Photosynthesis: Question 4

Worked solution

Part (a): Carbon fixation and formation of triose phosphate

Carbon dioxide diffuses into the stroma of the chloroplast, where it combines with the five-carbon compound ribulose bisphosphate (RuBP). This reaction, called carbon fixation, is catalysed by the enzyme rubisco (ribulose bisphosphate carboxylase).

The immediate product is an unstable six-carbon compound, which breaks down straight away into two molecules of glycerate 3-phosphate (GP), a three-carbon compound.

Each GP molecule is then reduced to triose phosphate (TP). This reduction step requires:

• ATP, which provides the energy (via phosphorylation), and
• reduced NADP, which provides the hydrogen (electrons) needed for reduction,

both of which are supplied by the light-dependent stage.

Part (b): The two fates of triose phosphate

Once formed, triose phosphate molecules can follow one of two paths:

1. Regeneration of RuBP. Most of the TP produced is used, together with further ATP from the light-dependent stage, to regenerate RuBP. This is essential, because without a continuous supply of RuBP the Calvin cycle would stop, as there would be nothing for incoming carbon dioxide to combine with.
2. Synthesis of organic molecules. The remaining TP leaves the cycle and is used to build glucose and other organic molecules that the plant needs, such as sucrose (for transport), starch (for storage), cellulose (for cell walls), lipids and amino acids.

Part (c): Limiting factors and the rate of photosynthesis

A limiting factor is the factor that, at a given moment, is in the shortest supply relative to what is needed, and so restricts (limits) the rate of a process, even while the other necessary factors are present in adequate amounts.

At low light intensities, light is the factor restricting the rate: as light intensity increases, the light-dependent reactions can proceed faster, producing ATP and reduced NADP more quickly, which in turn allows the Calvin cycle to run faster too, so the overall rate of photosynthesis increases roughly in proportion to light intensity.

However, carbon dioxide concentration is being held constant rather than increased alongside light intensity. As light intensity rises, the light-dependent reactions supply ATP and reduced NADP faster and faster, so the demand of the Calvin cycle for carbon dioxide also rises, but the fixed, constant supply of carbon dioxide cannot keep pace with this growing demand. Once light intensity is high enough, it is this constant carbon dioxide concentration (and the rate at which rubisco can fix it) that becomes the new limiting factor, not light. Beyond this point, supplying more light has no further effect, because the process is now being held back by carbon dioxide availability rather than by light, so the rate of photosynthesis levels off even as light intensity continues to rise.

Final answers

• (a) CO₂ + RuBP → (rubisco) → unstable 6-carbon compound → 2 GP; GP is reduced to TP using ATP and reduced NADP from the light-dependent stage.
• (b) Most TP regenerates RuBP (using more ATP); the rest is converted into glucose and other organic molecules.
• (c) Whichever factor is in shortest supply limits the rate at that time; light limits the rate at low intensities, but as light intensity rises the constant, unchanging supply of carbon dioxide cannot keep up with demand, so carbon dioxide concentration takes over as the limiting factor and the rate levels off.