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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Multiple Choice

What provides the energy for the synthesis of ATP in the light reactions of photosynthesis?

The proton gradient across the thylakoid membrane

Direct absorption of sunlight by ATP synthase

The breakdown of glucose

The reduction of NADP$^+$ to NADPH

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Verified step by step guidance

Understand the context: Photosynthesis occurs in two main stages—light reactions and the Calvin cycle. The light reactions take place in the thylakoid membranes of chloroplasts and involve the conversion of light energy into chemical energy.

Recognize the role of ATP synthesis: ATP is synthesized during the light reactions by a process called photophosphorylation, which is driven by the movement of protons (H$^+$ ions) across the thylakoid membrane.

Identify the source of energy: The energy for ATP synthesis comes from the proton gradient created by the electron transport chain. As electrons move through the chain, protons are pumped into the thylakoid lumen, creating a high concentration of protons inside the lumen compared to the stroma.

Explain the mechanism: The proton gradient drives ATP synthesis through chemiosmosis. Protons flow back into the stroma through ATP synthase, a protein complex embedded in the thylakoid membrane. This flow provides the energy needed to convert ADP and inorganic phosphate (Pi) into ATP.

Clarify incorrect options: Direct absorption of sunlight by ATP synthase does not occur; ATP synthase uses the proton gradient. The breakdown of glucose is part of cellular respiration, not photosynthesis. The reduction of NADP$^+$ to NADPH is another process in the light reactions but does not directly provide energy for ATP synthesis.