1. Where it starts:? Photosynthesis Before photosynthesis evolved, –Earth’s atmosphere had little free oxygen Oxygen released during photosynthesis changed the atmosphere –Favored evolution of new metabolic pathways, including aerobic respiration Then Now

2. Sunlight as an energy source Visible light –A small part of a spectrum of electromagnetic energy radiating from the sun Electromagnetic energy –Travels in waves –Is organized as photons

3. Photosynthetic Pigments Photosynthesis begins when photons are absorbed by photosynthetic pigment molecules Pigment molecules absorb only light of particular wavelengths –Photons not captured are reflected as color

4. Major Photosynthetic Pigments Chlorophyll a –Main photosynthetic pigment –Absorbs violet and red light (appears green) Chlorophyll b, carotenoids, phycobilins –Absorb additional wavelengths Collectively, photosynthetic pigments absorb almost all of wavelengths of visible light

5. Overview of photosynthesis Photosynthesis proceeds in two stages –Light-dependent reactions –Light-independent reactions 6H 2 O + 6CO 2 6O 2 + C 6 H 12 O 6

6. Sites of photosynthesis: chloroplasts Light-dependent reactions occur at a much- folded thylakoid membrane –a single, continuous compartment inside the stroma (chloroplast’s semifluid interior) Light-independent reactions occur in the stroma

7. Products of light-dependent reactions Typically, sunlight energy pass to photosystems (I, II) pull electrons from water –release them –drives formation of ATP and NADPH Oxygen is released from the chloroplast (and the cell)

8. Light independent reactions: the sugar factory Light-independent reactions proceed in the stroma Carbon fixation: Enzyme rubisco attaches carbon from CO 2 to RuBP to start the Calvin– Benson cycle

9. Calvin–Benson Cycle Cyclic pathway makes glucose –uses energy from ATP, hydrogen and electrons from NADPH –carbon and oxygen from CO 2, Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose)

10. Plant adaptations to environment Environments differ –Plants have different details of sugar production in light- independent reactions On dry days, plants conserve water by closing their stomata –O 2 from from photosynthesis cannot escape Photorespiration reduces efficiency of sugar production

11. Plant adaptations to environment C4 plants –Carbon fixation occurs twice –First reactions release CO 2 near rubisco, limit photorespiration when stomata are closed

12. Plant adaptations to environment CAM plants –Open stomata and fix carbon at night

13. A burning concern Photoautotrophs remove CO 2 from atmosphere; metabolic activity of organisms puts it back Human activities disrupt the carbon cycle –Add more CO 2 to the atmosphere than photoautotrophs can remove Imbalance contributes to global warming