Compare mitochondria and chloroplasts
Mitochondria and chloroplasts are double-membraned organelles with small circular DNA and 70S ribosomes, but they have different roles.
Section 1.2 · Cells as the basic units of living organisms
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 organelle has cristae?
What you will learn
- Recognise mitochondria and chloroplasts and outline their structures
- State that mitochondria and chloroplasts contain small circular DNA and 70S ribosomes
- Relate mitochondria to aerobic respiration and chloroplasts to photosynthesis
Mitochondria organise respiration
Mitochondria have an outer membrane and a folded inner membrane. The folds are called cristae and increase the inner membrane’s surface area. The matrix is the compartment enclosed by the inner membrane. Mitochondria are associated with aerobic respiration and ATP production.
Mitochondria contain small circular DNA and 70S ribosomes. These features distinguish them from the 80S ribosomes found in the eukaryotic cytoplasm. The quantity and appearance of mitochondria vary among cell types.
Chloroplasts organise photosynthesis
Chloroplasts have an envelope of two membranes. Inside, thylakoid membranes form stacks called grana, set in the stroma. Chloroplasts contain chlorophyll and are the site of photosynthesis in photosynthetic eukaryotic cells.
Chloroplasts contain small circular DNA and 70S ribosomes. Their membranes, compartments and genetic features should not be confused with mitochondria. A plant cell from a non-photosynthetic tissue may not contain chloroplasts.
Pause and explain
Which ribosome type is found in mitochondria and chloroplasts?
Worked example
An electron micrograph shows one organelle with folded inner membranes and another with stacks of internal membrane discs. Identify them and one shared feature.
Show the worked solution
- Cristae identify a mitochondrion.
- Grana identify a chloroplast.
- Both have their own small circular DNA.
Answer The organelle with cristae is a mitochondrion; the organelle with stacked thylakoid membranes is a chloroplast. Both have double-membrane envelopes, small circular DNA and 70S ribosomes.
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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.
State two features shared by mitochondria and chloroplasts and one feature that distinguishes them.
Give me a hint
Compare their internal membranes and functions.
Compare my reasoning
- Both have small circular DNA and 70S ribosomes.
- Mitochondria have cristae; chloroplasts have thylakoid stacks.
Both mitochondria and chloroplasts have double membranes, small circular DNA and 70S ribosomes. Mitochondria have folded inner membranes called cristae and are associated with respiration; chloroplasts contain thylakoid membranes arranged into grana and carry out photosynthesis.
Look for these in your work
- Two shared features
- Correct distinguishing structure or role
Why can a cell with many mitochondria have a high demand for ATP?
Give me a hint
Connect mitochondrial role to energy-requiring processes.
Compare my reasoning
- Mitochondria are associated with aerobic respiration.
- Respiration supplies ATP for energy-requiring processes.
Mitochondria are associated with aerobic respiration, which supplies ATP for energy-requiring processes. A high number is consistent with a high energy demand, though cell activity and context should be considered.
Look for these in your work
- Respiration linked
- Inference appropriately bounded
A student labels a stack of internal thylakoid membranes as cristae. Correct the label and identify the organelle.
Give me a hint
Distinguish the two internal membrane systems.
Compare my reasoning
- Thylakoid stacks form grana.
- Grana occur in chloroplasts; cristae are mitochondrial folds.
The stacks are grana in a chloroplast. Cristae are folds of the inner membrane in a mitochondrion.
Look for these in your work
- Correct label
- Organelle distinction
Common mistakes
- Confusing cristae with grana
- Saying chloroplasts carry out respiration instead of photosynthesis
- Calling mitochondrial or chloroplast ribosomes 80S
What can you explain now?
Which organelle has cristae?
Compare with the explanation
Mitochondrion
Cristae are folds of the mitochondrial inner membrane.
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Make it stick
Which structures and roles distinguish mitochondria from chloroplasts?
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