The light-independent reaction uses reduced NADP from the light- dependent…

The idea

The light-independent reaction occurs in the stroma of the chloroplast. It depends on the products of the light-dependent reaction, so it stops soon after light is removed even though light is not used directly in it. Reduced NADP is the reducing agent: it donates hydrogen to glycerate 3-phosphate (GP), converting it to triose phosphate (TP), and is oxidised back to NADP, which returns to the thylakoid membranes to be reduced again. ATP from the light-dependent reaction supplies energy for this reduction. Triose phosphate is the simple sugar produced; two triose phosphate molecules can be joined to give a hexose sugar such as glucose. In limiting-factor questions, less reduced NADP means less GP is reduced, so less triose phosphate and less sugar are formed.

What earns the marks

• States that reduced NADP reduces glycerate 3-phosphate to triose phosphate.
• Names triose phosphate, or the hexose formed from two triose phosphates, as the simple sugar produced.
• Explains that oxidised NADP returns to the light-dependent reaction to be reduced again.
• In a limiting-factor context, links less reduced NADP to less reduction of GP and therefore less triose phosphate and less sugar formed.
• Locates the light-independent reaction in the stroma of the chloroplast.
• Distinguishes the role of reduced NADP (hydrogen/reducing power) from that of ATP (energy) in the reduction of GP.

Common mistakes and how to put them right

• Calling it the dark reaction and claiming it happens only at night.

  Put it right: It is the light-independent reaction and can occur whenever its products are available, but it stops soon after light is removed because reduced NADP and ATP run out.

• Writing NADP where reduced NADP is required, so the reducing agent is never identified.

  Put it right: Reduced NADP is the reducing agent; NADP is the oxidised form that returns to the thylakoids.

• Using reduced NAD in place of reduced NADP.

  Put it right: Photosynthesis uses reduced NADP, not reduced NAD.

• Saying glucose is made directly from carbon dioxide, without passing through GP and triose phosphate.

  Put it right: Carbon dioxide combines with RuBP to form two molecules of GP, which is reduced to triose phosphate, and two triose phosphates can form a hexose sugar.

• Claiming that all triose phosphate becomes glucose.

  Put it right: Most triose phosphate is used to regenerate RuBP, and only some leaves the cycle to form organic substances.

Exam tips

• In any question about light falling, follow the chain: less reduced NADP, less GP reduced, less triose phosphate, less sugar.
• State where the oxidised NADP goes, because that recycling step is a marking point many students omit.
• When asked about the simple sugar, name triose phosphate and explain how two triose phosphate molecules form a hexose sugar such as glucose.