explain that energy transferred as ATP and reduced NADP from the light-dependent stage is used during the light-independent stage (Calvin cycle) of photosynthesis to produce complex organic molecules

Photosynthesis as an Energy Transfer Process

Learning Objective

Explain how the energy transferred as ATP and reduced NADP⁺ (NADPH) during the light‑dependent reactions is utilised in the light‑independent reactions (Calvin cycle) to synthesise complex organic molecules.

1. Overview of Photosynthesis

Photosynthesis can be divided into two linked stages:

1. Light‑dependent reactions – occur in the thylakoid membranes; convert solar energy into chemical energy (ATP and NADPH) and release O₂.
2. Light‑independent reactions (Calvin cycle) – occur in the stroma; use ATP and NADPH to fix CO₂ into carbohydrate precursors.

2. Light‑Dependent Reactions – Production of Energy Carriers

The main outputs are:

• ATP – generated by photophosphorylation via chemiosmosis.
• NADPH – produced when electrons from water reduce NADP⁺.

Key photochemical events (simplified):

\$\$

2 \, \text{H}2\text{O} \;\xrightarrow{\text{PSII}}\; O2 + 4 H^+ + 4 e^-

\$\$

Electrons travel through the electron transport chain, driving proton pumping and creating a proton motive force that powers ATP synthase.

3. Energy Carriers: ATP and NADPH

4. Light‑Independent Reactions (Calvin Cycle)

The Calvin cycle consists of three phases that repeat in a cyclic manner:

1. Carbon fixation – CO₂ is attached to ribulose‑1,5‑bisphosphate (RuBP) by the enzyme Rubisco, forming 3‑phosphoglycerate (3‑PGA).
2. Reduction – ATP phosphorylates 3‑PGA to 1,3‑bisphosphoglycerate; NADPH then reduces it to glyceraldehyde‑3‑phosphate (G3P).
3. Regeneration of RuBP – A portion of G3P is used, together with ATP, to regenerate RuBP, allowing the cycle to continue.

Overall stoichiometry for the synthesis of one G3P molecule (per three CO₂ fixed):

\$\$

3 \, CO2 + 9 \, ATP + 6 \, NADPH + 6 \, H2O \;\longrightarrow\; G3P + 9 \, ADP + 9 \, P_i + 6 \, NADP^+ + 6 \, H^+

\$\$

5. Integration of Light‑Dependent and Light‑Independent Stages

The two stages are tightly coupled:

• ATP and NADPH generated in the thylakoid lumen diffuse into the stroma where the Calvin cycle operates.
• The consumption of ATP and NADPH in the Calvin cycle maintains a gradient that drives continued electron flow and proton pumping in the light‑dependent reactions.
• O₂ released from water splitting is a by‑product; CO₂ uptake balances the carbon skeletons formed.

6. Production of Complex Organic Molecules

G3P is the primary output of the Calvin cycle. It can be:

• Converted into glucose and other carbohydrates via gluconeogenesis.
• Used as a precursor for starch, cellulose, lipids, and amino acids.

Thus, the chemical energy captured as ATP and NADPH is ultimately stored in the C‑H bonds of these organic compounds.

7. Summary Flowchart (Suggested Diagram)

8. Key Equations to Remember

• Overall photosynthetic reaction:
  

  \$6 \, CO2 + 6 \, H2O \xrightarrow{\text{light}} C6H{12}O6 + 6 \, O2\$

• ATP synthesis (photophosphorylation):
  

  \$ADP + P_i + \text{energy} \rightarrow ATP\$

• NADPH formation:
  

  \$NADP^+ + 2e^- + H^+ \rightarrow NADPH\$

• Carbon fixation (first step of Calvin cycle):
  

  \$RuBP + CO_2 \xrightarrow{\text{Rubisco}} 2 \, 3\text{-PGA}\$

9. Quick Revision Checklist

1. Identify where ATP and NADPH are produced.
2. State the role of each carrier in the Calvin cycle.
3. Describe how the regeneration of RuBP links back to the need for ATP.
4. Explain how the energy stored in ATP/NADPH ends up in glucose.