Lesson 2 of 6 / Cells and microscopy
Follow a protein through a cell
Why does a protein-secreting cell have extensive rough endoplasmic reticulum and Golgi body?
I can recognise and connect ribosomes, endoplasmic reticulum, Golgi body, lysosomes and membranes.
I can recognise the specified cell structures in drawings and micrographs; use a microscope and calibrated graticule.Syllabus
Syllabus 9477, Core 1(b). Recognise the specified cell structures in drawings and micrographs; use a microscope and calibrated graticule.
I can relate the specified cell structures to their functions.Syllabus
Syllabus 9477, Core 1(c). Relate the specified cell structures to their functions.
About 7 min
Make a guess. You are not marked.
A pancreatic cell secretes large amounts of digestive enzyme. Which organelle does the electron micrograph show most of?
Show the answer
Rough endoplasmic reticulum, packed with ribosomes.
Secreted proteins are made on ribosomes attached to rough endoplasmic reticulum, then pass to the Golgi body.
The key idea
Ribosomes make polypeptides; the endoplasmic reticulum and Golgi body process and route suitable products in membrane vesicles.
Explore the idea
Follow a protein through compartments
Rough endoplasmic reticulum: Ribosomes on the cytosolic surface make polypeptides that enter the endoplasmic reticulum. The endoplasmic reticulum lumen provides an environment for folding and early processing.
This tracks a selected protein cargo, not movement of whole organelles. Smooth endoplasmic reticulum lacks attached ribosomes and contributes to lipid synthesis and detoxification. Free ribosomes also synthesise proteins, including many used in the cytosol.
Try a real electron micrograph
Use the photograph on the right. Which internal pattern matches the diagram?
Different section angles change how an organelle looks. Match several visible features before naming it.
Identify a real lysosome
Read the real micrograph
Find the large, irregular dark compartment near the centre. Compare its varied contents with the internal folds of nearby mitochondria.
The source identifies the black bar as 0.5 micrometres. Peroxisomes provide context here; their detailed roles are outside this lesson. Display size changes with your screen; it is not a fixed microscope magnification.
Original schematic. Features, cell sizes and distances are not to scale. Use several visible clues when interpreting an unfamiliar section.
Explanation
The cytosol is the aqueous component of the cytoplasm in which many reactions occur. Ribosomes are non-membrane-bound assemblies of rRNA and protein that translate mRNA. Free ribosomes generally make proteins used in the cytosol; ribosomes attached to rough endoplasmic reticulum make many secreted and membrane proteins.
Rough endoplasmic reticulum consists of membrane cisternae with ribosomes on the cytosolic surface. A growing suitable polypeptide enters the endoplasmic reticulum, where folding and initial processing occur. Vesicles carry it to the Golgi body, a stack of flattened sacs. The Golgi body modifies, sorts and packages products for particular destinations.
A secretory vesicle fuses with the cell surface membrane and releases its contents by exocytosis. Lysosomes are membrane-bound compartments containing hydrolytic enzymes. Their membrane separates digestion of worn components or ingested material from the cytosol; they are not protein-making organelles.
Smooth endoplasmic reticulum is a membrane network without attached ribosomes and has roles in lipid synthesis and detoxification. In a micrograph, compare several clues: dots on cisternae suggest rough endoplasmic reticulum; a curved stack with nearby vesicles suggests Golgi body. Section angle and staining can change appearances, so a single dark dot is insufficient evidence. In a written answer, give each organelle its full name. Mark schemes that award rough endoplasmic reticulum reject rER and RER.
Step by step
- 1
Locate synthesis
Find ribosomes on the cytosolic endoplasmic reticulum surface.
- 2
Follow membrane traffic
Trace endoplasmic reticulum to vesicle to Golgi body to destination.
- 3
Support an identification
Use membrane organisation and nearby structures, not colour alone.
Worked example
Work through the evidence
A labelled amino acid appears first in rough endoplasmic reticulum, then Golgi body, then extracellular protein. What does this sequence support?
One way to explain it
It supports synthesis and processing through the secretory pathway: ribosomes at rough endoplasmic reticulum, vesicular transfer to Golgi body, then exocytosis.
Why this answer works
- The label tracks newly made protein, not an organelle moving bodily across the cell.
- Timing supports a route; appropriate controls are still needed to exclude alternative interpretations.
Watch out for this
A student says: "The Golgi body makes the polypeptide chain." What is wrong with this?
Show the answer
Ribosomes synthesise the chain. Golgi enzymes modify and sort products already made.
Check your understanding
Which feature best distinguishes rough endoplasmic reticulum from smooth endoplasmic reticulum?
Show the answer
Ribosomes attached to the cytosolic surface
Attached ribosomes give rough endoplasmic reticulum its characteristic appearance.
Quick recall
Why does a protein-secreting cell have extensive rough endoplasmic reticulum and Golgi body?