Photosynthesis
Can you recall? Page No. 138
1. Why energy is essential in different life processes?
Answer: Energy is essential in different life processes because it drives vital activities such as growth, reproduction, movement, and metabolism, enabling organisms to sustain life and perform functions necessary for survival. The document highlights that photosynthesis transforms solar energy into chemical energy, which is stored in carbohydrates and utilized by all living organisms directly or indirectly for these processes.
2. How do we get energy?
Answer: Plants obtain energy through photosynthesis, where solar energy is trapped by chlorophyll and converted into chemical energy stored in carbohydrates (glucose) using CO₂ and H₂O. Animals and other organisms acquire energy by consuming plants or other organisms that have stored this chemical energy. The document emphasizes that photosynthesis is the primary process providing energy for all life, as it is the foundation of the food chain.
Can you tell? Page No. 140
1. What made Hill to perform his experiment?
Answer: Robert Hill performed his experiment to investigate the source of oxygen evolved during photosynthesis. He aimed to determine whether oxygen comes from water or carbon dioxide, as the mechanism was unclear at the time. His experiment, conducted in 1937, involved culturing isolated chloroplasts in a medium devoid of CO₂ with a ferric compound, which changed color upon reduction, confirming that oxygen is released from water through photolysis.
2. Distinguish between action spectrum and absorption spectrum.
Answer: Absorption Spectrum: This is a graph showing the amount of light absorbed by different pigments at various wavelengths of the visible spectrum. It indicates the wavelengths (e.g., blue, violet, red for chlorophyll a and b) where pigments absorb light most effectively.
Action Spectrum: This is a graph showing the rate of photosynthesis at different wavelengths of light. It closely matches the absorption spectrum of chlorophyll a and b, demonstrating that the wavelengths absorbed by these pigments (red and blue) are the ones most effective for driving photosynthesis.
3. Draw well labelled diagram of chloroplast.
Answer:
Think about it Page No. 141
Does moon light support photosynthesis?
Answer: Moonlight does not support photosynthesis effectively because its intensity is too low to provide the energy required for the light-dependent reactions. The document notes that photosynthesis requires sufficient light intensity, with the highest rates in bright, diffused sunlight, and slows down in very low-intensity light.
Can you tell? Page No. 145
1. How chlorophyll – a is excited? Show it with a diagram.
Answer:
- Chlorophyll-a is an essential photosynthetic pigment as it converts light energy into chemical energy and acts as a reaction centre.
- Initially, it lies at ground state or singlet state but when it absorbs or receives photons (solar energy), it gets activated and goes in excited state or excited second singlet state.
- In the excited state, chlorophyll-a emits an electron. The emitted electron is energy rich, i.e. has extra amount of energy.
- Due to the loss of electron (e–), chlorophyll-a becomes positively charged. This is the ionized state.
- Chlorophyll-a molecule cannot remain in the ionized state for more than 10‘9 seconds. Hence the photo-chemical reaction or electron transfer occurs very fast.
- The energy rich electron is then transferred through various electron acceptors and donors (carriers).
- During the transfer, the electron emits energy which is utilized for the synthesis of ATP.
- This shows that light energy is converted into chemical energy in the form of ATP.
2. Describe Calvin’s cycle.
Answer: The Calvin cycle, also known as the C3 pathway, is the light-independent phase of photosynthesis occurring in the chloroplast stroma, where CO₂ is fixed into carbohydrates using ATP and NADPH from the light reaction. The document outlines three phases:
- Carboxylation: CO₂ reacts with ribulose-1,5-bisphosphate (RuBP, 5C) catalyzed by RuBisCO, forming an unstable 6C intermediate that splits into two 3-phosphoglyceric acid (3-PGA, 3C) molecules.
- Reduction (Glycolytic Reversal): 3-PGA is phosphorylated by ATP to 1,3-diphosphoglyceric acid, then reduced by NADPH to glyceraldehyde-3-phosphate (3-PGAL). Out of 12 3-PGAL molecules, 2 form one glucose molecule (C₆H₁₂O₆).
- Regeneration of RuBP: The remaining 10 3-PGAL molecules undergo complex reactions, consuming 6 ATP, to regenerate 6 RuBP molecules, ensuring continuous CO₂ fixation. The cycle requires 6 CO₂, 18 ATP, and 12 NADPH to produce one glucose, as detailed in Figure 12.10.
3. Draw a flowchart of non-cyclic photophosphorylation.
Answer: Flowchart Description (Text-Based for Guidance):
- Title: Flowchart of Non-Cyclic Photophosphorylation (based on Fig. 12.8 in the document).
- Steps and Components:
- Start: Light Absorption by PS-II (P680)
- Box: “PS-II (P680) absorbs light, excites electrons.”
- Arrow to: “Photolysis of H₂O.”
- Photolysis of Water
- Box: “H₂O → 2H⁺ + 2e⁻ + ½O₂↑.”
- Outputs:
- O₂ released (arrow labeled “O₂”).
- Electrons to PS-II (arrow labeled “e⁻”).
- Protons (H⁺) accumulate in thylakoid lumen (arrow labeled “H⁺”).
- Electron Transport from PS-II
- Box: “Electrons move from PS-II to Plastoquinone (PQ).”
- Arrow to: “Cytochrome b6-f complex.”
- Proton Gradient and ATP Synthesis
- Box: “H⁺ gradient drives ATP synthase: ADP + Pi → ATP.”
- Arrow labeled “ATP” as output.
- Arrow from Cytochrome b6-f to: “Plastocyanin (PC).”
- Electron Transfer to PS-I (P700)
- Box: “PC transfers electrons to PS-I (P700).”
- Arrow to: “PS-I absorbs light, excites electrons.”
- Electron Transport from PS-I
- Box: “Electrons move to Ferredoxin (FeS).”
- Arrow to: “NADP Reductase.”
- NADPH Formation
- Box: “NADP⁺ + 2e⁻ + H⁺ → NADPH.”
- Arrow labeled “NADPH” as output.
- Start: Light Absorption by PS-II (P680)
- Connections:
- Arrows indicate the flow of electrons (e⁻) through the chain: PS-II → PQ → Cytochrome b6-f → PC → PS-I → FeS → NADP Reductase.
- Show H⁺ accumulation in the thylakoid lumen and diffusion to stroma via ATP synthase.
- Outputs (O₂, ATP, NADPH) are shown as endpoints.
- Drawing Instructions: Create a linear or circular flowchart with boxes for each step, connected by arrows showing electron flow and outputs. Label each component and process clearly. Refer to Figure 12.8 in the document for a visual guide. Include thylakoid membrane context to show PS-II and PS-I locations.
Can you tell? Page No. 147
1. C4 plants are more productive. Why?
Answer: C4 plants are more productive because they minimize photorespiration through Kranz anatomy, which concentrates CO₂ in bundle sheath cells, enhancing Calvin cycle efficiency. The document explains that C4 plants fix CO₂ into a 4-carbon compound (oxaloacetic acid) in mesophyll cells, then transfer it to bundle sheath cells, preventing RuBisCO from using O₂. This allows efficient photosynthesis even in high light, temperature, and low CO₂ conditions, requiring 30 ATP per glucose but yielding higher productivity.
2. Xerophytic plants survive in high temperature. How?
Answer: Xerophytic plants, such as those with CAM (Crassulacean Acid Metabolism), survive high temperatures by opening stomata at night to fix CO₂ into malic acid, reducing water loss. During the day, stomata close, and malic acid releases CO₂ for the Calvin cycle, as described in the document. This adaptation, along with structural features like thick cuticles and reduced leaf surfaces, helps them conserve water and maintain photosynthesis in hot, arid conditions.
3. Summarise the photosynthetic reaction.
Answer: Photosynthesis converts 6 molecules of CO₂ and 12 molecules of H₂O into one molecule of glucose (C₆H₁₂O₆), 6 molecules of O₂, and 6 molecules of H₂O, using solar energy trapped by chlorophyll in chloroplasts. The process involves light-dependent reactions (producing ATP, NADPH, and O₂ via water photolysis) and light-independent reactions (Calvin cycle, fixing CO₂ into glucose). The overall reaction is:
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O (in the presence of light and chlorophyll)
4. Compare C4 plants and CAM plants.
Answer: C4 Plants:
- Occurrence: Found in tropical and subtropical grasses (e.g., sugarcane, maize, sorghum) and some dicotyledons.
- Pathway: Utilize the Hatch-Slack pathway, where CO₂ is initially fixed into a 4-carbon compound, oxaloacetic acid, in mesophyll cells, then transferred as malic acid to bundle sheath cells for the Calvin cycle (C3 pathway).
- Anatomy: Exhibit Kranz anatomy, with distinct mesophyll cells (initial CO₂ fixation, small chloroplasts with grana) and bundle sheath cells (Calvin cycle, large chloroplasts with minimal/no grana).
- CO₂ Fixation: Occurs during the day, with CO₂ concentrated in bundle sheath cells to minimize photorespiration, enhancing efficiency.
- Energy Requirement: Requires 30 ATP per glucose molecule (12 more than C3 plants) due to additional steps in CO₂ fixation and PEP regeneration.
- Advantages: Highly productive in high light, high temperature, and low CO₂ conditions due to no photorespiration, making them ideal for tropical environments.
CAM Plants:
- Occurrence: Found in desert plants (e.g., Kalanchoe, Opuntia, Aloe) adapted to arid conditions.
- Pathway: Utilize Crassulacean Acid Metabolism (CAM), fixing CO₂ at night into malic acid, which is stored in vacuoles, and releasing CO₂ for the Calvin cycle during the day.
- Anatomy: Lack Kranz anatomy; all reactions (CO₂ fixation and Calvin cycle) occur in mesophyll cells, with no specialized bundle sheath cells.
- CO₂ Fixation: Occurs at night when stomata are open (scotoactive) to reduce water loss, with daytime CO₂ release for the Calvin cycle when stomata are closed.
- Energy Requirement: Similar to C4 plants in terms of ATP use for CO₂ fixation, but specific ATP requirements are not detailed in the document; focuses on water conservation.
- Advantages: Adapted for water conservation in arid environments, allowing photosynthesis with minimal water loss, though less productive than C4 plants.
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