What you need to know
- 3.4Rough ER and Golgi in protein transport and extracellular enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. formation.
- 3.3(ii)Functions of ribosomeA small organelle made of rRNA and protein, where translation takes place., ER, Golgi and associated vesicles.
By the end, you should be able to…
- Put the secretory pathway in order.
- Distinguish polypeptideA chain of many amino acids joined by peptide bonds. synthesis from modification and packaging.
- Interpret a supplied time-course of labelled protein movement.
Build your understanding
Begin at a ribosome
A protein destined for secretion, such as a digestive enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction., is synthesised by a ribosomeA small organelle made of rRNA and protein, where translation takes place. associated with rough ER. The polypeptideA chain of many amino acids joined by peptide bonds. enters the ER as it is made, where it can fold and begin processing. The rough ER does not replace the ribosome as the site of translationMaking a polypeptide on a ribosome, using the sequence of codons on mRNA. tRNAs bring the amino acids..
The cell must first express the relevant geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. to supply messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome., linking this pathway to Unit 1 protein synthesis. The transport pathway described here concerns proteins entering the secretory system, not every protein made by a cell.
Transfer to the Golgi
Transport vesicles bud from the ER and carry protein to the Golgi apparatusA stack of flattened membrane sacs that modifies proteins and packages them into vesicles.. Their membranes fuse with the receiving compartment. The Golgi modifies and sorts the cargo and packages it into vesicles for particular destinations.
A vesicle has a membrane enclosing its contents; it is not a hole or an open channel through the cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles.. Compartmentalisation keeps proteins in suitable environments and allows different processing steps to occur in different places.
Export by exocytosis
Secretory vesicles move to the cell surface membraneThe partially permeable membrane around a cell that controls which substances enter and leave. and fuse with it, releasing their contents outside the cell by exocytosisReleasing material from a cell when a vesicle fuses with the cell surface membrane. It uses ATP.. The vesicle membrane becomes part of the cell-surface membrane. The protein does not have to diffuse directly through the hydrophobic bilayer.
For an extracellular enzyme, complete the explanation: synthesis at bound ribosomes → ER processing → transport vesicle → Golgi modification and sorting → secretory vesicle → exocytosis → enzyme acts outside the cell. Not all vesicles are secretory: some deliver cargo to other intracellular compartments.
Use timing as evidence
In a supplied tracer experiment, newly made labelled protein might be detected first near rER, then in the Golgi and later in secretory vesicles or outside the cell. A changing distribution supports a transport sequence. A single snapshot showing protein in two compartments cannot establish the order by itself.
A blocked step can cause accumulation before the block and reduced delivery afterwards. Interpret this conditionally: damage to energy supply or general cell function could also reduce secretion, so suitable controls are needed.
NOT: the Golgi does not synthesise the polypeptide. Free cytosolic proteins need not follow this route.
- Rough ER, then vesicle, then Golgi, then secretory vesicle
- Exocytosis moves bulk material out without crossing the bilayer
- Extracellular enzymes use this route
- The gene is not consumed to make the protein
Two traps this diagram blocks
- Do not start the secretory route on a free ribosome and skip the ER.
- Do not say the protein diffuses out through the membrane as individual molecules.
From the diagram alone
Question
Describe the route of a protein from its synthesis to release outside a eukaryotic cell. [4]
Show a model answer
Ribosomes on rough ER synthesise the polypeptide and it enters the ER. A vesicle carries it to the Golgi, where it is modified and sorted. A secretory vesicle then fuses with the cell-surface membrane and releases the protein by exocytosis.
Make your reasoning visible
Do not give the Golgi the wrong job
It modifies, sorts and packages proteins; ribosomeA small organelle made of rRNA and protein, where translation takes place. assemble the amino-acid sequence.
Explain the membrane event
In exocytosisReleasing material from a cell when a vesicle fuses with the cell surface membrane. It uses ATP., the vesicle membrane fuses with the cell surface membraneThe partially permeable membrane around a cell that controls which substances enter and leave. and contents are released.
Use a sequence, not a list
Link compartments with transport steps and explain why the protein reaches the next destination.
Key explanations
- A secretory vesicle leaves the Golgi and moves to the cell surface.
Avoid this misconception
The claim: Secreted enzymes pass directly through the cell-surface phospholipid bilayer.
The correction: They are delivered in vesicles and released by membrane fusion in exocytosis.
How does an enzyme get out of the cell?
Cells in your pancreas make digestive enzymes that work outside the cell. The enzyme is far too big to slip through the membrane. So how does it get out?
What the model shows
A secretory cell with its organelles, a single enzyme molecule you can follow, and a tracer experiment with a graph of where the labelled protein is over time.
What to look for
The order of the stops, and where the protein piles up when one step is blocked.
What it tells us
Secreted proteins follow a route: rough ER → Golgi apparatus → vesicles → exocytosis. Labelling over time is the evidence for the order.
Be careful what you claim
Illustrative times and simple constant rates.
Try it in the model: click the nucleus, then the next organelle.
Following a secreted enzyme
Click the organelles in order to follow one digestive enzyme out of a cell, then run a pulse-chase experiment and block one step at a time. Times are illustrative.
What you see
- A secretory cell (for example a pancreatic cell making a digestive enzyme) with its nucleus, rough ER, Golgi apparatus, vesicles, a mitochondrion and the cell-surface membrane.
- In the tracer experiment, each dot is labelled protein. The graph shows the share of the labelled protein in each place over 120 minutes.
How to use it
- Click the organelles in order to follow one enzyme, or run a pulse-chase experiment. Block one step to see where the protein builds up.
What it shows
- Secreted proteins are made at ribosomes on rough ER, carried in vesicles to the Golgi apparatus, modified and packaged, then released by exocytosis (3.3(ii), 3.4).
- Timed labelling shows the order: the label moves from compartment to compartment. A block causes a build-up before it and less delivery after it.
Limits of the model
- The times and rates are illustrative, following the pattern of classic pulse-chase experiments; real times vary between cells. Each transfer is treated as a simple constant-rate step.
- Only secreted proteins are followed. The Low ATP setting is a simplified general effect; real cells with too little ATP would also be damaged in other ways.
Guided tour · step 1 of 8
- In Cell, just below these steps, keep View on Follow one enzyme (options: Follow one enzyme, Tracer experiment).
- In the simulator, click the organelles in the order the enzyme visits them, starting with the nucleus.
- If you get stuck, press Next stop in Cell.
Nucleus (mRNA) → ribosome on rough ER (translation, into the ER) → transport vesicle → Golgi apparatus (modified, sorted, packaged) → secretory vesicle → exocytosis at the cell-surface membrane. The enzyme never crosses a membrane on its own: vesicles carry it.
Cell
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Order these locations for a secreted protein: Golgi, extracellular space, rough ER, secretory vesicle.
Show suggested answer
Rough ER → Golgi → secretory vesicle → extracellular space, with vesicular transport and exocytosis between the relevant steps.
Practice question
Retrieval and explanation · suggested answer
A treatment prevents ER-to-Golgi vesicle transport while synthesis initially continues. Predict an early effect.
Show suggested answer
Protein may accumulate in the ER while less reaches the Golgi and the extracellular space. This assumes the block is specific and other functions initially remain intact.
Practice question
Retrieval and explanation · suggested answer
Why is the term “extracellular” not the same as “outside the organism”?
Show suggested answer
It means outside a cell. A digestive enzyme can act in the intestine’s lumen while still being inside the body’s overall outline.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Labelled protein appears in both ER and Golgi at one time point. Give two reasons this does not prove that the Golgi sends newly made protein to the ER.
Show a hint
Consider direction, timing and ongoing synthesis.
Explore the reasoning
The snapshot gives locations but no order, and ongoing synthesis can place different molecules at different stages simultaneously. A timed labelling experiment and appropriate controls are needed to infer a route.
Recall this lesson
Lesson recall questions
Question 1 of 5: Eukaryotic ribosomes synthesise ________; centrioles organise the ________ in animal cells.
Eukaryotic ultrastructure
Practice question
Retrieval and explanation · suggested answer
Eukaryotic ribosomes synthesise ________; centrioles organise the ________ in animal cells.
Lesson: 3A 2: Eukaryotic Cells 1: Common Cellular Structures
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Cellular delivery addresses
3A · Eukaryotic Cells 2: Protein TransportThe 1999 Medicine Nobel Prize recognised work on signals within proteins that govern transport and localisation. Specific targeting signals extend the basic pathway studied here. A secreted protein…Specification 3.4Read & use the scienceClose article
Why this matters
The 1999 Medicine Nobel Prize recognised work on signals within proteins that govern transport and localisation. Specific targeting signals extend the basic pathway studied here. A secreted protein is made on ribosomes associated with rough endoplasmic reticulum, passes through the Golgi and leaves in vesicles. The Nobel research adds a question about routing: how does the cell avoid delivering a molecule to the wrong membrane? This matters to researchers studying inherited disorders and secretion. It does not replace the required route with a single magic address. At each step, a scientist can ask what evidence shows where the protein was made, where it travelled and where it finally acted.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.4 understand the role of the rough endoplasmic reticulum (rER) and the Golgi apparatus in protein transport within cells, including their role in the formation of extracellular enzymes
Use the science
Why might an otherwise active enzyme fail to perform its role if it reaches the wrong compartment?
Explore a possible response
Its substrate or suitable conditions may be absent there. Function depends on location as well as the amino-acid sequence.
Explore the source
- Nobel Prize 1999: protein targetingNobel Prize Outreach
Vesicles need the right destination
3A · Eukaryotic Cells 2: Protein TransportA cell making an extracellular enzyme must move a newly formed protein through rough endoplasmic reticulum and Golgi before secretion. Research recognised by the 2013 Medicine Nobel Prize…Specification 3.4 · 3.3(ii)Read & use the scienceClose article
Why this matters
A cell making an extracellular enzyme must move a newly formed protein through rough endoplasmic reticulum and Golgi before secretion. Research recognised by the 2013 Medicine Nobel Prize identified machinery that helps vesicles form and fuse at the right membrane. Without this organisation, cargo might stay inside a cell or reach the wrong compartment. A scientist can test the route by labelling a protein and following its location over time. The Nobel story enriches, rather than replaces, the syllabus pathway. In an exam explanation, name the organelles in order and connect vesicle movement to the protein’s eventual function outside the cell.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.4 understand the role of the rough endoplasmic reticulum (rER) and the Golgi apparatus in protein transport within cells, including their role in the formation of extracellular enzymes
- 3.3(ii) understand the function of the organelles listed in (i)
Use the science
If a protein is made normally but vesicles cannot fuse with the cell-surface membrane, what happens to its extracellular role?
Explore a possible response
The protein may accumulate inside the cell; it cannot act as an extracellular enzyme until released by exocytosis.
Explore the source
- 2013 Medicine Nobel Prize: vesicle trafficNobel Prize Outreach