Question

Total eclipse. An illuminated suspension of Chlorella is actively carrying out photosynthesis. Suppose that the light is suddenly switched off. How would the levels of 3 -phosphoglycerate and ribulose 1,5 -bisphosphate change in the next minute?

Short answer

3-Phosphoglycerate levels increase, while ribulose 1,5-bisphosphate levels decrease.

Step-by-step solution

Understanding Photosynthesis Light Reaction

In photosynthesis, the light reactions produce ATP and NADPH, which are used in the Calvin cycle. When light is present, ATP and NADPH are continuously supplied.

Effects of Switching Off Light

When the light is switched off, the light-dependent reactions stop generating ATP and NADPH, cutting off the energy supply necessary for the Calvin cycle to proceed.

Impact on 3-Phosphoglycerate (3-PGA)

3-Phosphoglycerate (3-PGA) is the primary product of carbon fixation in the Calvin cycle. With the halt in ATP and NADPH production, the conversion of 3-PGA to glyceraldehyde 3-phosphate (G3P) will slow down, causing 3-PGA levels to rise temporarily.

Impact on Ribulose 1,5-Bisphosphate (RuBP)

RuBP interacts with CO2 and is regenerated using ATP and NADPH in the Calvin cycle. With the amount of ATP and NADPH decreasing, RuBP regeneration slows down, eventually leading to a decrease in its levels.

Key concepts

Calvin cycle

The Calvin cycle is an essential component of photosynthesis, taking place in the stroma of chloroplasts. It is often referred to as the "dark reactions" or "light-independent reactions" because it does not directly rely on sunlight, but rather on ATP and NADPH produced during the light reactions.

These reactions convert atmospheric carbon dioxide into organic molecules. The Calvin cycle consists of three main phases: carbon fixation, reduction, and regeneration.

• **Carbon fixation**: Atmospheric CO2 is fixed into organic molecules using ribulose 1,5-bisphosphate (RuBP) as a starting point.
• **Reduction**: The fixed carbon molecules are then reduced to form triose phosphate molecules using ATP and NADPH.
• **Regeneration**: RuBP is regenerated to allow the cycle to continue.

The Calvin cycle is crucial for sustaining life on Earth as it is the primary pathway for carbon fixation in plants.

Light reactions

Light reactions are the initial phase of photosynthesis, occurring within the thylakoid membranes of chloroplasts. These reactions harness solar energy and convert it into chemical energy, producing the ATP and NADPH required for the Calvin cycle.

When light photons excite chlorophyll molecules, they set off a chain of reactions that result in water molecule splitting.

• **Photophosphorylation**: ATP is synthesized as electrons pass through the electron transport chain.
• **NADP+ Reduction**: Electrons ultimately reduce NADP+ to NADPH.

Together: ATP and NADPH fuel the Calvin cycle.

Without light, these reactions halt, stopping the production of energy necessary for the Calvin cycle and photosynthesis as a whole.

3-phosphoglycerate (3-PGA)

3-Phosphoglycerate (3-PGA) plays a central role in the Calvin cycle. It is the first stable product formed during CO2 fixation. RuBP binds with CO2 through the action of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (commonly known as RuBisCO), producing an unstable 6-carbon intermediate.

This quickly splits into two molecules of 3-PGA, which represent the entry into further reactions of the Calvin cycle.

When light intensity decreases or stops, the conversion process of 3-PGA to glyceraldehyde 3-phosphate slows down due to a lack of ATP and NADPH. As a result, 3-PGA levels rise momentarily as it cannot be processed at its normal rate until ATP and NADPH are replenished.

Ribulose 1,5-bisphosphate (RuBP)

Ribulose 1,5-bisphosphate (RuBP) is a critical 5-carbon sugar phosphate involved in the Calvin cycle, serving as the carbon acceptor for CO2. The enzyme RuBisCO catalyzes the carboxylation of RuBP, adding CO2 and forming 3-phosphoglycerate (3-PGA).

For RuBP to participate continuously in the Calvin cycle, a consistent supply of ATP and NADPH is needed for its regeneration. This step occurs in the regeneration phase of the cycle, where some of the glyceraldehyde 3-phosphate molecules are recycled back into RuBP.

When the light reactions cease due to a lack of light, there is a shortage of ATP and NADPH, ultimately hindering RuBP regeneration. The levels of RuBP in the chloroplasts diminish as they get progressively used up in carbon fixation without being replenished effectively.