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?

ATP and NADH Signup and view all the answers

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

Capture and harvest light Signup and view all the answers

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

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

What is photolysis in the context of photosynthesis?

The cleaving of water molecules into oxygen and hydrogen Signup and view all the answers

Where are photosystems located in the chloroplast?

Thylakoid membranes Signup and view all the answers

What role does chlorophyll A play in photosynthesis?

It is the primary photosynthetic pigment. Signup and view all the answers

What is the purpose of the antenna complex in photosystems?

To ensure efficient light energy capture through energy transfer. Signup and view all the answers

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

It becomes unstable and gets transferred to neighboring pigments. Signup and view all the answers

Which statement best describes the energy transfer in photosystems?

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

Why is the reaction center significant in the photosystem?

It is where the first redox reaction occurs. Signup and view all the answers

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

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

What occurs immediately after the light energy excites an electron?

The electron released becomes an electron carrier. Signup and view all the answers

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

Carotenoid Signup and view all the answers

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

To produce ATP from ADP Signup and view all the answers

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

RuBisCO Signup and view all the answers

What happens during the hydrolysis phase of the Calvin Cycle?

Cleaving of the intermediate into two 3-carbon molecules Signup and view all the answers

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

It is essential for ATP production. Signup and view all the answers

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

Reduction of 3-PGA Signup and view all the answers

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

To release protons in the thylakoid lumen Signup and view all the answers

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

Regeneration of RuBP Signup and view all the answers

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

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

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

It removes the remaining phosphate to create sucrose. Signup and view all the answers

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

It only involves sucrose synthaase during growth conditions. Signup and view all the answers

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

RUBISCO as a carboxylase. Signup and view all the answers

What is the effect of high temperatures on RuBisCO activity?

It increases oxygenase activity leading to photorespiration. Signup and view all the answers

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

Sucrose. Signup and view all the answers

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

ADP. Signup and view all the answers

Which enzyme activity is heightened in conditions of high irradiance?

RUBISCO as an oxygenase. Signup and view all the answers

What happens during photorespiration?

Production of phosphoglycerate. Signup and view all the answers

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

Deamination Signup and view all the answers

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

It is phosphorylated using ATP. Signup and view all the answers

What is the primary consequence of glycolate production in plants?

Decreased levels of oxygen Signup and view all the answers

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

To dissipate excess energy Signup and view all the answers

Which mechanism aids in carbon dioxide concentration in aquatic organisms?

CO2 and HCO3 pumps Signup and view all the answers

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

It results in some carbon being lost. Signup and view all the answers

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

Creation of carbonic acid Signup and view all the answers

What change occurs during the oxidation of serine?

Formation of hydroxypyruvate 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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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.