Photosynthesis Overview and Light Reactions

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Questions and Answers

What is the primary advantage of C4 plants over C3 plants in terms of photosynthesis?

  • C4 plants have specialized organelles called chloroplasts in their bundle-sheath cells that perform photosynthesis more efficiently.
  • C4 plants utilize PEP carboxylase, which is not inhibited by oxygen, leading to a more efficient carbon fixation process. (correct)
  • C4 plants have a higher concentration of RuBP carboxylase, allowing them to fix carbon dioxide more efficiently.
  • C4 plants are better at photorespiration which is the process of oxygen binding with RuBP carboxylase and releasing carbon dioxide.

What happens to the oxaloacetate produced by C4 plants during the night?

  • It is released from the plant as a waste product.
  • It is broken down into pyruvate and carbon dioxide.
  • It is stored in the vacuole as malic acid. (correct)
  • It is transported to the bundle-sheath cells and converted to glucose.

Which of the following statements accurately describes the spatial separation of the C4 pathway and the Calvin cycle in C4 plants?

  • Both the C4 pathway and the Calvin cycle occur in the bundle-sheath cells.
  • The C4 pathway occurs in the bundle-sheath cells while the Calvin cycle takes place in the mesophyll cells.
  • Both the C4 pathway and the Calvin cycle occur in the mesophyll cells.
  • The C4 pathway occurs in the mesophyll cells while the Calvin cycle occurs in the bundle-sheath cells. (correct)

What are the two major processes that occur during photosynthesis?

<p>Light reactions and carbon fixation (D)</p> Signup and view all the answers

How does CAM photosynthesis differ from C4 photosynthesis?

<p>CAM photosynthesis uses both the C4 pathway and the Calvin cycle within the same cells, while C4 photosynthesis spatially separates them. (B)</p> Signup and view all the answers

What is the role of water in photosynthesis?

<p>It acts as a source of electrons for the light reactions. (B)</p> Signup and view all the answers

What is the primary advantage of CAM plants over C3 plants under arid conditions?

<p>CAM plants can keep their stomata open at night, which is more efficient than C3 plants that keep them open during the day. (C)</p> Signup and view all the answers

How does light energy absorbed by pigment molecules in Photosystem II lead to ATP production?

<p>The light energy is used to create a chemical gradient across the thylakoid membrane, which drives ATP production. (C)</p> Signup and view all the answers

The process of cyclic photophosphorylation occurs in the absence of what?

<p>Water (A)</p> Signup and view all the answers

Which of the following statements is TRUE about the electron transport chain in photosynthesis?

<p>Electrons flow from Photosystem II to Photosystem I. (B)</p> Signup and view all the answers

What is the ultimate purpose of the light reactions in photosynthesis?

<p>To generate ATP and NADPH (C)</p> Signup and view all the answers

In photosynthesis, what is the role of NADP+?

<p>It is a carrier molecule that accepts high-energy electrons from Photosystem I. (B)</p> Signup and view all the answers

Which of the following is NOT a product of the light reactions?

<p>Glucose (B)</p> Signup and view all the answers

What is the primary function of cyclic electron flow in photosynthesis?

<p>To produce ATP only (D)</p> Signup and view all the answers

Which of the following accurately describes the relationship between the light-dependent reactions and the Calvin cycle?

<p>The light-dependent reactions provide NADPH and ATP to the Calvin cycle (C)</p> Signup and view all the answers

In the Calvin cycle, what molecule is initially fixed to carbon dioxide?

<p>Ribulose 1,5-bisphosphate (RuBP) (D)</p> Signup and view all the answers

What is the role of ATP hydrolysis in the Calvin cycle?

<p>Providing energy for carbon dioxide fixation (A)</p> Signup and view all the answers

Why are C4 plants better adapted to hot, dry environments compared to C3 plants?

<p>C4 plants minimize photorespiration, a wasteful process (C)</p> Signup and view all the answers

Which of the following accurately describes the process of photophosphorylation?

<p>The synthesis of ATP using light energy (A)</p> Signup and view all the answers

In the electron transport chain of photosynthesis, where does the potential energy for ATP synthesis originate?

<p>The movement of electrons down the electron transport chain (D)</p> Signup and view all the answers

Flashcards

Photosynthesis Equation

3CO2 + 6H2O ⎯→ C3H6O3 + 3O2 + 3H2O represents how sunlight converts water and CO2 into glucose and oxygen.

Role of Pigments

Pigment molecules like chlorophylls and carotenoids absorb light energy for photosynthesis.

Photosystems

Photosystems I and II are complexes that absorb light and convert it into chemical energy.

Cyclic Photophosphorylation

A process in Photosystem I that produces ATP without generating NADPH.

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Light Reactions

Convert light energy into chemical energy, producing ATP and NADPH and releasing oxygen.

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Electron Transport Chain

A series of proteins that transfer energized electrons from Photosystem II to Photosystem I.

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Water Photolysis

The splitting of water molecules to replace electrons in Photosystem II; produces oxygen as a byproduct.

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NADP+ Role

NADP+ accepts energized electrons from Photosystem I, converting it to NADPH for the Calvin cycle.

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Oxaloacetate function

Oxaloacetate is converted to malate or aspartate in photosynthesis.

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C4 pathway location

The C4 pathway occurs in the mesophyll cells, while the Calvin cycle happens in bundle-sheath cells.

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PEP Carboxylase advantage

PEP carboxylase in C4 plants is not inhibited by O2, allowing better CO2 utilization.

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CAM plants

CAM plants fix CO2 at night and store it as malic acid until daytime for Calvin cycle usage.

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Malate in CAM

Malate is stored in vacuoles overnight and used for CO2 fixation during the day in CAM plants.

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Photophosphorylation

A process where ATP is generated from ADP using light energy in chloroplasts.

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Cyclic Electron Flow

An electron transport process that produces ATP without Photosystem II.

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Calvin Cycle

A series of reactions in plants that fix carbon dioxide into organic compounds using ATP and NADPH.

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Chemiosmotic Mechanism

Process where proton gradient is used for ATP synthesis during photosynthesis or cellular respiration.

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Ribulose 1,5-bisphosphate (RuBP)

A five-carbon sugar that reacts with CO2 in the Calvin Cycle, forming 3-PGA.

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3-Phosphoglycerate (PGA)

A three-carbon compound produced in the Calvin cycle from RuBP and CO2.

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Glyceraldehyde 3-Phosphate (PGAL)

A three-carbon sugar produced from PGA in the Calvin cycle, used to form glucose.

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Study Notes

Photosynthesis Summary

  • Photosynthesis converts light energy to chemical energy, fixing carbon into organic compounds.
  • Balanced equation: 3COâ‚‚ + 6Hâ‚‚O → C₃H₆O₃ + 3Oâ‚‚ + 3Hâ‚‚O (light)
  • Photosynthesis begins with light absorption by pigments (chlorophylls and carotenoids) in photosystems within chloroplasts.
  • Light excites electrons, transferring energy to reaction center chlorophyll.
  • Two photosystems (I and II) work together, though Photosystem I can operate independently in cyclic photophosphorylation.
  • Electrons flow from water to Photosystem II, then down an electron transport chain to Photosystem I.
  • This flow generates a proton gradient, producing ATP through photophosphorylation.
  • Photosystem I transfers electrons to NADP+, producing NADPH.
  • Light-dependent reactions produce ATP and NADPH.
  • Cyclic photophosphorylation produces ATP without producing NADPH.

Light Reactions

  • Divided into two major processes: electron flow from water to Photosystem II, then down a transport chain to Photosystem I, finally to NADP+.
  • Light energy absorbed by pigment molecules in Photosystem II excites electrons, which are passed down to Photosystem I.
  • Water molecules are used to replace the lost electrons in Photosystem II, releasing oxygen as a byproduct.
  • Electron transport chain generates a proton gradient used to produce ATP.
  • Photosystem I uses light energy to produce energized electrons.
  • Ultimately, the energized electrons from Photosystem I are accepted by NADP+, producing NADPH.

Calvin Cycle (Carbon Fixation)

  • Calvin cycle takes place in the stroma of the chloroplast.
  • Uses ATP and NADPH from light reactions to reduce carbon dioxide into organic carbon.
  • Fixes carbon dioxide to RuBP, creating a three-carbon compound (3-PGA)
  • PGA is reduced to glyceraldehyde 3-phosphate (PGAL).
  • The cycle regenerates RuBP.
  • It involves three stages: carbon fixation, reduction, and regeneration.
  • C3 plants fix carbon dioxide directly into RuBP to form PGA.
  • C4 plants use a different pathway to initially fix CO2 to a four-carbon compound, for improved efficiency in hot environments.
  • CAM plants separate carbon fixation at night and the Calvin cycle during the day.

Additional Points

  • Energy from light-dependent reactions stored in NADPH and ATP.
  • Cyclic electron flow produces ATP, no NADPH is produced in this process.
  • ATP and NADPH from light reactions power the Calvin cycle.
  • Calvin cycle uses ATP and NADPH in a 3:2 ratio.

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