1. Proteins arrive at the Golgi: Proteins synthesized in the endoplasmic reticulum (ER) are transported to the Golgi apparatus in membrane-bound vesicles.
2. Modification and sorting: As proteins move through the Golgi stacks (compartments), they undergo a series of modifications, including:
* Glycosylation: Addition of sugar chains (glycans) to proteins. This can affect protein folding, stability, and function.
* Phosphorylation: Addition of phosphate groups, which can regulate protein activity.
* Sulfation: Addition of sulfate groups, also affecting protein function.
* Proteolysis: Removal of amino acid sequences, which can activate or inactivate proteins.
* Sorting: The Golgi sorts proteins based on their destination. Proteins destined for secretion are packaged into transport vesicles.
3. Packaging into vesicles: The Golgi packages the modified proteins into membrane-bound vesicles called secretory vesicles. These vesicles bud off from the Golgi and move towards the cell membrane.
4. Exocytosis: At the cell membrane, the secretory vesicles fuse with the plasma membrane, releasing their contents outside the cell.
Key Points:
* The Golgi apparatus is a stack of flattened, membrane-bound sacs called cisternae.
* It functions as a central processing and packaging plant for proteins.
* Proteins are modified and sorted in the Golgi before being exported out of the cell.
* The Golgi plays a crucial role in the secretion of hormones, enzymes, and other molecules.
In summary, the Golgi apparatus is responsible for the following steps in protein export:
* Transport: Receiving proteins from the ER.
* Modification: Adding sugar chains, phosphates, sulfates, and removing amino acid sequences.
* Sorting: Classifying proteins based on their destination.
* Packaging: Enclosing modified proteins in transport vesicles.
* Exocytosis: Releasing proteins from the cell.
