Photosynthesis
Understand how plants convert light energy into chemical energy through the light-dependent reactions and the Calvin cycle within the chloroplast.
Chloroplast Structure and the Photosynthesis Equation
Photosynthesis occurs in the chloroplasts of plant cells. Chloroplasts contain a green pigment called chlorophyll that absorbs light energy, primarily from the red and blue wavelengths of visible light. The overall equation for photosynthesis is:
Overall Equation
6CO2 + 6H2O ⟶ C6H12O6 + 6O2
(light energy required)
Carbon dioxide + Water → Glucose + Oxygen
Chloroplast Structure
Thylakoid Membranes
Flattened disc-like sacs where light reactions occur. Stacked into grana.
Stroma
The fluid-filled space surrounding the thylakoids. The Calvin cycle occurs here.
Double Membrane
Outer and inner membranes enclose the chloroplast, controlling what enters and exits.
Key point: Chlorophyll absorbs red and blue light, and reflects green light -- which is why plants appear green to our eyes.
Light-Dependent Reactions
The light-dependent reactions take place in the thylakoid membranes. They require light energy to split water molecules and generate the energy carriers ATP and NADPH, which power the Calvin cycle.
Steps of the Light Reactions
1. Light Absorption
Chlorophyll absorbs light energy in photosystem II (PSII)
2. Photolysis of Water
Water is split: 2H2O → 4H+ + 4e− + O2
3. Electron Transport Chain
Electrons pass through carriers, pumping H+ ions to generate ATP
4. NADPH Formation
Photosystem I (PSI) re-energises electrons to produce NADPH
Outputs of light reactions: ATP, NADPH (used in the Calvin cycle), and O2 (released as a by-product). The oxygen we breathe comes from the splitting of water, not from CO2.
The Calvin Cycle (Light-Independent Reactions)
The Calvin cycle occurs in the stroma of the chloroplast. It uses the ATP and NADPH produced during the light reactions to fix carbon dioxide into glucose. Although called "light-independent," it still depends indirectly on light because it needs ATP and NADPH from the light reactions.
The Three Stages of the Calvin Cycle
Carbon Fixation
CO2 combines with RuBP (a 5-carbon molecule) using the enzyme RuBisCO to form a 6-carbon compound that splits into two 3-carbon molecules (G3P).
Reduction
ATP and NADPH from the light reactions are used to convert G3P into glyceraldehyde-3-phosphate (G3P), a high-energy sugar.
Regeneration
Some G3P molecules are used to regenerate RuBP so the cycle can continue. One G3P exits to form glucose.
Connecting the Two Stages
Light Reactions
Thylakoids
Calvin Cycle
Stroma
Glucose
C6H12O6
Key Vocabulary
Chlorophyll
The green pigment found in chloroplasts that absorbs light energy (primarily red and blue wavelengths) for photosynthesis.
Photolysis
The splitting of water molecules using light energy during the light-dependent reactions, producing H+ ions, electrons, and O2.
Carbon Fixation
The process by which CO2 is incorporated into organic molecules (RuBP) during the Calvin cycle, catalysed by RuBisCO.
Stroma
The fluid-filled interior of the chloroplast surrounding the thylakoids, where the Calvin cycle takes place.
Worked Examples
Explain what would happen to the rate of photosynthesis if light intensity increased while temperature and CO2 remained constant.
Step 1: Increasing light intensity provides more energy for the light-dependent reactions in the thylakoids.
Step 2: More ATP and NADPH are produced, which drive the Calvin cycle faster.
Step 3: The rate of photosynthesis increases -- but only up to a point. Beyond a certain intensity, other factors (CO2 concentration or temperature) become limiting.
Answer: The rate increases proportionally until it reaches a plateau where another factor becomes limiting.
Where does the oxygen produced during photosynthesis come from?
Step 1: During the light-dependent reactions, water molecules are split (photolysis).
Step 2: 2H2O → 4H+ + 4e− + O2
Answer: The oxygen comes from the splitting of water (H2O), not from carbon dioxide. This was confirmed by isotope-tracing experiments using 18O-labelled water.
Why is RuBisCO considered the most important enzyme on Earth?
Step 1: RuBisCO catalyses carbon fixation -- the first step of the Calvin cycle where CO2 is incorporated into an organic molecule.
Step 2: Without this enzyme, plants could not convert atmospheric CO2 into glucose.
Answer: RuBisCO is the most abundant protein on Earth and is responsible for fixing approximately 100 billion tonnes of CO2 per year, making it essential for life and the global carbon cycle.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Where do the light-dependent reactions of photosynthesis occur?
Question 2
What are the products of the light-dependent reactions?
Question 3
Which enzyme catalyses carbon fixation in the Calvin cycle?
Question 4
The oxygen released during photosynthesis comes from which molecule?
Question 5
In the Calvin cycle, what is the first stable product formed after CO2 is fixed?
Key Concepts Summary
- ●Photosynthesis converts light energy into chemical energy: 6CO2 + 6H2O → C6H12O6 + 6O2.
- ●Light reactions occur in thylakoid membranes: water is split, producing ATP, NADPH, and O2.
- ●The Calvin cycle occurs in the stroma: CO2 is fixed into glucose using ATP and NADPH.
- ●RuBisCO is the key enzyme that catalyses carbon fixation in the Calvin cycle.
- ●Limiting factors (light intensity, CO2 concentration, temperature) affect the rate of photosynthesis.