Photosynthesis in Higher Plants
Welcome to HSLC Guru! In this chapter we explore one of the most important biological processes that sustains life on Earth — photosynthesis. This complete English-medium guide for ASSEB Class 11 Biology Chapter 13 covers early experiments, chloroplast structure, photosynthetic pigments, light and dark reactions, the C3 and C4 pathways, photorespiration, and factors influencing the rate of photosynthesis. You will also find detailed question answers, MCQs, fill in the blanks, true or false, and a glossary table for quick revision.
Summary
Photosynthesis is the physico-chemical process by which green plants, algae, and certain bacteria use light energy to convert carbon dioxide and water into carbohydrates, releasing oxygen as a by-product. The discovery of this process unfolded over centuries through landmark experiments. Joseph Priestley (1770) showed that plants restore air vitiated by burning candles or breathing animals. Jan Ingenhousz (1779) demonstrated that sunlight is essential and that only the green parts of plants release oxygen. Julius von Sachs (1854) proved that glucose is produced and stored as starch in chloroplasts. T. W. Engelmann (1888) used a prism and the alga Cladophora with aerobic bacteria to produce the first action spectrum, showing that blue and red light support the highest oxygen evolution. Cornelius van Niel studied purple and green sulphur bacteria and concluded that hydrogen from a suitable oxidisable compound (H2S in bacteria, H2O in plants) reduces carbon dioxide; this established that the oxygen released in photosynthesis comes from water, not from carbon dioxide.
Photosynthesis takes place in the chloroplast, a double-membrane plastid containing a fluid matrix called the stroma and stacks of flattened sacs called thylakoids. Stacks of thylakoids form grana, while interconnecting stromal lamellae link adjacent grana. The thylakoid membranes house the photosynthetic pigments and the machinery of the light reaction, while the stroma contains enzymes for the dark reaction. The principal pigments are chlorophyll a (the chief reaction-centre pigment, bright blue-green), chlorophyll b (yellow-green accessory pigment), carotenoids (orange to red carotenes), and xanthophylls (yellow). All pigments except chlorophyll a act as accessory pigments that broaden the range of wavelengths absorbed and protect chlorophyll a from photo-oxidation. The absorption spectrum shows the wavelengths of light absorbed by a pigment, while the action spectrum shows the rate of photosynthesis at different wavelengths; both peak in the blue and red regions, with chlorophyll a at the centre.
Photosynthesis has two stages. The light reaction (Hill reaction) takes place on the thylakoid membrane and includes light absorption, water splitting, oxygen release, and synthesis of ATP and NADPH. Pigments are organised into two photosystems: PS I (P700) and PS II (P680). In non-cyclic photophosphorylation, electrons flow from water through PS II, plastoquinone, the cytochrome b6-f complex, plastocyanin, PS I, and ferredoxin to NADP+, producing NADPH; this electron flow path resembles the letter Z and is called the Z-scheme. The splitting of water (photolysis) at PS II releases O2, protons, and electrons. In cyclic photophosphorylation, only PS I operates and electrons cycle back, producing ATP but no NADPH and no O2. The chemiosmotic hypothesis explains ATP synthesis: a proton gradient is created across the thylakoid membrane (protons accumulate inside the lumen) and protons flow back through ATP synthase (CF0–CF1 complex), driving phosphorylation of ADP to ATP.
The dark reaction (biosynthetic phase) takes place in the stroma and uses ATP and NADPH to fix CO2 into sugars. In C3 plants CO2 is fixed by the enzyme RuBisCO onto the 5-carbon acceptor RuBP, forming two molecules of 3-PGA, the first stable product. The Calvin cycle has three phases: carboxylation, reduction, and regeneration of RuBP. Six turns of the cycle produce one molecule of glucose. C4 plants (e.g., maize, sugarcane) use the Hatch and Slack pathway: PEPcase in mesophyll cells fixes CO2 into oxaloacetate (4-C); malate is transported to bundle-sheath cells where CO2 is released and refixed by RuBisCO. C4 plants exhibit Kranz anatomy and have higher productivity. Photorespiration occurs in C3 plants when RuBisCO fixes O2 instead of CO2, producing phosphoglycolate; it is wasteful as it releases CO2 without producing ATP, NADPH, or sugars. Major factors affecting photosynthesis include light intensity and quality, CO2 concentration, temperature, and water availability. Blackman’s law of limiting factors states that when a process is conditioned by several factors, the rate is determined by the slowest (limiting) factor.
Question Answers
1-Mark Questions
Q1. What is photosynthesis?
Answer: Photosynthesis is the process by which green plants synthesise organic food (glucose) from CO2 and water using light energy, releasing oxygen as a by-product.
Q2. Where in the chloroplast does the light reaction take place?
Answer: The light reaction takes place on the thylakoid membranes (grana) of the chloroplast.
Q3. Name the primary photosynthetic pigment.
Answer: Chlorophyll a is the primary or chief photosynthetic pigment.
Q4. What is the first stable product of the C3 cycle?
Answer: 3-phosphoglyceric acid (3-PGA), a 3-carbon compound, is the first stable product.
Q5. What is the reaction-centre wavelength of PS I?
Answer: PS I has its reaction centre at 700 nm and is therefore called P700.
Q6. Which enzyme is responsible for CO2 fixation in C4 plants in mesophyll cells?
Answer: Phosphoenolpyruvate carboxylase (PEPcase) fixes CO2 in mesophyll cells of C4 plants.
Q7. Define action spectrum.
Answer: Action spectrum is the graph showing the rate of photosynthesis at different wavelengths of light.
Q8. What is photolysis of water?
Answer: Photolysis is the splitting of water molecules in the presence of light at PS II, releasing O2, H+, and electrons.
Q9. Name the special leaf anatomy of C4 plants.
Answer: Kranz anatomy, in which bundle-sheath cells form a wreath-like ring around the vascular bundles.
Q10. Who proposed the chemiosmotic hypothesis?
Answer: The chemiosmotic hypothesis was proposed by Peter Mitchell in 1961.
2-3 Mark Questions
Q1. Differentiate between absorption spectrum and action spectrum.
Answer: The absorption spectrum represents the relative amount of light absorbed by a pigment at different wavelengths. The action spectrum represents the rate of photosynthesis (or any light-dependent process) at different wavelengths. When a pigment is the actual driver of a process, its absorption and action spectra closely match. Engelmann’s experiment first established the action spectrum of photosynthesis.
Q2. Distinguish between cyclic and non-cyclic photophosphorylation.
Answer: In cyclic photophosphorylation only PS I operates; electrons released from P700 return to it after passing through the electron transport chain, producing only ATP. In non-cyclic photophosphorylation both PS I and PS II operate; electrons travel from water to NADP+ in a Z-shaped path producing ATP, NADPH, and O2. Cyclic phosphorylation occurs when only longer wavelengths (>680 nm) are available or when NADP+ is unavailable.
Q3. Explain Engelmann’s experiment.
Answer: T. W. Engelmann (1888) split light using a prism and illuminated the filamentous green alga Cladophora placed in a suspension of aerobic bacteria. The bacteria accumulated mainly in the regions illuminated by blue and red light, where the alga released maximum oxygen. This produced the first action spectrum of photosynthesis and showed that blue and red light are most effective.
Q4. What is the Z-scheme of electron transport?
Answer: The Z-scheme depicts the path of electrons during non-cyclic photophosphorylation. Electrons flow from water → PS II (P680) → plastoquinone → cytochrome b6-f complex → plastocyanin → PS I (P700) → ferredoxin → NADP+ to form NADPH. When this pathway is plotted on the redox potential scale it resembles the letter “Z”, hence the name.
Q5. Why are C4 plants more efficient than C3 plants?
Answer: C4 plants concentrate CO2 in bundle-sheath cells, which keeps RuBisCO saturated with CO2 and minimises photorespiration. They tolerate higher temperatures, drier conditions, and high light intensities. PEPcase has a much higher affinity for CO2 than RuBisCO. As a result, C4 plants such as maize and sugarcane have higher productivity than C3 plants.
Q6. State Blackman’s law of limiting factors.
Answer: Blackman (1905) stated that “if a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor.” For example, when light intensity, CO2 concentration, and temperature are all suboptimal, the factor that is most below its optimum determines the overall rate of photosynthesis.
5-7 Mark Questions
Q1. Describe the structure of the chloroplast and its role in photosynthesis.
Answer: The chloroplast is a double-membrane-bound plastid found mainly in the mesophyll cells of leaves. The outer membrane is highly permeable, while the inner membrane is selectively permeable. The matrix inside is called the stroma. Suspended within the stroma are flattened, membrane-bound sacs called thylakoids. Stacks of thylakoids form grana, and adjacent grana are connected by stromal lamellae (intergranal lamellae). The thylakoid lumen lies inside each thylakoid. Chlorophyll and accessory pigments, electron transport components, and ATP synthase are embedded in the thylakoid membrane, where the light reaction takes place. The stroma contains enzymes (including RuBisCO), DNA, ribosomes, starch grains, and oil droplets, where the dark reaction (Calvin cycle) occurs. The chloroplast thus functions like a small biochemical factory: it captures light energy, splits water to release oxygen, and converts CO2 and water into carbohydrates that fuel almost all life on Earth.
Q2. Explain the Calvin cycle (C3 pathway) in detail.
Answer: The Calvin cycle, discovered by Melvin Calvin and co-workers, occurs in the stroma and consists of three phases. (i) Carboxylation: CO2 combines with the 5-carbon sugar ribulose 1,5-bisphosphate (RuBP) in a reaction catalysed by RuBisCO, producing two molecules of 3-phosphoglyceric acid (3-PGA). (ii) Reduction: Each 3-PGA is phosphorylated by ATP to 1,3-bisphosphoglycerate, which is then reduced by NADPH to glyceraldehyde-3-phosphate (G3P). For one molecule of glucose, six turns of the cycle are required, fixing six CO2, using 18 ATP and 12 NADPH. (iii) Regeneration: Of the 12 G3P molecules formed, 2 leave the cycle to form glucose/sucrose/starch and 10 are rearranged using 6 more ATP to regenerate 6 RuBP. The cycle is self-sustaining as long as ATP, NADPH, and CO2 are supplied.
Q3. Describe the Hatch and Slack pathway (C4 pathway) with the role of Kranz anatomy.
Answer: M. D. Hatch and C. R. Slack (1966) discovered an alternative CO2-fixation pathway in tropical grasses such as maize, sugarcane, and sorghum. C4 leaves show Kranz anatomy: large bundle-sheath cells, rich in chloroplasts but lacking grana, surround the vascular bundles in a wreath-like manner; mesophyll cells lie outside this ring. Step 1: In mesophyll cells, the enzyme PEPcase fixes atmospheric CO2 onto phosphoenolpyruvate (3-C) to form oxaloacetate (4-C), the first stable product. Step 2: Oxaloacetate is reduced to malate (or transaminated to aspartate) and transported to bundle-sheath cells. Step 3: In bundle-sheath cells malate is decarboxylated, releasing CO2 which is refixed by RuBisCO into the Calvin cycle; pyruvate returns to mesophyll cells and is regenerated to PEP using ATP. This CO2-pump keeps RuBisCO saturated, suppresses photorespiration, and allows high productivity even under hot, dry conditions.
Q4. What is photorespiration? Why is it considered a wasteful process?
Answer: Photorespiration is the light-dependent uptake of O2 and release of CO2 in C3 plants. RuBisCO has a dual nature: it acts as a carboxylase when CO2 is abundant and as an oxygenase when O2 is high (hot, bright, dry conditions). When RuBP combines with O2 instead of CO2, it forms one molecule of 3-PGA and one molecule of phosphoglycolate (2-C). Phosphoglycolate is metabolised through the chloroplast, peroxisome, and mitochondrion, releasing CO2. Photorespiration is wasteful because: (i) it does not produce sugars; (ii) it does not produce ATP or NADPH; (iii) it consumes ATP; (iv) it releases previously fixed CO2. C4 plants avoid photorespiration by concentrating CO2 around RuBisCO in the bundle-sheath cells, which is one major reason for their higher efficiency.
Q5. Discuss the major factors affecting the rate of photosynthesis.
Answer: Several external and internal factors influence photosynthesis. (1) Light: intensity, quality, and duration affect the rate. The rate increases with light intensity until light saturation is reached; beyond this, photo-oxidation may damage chlorophyll. Blue and red wavelengths are most effective. (2) CO2 concentration: the most important limiting factor under field conditions; increasing CO2 from 0.03% (atmospheric) to about 0.05% raises photosynthesis dramatically in C3 plants, while C4 plants saturate at lower concentrations. (3) Temperature: the dark reaction is enzyme-catalysed and very temperature sensitive; the optimum is generally 25–35 °C for temperate C3 plants and higher for C4 plants. (4) Water: water stress causes stomatal closure, reducing CO2 entry, and also affects leaf wilting and enzyme activity. (5) Internal factors: chlorophyll content, leaf age, accumulation of products, and mineral nutrition (Mg, Fe, N) also influence the rate. According to Blackman’s law, when several factors are involved, the slowest (limiting) factor controls the overall rate.
Multiple Choice Questions (MCQs)
Q1. The site of the light reaction in a chloroplast is the —
(a) Stroma (b) Thylakoid membrane (c) Outer membrane (d) Inter-membrane space
Answer: (b) Thylakoid membrane.
Q2. The first stable product of the Calvin cycle is —
(a) OAA (b) PEP (c) 3-PGA (d) RuBP
Answer: (c) 3-PGA.
Q3. The reaction centre of PS II absorbs light at —
(a) 700 nm (b) 680 nm (c) 660 nm (d) 720 nm
Answer: (b) 680 nm.
Q4. Which scientist proposed the chemiosmotic hypothesis?
(a) Calvin (b) Hatch (c) Mitchell (d) Hill
Answer: (c) Mitchell.
Q5. The C4 pathway was discovered by —
(a) Calvin and Benson (b) Hatch and Slack (c) Priestley and Ingenhousz (d) Engelmann and van Niel
Answer: (b) Hatch and Slack.
Q6. Photolysis of water occurs at —
(a) PS I (b) PS II (c) Cytochrome b6-f (d) Ferredoxin
Answer: (b) PS II.
Q7. The wreath-like arrangement of bundle-sheath cells in C4 leaves is called —
(a) Plasmodesmata (b) Kranz anatomy (c) Mesophyll wreath (d) Vasculature
Answer: (b) Kranz anatomy.
Q8. Which pigment is the chief photosynthetic pigment?
(a) Chlorophyll b (b) Carotene (c) Xanthophyll (d) Chlorophyll a
Answer: (d) Chlorophyll a.
Q9. The number of ATP and NADPH required to fix one molecule of CO2 in the Calvin cycle is —
(a) 2 ATP and 2 NADPH (b) 3 ATP and 2 NADPH (c) 4 ATP and 3 NADPH (d) 1 ATP and 1 NADPH
Answer: (b) 3 ATP and 2 NADPH.
Q10. Blackman’s law is also known as the law of —
(a) Mass action (b) Limiting factors (c) Action spectrum (d) Photolysis
Answer: (b) Limiting factors.
Fill in the Blanks
Q1. The dark reaction of photosynthesis takes place in the __________ of the chloroplast.
Answer: stroma.
Q2. __________ is the primary CO2 acceptor in C4 plants.
Answer: Phosphoenolpyruvate (PEP).
Q3. The Z-scheme describes the path of __________ during non-cyclic photophosphorylation.
Answer: electrons.
Q4. The enzyme that catalyses CO2 fixation in C3 plants is __________.
Answer: RuBisCO.
Q5. The accessory pigment that gives leaves their yellow colour is __________.
Answer: xanthophyll.
True or False
Q1. Oxygen released during photosynthesis comes from carbon dioxide.
Answer: False. It comes from the photolysis of water.
Q2. Cyclic photophosphorylation produces both ATP and NADPH.
Answer: False. It produces only ATP.
Q3. C4 plants exhibit Kranz anatomy.
Answer: True.
Q4. Photorespiration produces ATP and sugar.
Answer: False. It does not produce ATP, NADPH, or sugar.
Q5. Chlorophyll a is the chief reaction-centre pigment of photosynthesis.
Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Photosynthesis | Process of synthesising glucose from CO2 and water using light energy. |
| Chloroplast | Double-membrane plastid where photosynthesis occurs. |
| Stroma | Fluid matrix of the chloroplast where the dark reaction occurs. |
| Thylakoid | Flattened membrane sac inside the chloroplast that houses photosystems. |
| Granum | Stack of thylakoids; site of the light reaction. |
| Chlorophyll a | Chief photosynthetic pigment; reaction-centre pigment. |
| Chlorophyll b | Accessory pigment; transfers energy to chlorophyll a. |
| Carotenoids | Yellow-orange accessory pigments that protect chlorophyll from photo-oxidation. |
| Absorption spectrum | Plot of light absorbed by a pigment versus wavelength. |
| Action spectrum | Plot of rate of photosynthesis versus wavelength of light. |
| Photosystem (PS) | Light-harvesting unit on the thylakoid membrane; PS I and PS II. |
| Z-scheme | Z-shaped pathway of electron flow during non-cyclic photophosphorylation. |
| Photophosphorylation | Light-driven synthesis of ATP from ADP and Pi. |
| Photolysis | Light-dependent splitting of water at PS II. |
| Chemiosmotic hypothesis | Theory by Mitchell explaining ATP synthesis through a proton gradient. |
| Calvin cycle | C3 pathway of CO2 fixation in the stroma. |
| RuBisCO | Enzyme that fixes CO2 in C3 plants; also acts as oxygenase. |
| PEPcase | Primary CO2-fixing enzyme in C4 plants. |
| Kranz anatomy | Wreath-like arrangement of bundle-sheath cells in C4 leaves. |
| Photorespiration | Light-dependent O2 uptake and CO2 release in C3 plants. |
| Hill reaction | Light-driven reduction of an artificial electron acceptor by isolated chloroplasts. |
| Limiting factor | Factor at its lowest value that determines the rate of a process. |