Protein processing: rER and GolgiEdexcel A-Level Biology A: Revision notes
Section 1
Proteins for use outside the cell
Some proteins stay in the cell, but others are made for secretion, such as digestive enzymes and hormones. Proteins that work outside the cell are extracellular proteins. Their journey is a production line through membrane-bound organelles. Proteins made on free ribosomes in the cytoplasm stay in the cell; proteins made on ribosomes bound to the rough endoplasmic reticulum (rER) are for secretion or for membranes and lysosomes.
Section 2
Role of the rough endoplasmic reticulum
The rER is a system of flattened membrane sacs (cisternae) whose outer surface is covered in ribosomes. The mRNA for a secreted protein is translated on these ribosomes, and the growing polypeptide passes into the lumen of the rER.
In the lumen the polypeptide is folded into its 3D shape and may be modified. Small sections of rER then pinch off as transport vesicles carrying the protein. The large surface area of the rER lets many ribosomes work at once, so cells that secrete lots of protein have extensive rER.
Section 3
Role of the Golgi apparatus
The Golgi apparatus is a stack of flattened, curved sacs. Transport vesicles from the rER fuse with the Golgi on the side nearest the rER. In the Golgi, proteins are modified: for example, carbohydrate groups are added to make glycoproteins, and some proteins are trimmed.
The Golgi then sorts and packages the proteins into vesicles that bud off from the other side: secretory vesicles for export, and lysosomes for enzymes that stay in the cell.
Section 4
Secretion from the cell
Secretory vesicles move to the cell-surface membrane, helped by the cytoskeleton. They fuse with the membrane and release their contents outside the cell by exocytosis. The vesicle membrane becomes part of the cell-surface membrane.
The whole pathway: gene transcribed in the nucleus, mRNA to rER ribosomes, polypeptide into rER lumen, vesicle to Golgi, modification and packaging, secretory vesicle to cell-surface membrane, exocytosis. Every step needs ATP from mitochondria.
Ribosomes, not the rER itself, make the polypeptide. The Golgi does not make proteins: it modifies and packages them.
Section 5
Tracking the pathway: radioactive labelling
The pathway was shown by a pulse-chase experiment. Cells are given a short pulse of radioactive amino acids, then moved to a medium with unlabelled amino acids (the chase). Only proteins made during the pulse are labelled, so as time passes the radioactivity can be followed: first in the rER, then the Golgi, then in vesicles near the cell-surface membrane, and finally outside the cell.
If a toxin stops rER vesicles fusing with the Golgi, the labelled protein builds up in the rER and secretion falls.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Protein processing: rER and Golgi
- Acinar cells of the pancreas make and secrete large quantities of the digestive enzyme amylase into the pancreatic duct. Electron micrographs of these cells show extensive rough endoplasmic reticulum, a prominent Golgi apparatus and many vesicles close to the cell-surface membrane.Explain why these cells have extensive rough endoplasmic reticulum.2 marks
- Pancreatic cells were incubated for three minutes in a medium containing radioactively labelled amino acids and then transferred to a medium containing only unlabelled amino acids. Samples were taken at intervals and the position of the radioactivity in the cells was found. After 5 minutes most of the radioactivity was in the rough endoplasmic reticulum, after 20 minutes it was mostly in the Golgi apparatus, and after 60 minutes it was in vesicles near the cell-surface membrane and in the surrounding medium.Explain why radioactivity was found first in the rough endoplasmic reticulum.2 marks
- A researcher treats pancreatic cells with a toxin that prevents vesicles from the rough endoplasmic reticulum from fusing with the Golgi apparatus. The untreated cells normally secrete digestive enzymes.Predict and explain the effect of the toxin on the position of enzyme proteins in the cell and on the secretion of enzymes.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).