Today we are going to take a look at the Golgi apparatus (sometimes called the Golgi body or Golgi complex) and its role in the cell, followed by the lysosomes.
The Golgi apparatus
We first came across the Golgi apparatus in the article about the ribosomes and rough endoplasmic reticulum (RER). Protein synthesis begins at the ribosomes with the synthesis of a polypeptide chain from amino acids. The polypeptide chain is then transported into the RER to be folded and processed.
The next stop in the journey of a protein is the Golgi apparatus. Here, it is modified further and packaged up ready for transport out of the cell. For example, a polysaccharide could be attached to the protein to form a glycoprotein. As well as modifying and packaging proteins, the Golgi apparatus also modifies and process lipids.
The Golgi apparatus is a group of flattened sacs that are filled with fluid and bound by a membrane. Vesicles can be seen budding off from the edges of the sacs. Vesicles are just little bits of fluid-filled membrane that close around their contents to form a ball shape. Proteins are transported to and from the Golgi apparatus in vesicles. Vesicles that have left the Golgi apparatus can move to the cell-surface membrane and fuse with it to secrete the contents outside of the cell. This process is called exocytosis and is shown in the below diagram.
Of course not all proteins that a cell makes need to leave the cell (extracellular proteins); some need to stay in the cytoplasm for use within the cell (intracellular proteins). These are often made by the ribosomes that are free in the cytoplasm.
Lysosomes
The Golgi apparatus also makes lysosomes which are another organelle, so it makes sense to talk about them in this article as well. Lysosomes are a type of membrane-bound vesicle containing digestive enzymes, but they have no clear internal structure. The digestive enzymes are called lysozymes.
They are important in the immune response. When a pathogen is engulfed by a white blood cell in phagocytosis, the lysozyme enzymes digest the pathogen. Another role of the lysozyme enzymes is to break down worn out parts of the cell.
Summary
- The Golgi apparatus is a group of fluid-filled membrane-bound flattened sacs with vesicles budding off the edges.
- Proteins and lipids are processed, modified and packaged in the Golgi apparatus.
- Transport to and from the Golgi apparatus is via vesicles.
- Vesicles can fuse with the cell-surface membrane to secrete their contents in a process called exocytosis.
- Lysosomes are a type of vesicle produced by the Golgi apparatus which contain digestive enzymes called lysozymes.
