprotein receptors can be added, making the cell more sensitive to a particular stimulus. Alternatively, receptors can be removed and not replaced, making the cell less sensitive to specific ligands. Such changes can profoundly alter the sensitivity and functions of the cell.

3. Lysosomes. Vesicles called lysosomes that remain in the cytoplasm contain digestive enzymes.

In general, the Golgi acts as the cell's post office, receiving, sorting and delivering proteins and lipids. More specifically its functions include:

1. Producing glycoproteins such as mucin required in secretions, by adding the carbohydrate part to the protein.

2. Producing secretory enzymes, e.g. the digestive enzymes of the pancreas.

3. Secreting carbohydrates such as those involved in the production of new cell walls.

4. Transporting and storing lipids.

5. Forming lysosomes.

Lysosomes

Lysosomes (lysis – ‘splitting’, soma- 'body') are spherical bodies, some 0.1-1.0 µm in diameter. They contain around 50 enzymes, mostly hydrolases, in acid solution. They isolate these enzymes from the remainder of the cell and so prevent them from acting upon other chemicals and organelles within the cell. The functions of lysosomes are:

1. To digest material which the cell consumes from the environment. In the case of white blood cells, this may be bacteria or other harmful material. In protozoa, it is the food which has been consumed by phagocytosis. In either case the material is broken down within the lysosome, useful chemicals are absorbed into the cytoplasm and any debris is egested by the cell by exocytosis.

2. To digest parts of the cell, such as worn-out organelles. This is known as autophagy. After the death of the cell, the lysosomes are responsible for its complete breakdown, a process called autolysis (auto - ‘self’, lysis - ‘splitting’).

3. To release their enzymes outside the cell (exocytosis) in order to break down other cells.

In view of their functions, it is hardly surprising that lysosomes are especially abundant in secretory cells and in phagocytic white blood cells.