A level-readyAll subjects · Biology · WBI12 (Unit 2)
Organelles and the secretory pathway
Eukaryotic cells contain organelles, each with its own job. Some work together as a production line to make and export proteins.
Organelles and functions
Learn what each organelle looks like in an electron micrograph and what it does.
| Organelle | Structure (as seen in electron micrographs) | Function |
|---|---|---|
| Nucleus | Double membrane (nuclear envelope) with pores; contains chromatin | Holds DNA; controls the cell through gene expression |
| Nucleolus | Dense, dark region inside the nucleus | Makes ribosomal RNA and assembles ribosomes |
| Ribosomes | Tiny dots (80S), free or on rough ER | Protein synthesis |
| Rough ER | Flattened membrane sacs studded with ribosomes | Folds and transports proteins made on its ribosomes |
| Smooth ER | Membrane tubes with no ribosomes | Makes and transports lipids and steroids |
| Golgi apparatus | Stack of curved, flattened sacs with vesicles at the edges | Modifies proteins (for example adds carbohydrate) and packages them into vesicles |
| Mitochondria | Double membrane; inner membrane folded into cristae | Aerobic respiration, making ATP |
| Lysosomes | Small round vesicles with dark contents | Contain hydrolytic enzymes; break down old organelles and engulfed pathogens |
| Centrioles | Pair of short hollow cylinders of microtubules at right angles | Organise the spindle in animal cell division |
Making an extracellular enzyme
Extracellular enzymes, such as digestive enzymes, follow the secretory pathway:
1. The gene is transcribed in the nucleus; mRNA leaves through a nuclear pore.
2. Ribosomes on the rough ER translate it; the polypeptide enters the RER and folds.
3. The protein is carried in a vesicle to the Golgi apparatus.
4. The Golgi modifies it (for example adding carbohydrate to make a glycoprotein) and packages it into a secretory vesicle.
5. The vesicle moves to the cell surface membrane, fuses with it and releases the enzyme by exocytosis.
Spotting organelles
In electron micrographs, look for: double membranes (nucleus, mitochondria), folded inner membrane (mitochondria), ribosome dots on membranes (RER), a curved stack with vesicles (Golgi). Cells that secrete a lot of protein, such as pancreatic cells, have lots of RER and Golgi.
Worked example
A cell secretes large amounts of a protein hormone. Which organelles would you expect it to have in large numbers, and why?
- Rough ER: ribosomes make the protein and it is folded and transported.
- Golgi apparatus: modifies and packages it into vesicles.
- Mitochondria: supply ATP for synthesis and exocytosis.
Answer: Plenty of rough ER, Golgi apparatus and mitochondria.
Check yourself
- Which organelle modifies proteins and packages them into vesicles?
- Smooth ER
- Nucleolus
- Lysosome
- Golgi apparatus
Show answer
Golgi apparatus. The Golgi adds carbohydrate and packages proteins.
- What do lysosomes contain?
- DNA
- Hydrolytic (digestive) enzymes
- Chlorophyll
- Ribosomes
Show answer
Hydrolytic (digestive) enzymes. Lysosomes break down unwanted material using hydrolytic enzymes.
- What is the function of the nucleolus?
- Respiration
- Packaging proteins
- Forming the spindle
- Making ribosomal RNA and assembling ribosomes
Show answer
Making ribosomal RNA and assembling ribosomes. The nucleolus produces ribosomes.
- What is the role of centrioles?
- Organising the spindle during cell division in animal cells
- Making lipids
- Storing starch
- Making ATP
Show answer
Organising the spindle during cell division in animal cells. Centrioles help form the spindle that separates chromosomes.
- Which organelle makes lipids such as steroids?
- Rough endoplasmic reticulum
- Mitochondrion
- Smooth endoplasmic reticulum
- Nucleolus
Show answer
Smooth endoplasmic reticulum. Smooth ER has no ribosomes and synthesises lipids.
- Which shows the correct route of an extracellular enzyme after it is made?
- Rough ER, vesicle, Golgi apparatus, secretory vesicle, cell surface membrane
- Golgi apparatus, rough ER, nucleus, membrane
- Smooth ER, lysosome, nucleus
- Nucleus, mitochondrion, Golgi, ribosome
Show answer
Rough ER, vesicle, Golgi apparatus, secretory vesicle, cell surface membrane. Proteins go from RER to Golgi to secretory vesicles, then exocytosis.
- An organelle in an electron micrograph has a double membrane with the inner one folded into shelves. What is it?
- Golgi apparatus
- Rough ER
- Mitochondrion
- Lysosome
Show answer
Mitochondrion. Folded inner membrane = cristae of a mitochondrion.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.