1. Chapter 6 Table of Contents Section 1 The Light Reactions
Photosynthesis
Table of Contents
Section 1 The Light Reactions
Section 2 The Calvin Cycle

2. Chapter 6
Section 1 The Light Reactions
Objectives
Explain why almost all organisms depend on photosynthesis.
Describe the role of chlorophylls and other pigments in photosynthesis.
Summarize the main events of the light reactions.
Explain how ATP is made during the light reactions.

3. Chapter 6 A. Obtaining Energy
Section 1: The Light Reactions
A. Obtaining Energy
1. Photosynthesis converts light energy from the sun into chemical energy in the form of organic compounds through a series of reactions known as biochemical pathways.
2. Autotrophs (self feeder) use energy from sunlight or from chemical bonds in inorganic substances to make organic compounds.
a. examples: plants, algae and green bacteria
3. Animals and other organisms that must get energy from food instead of directly from sunlight or inorganic substances are called heterotrophs (other feeder).
b. examples: bugs, mice, whales, humans

4. B. Overview of Photosynthesis
Chapter 6
Section 1 The Light Reactions
B. Overview of Photosynthesis
1. Photosynthesis converts light energy from the sun into chemical energy in the form of organic compounds through a series of reactions known as biochemical pathways
6CO2 + 6H2O light energy C6H12O6 + 6O2
2. The oxygen (O2) and some of the organic compounds produced by photosynthesis are used by cells in a process called cellular respiration.
C6H12O6 + 6O  6CO2 + 6H2O + ATP

5. Chapter 6
Section 1 The Light Reactions

6. B. Overview of Photosynthesis, continued
Chapter 6
Section 1 The Light Reactions
B. Overview of Photosynthesis, continued
3. Photosynthesis can be divided into two stages:
In the light reactions, light energy is converted to chemical energy.
1. Uses sun, water, and chlorophyll
2. Produces stored in ATP and the energy carrier molecule NADPH
Occurs in the thylakoid mmb. of chloroplast
b. In the Calvin Cycle

7. B. Overview of Photosynthesis, continued
Chapter 6
Section 1 The Light Reactions
B. Overview of Photosynthesis, continued
3. Photosynthesis can be divided into two stages:
In the light reactions, light energy is converted to chemical energy.
b. In the Calvin Cycle, organic compounds are formed.
1. Using CO2, ATP, and NADPH.
2. Produces glucose and other organic molecules
3. Occurs in the stroma of the chloroplast

9. B. Overview of Photosynthesis, continued
Chapter 6
Section 1 The Light Reactions
B. Overview of Photosynthesis, continued
4. Equation of Photosynthesis:
SUN

10. C. Capturing Light Energy
Chapter 6
Section 1 The Light Reactions
C. Capturing Light Energy
The light reactions begin with the absorption of light in chloroplasts, organelles found in the cells of plants, and algae.
a. thylakoid
b. stroma

11. Chapter 6 Parts of a Chloroplast Section 1 The Light Reactions
Click below to watch the Visual Concept.
Visual Concept

12. C. Capturing Light Energy, continued
Chapter 6
Section 1 The Light Reactions
C. Capturing Light Energy, continued
2. Light and Pigments
a. White light from the sun is composed of an array of colors called the visible spectrum.
b. Pigments absorb certain colors of light and reflect or transmit the other colors.

13. C. Capturing Light Energy, continued
Chapter 6
Section 1 The Light Reactions
C. Capturing Light Energy, continued
3. Chloroplast Pigments
a. Located in the membrane of the thylakoids of chloroplasts are several pigments,
1) chlorophylls (such as chlorophyll a and chlorophyll b)
2) carotenoids.

14. D. Spectrum of Light and Plant Pigments
Chapter 6
Section 1 The Light Reactions
D. Spectrum of Light and Plant Pigments
Visual Concept
Light Waves

15. E. Converting Light Energy To Chemical Energy
Chapter 6
Section 1 The Light Reactions
E. Converting Light Energy To Chemical Energy
The pigments are grouped in clusters of a few hundred molecules in the thylakoid membrane.
a. Each cluster and the proteins that the pigment molecules are embedded in are referred to collectively as a photosystem.
2. By absorbing light, pigment molecules in photosystem I and photosystem II acquire some of the energy carried by the light.

16. E. Converting Light Energy To Chemical Energy,
Chapter 6
Section 1 The Light Reactions
E. Converting Light Energy To Chemical Energy,
3. In each photosystem, the acquired energy is passed quickly to other pigment molecules until it reaches a specific pair of chlorophyll a molecules.
4. The acquired energy forces electrons to enter a higher energy level in the two chlorophyll a molecules of photosystem II. These energized electrons are said to be “excited.” The excited electrons have enough energy to leave the chlorophyll a molecules.

17. E. Converting Light Energy To Chemical Energy
Chapter 6
Section 1 The Light Reactions
E. Converting Light Energy To Chemical Energy
5. The acceptor of these electrons from photosystem II is a molecule called the primary electron acceptor, which donates the electrons to the electron transport chain.
6. As the electrons move from molecule to molecule in this chain, they lose most of the acquired energy. The energy they lose is used to move protons into the thylakoid.

18. Chapter 6
Section 1 The Light Reactions

19. E. Converting Light Energy To Chemical Energy,
Chapter 6
Section 1 The Light Reactions
E. Converting Light Energy To Chemical Energy,
7. Light is absorbed by photosystem I at the same time it is absorbed by photosystem II. Electrons move from chlorophyll a molecules to another primary electron acceptor.
8. The electrons lost from photosystem I are replaced by electrons that have passed through the electron transport chain from photosystem II.
9. These electrons are then donated to another electron transport chain, which brings the electrons to the side of the thylakoid membrane that faces the stroma.
10. In the stroma, the electrons combine with a proton and NADP+. This causes NADP+ to be reduced to NADPH.

20. E. Converting Light Energy To Chemical Energy
Chapter 6
Section 1 The Light Reactions
E. Converting Light Energy To Chemical Energy
11. Replacing Electrons in Light Reactions
Electrons from photosystem II replace electrons that leave photosystem I. Replacement electrons for photosystem II are provided by the splitting of water molecules.
b. Oxygen produced when water molecules are split diffuses out of the chloroplast and then leaves the plant.

21. E. Converting Light Energy To Chemical Energy,
Chapter 6
Section 1 The Light Reactions
E. Converting Light Energy To Chemical Energy,
12. Making ATP in Light Reactions
An important part of the light reactions is the synthesis of ATP.
During chemiosmosis, the movement of protons through ATP synthase into the stroma releases energy, which is used to produce ATP.

22. F. Summary of Processes in Light Reactions
Chapter 6
Section 1 The Light Reactions
F. Summary of Processes in Light Reactions
Summary
Another Summary

24. Chapter 6 Table of Contents Section 1 The Light Reactions
Photosynthesis
Table of Contents
Section 1 The Light Reactions
Uses Water, Sunlight, Chlorophyll
Produces oxygen gas, ATP and NADPH
Occurs at Thylakoid and into Stroma
Section 2 The Calvin Cycle

25. Chapter 6 Objectives Summarize the main events of the Calvin cycle.
Section 2 The Calvin Cycle
Objectives
Summarize the main events of the Calvin cycle.
Describe what happens to the compounds that are made in the Calvin cycle.
Distinguish between C3, C4, and CAM plants.
Summarize how the light reactions and the Calvin cycle work together to create the continuous cycle of photosynthesis.
Explain how environmental factors influence photosynthesis.

26. Chapter 6 A. Carbon Fixation
Section 2 The Calvin Cycle
A. Carbon Fixation
1. The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, the Calvin cycle.
2. In the Calvin cycle, CO2 is incorporated into organic compounds, a process called carbon fixation.
3. The Calvin cycle, which occurs in the stroma of the chloroplast, is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar.

27. Chapter 6 A. Carbon Fixation
Section 2 The Calvin Cycle
A. Carbon Fixation
4. Most of the three-carbon sugars (G3P) generated in the Calvin cycle are converted to a five-carbon sugar (RuBP = ribulose bisphosphate) to keep the Calvin cycle operating.
5. But some of the three-carbon sugars leave the Calvin cycle and are used to make organic compounds (like GLUCOSE), in which energy is stored for later use.

28. Chapter 6
Section 2 The Calvin Cycle
The Calvin Cycle
Summary

29. Ongoing Cycle of Photosynthesis
Chapter 6
Section 2 The Calvin Cycle
Ongoing Cycle of Photosynthesis

31. B. Alternative Pathways
Chapter 6
Section 2 The Calvin Cycle
B. Alternative Pathways
1. The C4 Pathway
Some plants that evolved in hot, dry climates fix carbon through the C4 pathway. These plants have their stomata partially closed during the hottest part of the day.
b. Certain cells in these plants have an enzyme that can fix CO2 into four-carbon compounds even when the CO2 level is low and the O2 level is high. These compounds are then transported to other cells, where the Calvin cycle ensues.

32. B. Alternative Pathways
Chapter 6
Section 2 The Calvin Cycle
B. Alternative Pathways
2. The CAM Pathway
a. Some other plants that evolved in hot, dry climates fix carbon through the CAM pathway. These plants carry out carbon fixation at night and the Calvin cycle during the day to minimize water loss.

33. C. Factors That Affect Photosynthesis
Chapter 6
Section 2 The Calvin Cycle
C. Factors That Affect Photosynthesis
1. Light Intensity
The rate of photosynthesis increases as light intensity increases, because more electrons are excited in both photosystems.
b. However, at some point all of the available electrons are excited, and the maximum rate of photosynthesis is reached. The rate then stays level regardless of further increases in light intensity.
2. Carbon Dioxide Levels
a. As with increasing light intensity, increasing levels of carbon dioxide also stimulate photosynthesis until the rate levels off.

34. C. Factors That Affect Photosynthesis
Chapter 6
Section 2 The Calvin Cycle
C. Factors That Affect Photosynthesis
3. Temperature
a. As temperature increases, the rate of photosynthesis increases to a maximum and then decreases with further rises in temperature.
b. The rate peaks at a certain temperature, at which many of the enzymes that catalyze the reactions become ineffective.
c. Also, the stomata begin to close, limiting water loss and entry of carbon dioxide.

35. Environmental Influences on Photosynthesis
Chapter 6
Section 2 The Calvin Cycle
Environmental Influences on Photosynthesis
Click below to watch the Visual Concept.
Visual Concept

36. Chapter 6 Table of Contents Section 1 The Light Reactions
Photosynthesis
Table of Contents
Section 1 The Light Reactions
Uses Water, Sunlight, Chlorophyll
Produces oxygen gas, ATP and NADPH
Occurs at Thylakoid and into Stroma
Section 2 The Calvin Cycle
Uses carbon dioxide, ATP, NADPH and RuBP
Produces glucose, ADP, and NADP+
Occurse in the Stroma

37. Chapter 6
Section 1 The Light Reactions
SUN