Photosynthesis

1. Photosynthesis Capturing sunlight to produce organic compounds

2. Overview • Overall equation: • CO2 + H2O  C6H12O6 + O2 • 2 Main Stages: • I. Light Reactions: occurs in the membrane and interior of thylakoids. • II. Calvin Cycle (light-independent reactions): occurs in stroma • As in respiration, the products of earlier reactions and pathways are used in later reactions and pathways.

3. Light Reactions • “Big idea”: • energy from sunlight is used to excite e- of an electron transport chain (ETC). • ETC is used to generate ATP and NADPH. • These molecules “carry” energy to the next stage.

4. The second stage: Carbon fixation and the Calvin Cycle • “Big idea”: • energy from ATP and NADPH is used to build a simple organic compound that is a precursor of larger compounds, such as carbs, fats, proteins. • Carbon for organic compounds is acquired through carbon fixation (CO2 from atmosphere).

5. Leaf Cross-Section

6. Chloroplasts Leaf Cross-Section Chloroplasts

7. Photosynthetic Cell (3-D)

8. Inside a chloroplast Thylakoids: membrane-bound areas in the shape of flattened discs. Chloroplast Stroma: Fluid that surrounds the thylakoids.

9. ADP Stage 1 Stage 2 ATP CO2 H2O O2 NADP+ sunlight Thylakoid NADPH Carbohydrates

10. Light Absorption: How It Works • Requires the use of pigments (chlorophylls a and b and carotenoids) • Pigments are clustered together in the thylakoid membrane in groups of a few hundred • A cluster of pigments = “photosystem” • Pigments absorb LIGHT energy and convert it to CHEMICAL energy

11. Light Absorption: How It Works • Chlorophylls absorb violet, blue, and red but reflect green

12. Light Absorption: How It Works • Carotenoids absorb blue and some green but reflect yellow, orange, and brown Between the cholorphylls and the carotenoids, the majority of the visible spectrum is absorbed by the plant

13. Light Reactions: a closer look • Water is split to provide e- to ETC. H+ ions and oxygen are produced.

14. Light is absorbed by pigments in P.S. II. Each pigment absorbs a specific range of wavelengths. When light is absorbed by chlorophyll a, electrons become “excited” (gain energy). • The “excited” e- move to a primary e- acceptor and are then passed down the ETC.

15. Light is absorbed by pigments in P.S. II. Each pigment absorbs a specific range of wavelengths. When light is absorbed by chlorophyll a, electrons become “excited” (gain energy). • The “excited” e- move to a primary e- acceptor and are then passed down the ETC.

17. Light reactions (cont’d) • As e- move down the ETC they lose energy. The energy of the e- is used to pump H+ into the thylakoid.

18. When e- enter P.S. I, light is absorbed and e- become “excited” again. • Electrons move down another ETC and are taken by NADP+ to make NADPH. The e- in NADPH are still at a relatively high energy level. • i.e.: the energy of sunlight is now stored as chemical PE in an organic molecule.

19. The final step • Chemiosmosis: movement of H+ (protons) through ATP synthase transfers energy which is used to produce ATP. • The high concentration of H+ was built by e- moving down ETC = potential energy. • The energy of the e- moving down the ETC came from the sun. • The energy of sunlight is now stored as chemical PE in an organic molecule.

20. Chemiosmosis • The high concentration of H+ was built by e- moving down ETC = potential energy. • The energy of the e- moving down the ETC came from the sun. • The energy of sunlight is now stored as chemical PE in an organic molecule.

21. Overview of Photosynthesis Stroma ADP Stage 1 Stage 2 NADP+ sunlight Thylakoid (stack = granum)

22. Photosynthesis The “Dark” Reactions aka: Light-independent reactions

23. The Photosynthesis Equation • CO2 + H2O → C6H12O6 + O2 • Which reactant was used in the light reactions (stages 1 and 2)? • H2O (split to provide electrons for photosystem II) • Which product was produced in the light reactions? • O2 (a byproduct of splitting water molecules) • CO2 and C6H12O6 were not involved in the light reactions- must be involved the third stage

24. Stage 2: Calvin Cycle • During this stage, carbon is “fixed”. • What does “carbon fixation” refer to? • Changing inorganic carbon (like CO2) to organic carbon (molecules with C bonded to other C) • Summary of the Calvin Cycle: • CO2 molecules combine with an organic compound. Energy from NADPH and ATP is used to make PGAL. PGAL is then used to build organic compounds (like glucose).

25. Stage 2: The Details • A CO2 molecule combines with RuBP (5-C) to create a 6-C molecule. • 6-C molecule immediately splits into 2 3-C molecules (PGA) • Energy from ATP and NADPH is used to change PGA into PGAL • Some PGAL is used to regenerate RuBP to keep Calvin cycle going; the rest is used to make organic compounds (like glucose)

26. Stage 2: The Details 6-C compound splits 2 PGA 2 PGAL

27. Stage 2: The Starting Materials • Carbon fixation requires ATP, NADPH, RuBP, and CO2 • ATP and NADPH: made during the light reactions • RuBP: regenerated at the end of each cycle • CO2: Some is created by the plant during cell respiration and the rest is taken in from the atmosphere through openings in the plant leaves called stomata.

28. Leaf Cross-Section

29. Guard Cells and Stomata • CO2 enters plant leaves through openings called stomata. • Guard cells on either side of the stomata empty or fill with water to open and close the stomata.

30. Guard Cells and Stomata

31. The Problem with Stomata • When stomata open to allow CO2 in, it also allows H2O to escape. • So taking in CO2 comes at the expense of losing water. • Not an issue for C3 plants (most plants are this type) because they exist in temperate climates where water loss isn’t such a problem • Major problem for plants that exist in climates that are hot and/or dry • The solution: alternative carbon fixation

32. Alternative Carbon Fixation • The C4 Pathway • C4 plants partially close their stomata during the hottest part of the day (reduces the water loss, but also the amount of CO2 coming in) • Contain enzymes that “fix” CO2 into 4-C compounds when CO2 level is low • Breakdown 4-C compounds later on to release CO2 (which can then be used in Calvin cycle) • C4 plants include corn, sugar cane, and crabgrass

33. Alternative Carbon Fixation • The CAM Pathway • Plants living in the hottest and driest climates • Open stomata at night and close during day (opposite of other plants) • Take in CO2 at night and “fix” into various organic compounds • CO2 released from these compounds during the day and used in Calvin cycle • CAM plants include cacti and pineapple