Class 11 Biology NCERT Notes- Chapter 11: Photosynthesis in Higher Plants

Detailed Study Notes – Chapter 11: Photosynthesis in Higher Plants (Class 11 Biology, Notes, PDFs, Quizzes, MCQs)

1. Introduction to Photosynthesis

Photosynthesis is the fundamental physico-chemical process by which green plants, as autotrophs, synthesize their own food. All other life forms, known as heterotrophs, depend on plants for their energy needs. This process is the basis of life on Earth for two primary reasons: it is the primary source of all food, and it is responsible for releasing oxygen into the atmosphere. Photosynthesis uses light energy to drive the synthesis of organic compounds (carbohydrates) from carbon dioxide (CO₂) and water (H₂O). Simple experiments demonstrate that chlorophyll (the green pigment in leaves), light, and CO₂ are essential for this process to occur, which results in the production of starch.

2. Early Experiments and Discoveries

The scientific understanding of photosynthesis developed gradually through a series of key experiments.

• Joseph Priestley (1770): Through his experiments using a bell jar, a candle, a mouse, and a mint plant, Priestley discovered the essential role of air in the growth of plants. He observed that a burning candle or a breathing animal “damages” the air in a closed space, but a plant can “restore” this air. This led to his hypothesis that plants restore to the air whatever breathing animals and burning candles remove. In 1774, he discovered oxygen.
• Jan Ingenhousz (1730-1799): Building on Priestley’s work, Ingenhousz demonstrated that sunlight is essential for the plant’s process of purifying the air. In an experiment with an aquatic plant, he showed that small bubbles (which he later identified as oxygen) formed around the green parts only in bright sunlight, not in the dark.
• Julius von Sachs (1854): Sachs provided evidence that plants produce glucose during growth, which is typically stored as starch. He also discovered that the green substance, chlorophyll, is located within special bodies in plant cells, later named chloroplasts.
• T.W. Engelmann (1843-1909): Engelmann described the first action spectrum of photosynthesis. He used a prism to split light into its spectral components and illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria, which were used to detect sites of oxygen evolution, accumulated mainly in the regions of blue and red light, indicating that these wavelengths are most effective for photosynthesis.
• Cornelius van Niel (1897-1985): A microbiologist, van Niel made a milestone contribution by studying purple and green sulfur bacteria. He demonstrated that photosynthesis is a light-dependent reaction where hydrogen from a suitable oxidizable compound reduces CO₂ to carbohydrates. He proposed the general equation: 2H₂A + CO₂ → 2A + CH₂O + H₂O. In green plants, H₂O is the hydrogen donor (H₂A) and is oxidized to O₂ (A). In purple and green sulfur bacteria, the hydrogen donor is H₂S, and the oxidation product is sulfur or sulfate, not O₂. This led him to infer correctly that the oxygen evolved by green plants comes from the splitting of water, not from carbon dioxide. This was later proven using radioisotopic techniques.

The overall, correct chemical equation for photosynthesis in green plants is: 6CO₂ + 12H₂O --(Light)--> C₆H₁₂O₆ + 6H₂O + 6O₂

3. The Site of Photosynthesis: The Chloroplast

Photosynthesis occurs in the green parts of plants, primarily in the mesophyll cells of the leaves, which contain a large number of chloroplasts.

• Chloroplast Structure: Within each chloroplast, there is a membranous system consisting of:
  • Grana: Stacks of flattened, membranous sacs.
  • Stroma Lamellae: Membranes connecting the grana.
  • Stroma: The fluid-filled matrix surrounding the grana.
• Division of Labor:
  • Membrane System (Grana and Stroma Lamellae): This is the site of the light reactions. It is responsible for trapping light energy and synthesizing the high-energy intermediates ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
  • Stroma: This is the site of the dark reactions (or carbon reactions). It contains the enzymes that use the ATP and NADPH from the light reactions to synthesize sugars from CO₂. These reactions are not directly light-driven but depend on the products of the light reactions.

4. Photosynthetic Pigments

The color of leaves is due to four main pigments that can be separated by paper chromatography:

• Chlorophyll a: Bright or blue-green. This is the chief pigment associated with photosynthesis.
• Chlorophyll b: Yellow-green.
• Xanthophylls: Yellow.
• Carotenoids: Yellow to yellow-orange.

Chlorophyll a is the primary pigment that traps light energy. Its absorption spectrum shows maximum absorption in the blue and red regions of the visible spectrum, which corresponds to the highest rates of photosynthesis (the action spectrum). The other pigments—chlorophyll b, xanthophylls, and carotenoids—are called accessory pigments. They absorb light at different wavelengths and transfer the energy to chlorophyll a, thus broadening the spectrum of light used for photosynthesis and also protecting chlorophyll a from photo-oxidation.

5. The Light-Dependent Reactions

The “photochemical” phase includes light absorption, water splitting, oxygen release, and the formation of ATP and NADPH.

• Photosystems: Pigments are organized into two light-harvesting complexes (LHCs): Photosystem I (PS I) and Photosystem II (PS II).
  • Each photosystem contains hundreds of pigment molecules (antennae) that absorb light and funnel the energy to a single chlorophyll a molecule, the reaction center.
  • The reaction center of PS I is called P700 because it has an absorption peak at 700 nm.
  • The reaction center of PS II is called P680 because it has an absorption peak at 680 nm.
• The Electron Transport (Z-Scheme):
  1. The P680 reaction center in PS II absorbs 680 nm red light, exciting an electron.
  2. This high-energy electron is captured by a primary electron acceptor and passed down an electron transport system consisting of cytochromes.
  3. As the electron moves “downhill” in terms of redox potential, it is passed to the PS I complex.
  4. Simultaneously, the P700 reaction center in PS I absorbs 700 nm light, exciting an electron.
  5. This electron is captured by another acceptor and passed “downhill” to the energy-rich molecule NADP+, reducing it to NADPH + H⁺.
• Splitting of Water (Photolysis): To replace the electrons lost from PS II, water is split. This reaction is associated with PS II and occurs on the inner side of the thylakoid membrane. The reaction is: 2H₂O → 4H⁺ + O₂ + 4e⁻. The electrons are transferred to PS II, the protons (H⁺) accumulate in the thylakoid lumen, and oxygen is released as a byproduct.
• Photophosphorylation: This is the synthesis of ATP from ADP and inorganic phosphate (Pi) using light energy.
  • Non-cyclic Photophosphorylation: Involves both PS II and PS I operating in series (the Z-scheme). It produces both ATP and NADPH.
  • Cyclic Photophosphorylation: Occurs when only PS I is functional (e.g., in stroma lamellae, which lack PS II). The excited electron from P700 is cycled back to the PS I complex through the electron transport chain, producing only ATP and not NADPH. This likely occurs to meet the higher ATP demand of the Calvin cycle.
• Chemiosmotic Hypothesis: This explains the mechanism of ATP synthesis.
  1. A proton gradient (high concentration of H⁺) is created across the thylakoid membrane, with protons accumulating in the lumen.
  2. This gradient is formed by: (a) protons released from the splitting of water, (b) protons pumped from the stroma into the lumen as electrons move through the transport chain, and (c) protons removed from the stroma during the reduction of NADP+.
  3. The breakdown of this gradient, as protons move back into the stroma through a transmembrane channel in the ATP synthase enzyme (CF₀ part), releases energy.
  4. This energy causes a conformational change in the CF₁ part of the ATP synthase, which catalyzes the synthesis of ATP.

6. The Biosynthetic Phase (Calvin Cycle)

The ATP and NADPH produced in the light reactions are used in the stroma to fix CO₂ and synthesize sugars. This process is known as the Calvin cycle, discovered by Melvin Calvin using radioactive ¹⁴C.

• The Primary CO₂ Acceptor: The molecule that first combines with CO₂ is a 5-carbon ketose sugar called ribulose-1,5-bisphosphate (RuBP).
• The Enzyme: The reaction is catalyzed by the enzyme RuBP carboxylase-oxygenase (RuBisCO), the most abundant enzyme in the world.

The Calvin cycle occurs in all photosynthetic plants and proceeds in three stages:

1. Carboxylation: CO₂ combines with RuBP. This reaction, catalyzed by RuBisCO, forms an unstable 6-carbon intermediate that immediately splits into two molecules of the first stable product, a 3-carbon compound called 3-phosphoglyceric acid (3-PGA).
2. Reduction: This is a series of reactions where the 3-PGA molecules are converted into carbohydrate (triose phosphate). This step utilizes ATP for phosphorylation and NADPH for reduction.
3. Regeneration: The CO₂ acceptor, RuBP, is regenerated from the triose phosphates. This step also requires ATP.

Energy Requirements: To fix one molecule of CO₂, the cycle uses 3 ATP and 2 NADPH. Therefore, to produce one molecule of glucose (a 6-carbon sugar), the cycle must turn six times, consuming 6 CO₂, 18 ATP, and 12 NADPH.

Input	Output
6 CO₂	1 Glucose
18 ATP	18 ADP
12 NADPH	12 NADP⁺

7. The C₄ Pathway (Hatch and Slack Pathway)

Plants adapted to dry, tropical regions have evolved the C₄ pathway to minimize water loss and increase efficiency.

• Special Characteristics:
  • They have a special leaf anatomy called Kranz anatomy, where large bundle sheath cells form a wreath around the vascular bundles. These cells have thick walls, many chloroplasts, and no intercellular spaces.
  • They tolerate higher temperatures and high light intensities.
  • They lack photorespiration and have greater biomass productivity.
• Mechanism: The C₄ pathway involves two cell types: mesophyll and bundle sheath cells.
  1. In Mesophyll Cells: The primary CO₂ acceptor is a 3-carbon molecule, phosphoenolpyruvate (PEP). The fixation is catalyzed by PEP carboxylase (PEPcase), which has a high affinity for CO₂ and lacks oxygenase activity. The first product is a 4-carbon acid, oxaloacetic acid (OAA).
  2. Transport: OAA is converted to other 4-carbon acids like malic acid or aspartic acid, which are transported to the bundle sheath cells.
  3. In Bundle Sheath Cells: The 4-carbon acid is broken down (decarboxylation) to release a high concentration of CO₂ and a 3-carbon molecule.
  4. This CO₂ then enters the Calvin cycle (which occurs in the bundle sheath cells, rich in RuBisCO) to produce sugars. The high CO₂ concentration ensures RuBisCO functions as a carboxylase, preventing photorespiration.
  5. The 3-carbon molecule is transported back to the mesophyll cells to regenerate PEP, completing the cycle.

8. Photorespiration

Photorespiration is a process that occurs in C₃ plants when RuBisCO binds with O₂ instead of CO₂.

• Mechanism: RuBisCO’s active site can bind both CO₂ and O₂, making the binding competitive. When O₂ concentration is high relative to CO₂, RuBisCO acts as an oxygenase. It combines RuBP with O₂ to form one molecule of phosphoglycerate (3-C) and one molecule of phosphoglycolate (2-C).
• Consequences: This pathway is considered wasteful because:
  • It does not produce any sugars.
  • It consumes ATP.
  • It results in the release of previously fixed CO₂.
  • It does not synthesize ATP or NADPH.
• Absence in C₄ Plants: C₄ plants lack photorespiration because their anatomical and biochemical mechanism effectively pumps CO₂ into the bundle sheath cells, creating a high CO₂ concentration that ensures RuBisCO primarily functions as a carboxylase. This lack of photorespiration contributes to their higher productivity.

9. Factors Affecting Photosynthesis

The rate of photosynthesis is influenced by several internal (plant) and external factors. According to Blackman’s Law of Limiting Factors (1905), the rate of a process affected by multiple factors is determined by the factor nearest its minimal value.

• Light: At low intensities, the rate of photosynthesis is directly proportional to light intensity. At higher intensities, the rate plateaus as other factors (like CO₂) become limiting. Light saturation occurs at about 10% of full sunlight, so light is rarely a limiting factor in nature, except for plants in shade.
• Carbon Dioxide Concentration: CO₂ is the major limiting factor. The atmospheric concentration is low (0.03% to 0.04%).
  • C₃ plants show increased photosynthesis with higher CO₂ levels and saturate only beyond 450 µlL⁻¹.
  • C₄ plants show saturation at a lower concentration of about 360 µlL⁻¹. This means current atmospheric CO₂ levels are a limiting factor for C₃ plants.
• Temperature: The dark reactions are enzymatic and thus temperature-sensitive.
  • C₄ plants have a higher temperature optimum and show higher rates of photosynthesis at higher temperatures.
  • C₃ plants have a much lower temperature optimum.
• Water: Water is a reactant, but its main effect is indirect. Water stress causes stomata to close, reducing CO₂ availability. It also causes leaves to wilt, reducing the surface area for light absorption.

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Q&A Section

Short-Answer Questions (25 Questions)

Answer each question in 2-3 sentences.

1. What are the two main reasons photosynthesis is crucial for life on Earth?
2. Describe Joseph Priestley’s bell jar experiment and his resulting hypothesis.
3. What was Jan Ingenhousz’s key contribution to the understanding of photosynthesis?
4. How did T.W. Engelmann create the first action spectrum for photosynthesis?
5. Explain the significance of Cornelius van Niel’s work with purple and green sulfur bacteria.
6. What is the “division of labor” within a chloroplast?
7. Name the four main types of pigments involved in photosynthesis and identify the chief pigment.
8. What is the role of accessory pigments in photosynthesis?
9. Differentiate between Photosystem I (PS I) and Photosystem II (PS II) based on their reaction centers.
10. Briefly describe the path of an electron in the Z-scheme of the light reactions.
11. Where does the splitting of water occur, and what are its three products?
12. What is the difference between cyclic and non-cyclic photophosphorylation in terms of products and photosystems involved?
13. According to the chemiosmotic hypothesis, how is the proton gradient established across the thylakoid membrane?
14. What is the role of the ATP synthase enzyme in ATP production?
15. What is the primary acceptor of CO₂ in the Calvin cycle, and what enzyme catalyzes this reaction?
16. Name the three main stages of the Calvin cycle.
17. How many molecules of ATP and NADPH are required to produce one molecule of glucose via the Calvin cycle?
18. What is Kranz anatomy and in which type of plants is it found?
19. Describe the first step of CO₂ fixation in a C₄ plant, including the substrate, enzyme, and product.
20. Why do C₄ plants not exhibit photorespiration?
21. What is photorespiration, and why is it considered a wasteful process?
22. State Blackman’s Law of Limiting Factors.
23. Why is light rarely a limiting factor for photosynthesis in nature?
24. How do C₃ and C₄ plants respond differently to increasing CO₂ concentrations?
25. Explain the indirect effect of water stress on the rate of photosynthesis.

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Multiple-Choice Quiz (20 Questions)

Choose the single best answer for each question.

1. The scientist who first demonstrated that plants restore air that has been “damaged” by a burning candle was: a) Jan Ingenhousz b) Julius von Sachs c) Joseph Priestley d) T.W. Engelmann
2. The oxygen released during photosynthesis comes from: a) Carbon dioxide b) Water c) Glucose d) The atmosphere
3. The light-dependent reactions of photosynthesis occur in the: a) Stroma b) Grana and stroma lamellae c) Outer chloroplast membrane d) Cytoplasm
4. Which pigment appears bright or blue-green in a chromatogram and is the chief photosynthetic pigment? a) Chlorophyll b b) Xanthophyll c) Carotenoid d) Chlorophyll a
5. The reaction center of Photosystem I is known as: a) P680 b) P700 c) P540 d) P870
6. In the Z-scheme, the final acceptor of electrons is: a) ATP b) PS I c) NADP⁺ d) Water
7. Which of the following is produced ONLY during cyclic photophosphorylation? a) ATP b) NADPH c) Oxygen d) Both ATP and NADPH
8. The accumulation of protons for chemiosmosis occurs in the: a) Stroma b) Thylakoid lumen c) Intermembrane space d) Cytosol
9. The first stable product of CO₂ fixation in the Calvin cycle is: a) Ribulose-1,5-bisphosphate (RuBP) b) 3-phosphoglyceric acid (3-PGA) c) Oxaloacetic acid (OAA) d) Phosphoenolpyruvate (PEP)
10. The enzyme responsible for the carboxylation of RuBP is: a) PEP carboxylase b) ATP synthase c) RuBisCO d) NADP reductase
11. To produce one molecule of glucose, the Calvin cycle requires: a) 6 ATP and 6 NADPH b) 12 ATP and 18 NADPH c) 18 ATP and 12 NADPH d) 3 ATP and 2 NADPH
12. Plants adapted to dry tropical regions often use the: a) C₃ pathway b) C₄ pathway c) CAM pathway d) Photorespiration pathway
13. Kranz anatomy is a characteristic feature of the leaves of: a) All plants b) C₃ plants c) C₄ plants d) Shade-loving plants
14. In C₄ plants, the Calvin cycle takes place in the: a) Mesophyll cells b) Epidermal cells c) Stomata d) Bundle sheath cells
15. The primary CO₂ acceptor in C₄ plants is: a) RuBP b) PGA c) OAA d) PEP
16. Photorespiration occurs when RuBisCO binds with which molecule? a) O₂ b) CO₂ c) H₂O d) ATP
17. Which of the following is NOT a consequence of photorespiration? a) Release of CO₂ b) Utilization of ATP c) Synthesis of NADPH d) No synthesis of sugar
18. According to Blackman’s Law, if temperature is very low, the rate of photosynthesis will be limited by: a) Light intensity b) CO₂ concentration c) Temperature d) Water availability
19. C₄ plants show CO₂ saturation at about ______, while C₃ plants saturate beyond ______. a) 360 µlL⁻¹; 450 µlL⁻¹ b) 450 µlL⁻¹; 360 µlL⁻¹ c) 100 µlL⁻¹; 200 µlL⁻¹ d) 200 µlL⁻¹; 100 µlL⁻¹
20. Water stress primarily affects photosynthesis by: a) Directly stopping the light reaction b) Causing chlorophyll to break down c) Causing stomata to close, thus reducing CO₂ availability d) Increasing the rate of photorespiration

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Essay Questions (10 Questions)

Provide a detailed, paragraph-style answer for each question.

1. Trace the historical development of our understanding of photosynthesis by detailing the contributions of Priestley, Ingenhousz, von Sachs, and van Niel. Answer: Our understanding of photosynthesis was built upon a series of foundational experiments. In 1770, Joseph Priestley discovered that plants have a role in “restoring” air that had been “damaged” by burning candles or breathing animals, essentially discovering the role of gases in plant life. Building on this, Jan Ingenhousz showed that this process requires sunlight and occurs only in the green parts of the plant, identifying the output as bubbles of oxygen. In 1854, Julius von Sachs provided evidence that the product of this process is glucose, which is stored as starch, and that the green substance (chlorophyll) is located in chloroplasts. Finally, the work of Cornelius van Niel in the 1930s was a major milestone. By studying sulfur bacteria, he demonstrated that photosynthesis is a redox reaction where a hydrogen donor reduces CO₂, and he correctly inferred that the oxygen evolved by green plants comes from the splitting of H₂O, not CO₂.
2. Describe the structure of a chloroplast and explain how its internal organization facilitates the different stages of photosynthesis. Answer: The chloroplast is the site of photosynthesis and has a highly organized internal structure. It is enclosed by a double membrane. Inside, a membranous system consists of grana (stacks of thylakoids) and stroma lamellae (which connect the grana), all suspended in a fluid-filled matrix called the stroma. This structure creates a clear division of labor. The thylakoid membranes contain the photosynthetic pigments and protein complexes necessary for the light-dependent reactions, where light energy is trapped and converted into chemical energy in the form of ATP and NADPH. The stroma contains the enzymes, including RuBisCO, required for the light-independent reactions (the Calvin cycle), where the ATP and NADPH are used to fix CO₂ and synthesize sugars. The thylakoid membrane also encloses a space called the lumen, which is crucial for establishing the proton gradient needed for ATP synthesis via chemiosmosis.
3. Explain the Z-scheme of electron transport in the light reactions, detailing the roles of PS I, PS II, and the final products. Answer: The Z-scheme describes the pathway of non-cyclic electron flow during the light reactions of photosynthesis. The process begins when P680, the reaction center of Photosystem II, absorbs light and an electron becomes excited. This electron is passed to a primary acceptor and then down an electron transport chain, eventually reaching Photosystem I. To replace the electron lost from P680, a water molecule is split, releasing electrons, protons, and oxygen. Meanwhile, P700, the reaction center of Photosystem I, also absorbs light, exciting an electron that is passed to its own primary acceptor. This electron then moves down a second electron transport chain, where it is ultimately used to reduce NADP⁺ to NADPH + H⁺. The entire scheme, when plotted on a redox potential scale, resembles the letter ‘Z’, and its net products are the high-energy molecules ATP (produced via the associated proton gradient) and NADPH.
4. Detail the chemiosmotic hypothesis as it applies to ATP synthesis in chloroplasts. Be sure to describe how the proton gradient is generated and how it is used. Answer: The chemiosmotic hypothesis explains how ATP is synthesized during the light reactions. The process is linked to the development of a proton (H⁺) gradient across the thylakoid membrane, with a high concentration of protons accumulating in the thylakoid lumen. This gradient is created in three ways: (1) protons are released into the lumen from the splitting of water molecules at PS II; (2) as electrons are transported between PS II and PS I, a carrier molecule pumps protons from the stroma into the lumen; and (3) protons are consumed in the stroma during the reduction of NADP⁺ to NADPH. This electrochemical gradient represents stored potential energy. This energy is released when protons flow back down their concentration gradient from the lumen to the stroma through a specific channel in the ATP synthase enzyme. This flow of protons drives the enzyme to catalyze the phosphorylation of ADP to ATP.
5. Outline the three main stages of the Calvin cycle: carboxylation, reduction, and regeneration. What are the key inputs and outputs of the cycle for the synthesis of one glucose molecule? Answer: The Calvin cycle is the biosynthetic phase of photosynthesis, occurring in the stroma. It begins with carboxylation, where one molecule of CO₂ is fixed to a 5-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP), catalyzed by the enzyme RuBisCO. This forms two molecules of a 3-carbon compound, 3-PGA. The second stage is reduction, where the 3-PGA is converted into triose phosphate, the carbohydrate product. This step requires energy and reducing power, utilizing two molecules of ATP and two molecules of NADPH per CO₂ fixed. The final stage is regeneration, where the initial CO₂ acceptor, RuBP, is reformed from the triose phosphates, a process that requires one more molecule of ATP. To synthesize one molecule of glucose (C₆H₁₂O₆), the cycle must turn six times. The key inputs are 6 CO₂, 18 ATP, and 12 NADPH, and the key output is one molecule of glucose.
6. Compare and contrast the C₃ and C₄ photosynthetic pathways. Discuss differences in leaf anatomy, primary CO₂ acceptor, initial products, and efficiency. Answer: The C₃ and C₄ pathways are two different strategies for carbon fixation. C₃ plants use the Calvin cycle directly, where the primary CO₂ acceptor is the 5-carbon RuBP, and the first stable product is the 3-carbon 3-PGA. In contrast, C₄ plants have an initial fixation step where the primary acceptor is the 3-carbon PEP, and the first product is the 4-carbon oxaloacetic acid (OAA). Anatomically, C₄ plants possess specialized Kranz anatomy, with large bundle sheath cells surrounding the vascular tissue, which is absent in C₃ plants. The initial fixation in C₄ plants occurs in the mesophyll cells, while the Calvin cycle occurs exclusively in the bundle sheath cells; in C₃ plants, the entire process occurs in the mesophyll. C₄ plants are more efficient in hot, dry climates because their mechanism concentrates CO₂ in the bundle sheath cells, eliminating wasteful photorespiration and allowing them to have greater productivity and tolerate higher temperatures.
7. What is photorespiration? Explain the biochemical basis for this process and why C₄ plants have a mechanism to avoid it. Answer: Photorespiration is a wasteful metabolic pathway that occurs in C₃ plants when the enzyme RuBisCO binds with O₂ instead of CO₂. The biochemical basis for this lies in the dual carboxylase/oxygenase nature of RuBisCO’s active site, which binds competitively with both gases. When O₂ levels are high relative to CO₂, RuBisCO combines O₂ with RuBP, producing one molecule of 3-PGA and one molecule of the two-carbon phosphoglycolate. This pathway consumes ATP and results in the loss of fixed carbon as CO₂, without producing any sugar or NADPH. C₄ plants avoid photorespiration because they possess a CO₂-concentrating mechanism. They use the highly efficient PEP carboxylase to initially fix CO₂ in mesophyll cells, then transport the resulting 4-carbon acid to bundle sheath cells where it is decarboxylated. This releases a high concentration of CO₂ at the site of RuBisCO, ensuring it functions primarily as a carboxylase and effectively outcompetes O₂.
8. Explain Blackman’s Law of Limiting Factors and provide examples of how light, CO₂, and temperature can each be a limiting factor for photosynthesis. Answer: Blackman’s Law of Limiting Factors states that if a chemical process is affected by more than one factor, its rate will be determined by the factor that is nearest to its minimal value. For photosynthesis, this means that even if some conditions are optimal, the rate will be capped by the one factor that is in shortest supply. For example, on a bright, warm day, the rate of photosynthesis may be limited by the low atmospheric concentration of CO₂. In a dense forest or on a cloudy day, light intensity could be the limiting factor, even if CO₂ and temperature are optimal. Similarly, in cold conditions, temperature will be the limiting factor, as the enzymes of the Calvin cycle function sub-optimally, even with ample light and CO₂.
9. Discuss the roles of the different photosynthetic pigments (Chlorophyll a, b, carotenoids, and xanthophylls) and how their absorption spectra relate to the action spectrum of photosynthesis. Answer: Photosynthesis relies on pigments that absorb light energy. Chlorophyll a is the primary or chief pigment; it is found in the reaction centers of the photosystems and directly participates in converting light energy to chemical energy. Its absorption spectrum shows peaks in the blue and red regions of light. The other pigments—chlorophyll b, carotenoids, and xanthophylls—are known as accessory pigments. They absorb light at wavelengths that chlorophyll a does not absorb well, effectively broadening the range of light that can be used for photosynthesis. They then transfer this absorbed energy to chlorophyll a. The action spectrum of photosynthesis, which shows the rate of photosynthesis at different wavelengths, closely matches the absorption spectrum of chlorophyll a, but is broader due to the contribution of these accessory pigments. These pigments also play a protective role by preventing photo-oxidation of chlorophyll a.
10. Why are C₄ plants more productive than C₃ plants in tropical climates? Relate your answer to their anatomical and physiological adaptations. Answer: C₄ plants are more productive than C₃ plants in tropical climates due to a suite of anatomical and physiological adaptations that make them better suited to high light, high temperature, and dry conditions. Anatomically, they possess Kranz anatomy, which spatially separates initial carbon fixation from the Calvin cycle. Physiologically, they use the highly efficient PEP carboxylase enzyme, which has a high affinity for CO₂ and is not inhibited by O₂, to initially fix carbon. This allows them to effectively capture CO₂ even when stomata are partially closed to conserve water. This CO₂ is then “pumped” into the bundle sheath cells, creating a high internal CO₂ concentration. This high concentration prevents the wasteful process of photorespiration, which becomes more pronounced in C₃ plants at high temperatures. These adaptations—lack of photorespiration, high temperature optimum, and water use efficiency—give C₄ plants a significant productivity advantage in tropical environments.

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Answer Keys

Answer Key for Short-Answer Quiz

1. Photosynthesis is crucial because it is the primary source of all food on Earth and it is responsible for releasing oxygen into the atmosphere, which is essential for aerobic respiration.
2. Priestley placed a candle, a mouse, and a mint plant in a sealed bell jar. He found the plant could reverse the “damage” to the air caused by the burning candle or breathing mouse. He hypothesized that plants restore to the air whatever breathing animals and burning candles remove.
3. Ingenhousz demonstrated that sunlight is essential for the air-purifying process of plants. He showed that an aquatic plant produced oxygen bubbles only in the presence of light and only from its green parts.
4. Engelmann split light with a prism and shone it on the alga Cladophora. He used aerobic bacteria, which congregated in areas of high oxygen concentration, to show that the highest rates of photosynthesis (and thus O₂ release) occurred in the blue and red regions of the spectrum.
5. By studying bacteria that use H₂S instead of H₂O for photosynthesis, van Niel demonstrated that O₂ is produced from the splitting of water, not carbon dioxide. He proposed a general formula for photosynthesis as a light-dependent redox reaction.
6. The chloroplast has a division of labor where the membrane system (grana and stroma lamellae) conducts the light-dependent reactions, trapping light and making ATP/NADPH. The stroma is the site of the light-independent (dark) reactions, using those products to synthesize sugar.
7. The four pigments are Chlorophyll a (bright/blue-green), Chlorophyll b (yellow-green), Xanthophylls (yellow), and Carotenoids (yellow to yellow-orange). Chlorophyll a is the chief pigment.
8. Accessory pigments absorb light at wavelengths that chlorophyll a does not, widening the spectrum of light used for photosynthesis. They transfer this energy to chlorophyll a and also protect it from photo-oxidation.
9. The reaction center of PS I is called P700 because its absorption peak is at 700 nm. The reaction center of PS II is called P680 because its absorption maxima is at 680 nm.
10. In the Z-scheme, an electron is excited in PS II, passed down an electron transport chain to PS I, excited again in PS I, and finally transferred to NADP⁺ to form NADPH.
11. The splitting of water occurs on the inner side of the thylakoid membrane and is associated with PS II. Its products are electrons (e⁻), protons (H⁺), and oxygen ([O], which forms O₂).
12. Non-cyclic photophosphorylation involves both PS I and PS II and produces both ATP and NADPH. Cyclic photophosphorylation involves only PS I and produces only ATP, as the electron is cycled back to PS I.
13. The proton gradient is established by three processes: protons are released into the thylakoid lumen from the splitting of water, protons are pumped from the stroma to the lumen during electron transport, and protons are consumed in the stroma to reduce NADP⁺.
14. ATP synthase has a transmembrane channel (CF₀) that allows protons to diffuse from the lumen back to the stroma. This flow of protons provides energy that drives the catalytic site (CF₁) to synthesize ATP from ADP and Pi.
15. The primary CO₂ acceptor is a 5-carbon sugar, ribulose-1,5-bisphosphate (RuBP). The reaction is catalyzed by the enzyme RuBP carboxylase-oxygenase (RuBisCO).
16. The three main stages of the Calvin cycle are carboxylation, reduction, and regeneration.
17. The production of one molecule of glucose requires 6 turns of the cycle and consumes 18 ATP and 12 NADPH.
18. Kranz anatomy is a special leaf structure where large bundle sheath cells form a “wreath” around the vascular bundles. It is found in C₄ plants like maize and sorghum.
19. In a C₄ plant’s mesophyll cell, the 3-carbon substrate phosphoenolpyruvate (PEP) accepts CO₂. This reaction is catalyzed by PEP carboxylase (PEPcase) to form the 4-carbon product oxaloacetic acid (OAA).
20. C₄ plants do not exhibit photorespiration because they have a mechanism that pumps CO₂ into the bundle sheath cells. This increases the CO₂ concentration at the RuBisCO enzyme site, ensuring it functions as a carboxylase and minimizes its oxygenase activity.
21. Photorespiration is a process where RuBisCO binds with O₂ instead of CO₂, leading to the release of CO₂ and the utilization of ATP. It is wasteful because it produces no sugar or energy-rich compounds and loses previously fixed carbon.
22. Blackman’s Law of Limiting Factors states that if a chemical process is affected by more than one factor, its rate will be determined by the factor which is nearest to its minimal value.
23. Light saturation for most plants occurs at 10% of full sunlight. Therefore, except for plants in dense shade, the intensity of light is typically not the factor limiting the rate of photosynthesis.
24. C₄ plants show an increase in photosynthesis rates and then saturate at around 360 µlL⁻¹ CO₂. C₃ plants also respond to increased CO₂ but their saturation point is much higher, beyond 450 µlL⁻¹.
25. Water stress causes the stomata on the leaf surface to close in order to conserve water. This closure reduces the availability of CO₂ for photosynthesis, thereby limiting its rate.

Answer Key for Multiple-Choice Quiz

1. c) Joseph Priestley
2. b) Water
3. b) Grana and stroma lamellae
4. d) Chlorophyll a
5. b) P700
6. c) NADP⁺
7. a) ATP
8. b) Thylakoid lumen
9. b) 3-phosphoglyceric acid (3-PGA)
10. c) RuBisCO
11. c) 18 ATP and 12 NADPH
12. b) C₄ pathway
13. c) C₄ plants
14. d) Bundle sheath cells
15. d) PEP
16. a) O₂
17. c) Synthesis of NADPH
18. c) Temperature
19. a) 360 µlL⁻¹; 450 µlL⁻¹
20. c) Causing stomata to close, thus reducing CO₂ availability

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Comprehensive Glossary

• Accessory Pigments: Pigments like chlorophyll b, xanthophylls, and carotenoids that absorb light and transfer the energy to chlorophyll a.
• Action Spectrum: A graph showing the rate of a physiological activity (like photosynthesis) plotted against different wavelengths of light.
• ATP (Adenosine Triphosphate): A high-energy molecule that stores and transports chemical energy within cells.
• ATP Synthase: An enzyme complex embedded in the thylakoid membrane that uses the energy of a proton gradient to synthesize ATP. It consists of CF₀ and CF₁ parts.
• Autotroph: An organism that synthesizes its own food from inorganic substances using light or chemical energy; e.g., green plants.
• Blackman’s Law of Limiting Factors: A principle stating that the rate of a process governed by several factors is limited by the factor that is in shortest supply.
• Bundle Sheath Cells: Large cells surrounding the vascular bundles in C₄ plants, which are the site of the Calvin cycle.
• C₃ Pathway: The Calvin cycle, where the first stable product of carbon fixation is a 3-carbon compound (3-PGA).
• C₄ Pathway (Hatch and Slack Pathway): A photosynthetic pathway in plants of dry tropical regions where the first stable product of carbon fixation is a 4-carbon compound (OAA).
• Calvin Cycle: The set of chemical reactions that take place in the stroma of chloroplasts to fix CO₂ and synthesize sugars.
• Carboxylation: The first step of the Calvin cycle, where CO₂ is fixed to RuBP.
• Chemiosmosis: The process of ATP synthesis linked to the development of a proton gradient across a membrane.
• Chlorophyll: The green pigment in plants that absorbs light energy for photosynthesis. Chlorophyll a is the primary pigment.
• Chloroplast: The organelle in plant cells where photosynthesis takes place.
• Cyclic Photophosphorylation: A light-driven process involving only Photosystem I that produces ATP but not NADPH.
• Dark Reactions (Carbon Reactions): The light-independent phase of photosynthesis (Calvin Cycle) that occurs in the stroma and uses ATP and NADPH to produce sugars.
• Grana (singular: granum): Stacks of thylakoids within a chloroplast.
• Heterotroph: An organism that cannot manufacture its own food and instead obtains its food and energy by taking in organic substances.
• Kranz Anatomy: The special leaf structure of C₄ plants characterized by large bundle sheath cells arranged in a “wreath” around the vascular tissue.
• Light Harvesting Complex (LHC): An array of protein and chlorophyll molecules found in the thylakoid membranes which transfer light energy to a reaction center. Also called antennae.
• Light Reactions (Photochemical Phase): The phase of photosynthesis that is directly driven by light energy to produce ATP and NADPH.
• Lumen: The space inside the thylakoid sacs.
• Mesophyll: The tissue in the interior of a leaf where chloroplasts are mainly located.
• NADPH (Nicotinamide adenine dinucleotide phosphate): A high-energy electron carrier molecule used in the reduction phase of the Calvin cycle.
• Non-cyclic Photophosphorylation: The light-requiring part of photosynthesis in which electron flow occurs in a linear path (Z-scheme), producing ATP, NADPH, and oxygen.
• PEP Carboxylase (PEPcase): The enzyme in C₄ plants that catalyzes the addition of CO₂ to phosphoenolpyruvate (PEP).
• Phosphoenolpyruvate (PEP): The 3-carbon primary CO₂ acceptor in C₄ plants.
• Photo-oxidation: The damage or destruction of a molecule (like chlorophyll) by light.
• Photophosphorylation: The synthesis of ATP from ADP and phosphate that is driven by light energy.
• Photorespiration: A wasteful process in C₃ plants where RuBisCO binds with O₂, consuming ATP and releasing CO₂ without producing sugars.
• Photosystem (PS I and PS II): Discrete photochemical light-harvesting complexes within the thylakoid membrane, each with a specific reaction center chlorophyll (P700 for PS I, P680 for PS II).
• Pigments: Substances that absorb light at specific wavelengths.
• Reaction Center: A specific chlorophyll a molecule in a photosystem that receives energy from antenna pigments and initiates the process of electron transfer.
• Reduction: A stage in the Calvin cycle where 3-PGA is converted to carbohydrate using ATP and NADPH.
• Regeneration: The final stage of the Calvin cycle where the CO₂ acceptor, RuBP, is reformed.
• Ribulose-1,5-bisphosphate (RuBP): The 5-carbon sugar that is the primary CO₂ acceptor in the Calvin cycle.
• RuBisCO (Ribulose bisphosphate carboxylase-oxygenase): The enzyme that catalyzes the first step of the Calvin cycle (carboxylation) and also the oxygenation step in photorespiration.
• Stroma: The fluid-filled matrix of a chloroplast, which is the site of the Calvin cycle.
• Stroma Lamellae: Membranous channels connecting the grana in a chloroplast.
• Thylakoid: A flattened, membrane-bound sac inside the chloroplast, which is the site of the light reactions.
• Z-Scheme: A model depicting the pathway of electron flow from PS II to PS I during the light-dependent reactions of photosynthesis.