Lesson 2 of 6 / Photosynthesis and respiration
Use carbon dioxide to build carbohydrate
What returns to the light-dependent reactions?
I can identify light-independent inputs and outputs and explain storage of captured energy in carbohydrate.
I can identify initial reactants and final products of both photosynthetic stages.Syllabus
Syllabus 8876, Core 3(b). Identify initial reactants and final products of both photosynthetic stages.
I can explain light-to-chemical energy conversion and carbohydrate storage.Syllabus
Syllabus 8876, Core 3(c). Explain light-to-chemical energy conversion and carbohydrate storage.
About 5 min
Make a guess. You are not marked.
The light-independent reactions of photosynthesis: do they happen only at night?
Show the answer
They need products made in light.
They do not use light directly, but they depend on products made in the light, so they mainly run by day.
The key idea
The light-independent stage uses carbon dioxide, ATP and reduced NADP to form carbohydrate, returning ADP, phosphate and oxidised NADP for reuse.
Explore the idea
Connect the two stages
Light-independent reactions in the stroma use carbon dioxide together with these chemical supplies. They do not directly absorb light, but depend on the linked stage.
Original models of required stage inputs and outputs. Shapes are not full chemical formulae; exact pathway ATP yields are not required here.
Explanation
In the chloroplast stroma, carbon dioxide is incorporated into organic molecules through the light-independent reactions. ATP supplies energy and reduced NADP supplies reducing power. Carbohydrate is produced; a small sugar product can be used to build larger carbohydrates.
As ATP is used, ADP and inorganic phosphate become available again. As reduced NADP donates reducing power, NADP is regenerated. These return to the light-dependent stage, linking the processes in a continuing exchange rather than a one-way use of disposable carriers.
The overall result is conversion of light energy into chemical energy stored in carbohydrate. Carbon atoms enter from carbon dioxide; the energy to build the organic product is not created from nothing. Carbohydrates can later be used in respiration or to build and store material.
Light-independent means the reactions do not directly use absorbed light. They still depend on supplies generated by the light-dependent stage. In darkness, existing supplies may support activity briefly, but indefinite production cannot be inferred. Other factors such as carbon dioxide availability can also limit the linked process.
Step by step
- 1
Supply carbon
Carbon dioxide contributes the carbon incorporated into organic products.
- 2
Use energy and reducing power
ATP and reduced NADP support the conversion to carbohydrate.
- 3
Recycle carriers
ADP, phosphate and NADP can return to the light-dependent reactions.
Worked example
Light without carbon dioxide
A chloroplast receives light but no usable carbon dioxide for an extended period. Why is light alone insufficient for sustained carbohydrate production?
One way to explain it
Light can provide an energy input, but the light-independent stage also needs carbon dioxide as its carbon source. Without that substrate, light energy alone cannot supply the missing carbon atoms for continued carbohydrate synthesis.
Why this answer works
- Separate matter input from energy input.
- Identify which required input is missing.
Watch out for this
A student says: "Light-independent reactions normally run only at night." What is wrong with this?
Show the answer
They are named for not requiring light directly. They can operate in light and depend on ATP and reduced NADP from the light-dependent stage.
You may meet this in school papers
Not required by the 2027 syllabus, but school papers still ask it. Learn it after the main idea.
RuBP, rubisco and GP (the Calvin cycle in detail)
Some school papers name the molecules of the light-independent stage. The enzyme rubisco fixes CO2 to a five-carbon sugar, RuBP. The product splits into two three-carbon molecules of GP (glycerate 3-phosphate). ATP and reduced NADP convert GP to triose phosphate; most triose phosphate is used to regenerate RuBP, and some makes glucose and other carbohydrates. The 2027 H1 syllabus asks only for the inputs and outputs of each stage, not these names.
Check your understanding
Which is returned for reuse after reduced NADP donates reducing power?
Show the answer
Oxidised NADP.
It can be reduced again in the light-dependent stage.
Quick recall
What returns to the light-dependent reactions?