9.2 Section Objectives – page 225

1. 9.2 Section Objectives – page 225 Section Objectives: • 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.

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.

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

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

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.

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 electrontransport chain, a series of proteins embedded in the thylakoid membrane.

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 to ADP bonds P forming ATP oxygen released H+ NADP+ NADPH for the use in light-independent reactions

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.

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.

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 O2 + 2H+ Chlorophyll 2 2e- _ 1 O2+ 2e- H2O ®2H++ H2O 2

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.

12. (CO2) Section 9.2 Summary – pages 225-230 (CO2) The Calvin Cycle (Unstable intermediate) (RuPB) ADP + ATP ATP ADP + NADPH NADP+ (PGAL) (PGAL) (PGAL) (Sugars and other carbohydrates)

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)

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)

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)

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)

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. P ADP+ ATP (PGAL)

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

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

20. Section 2 Check 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

21. Section 2 Check 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 to ADP bonds P forming ATP oxygen released H+ NADP+ NADPH for the use in light-independent reactions

22. Section 2 Check 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

23. Section 2 Check 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.

24. Section 2 Check 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

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