Relate chloroplast structure to photosynthesis
Use chloroplast diagrams and electron micrographs to locate light-dependent and light-independent stages.
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- Chloroplasts contain photosynthetic pigments, and enzymes catalyse metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which chloroplast structures contain the site of the light-dependent stage?
What you will learn
- Locate grana, thylakoid membranes, thylakoid spaces and stroma
- Relate chloroplast compartments to photosynthesis stages
Build the explanation
A chloroplast contains thylakoids: flattened membrane-bound sacs with a thylakoid space inside. Thylakoids are stacked into grana and linked by membranes. The light-dependent stage occurs at thylakoid membranes, with proton movement involving the thylakoid spaces. The light-independent stage, including the Calvin cycle, occurs in the stroma. Chloroplast structure therefore separates the membrane-based light reactions from enzyme-controlled carbon fixation in the stroma.
Apply it carefully
In a diagram or electron micrograph, distinguish a thylakoid membrane from the enclosed thylakoid space and from the surrounding stroma. A granum is a stack of thylakoids, not a separate organelle. Images have different magnifications and contrast; do not infer a functional rate from the number of visible stacks.
Pause and explain
Where is the light-independent stage located?
Worked example
Where does the Calvin cycle occur in a chloroplast?
Show the worked solution
- Locate the fluid region around the grana.
- The stroma surrounds the thylakoids.
Answer The stroma.
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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.
Use chloroplast structure to compare the locations of the light-dependent stage and the Calvin cycle.
Give me a hint
Name the membrane system and the surrounding fluid region.
Compare my reasoning
- Light-dependent reactions occur on thylakoid membranes in grana.
- The Calvin cycle occurs in the surrounding stroma.
The light-dependent stage occurs at thylakoid membranes, including those stacked in grana. The Calvin cycle occurs in the stroma, the fluid region around the thylakoids.
Look for these in your work
- Thylakoid membrane location correct
- Stroma location correct
- Stages distinguished
An electron micrograph shows stacks of flattened membrane sacs surrounded by stroma. Identify a granum and state one functional link.
Give me a hint
A granum is a stack; connect its thylakoid membranes to the light-dependent stage.
Compare my reasoning
- A granum is a stack of thylakoids.
- Thylakoid membranes are the site of the light-dependent stage.
The stack is a granum. Its thylakoid membranes provide the site of the light-dependent stage; the surrounding stroma is where the Calvin cycle occurs.
Look for these in your work
- Structure named
- Stage linked to membrane
- Stroma distinguished
A student labels the internal fluid of a thylakoid as stroma. Correct the label and give the correct names for both regions.
Give me a hint
Distinguish the inside of a thylakoid from the surrounding chloroplast fluid.
Compare my reasoning
- Inside a thylakoid is the thylakoid space.
- The fluid surrounding the thylakoids is the stroma.
The internal compartment is the thylakoid space. The surrounding chloroplast fluid is the stroma, where the Calvin cycle occurs.
Look for these in your work
- Thylakoid space correctly named
- Stroma correctly named
- Calvin-cycle location stated
Common mistakes
- Calling a granum one thylakoid
- Confusing thylakoid space with stroma
- Putting the Calvin cycle on grana
What can you explain now?
Which chloroplast structures contain the site of the light-dependent stage?
Compare with the explanation
Thylakoid membranes
Light-dependent reactions occur on thylakoid membranes in grana and connecting membranes.
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Make it stick
Where are the two photosynthesis stages located in a chloroplast?
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