IB Biology Topic B2.2.8: Structure and Function of the Golgi Apparatus Notes

The Golgi Apparatus: Processing and Sorting Proteins

1. The Golgi apparatus (also called the Golgi body or Golgi complex) is a stack of flattened, membrane-bound sacs called cisternae.
   1. It receives proteins from the rER, modifies them, and sorts them into vesicles for delivery to their final destinations.
2. The Golgi has a distinct polarity.
   1. The cis face (nearest the rER) receives transport vesicles carrying newly synthesised proteins.
   2. The trans face (nearest the plasma membrane) dispatches vesicles carrying finished products.
   3. Proteins move through the stack from cis to trans, modified at each stage by enzymes specific to that cisterna.

Protein Modification in the Golgi

1. As proteins pass through the cisternae, they undergo several modifications.
   1. Glycosylation (adding or trimming carbohydrate chains) produces glycoproteins used in cell recognition and signalling.
   2. Phosphorylation (adding phosphate groups) targets certain proteins to lysosomes, because lysosomal enzymes carry a mannose-6-phosphate tag that acts as an address label.
   3. The Golgi can also cleave inactive precursor proteins into their active forms.
2. Each cisterna contains a different set of enzymes, so modifications happen in a set sequence as proteins move from cis to trans.
   1. This works like a production line: proteins enter one end unfinished and leave the other end fully modified and ready for use.

Sorting and Secretion from the Trans Face

1. At the trans face, the Golgi sorts proteins into different vesicles depending on their destination.
   1. Secretory vesicles carry proteins to the plasma membrane for release by exocytosis, for example insulin from pancreatic beta cells or antibodies from plasma cells.
   2. Other vesicles deliver membrane proteins, such as receptors and ion channels, to the plasma membrane, where they are inserted directly.
   3. A third set of vesicles transports hydrolytic enzymes to lysosomes for intracellular digestion.
2. The cisternal maturation model describes how the cisternae themselves move through the stack from cis to trans, carrying their cargo with them.
   1. As a cisterna matures and moves toward the trans face, its enzyme composition changes through vesicle-mediated recycling of Golgi enzymes back toward the cis side.