Photosynthesis Overview and Process
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Questions and Answers

What are the primary reactants required for photosynthesis?

  • Oxygen and Glucose
  • Hydrogen and Carbon Dioxide
  • Carbon Dioxide and Water (correct)
  • Glucose and Water

    • Which process describes the loss of electrons during photosynthesis?

  • Phosphorylation
  • Reduction
  • Photolysis
  • Oxidation (correct)

    • In which part of the chloroplast does the Calvin Cycle occur?

  • Inner Membrane
  • Outer Membrane
  • Thylakoid Membranes
  • Stroma (correct)

    • Which of the following is produced during the light-dependent reactions of photosynthesis?

    <p>ATP and NADH</p> Signup and view all the answers

    What is the main function of accessory pigments like carotenoids and xanthophylls in photosynthesis?

    <p>Capture and harvest light</p> Signup and view all the answers

    Which statement accurately describes the role of NADH in the Calvin Cycle?

    <p>It serves as an electron donor for reduction processes.</p> Signup and view all the answers

    What is photolysis in the context of photosynthesis?

    <p>The cleaving of water molecules into oxygen and hydrogen</p> Signup and view all the answers

    Where are photosystems located in the chloroplast?

    <p>Thylakoid membranes</p> Signup and view all the answers

    What role does chlorophyll A play in photosynthesis?

    <p>It is the primary photosynthetic pigment.</p> Signup and view all the answers

    What is the purpose of the antenna complex in photosystems?

    <p>To ensure efficient light energy capture through energy transfer.</p> Signup and view all the answers

    What happens after light energy excites the electron in the chlorophyll?

    <p>It becomes unstable and gets transferred to neighboring pigments.</p> Signup and view all the answers

    Which statement best describes the energy transfer in photosystems?

    <p>There is a gradual decrease in energy based on thermodynamic laws.</p> Signup and view all the answers

    Why is the reaction center significant in the photosystem?

    <p>It is where the first redox reaction occurs.</p> Signup and view all the answers

    What type of energy levels do the pigments in the antenna complex exhibit?

    <p>They have varying energy levels which aid in efficiency.</p> Signup and view all the answers

    What occurs immediately after the light energy excites an electron?

    <p>The electron released becomes an electron carrier.</p> Signup and view all the answers

    Which pigment has the highest energy level in the antenna complex?

    <p>Carotenoid</p> Signup and view all the answers

    What is the primary function of ATP synthase in the thylakoid lumen?

    <p>To produce ATP from ADP</p> Signup and view all the answers

    During the Calvin Cycle, which enzyme is responsible for incorporating carbon dioxide into organic molecules?

    <p>RuBisCO</p> Signup and view all the answers

    What happens during the hydrolysis phase of the Calvin Cycle?

    <p>Cleaving of the intermediate into two 3-carbon molecules</p> Signup and view all the answers

    Which of the following statements about the proton gradient in the thylakoid lumen is true?

    <p>It is essential for ATP production.</p> Signup and view all the answers

    Which of the following processes results in the formation of triode phosphate (G3P)?

    <p>Reduction of 3-PGA</p> Signup and view all the answers

    What is the role of Plastoquinone in the electron transport chain?

    <p>To release protons in the thylakoid lumen</p> Signup and view all the answers

    What is the primary outcome of the regeneration stage in the Calvin Cycle?

    <p>Regeneration of RuBP</p> Signup and view all the answers

    What is the significance of light absorption at the peak wavelengths by P680 and P700?

    <p>It ensures maximum efficiency in the capture of solar energy.</p> Signup and view all the answers

    What role does the enzyme sucrose phosphate phosphatase play in sucrose synthesis?

    <p>It removes the remaining phosphate to create sucrose.</p> Signup and view all the answers

    Which of the following statements about the alternative pathway in sucrose synthesis is true?

    <p>It only involves sucrose synthaase during growth conditions.</p> Signup and view all the answers

    Which enzyme is responsible for fixing carbon from carbon dioxide during photosynthesis?

    <p>RUBISCO as a carboxylase.</p> Signup and view all the answers

    What is the effect of high temperatures on RuBisCO activity?

    <p>It increases oxygenase activity leading to photorespiration.</p> Signup and view all the answers

    What is the primary product of the reaction catalyzed by sucrose synthase?

    <p>Sucrose.</p> Signup and view all the answers

    In the synthesis of starch, which molecule is preferred over others for the reaction?

    <p>ADP.</p> Signup and view all the answers

    Which enzyme activity is heightened in conditions of high irradiance?

    <p>RUBISCO as an oxygenase.</p> Signup and view all the answers

    What happens during photorespiration?

    <p>Production of phosphoglycerate.</p> Signup and view all the answers

    What is the process in which serine releases its nitrogen group?

    <p>Deamination</p> Signup and view all the answers

    What happens to glycerate after it is formed from serine's oxidation?

    <p>It is phosphorylated using ATP.</p> Signup and view all the answers

    What is the primary consequence of glycolate production in plants?

    <p>Decreased levels of oxygen</p> Signup and view all the answers

    What is a 'safety valve' function of photorespiration in plants?

    <p>To dissipate excess energy</p> Signup and view all the answers

    Which mechanism aids in carbon dioxide concentration in aquatic organisms?

    <p>CO2 and HCO3 pumps</p> Signup and view all the answers

    What is one reason why photorespiration is considered less efficient than other processes?

    <p>It results in some carbon being lost.</p> Signup and view all the answers

    What is the primary result of the carbon dioxide being soluble in water?

    <p>Creation of carbonic acid</p> Signup and view all the answers

    What change occurs during the oxidation of serine?

    <p>Formation of hydroxypyruvate</p> Signup and view all the answers

    Study Notes

    Photosynthesis

    • Photosynthesis is a metabolic reaction, similar to cellular respiration.
    • Reactants: Carbon Dioxide and Water
    • Light is required, necessitating light-absorbing pigments like chlorophyll.
    • Products: Atomic Oxygen and sugars (Glucose, etc.)
      • Photosynthesis produces G3P (glyceraldehyde-3-phosphate), a 3-carbon triose sugar.
    • This process involves a significant REDOX reaction.
      • Reduction of Carbon Dioxide (gains electrons via H addition).
      • Oxidation of Water (loses electrons becoming atomic oxygen).
    • Occurs in algae, phytoplankton, and bacteria.

    Components of Photosynthesis

    • Light-Dependent Reaction
      • Occurs in the thylakoid membranes where chlorophyll and other components are embedded.
      • Light facilitates photolysis of water:
        • Water molecules are cleaved using light energy into ½ O2 (monatomic oxygen) and 2 Hydrogen+ + 2e-.
        • The oxygen is released by plants during photosynthesis.
      • Elevation of Light Energy:
        • Excited chlorophyll electrons lose or give away energy.
        • To regain stability, they need to replenish the lost electron from photolysis.
      • Production of ATP and NADH:
        • ATP is used in the Calvin Cycle (energy-consuming reaction) from phosphorylation of ADP.
        • NADH serves as a strong reducing agent in the Calvin Cycle:
          • Important for REDOX reactions to proceed.
          • Donor of electrons.
    • Calvin Cycle (Light Independent Reaction)
      • Occurs in the stroma (fluid environment surrounding the thylakoid).

    Photosystems

    • Complex of light-capturing proteins and pigments (molecules) embedded in the thylakoid membrane.
    • Function: To harvest light due to the presence of pigments.
    • Parts:
      • Light Harvesting Complex (Antenna Complex)
        • Main function is to harvest and collect light.
        • Composed primarily of pigments with accessory pigments (carotenoids and xanthophylls).
          • Accessory pigments: Carotenoids and Xanthophyll.
          • Primary Pigment: Chlorophyll A (primary photosynthetic pigment)
        • Ensures efficient light capture.
        • Forster resonance energy transfer (FRET):
          • Light energy excites electrons, making them unstable.
          • High excitation is transferred to neighboring pigments until it reaches the reaction center.
          • Dependent on distances and orientation.
          • Requires spectral overlap (similarity in light absorbed wavelengths).
          • Pigments in the antenna complex have different energy levels or states:
            • Carotenoid (highest energy)
            • Xanthophyll
            • Chlorophyll B
          • Photosystems sacrifice a bit of energy to ensure the energy reaches the reaction center.
          • There is a gradual decrease in energy via thermodynamic laws.
          • Funneling effect in the photosystems.
          • Not a random arrangement of pigments in the antenna complex.
          • Sacrificed energy results in 95-99% efficiency.
      • Reaction Center
        • Not directly involved in light transfer.
        • Called the reaction center because the first REDOX reaction happens here.
        • Structures:
          • Contains a dimer (of chlorophyll a)
            • Chlorophyll a has the lowest energy level.
            • Ensures the endpoint of energy transfer.
          • Photoact or photoionization occurs once energy reaches the reaction center.
          • Light energy excitation results in the release of an electron.
          • The released electron becomes an electron carrier.
          • Gets another electron from photolysis of water.
        • Transfer of Electrons:
          • Photosystem II (PSII) to Plastoquinone, Cytochrome b6/f, Plastocyanin, and Photosystem I (PSI), lastly, Ferredoxin.
          • No direct transfer from PSII to PSI due to thylakoid compartmentalization and distance between photosystems.
          • Electron passing establishes a proton gradient:
            • Plastoquinone releases hydrogen atoms into the thylakoid lumen upon accepting electrons.
            • Cytochrome b6/f also releases hydrogen atoms.
            • Ferredoxin uses hydrogen atoms in the chloroplast stroma.
          • ATP synthase produces ATP when there is a steep proton gradient in the thylakoid lumen compared to the chloroplast stroma.
            • Protons move out of the stroma.
            • Passing through the synthase, ADP is phosphorylated into ATP.
        • P680 and P700 are the names of the reaction centers at PSII and PSI, respectively.
          • These names correspond to the peak wavelength of light they can absorb.

    Carbon Dioxide Assimilation

    • Plants absorb carbon dioxide and incorporate it into organic compounds.
    • Calvin Cycle
      • Divided into three stages.
      • Cyclical in nature with some products used to regenerate RuBP for further carbon dioxide assimilation.

    Calvin Cycle Stages

    • Carbon Fixation
      • RuBisCO - Ribulose bisphosphate carboxylase.
        • As a carboxylase, it incorporates carbon from carbon dioxide into organic molecules.
        • RuBP is a five-carbon molecule.
      • Two phases:
        • Carboxylation of RuBP: Creation of an unstable intermediate product (6-carbon molecule).
        • Hydrolysis: Cleaving the intermediate product into two 3-carbon molecules (3-PGA process).
      • Reduction of 3-PGA:
        • Phosphorylation: 6 ATP are used to add another phosphate group via phosphoglycerate kinase.
        • Reduction: 6 NADPH are used by dehydrogenases to create triose phosphate (G3P and DHAP).
          • DHAP is an isomer of G3P.
        • 3 molecules of carbon dioxide result in 6 G3P.
          • 1 G3P is used for other pathways.
          • 5 G3P are used to regenerate RuBP.
    • Regeneration of RuBP:
      • Sugar Shuffle: Triose phosphate cleaves and reforms into several sugar forms, requiring Q-enzyme (alpha 1,6) link, even with starch synthase.
        • Addition at the 6th carbon.
        • The branching structure responsible for the sticky texture of rice requires more water.
      • Sucrose Synthesis: Requires a transporter and another set of enzymes.
        • Principal pathways: Suc phos synthase and Suc phos phosphatase
        • Pathways:
          • Phosphate translocator enables transport of phosphate and triose phosphate.
          • Aldolase activity and phosphatase.
          • Isomerization to form glucose 6 phosphate.
          • Mutase activity.
          • Instead of ATP, U(racil)TP is utilized.
          • Results in UDP-glucose.
          • Sucrose phosphate synthase activity joins UDP-glucose to form sucrose phosphate.
          • Enzyme sucrose phosphate phosphatase removes the remaining phosphate, ultimately creating sucrose.
    • Secondary Pathway:
      • Alternative pathway during active growth or abiotic stress.
      • Only sucrose synthase is present (catalyzes a reversible reaction).
      • Commonly found in plant sinks.

    Additional Info

    • RuBisCO as Carboxylase:
      • Fixations of Carbon from carbon dioxide to RuBP.
    • RuBisCO as Oxygenase:
      • Condensation of O2 with RuBP.
        • Results in a 5-carbon molecule to create only 1 PGA (phosphoglycerate).
      • Goal of photorespiration.

    Photorespiration

    • Release of carbon dioxide.
    • Lowers the efficiency of photosynthesis.
    • Occurs at high temperatures and/or high irradiance.
      • Irradiance is the number of photons hitting a certain unit area (essentially higher light intensity).
      • RuBisCO oxygenase activity heightens with the higher concentration of oxygen in plant tissue.
    • Different structures due to phosphate groups, both are reducing agents.
    • Oxidation of glycine.
      • Decarboxylation (removal of a carbon atom from a molecule).
      • Carboxylation into carbon dioxide.
    • Serine undergoes deamination (releasing the N group) to form hydroxypyruvate.
      • Hydroxypyruvate (negatively charged) undergoes redox reaction to become neutral through the utilization of NADH to NAD+ (oxidized).
        • Forms glycerate.
    • Glycerate returns to the chloroplast.
      • Phosphorylation using ATP to become 3 phosphoglycerate.

    Significance of Photorespiration

    • Internal cellular recycling of carbon dioxide (in the mitochondrion), oxygen (in the peroxisome), and NH3 (formation of other amino acids).
    • Glycolate production decreases oxygen levels.
    • Physiological defense against high irradiance and thermal load excess.
      • Acts as a safety valve for dissipating excess energy (ATP, NADH, and NADPH).
    • Contributes to the cell's amino pool (glycerine and serine).
    • Response to niche diversification scheme:
      • Photorespiration occurs in high-temperature areas allowing plants to survive these conditions and occupy specific niches.
    • Salvages or maximizes 75% of carbon from glycolate for the Calvin Cycle.
      • Not 100% efficient, some carbon is lost.
      • Results in less biomass than plants that don't undergo photorespiration.
    • Not all plants photorespire or only for a short time.

    Plants that do not Photorespire

    • Carbon Dioxide Concentration Mechanism
      • CO2 and HCO3 pumps: Pumping carbon dioxide into the chloroplast. - Common in aquatic organisms.
        • Pathway:
          • Carbon dioxide is soluble in water, forming carbonic acid (H2CO3).
            • Further broken down into bicarbonate ions and protons.
            • Bicarbonate ions are negatively charged, making it harder to diffuse through the membrane.

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    Photosynthesis Lecture Notes PDF

    Description

    Explore the intricate process of photosynthesis, including its reactants and products. This quiz delves into the light-dependent reactions and the metabolic roles of chlorophyll. Test your knowledge on how this essential process fuels life on Earth.

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