1. 9.2 Section Objectives – page 225
Relate the structure of chloroplasts to the events in photosynthesis.
Describe light-dependent reactions.
Explain the reactions and products of the light-independent Calvin cycle.
9.2 Section Objectives – page 225

2. Section 9.2 Summary – pages 225-230
Trapping Energy from Sunlight
The process that uses the sun’s energy to make simple sugars is called photosynthesis.
Section 9.2 Summary – pages

3. Section 9.2 Summary – pages 225-230
Trapping Energy from Sunlight
Photosynthesis happens in two phases.
The light-dependent reactions convert light energy into chemical energy.
2. The molecules of ATP produced in the light-dependent reactions are then used to fuel the light-independent reactions that produce simple sugars.
The general equation for photosynthesis is written as 6CO2 + 6H2O→C6H12O6 + 6O2
Section 9.2 Summary – pages

4. Section 9.2 Summary – pages 225-230
Trapping Energy from Sunlight
Click image to view movie.
Section 9.2 Summary – pages

5. Section 9.2 Summary – pages 225-230
The chloroplast and pigments
To trap the energy in the sun’s light, the thylakoid membranes contain pigments, molecules that absorb specific wavelengths of sunlight.
Although a photosystem contains several kinds of pigments, the most common is chlorophyll.
Chlorophyll absorbs most wavelengths of light except green.
Section 9.2 Summary – pages

6. Section 9.2 Summary – pages 225-230
Light-Dependent Reactions
As sunlight strikes the chlorophyll molecules in a photosystem of the thylakoid membrane, the energy in the light is transferred to electrons.
These highly energized, or excited, electrons are passed from chlorophyll to an electron transport chain, a series of proteins embedded in the thylakoid membrane.
Section 9.2 Summary – pages

7. Section 9.2 Summary – pages 225-230
Sun
Light-Dependent Reactions
Light energy transfers to chlorophyll.
At each step along the transport chain, the electrons lose energy.
Chlorophyll passes energy down through the electron transport chain.
Energized electrons provide energy that
splits
H2O
bonds P
to ADP
forming ATP H+
oxygen
released
NADP+
NADPH
for the use in light-independent reactions
Section 9.2 Summary – pages

8. Section 9.2 Summary – pages 225-230
Light-Dependent Reactions
This “lost” energy can be used to form ATP from ADP, or to pump hydrogen ions into the center of the thylakoid disc.
Electrons are re-energized in a second photosystem and passed down a second electron transport chain.
Section 9.2 Summary – pages

9. Section 9.2 Summary – pages 225-230
Light-Dependent Reactions
The electrons are transferred to the stroma of the chloroplast. To do this, an electron carrier molecule called NADP is used.
NADP can combine with two excited electrons and a hydrogen ion (H+) to become NADPH.
NADPH will play an important role in the light-independent reactions.
Section 9.2 Summary – pages

10. Section 9.2 Summary – pages 225-230
Restoring electrons
To replace the lost electrons, molecules of water are split in the first photosystem. This reaction is called photolysis.
Sun _ 1
Chlorophyll
O2 + 2H+ 2 _
2e- 1
H2O ®2H+ +
O2 + 2e-
H2O 2
Section 9.2 Summary – pages

11. Section 9.2 Summary – pages 225-230
Restoring electrons
The oxygen produced by photolysis is released into the air and supplies the oxygen we breathe.
The electrons are returned to chlorophyll.
The hydrogen ions are pumped into the thylakoid, where they accumulate in high concentration.
Section 9.2 Summary – pages

12. Section 9.2 Summary – pages 225-230
(CO2)
The Calvin Cycle
Section 9.2 Summary – pages

13. Section 9.2 Summary – pages 225-230
The Calvin Cycle
Carbon fixation The carbon atom from CO2 bonds with a five-carbon sugar called ribulose biphosphate (RuBP) to form an unstable six-carbon sugar.
(CO2)
The stroma in chloroplasts hosts the Calvin cycle.
(RuBP)
Section 9.2 Summary – pages

14. Section 9.2 Summary – pages 225-230
The Calvin Cycle
Formation of 3-carbon molecules The six-carbon sugar formed in Step A immediately splits to form two three-carbon molecules.
(Unstable intermediate)
Section 9.2 Summary – pages

15. Section 9.2 Summary – pages 225-230
The Calvin Cycle
Use of ATP and NADPH A series of reactions involving ATP and NADPH from the light-dependent reactions converts the three-carbon molecules into phosphoglyceraldehyde (PGAL), three-carbon sugars with higher energy bonds.
ATP
ADP +
NADPH
NADP+
(PGAL)
Section 9.2 Summary – pages

16. Section 9.2 Summary – pages 225-230
The Calvin Cycle
Sugar production One out of every six molecules of PGAL is transferred to the cytoplasm and used in the synthesis of sugars and other carbohydrates. After three rounds of the cycle, six molecules of PGAL are produced.
(PGAL)
(Sugars and other carbohydrates)
Section 9.2 Summary – pages

17. Section 9.2 Summary – pages 225-230
The Calvin Cycle
RuBP is replenished Five molecules of PGAL, each with three carbon atoms, produce three molecules of the five-carbon RuBP. This replenishes the RuBP that was used up, and the cycle can continue.
ADP+ P
ATP
(PGAL)
Section 9.2 Summary – pages

18. Question 1
The process that uses the sun’s energy to make simple sugars is ________.
A. cellular respiration
B. glycolysis
C. photosynthesis
D. photolysis
Section 2 Check

19. The answer is C. Photosynthesis happens in two phases to make simple sugars and convert the sugars into complex carbohydrates for energy storage.
Section 2 Check

20. Question 2
The function accomplished by the light-dependent reactions is ________.
A. energy storage
B. sugar production
C. carbon fixation
D. conversion of sugar to PGAL
Section 2 Check

21. Sun
The answer is A. The light-dependent reactions transfer energy from the sun to chlorophyll, and pass energized electrons to proteins embedded in the thylakoid membrane for storage in ATP and NADPH molecules.
Light energy transfers to chlorophyll.
Chlorophyll passes energy down through the electron transport chain.
Energized electrons provide energy that
splits
H2O
bonds P
to ADP
forming ATP H+
oxygen
released
NADP+
NADPH
for the use in light-independent reactions
Section 2 Check

22. Question 3 The first step in the Calvin cycle is the ________.
A. replenishing of ribulose biphosphate
B. production of phosphoglyceraldehyde
C. Splitting of six-carbon sugar into two three-carbon molecules
D. Bonding of carbon to ribulose biphosphate
Section 2 Check

23. The answer is D. The carbon atom from CO2 bonds with a five-carbon sugar to form an unstable six-carbon sugar. This molecule then splits to form two three-carbon molecules.
Section 2 Check

24. Question 4
How many rounds of the Calvin cycle must occur in order for one molecule of PGAL to be transferred to the cell’s cytoplasm?
A. 1
B. 2
C. 3
D. 4
Section 2 Check

25. The answer is C. Each round of the Calvin cycle produces two molecules of PGAL.
Section 2 Check