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1. Describe the structure of the chloroplast.

2. Describe the relationship between action spectrum and absorption spectrum .
3. Trace the movement of electrons in linear electron flow.
4. Trace the movement of electrons in cyclic electron flow.
5. Explain the stages of Calvin cycle.
6.Describe the role of ATP and NADPH in the Calvin cycle.
7. Describe the major consequences of phodphorespiration

8. Describe two important photosynthesic adaptation that minimize phodphorespiration

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1. Structure of the Chloroplast

Chloroplasts are double-membrane-bound organelles found in plant and algal cells. Their structure includes:

  • Outer Membrane: A smooth membrane that is permeable to small molecules and ions.
  • Inner Membrane: Less permeable and contains transport proteins that regulate the passage of substances.
  • Intermembrane Space: The space between the outer and inner membranes.
  • Stroma: The fluid-filled space inside the inner membrane, which contains enzymes, starch granules, DNA, and ribosomes. It is where the Calvin cycle takes place.
  • Thylakoids: Flattened sacs organized into stacks called grana (singular: granum). Thylakoids contain chlorophyll and other pigments necessary for capturing light energy during photosynthesis.
  • Thylakoid Membrane: Contains the proteins and pigments necessary for the light-dependent reactions of photosynthesis.

2. Relationship Between Action Spectrum and Absorption Spectrum

  • Absorption Spectrum: This graph shows the wavelengths of light that are absorbed by chlorophyll and other pigments in a plant. Different pigments absorb light at different wavelengths, with peaks at particular colors corresponding to their specific absorption properties.
  • Action Spectrum: This graph shows the effectiveness of different wavelengths of light in driving photosynthesis. It indicates which wavelengths are most effective at promoting the photosynthetic process.

The relationship between them is that the action spectrum usually resembles the absorption spectrum; wavelengths of light that are absorbed effectively by pigments tend to also be the ones that drive the photosynthetic process most efficiently.

3. Movement of Electrons in Linear Electron Flow

  1. Photon Absorption: Chlorophyll absorbs light, exciting electrons to a higher energy state.
  2. Water Splitting: An enzyme complex absorbs sunlight and splits water (H₂O), releasing electrons, protons (H⁺), and O₂.
  3. Electron Transport Chain (ETC): Excited electrons move through a series of proteins in the thylakoid membrane (e.g., plastoquinone, cytochrome b6f, plastocyanin).
  4. Creation of Proton Gradient: As electrons pass through the chain, protons are pumped into the thylakoid lumen, creating a proton gradient.
  5. ATP Synthesis: Protons flow back into the stroma through ATP synthase, driving the conversion of ADP and inorganic phosphate into ATP.
  6. NADPH Formation: Electrons continue down the chain and eventually reduce NADP⁺ to form NADPH in a reaction catalyzed by NADP⁺ reductase.

4. Movement of Electrons in Cyclic Electron Flow

  1. Photon Absorption: Similar to linear flow, chlorophyll absorbs light, exciting electrons.
  2. Electron Transport: The excited electrons are transferred to the primary electron acceptor but instead of moving to NADP⁺, they are transported back into the ETC.
  3. ATP Synthesis: As electrons cycle back through the ETC, they again contribute to the proton gradient and ultimately lead to the production of ATP through ATP synthase.
  4. Recycling of Electrons: This process allows the electron to return to Photosystem I, where it can again be excited by light, continuing the cycle.

5. Stages of the Calvin Cycle

  1. Carbon Fixation: CO₂ is incorporated into ribulose-1,5-bisphosphate (RuBP) using the enzyme ribulose bisphosphate carboxylase/oxygenase (Rubisco), forming 3-phosphoglycerate (3-PGA).
  2. Reduction Phase: ATP and NADPH generated in the light-dependent reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).
  3. Regeneration of RuBP: Some G3P molecules exit the cycle to form glucose and other carbohydrates, while others are used to regenerate RuBP, requiring ATP.

6. Role of ATP and NADPH in the Calvin Cycle

  • ATP: Provides the energy needed for the conversion of 3-PGA to G3P and the regeneration of RuBP.
  • NADPH: Provides the reducing power (electrons) necessary to convert 3-PGA into G3P. It plays a critical role in reducing carbon compounds during the reduction phase.

7. Major Consequences of Photorespiration

Photorespiration is a process that occurs when Rubisco fixes O₂ instead of CO₂, leading to:

  • Reduced Efficiency: It reduces the overall efficiency of photosynthesis by diverting energy and carbon away from sugar production.
  • Energy Loss: It consumes ATP and NADPH without producing glucose, leading to loss of energy that could have been used for plant growth.
  • Production of Toxic Compounds: Photorespiration produces glycolate, which can be toxic, and requires additional energy to recycle.

8. Two Important Photosynthetic Adaptations That Minimize Photorespiration

  1. C4 Pathway: Plants like maize and sugarcane utilize a C4 pathway where CO₂ is initially fixed to a 4-carbon compound (oxaloacetate) in mesophyll cells, which is then transported to bundle-sheath cells where it is released for the Calvin cycle. This spatial separation significantly reduces the likelihood of Rubisco binding with O₂.

  2. CAM (Crassulacean Acid Metabolism): Plants such as succulents open their stomata at night to fix CO₂ into organic acids, which are stored and used during the day when the stomata are closed. This temporal separation minimizes water loss and reduces photorespiration by ensuring that CO₂ is available when the Calvin cycle operates.