Photosynthesis has two linked stages inside the chloroplast. The first captures light energy, and the second uses that energy to build glucose from carbon dioxide.
This lesson belongs to leaves and photosynthesis. It follows interpreting limiting-factor graphs. The two stages explain why different factors limit the rate.
How do the two stages compare?
Learn them as a pair by asking the same four questions of each.
| Question | Light-dependent stage | Light-independent stage |
|---|---|---|
| Where? | Grana (thylakoid membranes) | Stroma |
| What goes in? | Light, water | Carbon dioxide, energy-carrying molecules from stage 1 |
| What comes out? | Oxygen, energy-carrying molecules | Glucose |
| What limits it? | Light intensity, chlorophyll | Carbon dioxide, temperature |
The energy-carrying molecules are the link. Stage 1 makes them and stage 2 uses them up, so neither stage can run for long without the other.
Why does water matter in the first stage?
Light energy splits water. This releases oxygen, which leaves the leaf as a by-product, and supplies the electrons and hydrogen that stage 2 needs.
The oxygen released in photosynthesis comes from water, not from carbon dioxide. Some students write that carbon dioxide is split, so check this line in your notes.
Worked example: predict what limits the rate
Question: A scientist grows plants in a greenhouse with bright light, but the carbon dioxide level is low. Which stage is slowed, and why?
The stage that needs carbon dioxide is the light-independent stage. With little carbon dioxide, glucose is made slowly, so the energy-carrying molecules from stage 1 are not used up.
Because those molecules build up, the light-dependent stage also slows down. The bright light cannot help, since the plant cannot use the energy it captures.
The same reasoning explains a flat section on a graph. When the curve stops rising with more light, the light-independent stage has reached its own limit, set by carbon dioxide or temperature.
Now reverse it. A plant gets plenty of carbon dioxide at a warm temperature but is kept in dim light. Here stage 1 is slow, so few energy-carrying molecules are made, and stage 2 has no energy to use. The limiting factor is light.
The mistake that costs marks
A common mistake is to say that the light-independent stage “takes place at night”. It needs products of the light stage, so in a real plant it slows down soon after the light is gone.
Another mistake is to put carbon dioxide into stage 1 and oxygen into stage 2. A memory tag helps: the light stage makes oxygen, and the other stage makes glucose.
| Student wrote | Problem | Correct |
|---|---|---|
| “Light-independent happens in the dark” | The name means no direct light is needed | “It does not use light directly but needs products of the light stage” |
| “Oxygen comes from carbon dioxide” | Oxygen comes from splitting water | “Oxygen is released when water is split” |
| “Both stages happen in the grana” | Stage 2 is in the stroma | “Stage 1 in the grana, stage 2 in the stroma” |
Check yourself
A student says: “If I raise the temperature from 20 °C to 30 °C, the light-dependent stage will speed up the most.” Decide whether this is the best prediction, and explain.
Answer
It is not the best prediction. The light-independent stage is controlled by enzymes, so it responds strongly to temperature.
The light-dependent stage depends mainly on light intensity and chlorophyll. The rate of the light-independent stage is more likely to rise between 20 °C and 30 °C.
What to study next
Practise the whole chapter with the leaves and photosynthesis practice set. The temperature idea links to interpreting temperature and pH graphs, so read that lesson next.
If you want a teacher to quiz you on the inputs and outputs until they stay separate, see online one-to-one Biology tuition or the one-hour trial class (from RM50).