Photosynthesis
Photosynthesis is the endothermic reaction inside plant and algal chloroplasts that converts light energy into chemical energy stored in glucose. Understanding how the light-dependent reactions on the thylakoid membranes supply ATP and NADPH to the light-independent Calvin cycle in the stroma, alongside how light intensity, CO₂, and temperature act as limiting factors, covers the core exam questions on this topic.
- Photosynthesis (overall reaction)
- The conversion of light energy into chemical energy stored in glucose. Word equation: carbon dioxide + water → glucose + oxygen, requiring light and chlorophyll; balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Chloroplast and chlorophyll
- Chloroplasts are the organelles where photosynthesis occurs. Chlorophyll, the green pigment inside them, absorbs mainly red and blue light and reflects green, capturing the light energy that drives the reaction.
- Light-dependent reactions
- The first stage, taking place in the thylakoid membranes. Light energy splits water (photolysis), releasing oxygen, and is used to make the energy carriers ATP and NADPH.
Photosynthesis: Enzyme Collision, Active Site & Rate Curve Lab
Drag the temperature, pH, and substrate sliders to watch how kinetic collisions, active-site binding, and denaturation change the reaction rate in real time.
Key Concepts & Exam Definitions (5)
- Photosynthesis (overall reaction)
- The conversion of light energy into chemical energy stored in glucose. Word equation: carbon dioxide + water → glucose + oxygen, requiring light and chlorophyll; balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Chloroplast and chlorophyll
- Chloroplasts are the organelles where photosynthesis occurs. Chlorophyll, the green pigment inside them, absorbs mainly red and blue light and reflects green, capturing the light energy that drives the reaction.
- Light-dependent reactions
- The first stage, taking place in the thylakoid membranes. Light energy splits water (photolysis), releasing oxygen, and is used to make the energy carriers ATP and NADPH.
- Light-independent reactions (Calvin cycle)
- The second stage, taking place in the stroma. It uses ATP and NADPH from the light reactions and the enzyme RuBisCO to fix carbon dioxide, ultimately building glucose. It does not require light directly but depends on the products of the light reactions.
- Limiting factors
- Environmental conditions that cap the rate of photosynthesis when in short supply. The three main ones are light intensity, carbon dioxide concentration, and temperature.
Quick-Fire Flashcards (5)
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What is the word equation for photosynthesis?
Where in the cell does photosynthesis take place?
Where does the oxygen released in photosynthesis come from?
Which two energy-carrying molecules do the light-dependent reactions supply to the Calvin cycle?
Name the three main limiting factors of photosynthesis.
Exam-Style Questions & Mark Schemes (3)
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- Q1
Why is the oxygen produced during photosynthesis derived from water rather than from carbon dioxide?
Click “Check Mark Scheme” to compare your answer, During the light-dependent reactions, water molecules are split by photolysis to supply electrons and protons (H⁺), and oxygen is released as a by-product. Isotope-labelling experiments (using water containing heavy oxygen-18) showed the released O₂ carries the label from water, not from CO₂, proving water is the source.
- Q2
Explain why increasing light intensity eventually stops increasing the rate of photosynthesis.
Click “Check Mark Scheme” to compare your answer, At low light intensity, light is the limiting factor, so the rate rises as intensity increases. Beyond a certain point, another factor (such as carbon dioxide concentration or temperature) becomes limiting, so the rate levels off (plateaus) even as light continues to increase.
- Q3
State the role of the enzyme RuBisCO in the Calvin cycle.
Click “Check Mark Scheme” to compare your answer, RuBisCO catalyses carbon fixation: it attaches CO₂ to the five-carbon molecule RuBP, the first step of the Calvin cycle. This forms an unstable six-carbon compound that immediately splits into two three-carbon molecules (glycerate-3-phosphate), which are later converted using ATP and NADPH toward glucose.
Common Exam Questions (3)
What is the equation for photosynthesis?
The word equation is: carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll). The balanced symbol equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Where does photosynthesis take place?
Photosynthesis takes place inside chloroplasts: the light-dependent reactions occur across the thylakoid membranes (where chlorophyll absorbs light energy), and the light-independent Calvin cycle occurs in the fluid-filled stroma.
What are the three limiting factors of photosynthesis?
The three main limiting factors of photosynthesis are light intensity, carbon dioxide concentration, and temperature. Whichever factor is in shortest supply limits the overall rate of reaction.
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