Distinguish eukaryotic ribosomes and microtubule structures
Eukaryotic cytoplasm contains 80S ribosomes, while centrioles and microtubules help organise cell structures and movement.
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
Which ribosomes are found in eukaryotic cytoplasm?
What you will learn
- Distinguish 80S cytoplasmic ribosomes from 70S ribosomes in mitochondria and chloroplasts
- Recognise centrioles and microtubules and outline their roles
Ribosome size depends on location
Ribosomes are the sites of protein synthesis. Eukaryotic cytoplasm contains 80S ribosomes. Mitochondria and chloroplasts contain 70S ribosomes. The “S” values describe sedimentation coefficients and are not ordinary size units that can be added.
When identifying a ribosome in an image, its location matters. A free ribosome or one attached to rough ER in eukaryotic cytoplasm is 80S; a ribosome inside a mitochondrion or chloroplast is 70S.
Centrioles and microtubules organise cell structures
Centrioles are cylindrical structures made from microtubules, typically found in pairs in many animal cells. They help organise microtubules and are associated with formation of the spindle during cell division.
Microtubules are protein-based components of the cytoskeleton. They help maintain cell organisation and provide structures involved in intracellular movement. Plants and animals do not show identical centriole patterns; do not state that all eukaryotic cells have centrioles.
Pause and explain
What are centrioles made from?
Worked example
A ribosome is located in the cytoplasm of a eukaryotic cell. State its type. A second is inside a chloroplast. State that type.
Show the worked solution
- Cytoplasmic eukaryotic ribosomes are 80S.
- Chloroplast ribosomes are 70S.
Answer The cytoplasmic ribosome is 80S. The ribosome inside the chloroplast is 70S.
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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.
A ribosome is found inside a mitochondrion. Explain why 70S is the expected label.
Give me a hint
Use location rather than the whole-cell domain.
Compare my reasoning
- Mitochondria contain 70S ribosomes.
- The surrounding cytoplasm contains 80S ribosomes.
The ribosome is inside a mitochondrion, where 70S ribosomes occur. Eukaryotic cytoplasm has 80S ribosomes, so location distinguishes the types.
Look for these in your work
- Location interpreted
- Both ribosome types contrasted
Outline one role of microtubules and one role associated with centrioles.
Give me a hint
Separate the microtubule network from its organising structures.
Compare my reasoning
- Microtubules help organise cell structure and intracellular movement.
- Centrioles help organise microtubules and are associated with spindle formation.
Microtubules help maintain cell organisation and support intracellular movement. Centrioles organise microtubules and are associated with forming the spindle during cell division in cells that contain centrioles.
Look for these in your work
- Microtubule role
- Centriole role bounded
A diagram shows a pair of cylindrical structures near the nucleus. Give a likely identification and one limitation.
Give me a hint
Use a cautious identification and note cell-type differences.
Compare my reasoning
- The structures are consistent with centrioles.
- Not every eukaryotic cell contains centrioles.
The pair is consistent with centrioles, which are cylindrical microtubule structures. Their presence should not be generalised to every eukaryotic cell, especially typical plant cells.
Look for these in your work
- Likely identification
- Distribution limitation
Common mistakes
- Calling every eukaryotic ribosome 70S
- Treating “S” as a length unit
- Claiming all eukaryotic cells contain centrioles
What can you explain now?
Which ribosomes are found in eukaryotic cytoplasm?
Compare with the explanation
80S
Eukaryotic cytoplasm contains 80S ribosomes.
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Make it stick
How do ribosome type and centriole presence depend on cellular location or cell type?
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