The secretory pathway and viruses
Watch the whole lecture free
Make a free account to watch every lecture to the end: 3 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
That was the teaching part. The rest of this lecture is free with an account: 3 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
No password needed now: we email you a link to set one. Already have an account? Sign in, then come back to this page.
The teaching part (2 min) is free to watch. A free account opens the whole lecture: 3 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Making a protein for export
- Step one: making the polypeptide
- Step two: folding and transport
- Step three: the Golgi body
- The whole route in order
Read the transcript of the teaching part
Last time
Before we start: last time, in Cell structures, plant and animal cells, and prokaryotes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Making a protein for export
A cell in your windpipe makes mucus all day. The mucus is a protein with sugar attached, and it must leave the cell. It does not simply float out. It travels a fixed route through the cell, and Cambridge asks you to put that route in order.
Step one: making the polypeptide
The gene for the protein is copied into messenger RNA in the nucleus. The messenger RNA leaves through a nuclear pore. A ribosome attaches to the rough endoplasmic reticulum, which is flattened sacs studded with ribosomes. Amino acids are joined into a polypeptide, which is threaded into the space inside the sac. Proteins for use in the cytoplasm are made on free ribosomes. Proteins to be exported are made on the rough ER.
Step two: folding and transport
Inside the rough ER the polypeptide folds into its three dimensional shape. A small piece of the ER pinches off as a transport vesicle, carrying the protein with it. The vesicle moves to the Golgi body and fuses with its first face. Energy for the movement comes from A T P, made by mitochondria.
Step three: the Golgi body
The Golgi body is a stack of flattened sacs. It modifies the protein, for example by adding sugar chains to make a glycoprotein. It then packages the finished protein into secretory vesicles. The secretory vesicles move to the cell surface membrane and fuse with it. The contents are released by exocytosis.
The whole route in order
The order is: nucleus, rough endoplasmic reticulum, transport vesicle, Golgi body, secretory vesicle, cell surface membrane. If radioactive amino acids are supplied, the label appears first in the rough ER, then the Golgi body, then the vesicles. Smooth ER is not on this route. It makes lipids, not protein.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Protein synthesis and secretion: the route through the cell · Viruses.
The whole chapter: Cell Structure, Cell Structure.
Chapter 1: Cell structure
- Microscopy: resolution, magnification and graticules 7 min
- Cell structures, plant and animal cells, and prokaryotes 5 min
- The secretory pathway and viruses 6 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.