Intro to Photosynthesis (chpt 10)

1. Intro to Photosynthesis (chpt 10) • A. Breakdown orgs into 2 categories based on how they get their nutrients • 1. autotrophs – can make their own food aka producers • a. photoautotroph – uses light as a source of E to make food • b. chemoautotroph – uses chemicals like sulfur or ammonia to make food (usually bacteria)

2. 2. heterotrophs – can’t make their own food aka consumers

3. II. Where does photosyn take place? chloroplasts! • A. All green parts of a plant contain chloroplasts which contain chlorophyll – leaves contain chloroplasts in the mesophyll

4. B. inside photosynthetic plant cell • 1. stomata – “pores” on underside of leaf that allow CO2 in & let O2 (& water) out • 2. chlorophyll – green pigment able to absorb light E

5. 3. thylakoid – membranous flattened sacks in chloroplast – stacks of thylakoids are called grana

6. III. pathways of photosyn(in C3 plants – most common) • A. Equation 6CO2 + 6H2O >>> C6H12O6 + 6O2 B. looks like backwards cell resp.

7. C. overview of photosynthesis • 1. 2 parts • a. light reaction • 1) purpose – to make NADPH (another e- carrier) & ATP • 2) occurs in the thylakoid membranes of the chloroplast

8. b. dark rxn aka Calvin cycle • 1) purpose – to change CO2 into glucose (or other carbs) • 2) occurs in the stroma of the chloroplast

10. IV. quick review: light properties • A. light acts as both a particle & a wave – we’re interested in the wave aspect • 1. electromagnetic spectrum

11. 2. visible light – ROY G BIV

12. 3. “pieces” of light = photons • a. Photons have a fixed quantity of E. The amt of E is related to the wavelength of light. • 1. short wavelength has high E • 2. long wavelength has low E

13. V. Light rxn of photosynthesis • A. excitation of chlorophyll • 1. Photons are absorbed by atoms & e- jumps to a higher energy level farther away from the nucleus. The e- quickly loses this E in the form of light/color & falls back down to its original energy level

14. 2. photosystems – complex made of chlorophyll A, proteins & other mlcls • a. photosystem I (aka P700) – 1st discovered, can absorb light in the 700nm range ---- if it absorbs light in that range that means we don’t see that color!

15. b. photosystem II (aka p680) – 2nd discovered, absorbs light in the 680nm range • picture p 185?

16. note – there are other chlorophylls & pigments, but chlorophyll A is the only one that participates DIRECTLY in photosyn. – the others can funnel light E into chlorophyll A

17. B. noncyclic electron flow – predominant route of e- flow • 1. steps of noncyclic flow • a. Light hits P II (P680) & an e- is excited & captured by the primary e- acceptor. A “hole” now exists in the P680 system • b. water is split & an e- from it is used to replace the e- lost from P680 ---- O2 is released!!

18. c. the excited e- from P680 is passed down to the PI (P700) system along an ETC– like the one in cell resp. • d. as the e- “falls” down the ETC, ATP is produced for use in the Calvin Cycle • e. the e- fills a “hole” in the P700 (created when light hit the P700 system, excited an e-, which was then captured by the primary e- acceptor)

20. f. the e- from the P700 is then passed along a second ETC where NADP+ is reduced to NADPH --- the NADPH will be used in the Calvin cycle (aka dark rxn)

21. 3. cyclic knows to turn on when there is an abundance of NADPH in the chloroplast • Noncyclic electron flow produces ATP and NADPH in roughly equal quantities. P 187 bottom (Ferrodoxin) • However, the Calvin cycle consumes more ATP (9) than NADPH (6). • Cyclic electron flow allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle.

23. VI. The Calvin Cycle (aka dark rxn) • A. general info • 1. ATP & NADPH from the light rxn are used in the Calvin cycle • 2. similar to Kreb’scyc in cell resp.. because there is a regeneration of mlcls • 3. overall rxn of the Calvin cycle (see paper)

24. 4. actual output of Calvin cycle is PGAL 3-phosphoglyceraldehyde • 5. the cycle must occur TWICE to produce 1 molecule of sugar

25. B. Calvin cycle in 3 parts

26. 1. carbon fixation • a. CO2 bonds to RuBP via the enzyme rubiscoto form a 6-carbon intermediate • b. the 6-C intermediate is unstable & promptly breaks down into 2 3-C mlcls (for each CO2) this 3-C mlcl is PGA (3-phosphoglycerate)

27. carbon fixation

28. 2. reduction • a. ATPs used to add phosphate groups to the carbon chain • b. NADPHs are used to reduce the carbon chain to G3P • c. CO2 (3 of them) used to produce 6 mlcls of G3P • d. only 1 G3P can be used – the other 5 are reused in the cycle

29. reduction

30. 3. regeneration • a. carbon skeletons are rearranged to form 3 mlcls of RuBP • b. uses 3 more ATPs

32. VII. Alternatives to carbon fixation • A. C3 plants carry out photorespiration – • occurs on hot dry days • occurs in light • consumes O2

33. 1. CO2 enters the leaf through stomata • 2. stomata also function in transpiration (water loss through evap) so if it’s hot & dry the stomata stay closed to preserve water & thus have a problem taking in CO2 • 3. HOT climate??? can carry out photorespiration

34. 4. photorespiration consumes O2 • 5. it releases CO2 • 6. it makes no ATP • 7. since it uses up CO2 there is not photosynthetic output – everything decreases (but the plant conserves water)

35. B. C4 plants us alternative mode of C fixation so their rate of photosyn doesn’t decrease! • 1. They use an alternate 4-C cmpd that doesn’t break down as easily as RuBP • 2. the enzyme PEP (phosphoenolpyruvate) is used instead of rubisco. PEP has a higher affinity for CO2 & is able to capture CO2 even at really low amts.

36. 3. example: corn, pineapple, & sugar cane

37. C. CAM plants – succulents/cacti • 1. stomata open at night to take in CO2 • 2. Calvin cycle done at night when it’s cooler.