Table of contents

1. Introduction to Biology2h 42m
2. Chemistry3h 37m
3. Water1h 26m
4. Biomolecules2h 23m
5. Cell Components2h 26m
6. The Membrane2h 31m
7. Energy and Metabolism2h 0m
8. Respiration2h 40m
9. Photosynthesis2h 49m
10. Cell Signaling59m
11. Cell Division2h 47m
12. Meiosis2h 0m
13. Mendelian Genetics4h 44m
14. DNA Synthesis2h 27m
15. Gene Expression3h 6m
16. Regulation of Expression3h 31m
17. Viruses37m
- Viruses
  37m
18. Biotechnology2h 58m
19. Genomics17m
- Genomes
  17m
20. Development1h 5m
21. Evolution3h 1m
22. Evolution of Populations3h 53m
23. Speciation1h 37m
24. History of Life on Earth2h 6m
25. Phylogeny2h 31m
26. Prokaryotes4h 59m
27. Protists1h 12m
28. Plants1h 22m
29. Fungi36m
- Fungi
  19m
- Fungi Reproduction
  17m
30. Overview of Animals34m
- Overview of Animals
  34m
31. Invertebrates1h 2m
32. Vertebrates50m
33. Plant Anatomy1h 3m
34. Vascular Plant Transport1h 2m
- Water Potential
  1h 2m
35. Soil37m
- Soil and Nutrients
  20m
- Nitrogen Fixation
  16m
36. Plant Reproduction47m
- Flowers
  33m
- Seeds
  14m
37. Plant Sensation and Response1h 9m
38. Animal Form and Function1h 19m
39. Digestive System1h 10m
- Digestion
  1h 3m
- Blood Sugar Homeostasis
  7m
40. Circulatory System1h 49m
41. Immune System1h 12m
42. Osmoregulation and Excretion50m
- Osmoregulation and Excretion
  50m
43. Endocrine System1h 4m
- Endocrine System
  1h 4m
44. Animal Reproduction1h 2m
- Animal Reproduction
  1h 2m
45. Nervous System1h 55m
- Neurons and Action Potentials
  1h 8m
- Central and Peripheral Nervous System
  46m
46. Sensory Systems46m
- Sensory System
  46m
47. Muscle Systems23m
- Musculoskeletal System
  23m
48. Ecology3h 11m
49. Animal Behavior28m
- Animal Behavior
  28m
50. Population Ecology3h 41m
51. Community Ecology2h 46m
52. Ecosystems2h 36m
53. Conservation Biology24m
- Conservation Biology
  24m

9. Photosynthesis

Light Reactions of Photosynthesis

General Biology

9. Photosynthesis

Light Reactions of Photosynthesis

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5:10 minutes

Problem 10

Textbook Question

An investigator exposes chloroplasts to 700-nm photons and observes low O₂ production, but high ATP production. Which of the following best explains this observation?
a. The electrons from water are directly transferred to NADP⁺, which is used to generate ATP.
b. Photosystem II is not splitting water, and the ATP is being produced by cycling electrons via photosystem I.
c. The O₂ is being converted to water as a terminal electron acceptor in the production of ATP.
d. Electron transport has stopped and ATP is being produced by the Calvin cycle.

Verified step by step guidance

Understand the role of photosystems in photosynthesis: Photosystem II (PSII) is responsible for splitting water molecules to release oxygen (O2), electrons, and protons. Photosystem I (PSI) is primarily involved in the production of NADPH and can also contribute to ATP production through cyclic electron flow.

Analyze the wavelength of light: The 700-nm photons correspond to the absorption peak of Photosystem I (PSI), not Photosystem II (PSII). This suggests that PSI is active, while PSII might not be effectively engaged.

Consider the implications of low O2 production: Low oxygen production indicates that water splitting by PSII is not occurring efficiently. This suggests that PSII is not functioning optimally or is bypassed.

Evaluate ATP production: High ATP production in the absence of significant O2 production suggests that ATP is being generated through cyclic photophosphorylation, a process where electrons are cycled back to PSI, allowing for ATP synthesis without the production of NADPH or O2.

Conclude the explanation: The observation of low O2 but high ATP production is best explained by the fact that Photosystem II is not splitting water, and ATP is being produced by cycling electrons via Photosystem I, which aligns with option b.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Photosystems in Photosynthesis

Photosynthesis involves two main photosystems: Photosystem I (PSI) and Photosystem II (PSII). PSII absorbs light at 680 nm and is responsible for splitting water molecules to release oxygen. PSI absorbs light at 700 nm and primarily facilitates the production of NADPH. Understanding the roles of these photosystems is crucial for explaining the observed phenomena in the question.

Cyclic Electron Flow

Cyclic electron flow is a process in photosynthesis where electrons are recycled around Photosystem I, leading to the production of ATP without the generation of NADPH or oxygen. This process is significant when PSII is inactive or when the plant requires more ATP than NADPH, explaining the high ATP production and low O2 output in the scenario described.

Role of Water in Photosynthesis

Water is a crucial electron donor in photosynthesis, particularly in Photosystem II, where it is split to release oxygen, electrons, and protons. If PSII is not functioning, as suggested in the question, water is not split, resulting in low oxygen production. This concept helps clarify why O2 levels remain low despite active ATP synthesis.

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Related Practice

Textbook Question

Predict how the following conditions would affect the production of O₂, ATP, and NADPH and state whether noncyclic or cyclic electron flow would occur in each:

(3) blue and red photons hit a chloroplast, but a proton channel has been introduced into the thylakoid membrane, so it is fully permeable to protons.

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Textbook Question

Predict how the following conditions would affect the production of O₂, ATP, and NADPH and state whether noncyclic or cyclic electron flow would occur in each:

(2) blue and red photons hit a chloroplast, but no NADP⁺ is available

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views

Textbook Question

Predict how the following conditions would affect the production of O2, ATP, and NADPH and state whether noncyclic or cyclic electron flow would occur in each: (1) Only blue photons hit a chloroplast

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Textbook Question

Predict how the following conditions would affect the production of O₂, ATP, and NADPH and state whether noncyclic or cyclic electron flow would occur in each:

(1) Only blue photons hit a chloroplast

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Textbook Question

Explain what is meant by saying the light reactions convert solar energy to chemical energy.

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Textbook Question

The following diagram compares the chemiosmotic synthesis of ATP in mitochondria and chloroplasts. Identify the components that are shared by both organelles and indicate which side of the membrane has the higher H+ concentration. Then label on the right the locations within the chloroplast.

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Multiple Choice

Where do the light-dependent reactions of photosynthesis take place?

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Multiple Choice

Which of the following statements correctly describes a reaction associated with Photosystem II?

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