Identify the nucleus, nuclear envelope and nucleolus
The nucleus is a membrane-bound compartment; the nucleolus is a region inside it involved in ribosome assembly.
AS Level · Cambridge International Biology 9700, 2028–2030. Interpret only the structures supported by the provided image or description. Written practice is self-checked; it does not demonstrate practical microscopy competence.
Before you begin
- Recognise that organisms are made of cells and that a light microscope produces a magnified image.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where is the nucleolus located?
What you will learn
- Recognise the nucleus, nuclear envelope and nucleolus in eukaryotic cells
- Relate the nuclear envelope and nucleolus to their stated cellular roles
A compartment containing genetic material
The nucleus is enclosed by a nuclear envelope and contains chromosomes made from DNA associated with proteins. In many images it is a relatively large compartment in the cytoplasm. The nuclear envelope separates nuclear contents from the cytoplasm.
The nucleolus is a region within the nucleus. It is not a separate membrane-bound organelle. Its visible appearance can vary with cell activity and the preparation of the sample.
Connect structure and role without overclaiming
The nuclear envelope provides a boundary around the nucleus. The nucleolus is associated with the production of ribosomal components and assembly of ribosomes. Cells vary in nuclear shape and position, so identification should use the image and surrounding features.
Do not identify every dark region as a nucleolus. A nucleolus lies within a nucleus; a dark patch elsewhere in the cytoplasm requires different evidence.
Pause and explain
Which structure surrounds the nucleus?
Worked example
A micrograph shows a large compartment with a distinct boundary and a dense region inside it. Identify the two structures.
Show the worked solution
- The larger enclosed compartment is the nucleus.
- The dense internal region may be the nucleolus.
Answer The large membrane-bounded compartment is the nucleus, and the dense region within it is consistent with a nucleolus. The nucleolus is inside the nucleus, not an independent organelle.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the relationship among the nucleus, nuclear envelope and nucleolus.
Give me a hint
Name the compartment, boundary and internal region.
Compare my reasoning
- The nuclear envelope surrounds the nucleus.
- The nucleolus is within the nucleus.
The nuclear envelope forms the boundary around the nucleus, and the nucleolus is a region within the nucleus.
Look for these in your work
- Envelope identified
- Nucleolus located correctly
A dense body lies inside a large membrane-bounded compartment. Give a cautious interpretation.
Give me a hint
Use “consistent with” if the image is not definitive.
Compare my reasoning
- The compartment is consistent with a nucleus.
- The dense internal body is consistent with a nucleolus.
The membrane-bounded compartment is consistent with a nucleus, and the dense region inside it is consistent with a nucleolus. The image alone may not establish every function.
Look for these in your work
- Both structures located
- Evidence limits respected
Why is it inaccurate to call the nucleolus a membrane-bounded organelle?
Give me a hint
Recall its location and boundary.
Compare my reasoning
- It is a region inside the nucleus.
- The nuclear envelope surrounds the nucleus, not a separate nucleolus compartment.
The nucleolus is a region within the nucleus and is not surrounded by its own membrane. The nuclear envelope encloses the whole nucleus.
Look for these in your work
- Internal location stated
- No separate membrane claimed
Common mistakes
- Placing the nucleolus outside the nucleus
- Calling the nuclear envelope the cell-surface membrane
- Treating the nucleolus as a membrane-bounded organelle
What can you explain now?
Where is the nucleolus located?
Compare with the explanation
Inside the nucleus
It is a region within the nucleus.
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Make it stick
Where are the nucleolus and nuclear envelope relative to the nucleus?
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