Light-independent (dark) reactions occur in the stroma of the chloroplast (pH 8)

2. Light-independent (dark) reactions occur in the stroma of the chloroplast (pH 8) Consumes ATP & NADPH from light reactions regenerates ADP, Pi and NADP+

3. fixing CO2 1) RuBP binds CO2

4. fixing CO2 • RuBP binds CO2 • 2) rapidly splits into two 3-Phosphoglycerate • therefore called C3 photosynthesis

5. Reversing glycolysis • G3P has 2 possible fates • 1) 1 in 6 becomes (CH2O)n • 2) 5 in 6 regenerate RuBP

6. Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) or is converted to DHAP & exported to cytoplasm to make sucrose Pi/triosePO4 antiporter only trades DHAP for Pi

7. Regenerating RuBP Basic problem: converting a 3C to a 5C compound must assemble intermediates that can be broken into 5 C sugars after adding 3C subunit

8. Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • a) pH • b) [Mg2+] • c) CO2 is an allosteric activator of rubisco that only binds at high pH and high [Mg2+] • also: stomates open in the light

9. Regulating the Calvin Cycle • Several Calvin cycle enzymes (e.g.Fructose-1,6-bisphosphatase) are also regulated by thioredoxin • contain disulfide bonds which get oxidized in the dark • in light, ferredoxin reduces thioredoxin, thioredoxin reduces these disulfide bonds to activate the enzyme • How light reactions talk to the Calvin cycle SH SH light oxidized enzyme (inactive) S - S reduced thioredoxin 2Fdox 2e- PSI + PSII S - S SH SH oxidized thioredoxin reduced enzyme (active) 2Fdred

10. PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + phosphoglycolate

11. PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + Phosphoglycolate Releases CO2 without making ATP or NADH

12. PHOTORESPIRATION • Releases CO2 without making ATP or NADH • Called photorespiration : undoes photosynthesis

13. PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + Phosphoglycolate C3 plants can lose 25%-50% of their fixed carbon

14. PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + Phosphoglycolate C3 plants can lose 25%-50% of their fixed carbon Both rxns occur at same active site

15. PHOTORESPIRATION C3 plants can lose 25%-50% of their fixed carbon phosphoglycolate is converted to glycolate : poison!

16. Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes

17. Detoxifying Glycolate • 1) glycolate is shuttled to peroxisomes • 2) peroxisomes convert it to glycine • produce H2O2

18. Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes 2) peroxisomes convert it to glycine 3) glycine is sent to mitochondria

19. Detoxifying Glycolate • 1) glycolate is shuttled to peroxisomes • 2) peroxisomes convert it to glycine • 3) glycine is sent to mitochondria • 4) mitochondria convert 2 glycine to 1 serine + 1 CO2 • Why photorespiration loses CO2

20. Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes 2) peroxisomes convert it to glycine 3) glycine is sent to mitochondria 4) mitochondria convert 2 glycine to 1 serine + 1 CO2 5) serine is returned to peroxisome

21. Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes 2) peroxisomes convert it to glycine 3) glycine is sent to mitochondria 4) mitochondria convert 2 glycine to 1 serine + 1 CO2 5) serine is returned to peroxisome 6) peroxisome converts it to glycerate & returns it to chloroplast

22. Detoxifying Glycolate Why peroxisomes are next to cp and mito in C3 plants Mitochondrion

23. Lipid metabolism Most are glycerolipids: fatty acids bonded to glycerol

24. GLYCEROLIPIDS • Triacylglycerols = FAs on all 3 C • store energy

25. GLYCEROLIPIDS Bond FA to glycerol Diacylglycerols = FAs on 2 Cs, headgroup on C 3

26. GLYCEROLIPIDS • Diacylglycerols = FAs on 2 Cs, headgroup on C 3 • Form bilayers in water

27. Lipid metabolism Unique aspects in plants Make fatty acids by same set of reactions, but in plastids with a prokaryotic fatty acid synthase 12 proteins, cf one multifunctional protein

28. Lipid metabolism Make fatty acids in plastids with a prokaryotic FAS • 12 proteins, instead of one multifunctional protein • Assemble some lipids in CP, others in ER

29. Lipid metabolism Make fatty acids in plastids with a prokaryotic FAS • 12 proteins, instead of one multifunctional protein • Assemble some lipids in CP, others in ER • Acetyl-CoA carboxylase is also prokaryotic = 4 subunits, except in grasses (profoxydim & other grass herbicides inhibit ACCase)

30. Lipid metabolism • Acetyl-CoA carboxylase is also prokaryotic = 4 subunits, except in grasses (profoxydim & other grass herbicides inhibit ACCase) • Same biochem, but diff location and enzymes

31. Lipid metabolism • Acetyl-CoA carboxylase is also prokaryotic = 4 subunits, except in grasses (profoxydim & other grass herbicides inhibit ACCase) • Same biochem, but diff location and enzymes • In light cp make lots of NADPH, and leaves are main sinks for FA

32. Lipid metabolism • Acetyl-CoA carboxylase is also prokaryotic = 4 subunits, except in grasses (profoxydim & other herbicides inhibit ACCase) • Same biochem, but diff location and enzymes • In light cp make lots of NADPH, and leaves are main sinks for FA • But, each cell makes its own FA, so NADPH in other cells comes from Pentose-Pi shunt

33. Lipid metabolism Source of acetyl-CoA is controversial • Most comes from plastid PDH

34. Lipid metabolism Source of acetyl-CoA is controversial • Most comes from plastid PDH • Some comes from cytoplasmic acetate; activated in cp • Also used to make sterols, some amino acids, many others

35. Lipid metabolism Source of acetyl-CoA is controversial • Most comes from plastid PDH • Some comes from cytoplasmic acetate; activated in cp • Also used to make sterols, some amino acids, many others • Why ACCase is “committed step”

36. Lipid metabolism Assemble some lipids in CP, others in ER • “16:3 plants” assemble lipids in cp using FA-ACP = prokaryotic pathway (“primitive”)

37. Lipid metabolism “16:3 plants” assemble lipids in cp using FA-ACP = prokaryotic pathway (“primitive”) “18:3 plants” export FA, assemble lipids in ER using FA-CoA = eukaryotic pathway (“advanced”)

38. Lipid metabolism “16:3 plants” assemble lipids in cp using FA-ACP = prokaryotic pathway (“primitive”) “18:3 plants” export FA, assemble lipids in ER using FA-CoA = eukaryotic pathway (“advanced”) Substrates for most desaturases are lipids, not FA!

39. Lipid metabolism Unique aspects in plants Chloroplasts have lots of galactolipids: sugar linked directly to diacylglycerol

40. Lipid metabolism Unique aspects in plants Chloroplasts have lots of galactolipids: sugar linked directly to diacylglycerol: saves PO4

41. Lipid metabolism Unique aspects in plants • Chloroplasts have lots of galactolipids: sugar linked directly to diacylglycerol : saves PO4 • A) MGDG (Monogalactosyl diacylglycerol) 50% cp • -> most abundant lipid on earth!

42. Lipid metabolism Unique aspects in plants • Chloroplasts have lots of galactolipids: sugar linked directly to diacylglycerol : saves PO4 • A) MGDG (Monogalactosyl diacylglycerol) 50% cp • B) DGDG (Digalactosyl diacylglycerol) 28% cp

43. Lipid metabolism • Chloroplasts have lots of galactolipids: sugar linked directly to diacylglycerol : saves PO4 • A) MGDG (Monogalactosyl diacylglycerol) 50% cp • B) DGDG (Digalactosyl diacylglycerol) 28% cp • C) SQDG( Sulphoquinovosyldiacylglycerol) 16% cp

44. Lipid metabolism • Chloroplasts have lots of galactolipids: sugar linked directly to diacylglycerol : saves PO4 • A) MGDG (Monogalactosyl diacylglycerol) 50% cp • B) DGDG (Digalactosyl diacylglycerol) 28% cp • C) SQDG( Sulphoquinovosyldiacylglycerol) 16% cp • Very unsaturated!

45. Lipid metabolism • Chloroplasts have lots of galactolipids: sugar linked directly to diacylglycerol : saves PO4 • A) MGDG (Monogalactosyl diacylglycerol) 50% cp • B) DGDG (Digalactosyl diacylglycerol) 28% cp • C) SQDG( Sulphoquinovosyldiacylglycerol) 16% cp • Very unsaturated! • Makes membranes • very fluid

46. Lipid metabolism • Chloroplasts have lots of galactolipids: sugar linked directly to diacylglycerol : saves PO4 • A) MGDG (Monogalactosyl diacylglycerol) 50% cp • B) DGDG (Digalactosyl diacylglycerol) 28% cp • C) SQDG( Sulphoquinovosyldiacylglycerol) 16% cp • Very unsaturated! • Makes membranes • very fluid • Source of 3 FA

47. Lipid metabolism Unique aspects in plants Make fatty acids in plastids large amounts of galactolipids Oleosomes: oil-storing organelles with only outer leaflet

48. Lipid metabolism Oleosomes: oil-storing organelles with only outer leaflet • Put oils between the leaflets as they are made

49. Lipid metabolism Oleosomes: oil-storing organelles with only outer leaflet • Put oils between the leaflets as they are made • Add oleosin proteins to outside: curve the membrane

50. Lipid metabolism Oleosomes: oil-storing organelles with only outer leaflet • Put oils between the leaflets as they are made • Add oleosin proteins to outside: curve the membrane • Oils often have unusual fatty acids