What this chapter is about
Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy stored in glucose. This chapter explains how higher plants carry out this fundamental life process in their chloroplasts. You will learn about the site of photosynthesis, the pigments involved, and the two main stages: the light reactions and the carbon-fixation reactions.
At the Class 11 level, this chapter builds on your earlier understanding of cell organelles and introduces you to the biochemistry of energy conversion. You will study how light energy is captured by chlorophyll, how water molecules are split to release oxygen, and how carbon dioxide is reduced to form sugars. The chapter also covers different pathways of carbon fixation found in C3 plants, C4 plants and plants that use Crassulacean Acid Metabolism (CAM).
After studying this chapter, you should be able to describe the structure of the chloroplast, explain the light and dark reactions, compare the C3 and C4 pathways, list the factors affecting the rate of photosynthesis, and understand the significance of this process for life on Earth.
Key ideas
- Chloroplast as the site: Photosynthesis occurs in chloroplasts, which contain thylakoid membranes (where light reactions occur) and stroma (where the Calvin cycle takes place).
- Photosynthetic pigments: Chlorophyll a is the primary pigment; chlorophyll b, carotenoids and xanthophylls are accessory pigments that absorb light of different wavelengths and pass energy to chlorophyll a.
- Two photosystems: Photosystem I (PS I, absorption peak at 700 nm) and Photosystem II (PS II, absorption peak at 680 nm) work together during non-cyclic electron flow to produce ATP and NADPH, and release oxygen from water.
- Light reactions produce ATP and NADPH: The splitting of water (photolysis) releases electrons, protons and oxygen; electron transport through the thylakoid membrane creates a proton gradient that drives ATP synthesis (photophosphorylation).
- Calvin cycle fixes carbon: In the stroma, the enzyme RuBisCO catalyses the fixation of carbon dioxide into a three-carbon compound (3-phosphoglyceric acid, or 3-PGA) in C3 plants; this cycle uses ATP and NADPH to produce sugars.
- C4 pathway avoids photorespiration: Plants like maize and sugarcane first fix carbon dioxide into a four-carbon compound (oxaloacetic acid) in mesophyll cells using PEP carboxylase, then release carbon dioxide in bundle-sheath cells for the Calvin cycle.
- CAM plants conserve water: Succulents like cacti and bryophyllum open stomata at night to take in carbon dioxide, store it as malic acid, and use it during the day when stomata remain closed.
- Factors affecting photosynthesis: Light intensity, carbon dioxide concentration, temperature and water availability influence the rate; the concept of limiting factors explains why only one factor controls the rate at a given time.
Formulas and facts to remember
1. Overall equation of photosynthesis: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O (in presence of light and chlorophyll). Oxygen released comes from water, not carbon dioxide.
2. Photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂. This occurs in Photosystem II and is the source of electrons for the light reactions.
3. ATP synthesis in chloroplasts: ADP + Pi → ATP (driven by the proton gradient across the thylakoid membrane; called photophosphorylation).
4. RuBisCO: Ribulose bisphosphate carboxylase-oxygenase is the enzyme that fixes carbon dioxide in the Calvin cycle; it is the most abundant protein on Earth.
5. Calvin cycle requirement: To fix three molecules of carbon dioxide and produce one molecule of glyceraldehyde-3-phosphate (G3P), the cycle uses 9 ATP and 6 NADPH.
6. Compensation point: The light intensity at which the rate of photosynthesis equals the rate of respiration; net gas exchange is zero.
7. Kranz anatomy: C4 plants show a special leaf anatomy with prominent bundle-sheath cells containing chloroplasts, surrounding the vascular bundles.
8. Action spectrum vs absorption spectrum: Action spectrum shows the rate of photosynthesis at different wavelengths; absorption spectrum shows how much light each pigment absorbs at different wavelengths.
Worked examples
### Example 1: Calculating oxygen evolution
A small aquatic plant produces 36 mL of oxygen gas in two hours under bright light at 25 °C. Calculate the rate of oxygen evolution per hour.
Solution: Rate of oxygen evolution = Total volume / Time Rate = 36 mL / 2 h = 18 mL per hour.
This tells us the plant releases 18 mL of oxygen every hour under these conditions.
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### Example 2: Understanding the Calvin cycle
A student wants to know how many turns of the Calvin cycle are needed to produce one molecule of glucose (C₆H₁₂O₆).
Solution: Each turn of the Calvin cycle fixes one molecule of carbon dioxide. Glucose contains six carbon atoms. Therefore, six turns of the Calvin cycle are needed to fix six carbon dioxide molecules. The six molecules of G3P produced (three-carbon each) combine to form one glucose molecule (though technically two G3P molecules join to form one hexose).
Number of turns = 6.
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### Example 3: Identifying the limiting factor
In a laboratory, a student exposes a potted plant to increasing light intensity while keeping carbon dioxide concentration constant at 0.03 percent and temperature at 28 °C. The rate of photosynthesis increases initially but then levels off despite further increase in light. Identify the limiting factor after the rate becomes constant.
Solution: Initially, light is the limiting factor, so increasing light increases photosynthesis. Once sufficient light is available, another factor limits the rate. Since temperature is optimal and held constant, the most likely limiting factor is carbon dioxide concentration, which remains at 0.03 percent (normal atmospheric level). Providing additional carbon dioxide would increase the rate further.
Limiting factor at plateau: Carbon dioxide concentration.
Common mistakes
- Thinking oxygen in photosynthesis comes from carbon dioxide → Oxygen comes from the splitting of water molecules in Photosystem II, as shown by isotope experiments.
- Confusing the site of light and dark reactions → Light reactions occur in thylakoid membranes; the Calvin cycle (dark reactions) occurs in the stroma.
- Believing dark reactions occur only at night → The Calvin cycle does not require light directly but depends on ATP and NADPH from light reactions, so it runs mainly during the day.
- Assuming all plants use the same carbon-fixation pathway → C3, C4 and CAM plants have distinct pathways adapted to their environments.
- Forgetting that RuBisCO can also act as an oxygenase → In C3 plants under high oxygen and low carbon dioxide, RuBisCO catalyses photorespiration, which wastes fixed carbon.
Quick revision
- Photosynthesis converts light energy into chemical energy stored in glucose; site is the chloroplast.
- Light reactions in thylakoids produce ATP, NADPH and oxygen; the Calvin cycle in stroma uses these to fix carbon dioxide into sugar.
- Chlorophyll a is the primary pigment; accessory pigments expand the range of light absorbed.
- C4 plants use PEP carboxylase first, avoiding photorespiration; CAM plants open stomata at night to conserve water.
- Factors like light intensity, carbon dioxide concentration and temperature affect the rate; the slowest factor at any moment is the limiting factor.
- Six turns of the Calvin cycle are needed to produce one glucose molecule.