How Organelles Work Together
No organelle works alone. To make and secrete a single protein, the nucleus, ribosomes, rough endoplasmic reticulum, Golgi apparatus, vesicles and cell-surface membrane must all work together — powered by ATP from the mitochondria. This page follows a protein on its journey through the cell, with clear labelled diagrams for A-level Biology across every exam board.
OCR A
OCR B
Edexcel
WJEC / Eduqas
Organelles working together: the big picture
A cell is not just a bag of separate parts. Its membrane-bound organelles are organised into pathways, and one of the clearest examples is the production and secretion of proteins — such as enzymes, antibodies and hormones like insulin. OCR A even lists this explicitly as “the interrelationship between the organelles involved in the production and secretion of proteins.” Follow a protein from the nucleus to the outside of the cell, and you pass through almost the whole endomembrane system.
Examiners love to test this as a sequence. If you are asked to “describe how a protein is made and secreted”, work through the organelles in order and say what each one does — that structure alone earns most of the marks.
The organelles involved — and what each one does
Before following the pathway, meet the “team”. Each organelle has one clear job in making and exporting a protein. Notice how the product is handed on from one organelle to the next, like a production line.
Nucleus & nucleolus
Holds the instructionsThe nucleus contains the DNA — the gene for the protein. The gene is transcribed into a short mRNA copy, which leaves through a nuclear pore. The nucleolus inside makes the ribosomes.
Ribosomes
Assemble the proteinRibosomes read the mRNA and join amino acids together to build the polypeptide (translation). Ribosomes that make proteins for export sit on the surface of the rough endoplasmic reticulum.
Rough endoplasmic reticulum (RER)
Folds & transportsThe new protein passes straight into the cavity of the RER, where it folds into its 3-D shape. The RER then buds off a transport vesicle carrying the protein towards the Golgi.
Golgi apparatus
Modifies & packagesThe Golgi modifies the protein — for example adding carbohydrates to make a glycoprotein — then sorts and packages it into a secretory vesicle.
Secretory vesicle
Delivers the productA membrane-bound secretory vesicle pinches off the Golgi and carries the finished protein across the cytoplasm to the cell-surface membrane.
Cell-surface membrane
Releases the proteinThe vesicle fuses with the cell-surface membrane and the protein is released outside the cell by exocytosis.
Mitochondria
Powers the processThe mitochondria don’t touch the protein, but they carry out aerobic respiration to supply the ATP needed to move vesicles and to drive exocytosis. Without them, the pathway stops.
Lysosomes (linked)
Golgi vesiclesNot part of secretion, but worth knowing: lysosomes are a type of Golgi vesicle. They contain digestive (hydrolytic) enzymes — themselves made and packaged by the very same pathway.
The journey step-by-step
Now follow the protein through the cell in four stages. Click each stage to expand its labelled diagram and the key events. This is the exact sequence you should be able to describe in an exam.
1Stage 1 — From gene to ribosomeNucleus → mRNA → ribosome on the RER
Key events
- The gene in the DNA is transcribed into a copy called messenger RNA (mRNA).
- The mRNA leaves the nucleus through a nuclear pore and enters the cytoplasm.
- The mRNA attaches to a ribosome on the surface of the rough endoplasmic reticulum.
Why is mRNA needed? DNA is too large to leave the nucleus, so a small mRNA copy of the gene carries the instructions out to the ribosome.
2Stage 2 — Building & transporting the proteinRibosome → RER cavity → transport vesicle → Golgi
Key events
- The protein is assembled on the ribosome and released into the cavity of the cisternae of the RER, where it folds into its correct shape.
- The protein is transported through the cisternae of the RER.
- At the end of a cisterna a vesicle buds off — a transport vesicle — carrying the protein to the Golgi apparatus.
The transport vesicle is made from a piece of the RER’s own membrane that pinches off. This is how the protein stays enclosed and separate from the cytoplasm the whole way.
3Stage 3 — Modifying the protein in the GolgiTransport vesicle fuses → protein modified → secretory vesicle
Key events
- The transport vesicle fuses with the bottom cisterna of the Golgi and the protein enters the cavity.
- The protein moves up through the stack of cisternae. As it does, it is modified — for example carbohydrates are added to make a glycoprotein.
- Because the cisternae are not connected, the protein moves between them in vesicles that bud off and fuse to the cisterna above.
- At the top, the finished protein is packaged into a secretory vesicle.
“Modify, sort, package” is a good three-word summary of the Golgi’s job. The green star on the diagram marks the modified protein.
4Stage 4 — Secretion by exocytosisSecretory vesicle → cell membrane → protein released
Key events
- The secretory vesicle transports the modified protein through the cytoplasm to the cell-surface membrane.
- The vesicle membrane fuses with the cell-surface membrane.
- The protein is released outside the cell by exocytosis.
Exocytosis needs ATP — supplied by respiration in the mitochondria. That is why cells that secrete a lot of protein (e.g. pancreatic cells) have many mitochondria as well as lots of RER and Golgi.
Rough vs smooth endoplasmic reticulum
The endoplasmic reticulum comes in two forms. Only the rough ER is part of the protein-secretion pathway — a common point examiners test — so be clear on the difference.
| Rough ER (RER) | Smooth ER (SER) | |
|---|---|---|
| Ribosomes on surface? | Yes — studded with ribosomes (this is what makes it “rough”). | No ribosomes. |
| Main function | Folds and transports proteins destined for secretion or membranes. | Synthesises and transports lipids and steroids; stores calcium ions. |
| Role in secretion | Central — receives the new protein and sends it on in a transport vesicle. | Not directly involved in protein secretion. |
| Abundant in | Cells making lots of protein (e.g. antibody-secreting plasma cells, pancreatic cells). | Cells making lots of lipid/steroid (e.g. liver cells, cells of the ovaries and testes). |
Both types of ER are continuous with the nuclear envelope, which is why the pathway flows so smoothly out of the nucleus.
Calculations you might be asked
Organelles are studied under the microscope, so questions often combine the pathway with a magnification or size calculation. Click each to expand a worked example. Remember the golden rule: work in the same units throughout.
1Magnification of an organelle
The formula
The relationship between the size of an image, the real (actual) size of the object, and the magnification is:
A handy way to remember and rearrange it is the triangle I = A × M (Image = Actual × Magnification).
Worked example
A Golgi apparatus appears 30 mm wide in an electron micrograph. Its actual width is 1.5 µm. What is the magnification?
- Convert to the same units: 30 mm = 30 × 1000 = 30 000 µm.
- Apply the formula:
1 mm = 1000 µm and 1 µm = 1000 nm. Most organelles are measured in µm; membranes and ribosomes in nm.
2Actual size of an organelle from an image
If you know the image size and the magnification, rearrange the formula to find the real size:
Worked example
A mitochondrion measures 40 mm long on a micrograph taken at a magnification of × 50 000. What is its real length?
Always convert the image measurement into the unit the answer is wanted in before or after dividing — and show the conversion. Examiners award marks for the working, not just the final number.
Test yourself
Choose a puzzle below and click an answer to check it. Use the sections above if you get stuck.
Frequently asked questions
Which organelles are involved in making and secreting a protein?
The nucleus (holds the DNA and makes mRNA), ribosomes on the rough endoplasmic reticulum (assemble the protein), the rough endoplasmic reticulum (folds and transports it), the Golgi apparatus (modifies and packages it), secretory vesicles (carry it to the membrane) and the cell-surface membrane (releases it by exocytosis). Mitochondria supply the ATP needed throughout.
What is the correct order of organelles in the secretory pathway?
Nucleus → ribosome on the rough endoplasmic reticulum → rough endoplasmic reticulum → transport vesicle → Golgi apparatus → secretory vesicle → cell-surface membrane (exocytosis).
What does the Golgi apparatus do to a protein?
The Golgi apparatus modifies the protein — for example by adding carbohydrates to make glycoproteins — and then sorts and packages it into vesicles ready for secretion or delivery elsewhere in the cell.
Why does the rough endoplasmic reticulum have ribosomes on it?
The ribosomes on the rough endoplasmic reticulum assemble proteins that are destined to be secreted from the cell or inserted into membranes. The newly made protein passes straight into the cavity of the rough ER to be folded and transported, which is why the rough ER appears studded with ribosomes.
How does a protein finally leave the cell?
A secretory vesicle carries the finished protein to the cell-surface membrane. The vesicle membrane fuses with the cell-surface membrane and the protein is released outside the cell by a process called exocytosis, which requires ATP from respiration in the mitochondria.
How Organelles Work Together
No organelle works alone. To make and secrete a single protein, the nucleus, ribosomes, rough endoplasmic reticulum, Golgi apparatus, vesicles and cell-surface membrane must all work together — powered by ATP from the mitochondria. This page follows a protein on its journey through the cell, with clear labelled diagrams for A-level Biology across every exam board.
Organelles working together: the big picture
A cell is not just a bag of separate parts. Its membrane-bound organelles are organised into pathways, and one of the clearest examples is the production and secretion of proteins — such as enzymes, antibodies and hormones like insulin. OCR A even lists this explicitly as “the interrelationship between the organelles involved in the production and secretion of proteins.” Follow a protein from the nucleus to the outside of the cell, and you pass through almost the whole endomembrane system.
Examiners love to test this as a sequence. If you are asked to “describe how a protein is made and secreted”, work through the organelles in order and say what each one does — that structure alone earns most of the marks.
The organelles involved — and what each one does
Before following the pathway, meet the “team”. Each organelle has one clear job in making and exporting a protein. Notice how the product is handed on from one organelle to the next, like a production line.
Nucleus & nucleolus
Holds the instructionsThe nucleus contains the DNA — the gene for the protein. The gene is transcribed into a short mRNA copy, which leaves through a nuclear pore. The nucleolus inside makes the ribosomes.
Ribosomes
Assemble the proteinRibosomes read the mRNA and join amino acids together to build the polypeptide (translation). Ribosomes that make proteins for export sit on the surface of the rough endoplasmic reticulum.
Rough endoplasmic reticulum (RER)
Folds & transportsThe new protein passes straight into the cavity of the RER, where it folds into its 3-D shape. The RER then buds off a transport vesicle carrying the protein towards the Golgi.
Golgi apparatus
Modifies & packagesThe Golgi modifies the protein — for example adding carbohydrates to make a glycoprotein — then sorts and packages it into a secretory vesicle.
Secretory vesicle
Delivers the productA membrane-bound secretory vesicle pinches off the Golgi and carries the finished protein across the cytoplasm to the cell-surface membrane.
Cell-surface membrane
Releases the proteinThe vesicle fuses with the cell-surface membrane and the protein is released outside the cell by exocytosis.
Mitochondria
Powers the processThe mitochondria don’t touch the protein, but they carry out aerobic respiration to supply the ATP needed to move vesicles and to drive exocytosis. Without them, the pathway stops.
Lysosomes (linked)
Golgi vesiclesNot part of secretion, but worth knowing: lysosomes are a type of Golgi vesicle. They contain digestive (hydrolytic) enzymes — themselves made and packaged by the very same pathway.
The journey step-by-step
Now follow the protein through the cell in four stages. Click each stage to expand its labelled diagram and the key events. This is the exact sequence you should be able to describe in an exam.
1Stage 1 — From gene to ribosomeNucleus → mRNA → ribosome on the RER
Key events
- The gene in the DNA is transcribed into a copy called messenger RNA (mRNA).
- The mRNA leaves the nucleus through a nuclear pore and enters the cytoplasm.
- The mRNA attaches to a ribosome on the surface of the rough endoplasmic reticulum.
Why is mRNA needed? DNA is too large to leave the nucleus, so a small mRNA copy of the gene carries the instructions out to the ribosome.
2Stage 2 — Building & transporting the proteinRibosome → RER cavity → transport vesicle → Golgi
Key events
- The protein is assembled on the ribosome and released into the cavity of the cisternae of the RER, where it folds into its correct shape.
- The protein is transported through the cisternae of the RER.
- At the end of a cisterna a vesicle buds off — a transport vesicle — carrying the protein to the Golgi apparatus.
The transport vesicle is made from a piece of the RER’s own membrane that pinches off. This is how the protein stays enclosed and separate from the cytoplasm the whole way.
3Stage 3 — Modifying the protein in the GolgiTransport vesicle fuses → protein modified → secretory vesicle
Key events
- The transport vesicle fuses with the bottom cisterna of the Golgi and the protein enters the cavity.
- The protein moves up through the stack of cisternae. As it does, it is modified — for example carbohydrates are added to make a glycoprotein.
- Because the cisternae are not connected, the protein moves between them in vesicles that bud off and fuse to the cisterna above.
- At the top, the finished protein is packaged into a secretory vesicle.
“Modify, sort, package” is a good three-word summary of the Golgi’s job. The green star on the diagram marks the modified protein.
4Stage 4 — Secretion by exocytosisSecretory vesicle → cell membrane → protein released
Key events
- The secretory vesicle transports the modified protein through the cytoplasm to the cell-surface membrane.
- The vesicle membrane fuses with the cell-surface membrane.
- The protein is released outside the cell by exocytosis.
Exocytosis needs ATP — supplied by respiration in the mitochondria. That is why cells that secrete a lot of protein (e.g. pancreatic cells) have many mitochondria as well as lots of RER and Golgi.
Rough vs smooth endoplasmic reticulum
The endoplasmic reticulum comes in two forms. Only the rough ER is part of the protein-secretion pathway — a common point examiners test — so be clear on the difference.
| Rough ER (RER) | Smooth ER (SER) | |
|---|---|---|
| Ribosomes on surface? | Yes — studded with ribosomes (this is what makes it “rough”). | No ribosomes. |
| Main function | Folds and transports proteins destined for secretion or membranes. | Synthesises and transports lipids and steroids; stores calcium ions. |
| Role in secretion | Central — receives the new protein and sends it on in a transport vesicle. | Not directly involved in protein secretion. |
| Abundant in | Cells making lots of protein (e.g. antibody-secreting plasma cells, pancreatic cells). | Cells making lots of lipid/steroid (e.g. liver cells, cells of the ovaries and testes). |
Both types of ER are continuous with the nuclear envelope, which is why the pathway flows so smoothly out of the nucleus.
Calculations you might be asked
Organelles are studied under the microscope, so questions often combine the pathway with a magnification or size calculation. Click each to expand a worked example. Remember the golden rule: work in the same units throughout.
1Magnification of an organelle
The formula
The relationship between the size of an image, the real (actual) size of the object, and the magnification is:
A handy way to remember and rearrange it is the triangle I = A × M (Image = Actual × Magnification).
Worked example
A Golgi apparatus appears 30 mm wide in an electron micrograph. Its actual width is 1.5 µm. What is the magnification?
- Convert to the same units: 30 mm = 30 × 1000 = 30 000 µm.
- Apply the formula:
1 mm = 1000 µm and 1 µm = 1000 nm. Most organelles are measured in µm; membranes and ribosomes in nm.
2Actual size of an organelle from an image
If you know the image size and the magnification, rearrange the formula to find the real size:
Worked example
A mitochondrion measures 40 mm long on a micrograph taken at a magnification of × 50 000. What is its real length?
Always convert the image measurement into the unit the answer is wanted in before or after dividing — and show the conversion. Examiners award marks for the working, not just the final number.
Test yourself
Choose a puzzle below and click an answer to check it. Use the sections above if you get stuck.
Frequently asked questions
Which organelles are involved in making and secreting a protein?
The nucleus (holds the DNA and makes mRNA), ribosomes on the rough endoplasmic reticulum (assemble the protein), the rough endoplasmic reticulum (folds and transports it), the Golgi apparatus (modifies and packages it), secretory vesicles (carry it to the membrane) and the cell-surface membrane (releases it by exocytosis). Mitochondria supply the ATP needed throughout.
What is the correct order of organelles in the secretory pathway?
Nucleus → ribosome on the rough endoplasmic reticulum → rough endoplasmic reticulum → transport vesicle → Golgi apparatus → secretory vesicle → cell-surface membrane (exocytosis).
What does the Golgi apparatus do to a protein?
The Golgi apparatus modifies the protein — for example by adding carbohydrates to make glycoproteins — and then sorts and packages it into vesicles ready for secretion or delivery elsewhere in the cell.
Why does the rough endoplasmic reticulum have ribosomes on it?
The ribosomes on the rough endoplasmic reticulum assemble proteins that are destined to be secreted from the cell or inserted into membranes. The newly made protein passes straight into the cavity of the rough ER to be folded and transported, which is why the rough ER appears studded with ribosomes.
How does a protein finally leave the cell?
A secretory vesicle carries the finished protein to the cell-surface membrane. The vesicle membrane fuses with the cell-surface membrane and the protein is released outside the cell by a process called exocytosis, which requires ATP from respiration in the mitochondria.