The Golgi complex is a central processing and distribution center for many cell products. It receives materials from other internal membranes, modifies them into functional forms, and sorts them into vesicles headed to precise destinations.

Core structure of the Golgi complex

Cisternae and polarity

• The Golgi is typically described as a series of flattened sacs arranged in a stack.

Textbook-style figure and explanation emphasizing the Golgi as a stack of flattened membranous sacs with distinct cis (receiving) and trans (releasing) faces. This supports the idea that proteins and lipids are modified as they move through the stack before being packaged into vesicles for specific destinations. Source

• It is polarized, meaning different regions perform different tasks:

• cis face: the “receiving” side, generally closest to incoming transport vesicles

• trans face: the “shipping” side, where products are packaged for delivery

• Small vesicles continually bud from and fuse with Golgi membranes, supporting high-volume intracellular trafficking.

What the Golgi does to proteins

Folding and quality control (functional emphasis)

The specification highlights that the Golgi folds and modifies proteins. In practice, many proteins arrive already folded, but the Golgi contributes to functional maturation by:

• maintaining conditions that support proper protein conformation

• enabling final processing steps that stabilise structure or activate function

• sorting improperly processed cargo away from standard export routes (a key idea: only correctly processed products are efficiently shipped)

Chemical modification (major AP focus)

The Golgi performs chemical modifications that can change a protein’s stability, activity, and destination. Common categories include:

• Glycosylation (addition or trimming of carbohydrate chains)

A sentence-level way to think about this: the Golgi changes the “labels” and “finishing details” on proteins so they work correctly and reach the correct location.

• Phosphorylation of specific residues or carbohydrate groups can contribute to sorting signals.

• Proteolytic processing can activate proteins by cutting them into mature forms.

• Sulfation of certain amino acids or sugars can alter interactions and stability.

Packaging and vesicle-based trafficking

Sorting: deciding where proteins go

A key Golgi role is sorting, which depends on molecular “address labels” on cargo:

• Some proteins are destined for the plasma membrane (often embedded as membrane proteins).

• Some are destined for secretion outside the cell.

• Some are sent to other internal destinations via distinct vesicle routes.

Vesicle formation and targeting

The Golgi “ships” cargo by producing vesicles with selective contents.

Secretory pathway diagram showing how transport vesicles move cargo from the endoplasmic reticulum to the Golgi apparatus (entering at the cis face and exiting at the trans face). It also depicts formation of secretory vesicles and fusion with the plasma membrane, connecting Golgi sorting/packaging to delivery and exocytosis. Source

• Cargo selection: Golgi membranes concentrate specific proteins into budding regions.

• Budding: a vesicle forms and pinches off, enclosing cargo inside its lumen or within its membrane.

• Targeting and fusion: vesicles recognise the correct target membrane and fuse, delivering cargo for use or further transport.

Why stacking matters

The stacked arrangement supports stepwise processing:

• Different cisternae contain different sets of enzymes.

• As cargo progresses from cis to trans regions, it can undergo a predictable sequence of modifications.

• This organisation increases efficiency and reduces random mixing of unfinished and finished products.