Protein transport and extracellular enzymes

Cell Structure, Reproduction and Development · Cells, tissues and organelles · note 4 of 6

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Protein transport and extracellular enzymesSpec 3.4

In short

Proteins made on ribosomes of the rough endoplasmic reticulum enter its lumen and are packaged into transport vesicles. These fuse with the Golgi apparatus, where the proteins are modified and packaged into secretory vesicles. The vesicles move to the cell surface membrane and fuse with it, releasing the protein, for example an extracellular enzyme, by exocytosis.

Proteins that are to leave the cell, such as extracellular enzymes (digestive enzymes released by pancreatic cells are an example), follow a set route through the cell.

The gene is transcribed in the nucleus and the mRNA leaves through a nuclear pore.
The mRNA is translated by ribosomes on the rough ER. The new polypeptide passes into the lumen of the rER, where it is folded into its three-dimensional shape.
Small sections of rER bud off to form transport vesicles, carrying the protein to the Golgi apparatus.
The vesicles fuse with the Golgi apparatus. Here the protein is modified (for example carbohydrate is added to make a glycoprotein, or the protein is trimmed) and sorted.
The modified protein is packaged into secretory vesicles that pinch off from the Golgi apparatus.
Secretory vesicles move to the cell surface membrane (along the cytoskeleton), fuse with it and release the protein outside the cell by exocytosis

The Golgi apparatus also packages hydrolytic enzymes into vesicles that remain in the cell as lysosomes. Cells that secrete a lot of protein, such as enzyme-secreting cells of the pancreas, have extensive rER and many Golgi stacks.

Diagram of the route of an extracellular enzyme: made on ribosomes on the rough ER, carried in a transport vesicle to the receiving face of the Golgi apparatus, packaged into secretory vesicles at the releasing face and released by exocytosis at the cell surface membrane; another vesicle of hydrolytic enzymes forms a lysosome. — Route of a secreted protein: rough ER → transport vesicle → Golgi apparatus → secretory vesicle → exocytosis.

Exam tip:

Give the route in the correct order (rER, transport vesicle, Golgi apparatus, secretory vesicle, cell surface membrane) and say what happens at each stage.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between a tissue, an organ and an organ system?

A tissue is a group of similar cells doing the same job. An organ is a group of different tissues working together for one function, such as the stomach. An organ system is several organs working together for a major body function, such as the digestive system.

What does the Golgi apparatus do?

The Golgi apparatus receives proteins from the rough ER in transport vesicles, modifies them (for example by adding carbohydrate to make glycoproteins), then packages them into secretory vesicles. It also makes lysosomes, which contain hydrolytic enzymes, so it acts as the cell's processing and packaging centre.

How do prokaryotic cells differ from eukaryotic cells?

Prokaryotic cells have no nucleus or membrane-bound organelles. Their DNA is a circular loop lying free in the cytoplasm, they have smaller 70S ribosomes, and they have a peptidoglycan cell wall. Eukaryotic cells have a nucleus with linear DNA, larger 80S ribosomes and membrane-bound organelles such as mitochondria.

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