Desert plants such as cacti open their stomata only at night to absorb carbon dioxide, which is stored as an organic acid until daytime, when stomata close and photosynthesis is completed using sunlight. Using this example, explain how the process of photosynthesis can be separated into distinct stages, and identify what each stage requires.
Generated by claude-sonnet-4-6 · 2026-06-26 01:00 · grounding rag
Model Answer
Photosynthesis occurs in two distinct stages that can be separated in time:
Stage 1 (Light-dependent): Absorption of light energy by chlorophyll, conversion of light energy to chemical energy, and splitting of water molecules. This requires sunlight and chlorophyll.
Stage 2 (Light-independent): Reduction of carbon dioxide to carbohydrates using the chemical energy stored in Stage 1. This requires CO₂ (stored as an organic acid in cacti at night).
In cacti, Stage 2 occurs during the day (stomata closed, sunlight available), while CO₂ is absorbed and stored at night — proving the two stages need not occur simultaneously.
Source: Chapter 5, Section 5.2.1
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Explanation
- Examiners want you to clearly name/describe two stages and state what each stage requires — this covers all 3 marks.
- The cactus example directly shows separation of stages: CO₂ fixation (night) and light reactions (day).
- Key terms: light energy, chlorophyll, chemical energy, splitting of water, reduction of CO₂ — use at least a few of these.
- Don't write "light reaction" and "dark reaction" as labels unless you define them — just describe what each stage does and needs.