Chapter 7

2. Chapter 7 Photosynthesis: Using Light to Make Food

3. Sustain Life • Energy classification • Autotrophs—self nourishing • Obtain carbon from CO2 • Obtain energy from light (photosynthesis) or chemical reactions (chemosynthesis) • Heterotrophs—use others for energy source • Obtain carbon from autotrophs • Obtain energy from autotrophs • Even if ingest other heterotrophs, at some point the original carbon & energy came from an autotroph • Carbon & Energy • Enter life through photosynthesis (autotrophs) • Released through glycolysis & cellular respiration (heterotrophs)

4. Photosynthesis • Chlorophyll • Plants • Algae • Some bacteria • Transfer sun’s energy into chemical bonds • Converts energy of photons to energy stored in ATP • Oxygen production is a byproduct

5. Photosynthesis • Three stages • Light-capturing • Light-dependent • Convert light energy into chemical energy • Light-independent • Form organic compounds (glucose) • CO2 + H2O => C6H12O6 (glucose) + O2 • Remember that this is the opposite direction but the same basic reaction as cellular respiration.

6. Properties of light • Wavelength • Spectrum

7. Properties of light • Photons • Packets of particle-like light • Fixed energy (each photon a specific energy wavelength) • Think of them as bundles of energy, like an electrified rubber ball • Energy level • Low energy = long wavelength • Microwaves, radio waves • High energy = short wavelength • Gamma rays, x-rays • Only a small part of spectrum (400-750 nm) is used for vision & photosynthesis

8. Properties of light • The light that you see is REFLECTED, not absorbed. • Therefore, a green plant is reflecting the green part of the spectrum (and photons of that energy), not absorbing them; it absorbs all parts of the spectrum except green.

9. pigments • Molecules that absorb photons of only a particular wavelength • Chlorophyll a • Absorbs red, blue, violet light • Reflects green, yellow light • Major pigment in almost all photoautotrophs • Chlorophyll b • Absorbs red-orange, some blue • Reflects green, some blue

10. Pigments • Carotenoids • Absorb blue-violet, blue-green light • Reflect red, orange, yellow light • Give color to many flowers, fruits, vegetables • Color leaves in Autumn

11. Pigments • Anthocyanins • Absorb green, yellow, some orange light • Reflect red, purple light • Cherries, many flowers • Color leaves in Autumn • Phycobilins • Absorb green, yellow, orange light • Reflect red, blue-green light • Some algae & bacteria

12. Electron energy • Pigment absorbs light of specific wavelentgh • Corresponds to energy of photon • Electron absorbs energy from photon • Energy boosts electron to higher level • Electron then returns to original level • When it returns, emits some energy (heat or photon)

13. overview • Stage 1 (Light-Dependent) • Light energy converted to bond energy of ATP • Water molecules split, helping to form NADPH • Oxygen atoms escape • Stage 2 (Light-Independent) • ATP energy used to synthesize glucose & other carbohydrates

15. chloroplasts

19. Light-Dependent reactions • Occurs in thylakoids • Electrons transfer light energy in electron transport chain in photosystems

20. Light-Dependent reactions • Photosystems—Clusters of chlorophyll, pigments, proteins • Light-gathering “antennae” • Photosystem I (P680)—absorbs red light at 680nm • Photosystem II (P700)—absorbs far-red light at 700nm

21. Light-Dependent reactions • Electrons transfer from photosystems • Electron transfers pump H+ into inner thylakoid compartment • Repeats, building up concentration and electric gradients • Chemiosmosis!

22. Light-Dependent reactions • H+ can only pass through channels inside ATP Synthase • Ion flow through channel makes protein turn, forcing Phosphate onto ADP • Phosphorylation!

23. Light-Dependent reactions • Electrons continue until bonding NADP+ to form NADPH • NADPH used in next part of cycle • Process is very similar to cellular respiration!!!! • Oxidative phosphorylation

24. Light-dependent reaction

25. Light-dependent reaction

26. Light-dependent reaction

27. Light-independent reactions

28. Light-independent reactions • ATP provides energy for bond formation • NADPH provides hydrogen & electrons • CO2 provides carbon & oxygen

29. Light-independent reactions • CO2 in air diffuses into stroma • CO2 attaches to rubisco (RuBP) • Enters Calvin cycle (also called Calvin-Benson) • RuBP splits to form PGA • PGA gets phosphate from ATP, then H+ and electrons from NADPH • Forms PGAL • Two PGAL combine to form glucose plus phosphate group

30. Light-independent reactions • Some PGAL recycles to form more RuBP • Takes 6 “turns” of cycle to form one glucose molecule • 6 CO2 must be fixed and 12 PGAL must form to produce one glucose molecule and keep the cycle running

31. *(G3P = PGAL)

32. Photosynthesis summary

34. stomata • Microscopic openings in leaves • Close when hot & dry • Keeps water inside • Prevents CO2 & O2 exchange

35. C3 Plants • Basswood, beans, peas, evergreens • 3-Carbon PGA is first stable intermediate in Calvincycle • Stomata close, O2 builds up • Increased O2 levels compete w/ CO2 in cycle • Rubisco attaches oxygen, NOT carbon to RuBP • This yields 1 PGA rather than 2 • Lowers sugar production & growth of plant • 12 “turns” rather than 6 to make sugars • Better adapted to cold & wet

36. C4 Plants • Corn, sugar cane, tropical plants • Adapted to hot, dry climates • Close stomata to conserve water • This limits CO2 entry and allows O2 to accumulate • This allows CO2 to remain high for Calvin cycle • Carbon stored in special cells, can be donated to Calvin cycle later • Requires 1 more ATP than C3, but less water lost & more sugar produced

37. C4 Plants

38. CAM Plants • Desert plants (cactus) • Crassulcean Acid Metabolism (CAM) • Opens stomata at night, uses C4 cycle • Cells store malate & organic acids • During day when stomata close, malate releases CO2 for Calvin cycle