Light-Independent Reaction

The light-independent reaction is the second stage of photosynthesis and occurs after the light-dependent reaction.

The light-independent reaction has two alternative names. It is often referred to as the dark reaction due to it not necessarily needing light energy to occur. However, this name is often misleading since it suggests that the reaction exclusively occurs in the dark. This is false; while the light-independent reaction can occur in the dark, it also occurs during the day. It is also referred to as the Calvin cycle, as the reaction was discovered by a scientist named Melvin Calvin.

The light-independent reaction is a self-sustaining cycle of different reactions that allows carbon dioxide to be converted into glucose. It occurs in the stroma, which is a colourless fluid found in the chloroplast (find the structure in the photosynthesis article). The stroma surrounds the membrane of the thylakoid discs, which is where the light-dependent reaction occurs.

The overall equation for the light-independent reaction is:

$$ \text{6 CO}_{2} \text{ + 12 NADPH + 18 ATP} \longrightarrow \text{C}_{6} \text{H}_{12} \text{O}_{6} \text{ + 12 NADP}^{+ }\text{ + 18 ADP + 18 P}_{i} $$

What are the reactants in the light-independent reaction?

There are three main reactants in the light-independent reaction:

Carbon dioxide is used during the first stage of the light-independent reaction, which is called carbon fixation. Carbon dioxide is incorporated into an organic molecule (is "fixed"), which is then converted into glucose.

NADPH acts as an electron donor during the second stage of the light-independent reaction. This is called phosphorylation (addition of phosphorus) and reduction. NADPH was produced during the light-dependent reaction, and is split into NADP+ and electrons during the light-independent reaction.

ATP is used to donate phosphate groups at two stages during the light-independent reaction: phosphorylation and reduction and regeneration. It is then split into ADP and inorganic phosphate (which is referred to as Pi).

The light-independent reaction in stages

There are three stages:

1. Carbon fixation.
2. Phosphorylation and reduction.
3. Regeneration of the carbon acceptor.

Six cycles of the light-independent reaction are required to produce one glucose molecule.

Carbon fixation

Carbon fixation refers to the incorporation of carbon into organic compounds by living organisms. In this case, the carbon from carbon dioxide and ribulose-1,5-biphosphate (RuBP) will be fixed into something called 3-phosphoglycerate (G3P). This reaction is catalysed by an enzyme called ribulose-1,5-biphosphate carboxylase oxygenase (RUBISCO).

The equation for this reaction is:

$$ 6 \text{ RuBP + 6CO}_{2}\text{ } \underrightarrow{\text{ Rubisco }} \text{ 12 G3P} $$

Phosphorylation

We now have G3P, which we need to convert into 1,3-biphosphoglycerate (BPG). It might be hard to gather from the name, but BPG has one more phosphate group than G3P - hence why we call this the phosphorylation stage.

Where would we get the extra phosphate group? We use the ATP that has been produced in the light-dependent reaction.

The equation for this is:

$$ \text{12 G3P + 12 ATP} \longrightarrow \text{12 BPG + 12 ADP} $$

Reduction

Once we have BPG, we want to turn it into glyceraldehyde-3-phosphate (GALP). This is a reduction reaction and therefore needs a reducing agent.

Remember the NADPH produced during the light-dependent reaction? This is where it comes in. NADPH is converted into NADP+ as it donates its electron, allowing for BPG to be reduced to GALP (by gaining electron from NADPH). An inorganic phosphate also splits from BPG.

$$ \text{12 BPG + 12 NADPH} \longrightarrow \text{12 NADP}^{+}\text{ + 12 P}_{i}\text{ + 12 GALP} $$

Gluconeogenesis

Two of the twelve GALPs produced are then removed from the cycle to make glucose via a process called gluconeogenesis. This is possible because of the number of carbons present - 12 GALP has a total of 36 carbons, with each molecule being three carbons long.

If 2 GALP leave the cycle, six carbon molecules overall leave, with 30 carbons remaining. 6RuBP also contains a total of 30 carbons, as each RuBP molecule is five carbons long.

Regeneration

In order to ensure that the cycle continues, RuBP has to be regenerated from GALP. This means we need to add another phosphate group, as GALP has only one phosphate attached to it whilst RuBP has two. Therefore, one phosphate group needs to be added for every RuBP generated. This means that six ATPs need to be used to create six RuBP from ten GALP.

The equation for this is:

$$ \text{12 GALP + 6 ATP }\longrightarrow \text{ 6 RuBP + 6 ADP} $$

RuBP can now be used again to combine with another${\mathrm{CO}}_2$molecule, and the cycle continues!

Overall, the entire light-independent reaction looks like this:

What are the products of the light-independent reaction?

What are the products of light independent reactions? The products of the light-independent reaction are glucose, NADP⁺, and ADP, whereas the reactants are CO₂, NADPH and ATP.

Glucose: glucose is formed from 2GALP, which leaves the cycle during the second stage of the light-independent reaction. Glucose is formed from GALP via a process called gluconeogenesis, which is separate from the light-independent reaction. Glucose is used to fuel multiple cellular processes within the plant.

NADP+: NADP is NADPH without the electron. After the light-independent reaction, it is reformed into NADPH during the light-dependent reactions.

ADP: Like NADP+, after the light-independent reaction ADP is re-used in the light-dependent reaction. It is converted back to ATP to be used again in the Calvin cycle. It is produced in the light-independent reaction alongside inorganic phosphate.