1. Plants and Photosynthesis

5. Photosynthesis Organisms Autotrophs: “Self Feeders” Photo-: Light
Chemo-: Oxidize inorganics (Ex: Sulfur, Ammonia), unique to bacteria
Heterotrophs: “Other Feeders”

6. History Jean-Baptiste van Helmont (1600’s) grew willow tree
Weighed soil before and after
Added only water
Tree gained 75 kg
No change in mass of soil
Concluded: mass in plants comes from water

7. Site of Photosynthesis
Upper Epidermis
Mesophyll Cells
Lower Epidermis
Vein
Stoma

8. Site of Photosynthesis
Thylakoids
Stroma
Granum
Inner & Outer Membranes

9. Photosynthesis Conversion of Light E into Chem E Light E
Travels in waves (photons)
Wavelength (): crest to crest (measured in nm)
 inversely related to frequency
Higher frequency = more E
Different  = different properties

10. Wavelength (nanometers)
Nature of Light
Visible spectrum is ~380–750 nm
Gamma Rays
X-Rays UV
Infrared
Micro- waves
Radio Waves
Visible Light
400
450
500
550
600
650
700
750
Wavelength (nanometers)

11. Nature of Light
Pigments absorb certain  and reflect or transmit others

12. Nature of Light
Spectrophotometers measure amount of Light pigments absorb or reflect

13. Nature of Light Pigments Absorb and reflect light
Specific pigment = specific light
Chlorophylls
a and b – both absorb blues and reds
a is 1 pigment for photosynthesis – focuses solar E onto a pair of e-s

14. Nature of Light
Accessory pigments – funnel the E they collect to a central Chlorophyll A
Carotenoids
Carotenes – reflect oranges
Xanthophylls – reflect yellows
Phycocyanins – reflect blues
Some accessory pigments provide photoprotection against excess light
Carotenoids in human eyes serve same function

15. Absorption/Action Spectra
Visible Light
400
450
500
550
600
650
700
750 20 40 60 80
100
% Light Absorption
Collectively
Chlorophyll
Carotenoids
Phycocyanin
400
450
500
550
600
650
700
750
Wavelength (nanometers)

16. Engelmann’s Experiment
Simple experiment in 1883
Compare to action spectrum

17. Photosynthesis Can be divided into Light-dependent rxn
Makes E storing compounds NADPH and ATP to fuel L-i rxn
Occurs in thylakoids
Light-independent rxn
Uses NADPH and ATP to produce glucose, a more stable form of E
Occurs in stroma

18. Photosynthesis

19. Light-dependent rxn

20. Light-dependent rxn
Light is absorbed in photosystem II, an “antenna complex” of hundreds of pigments that funnel E to a reaction center
Rxn Center: central chlorophyll a molecule next to a protein, the 1° e- acceptor

22. Light-dependent rxn
Chemi osmosis

23. Photosynthesis

24. Light-dependent rxn

27. Light-dependent rxn
The e-s from the broken bonds slide down the ETC, slowly losing E
The e-s are recharged by sunlight in photosystem I and are passed along more carrier proteins to NADP+, reducing it to NADPH

28. sun O2
Light-dependent H+ H+
H20 H+

29. Light-dependent
sun
sun O2
ADP
ATP H+ H+
H20 H+

30. Light-dependent rxn summary
H2O is broken up by sunlight
O2 is released as waste
e-s flow down ETC, pump H+ ions, and finally make NADPH
H+ ions diffuse across thylakoid membrane and help form ATP
ATP and NADPH move on to the light-independent rxn

31. Photosynthesis

33. L-i rxn – C fixation

34. L-i rxn – Reduction
12 ATPs phosphorylate the 12 3PGs to form 12 1,3 bisphosphoglycerates
A pair of e-s from NADPH reduces each 1,3 bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P)
The electrons reduce a carboxyl group to a carbonyl group

35. L-i rxn – Reduction

36. L-i rxn – Reduction
Two G3Ps can now be removed from the cycle to make glucose or be used for as any other carb the plant cell needs

38. Light-independent rxn summary
Carbon Fixation
CO2 needed to begin the process
Synthesis of G3P (Glyceraldehyde 3 phosphate)
ATP and NADPH are used
Regeneration of 5C compound
Need more ATP to reset the cylce

39. Photorespiration
Stomata not only allow gas exchange, but transpiration also
Hot, dry day – stomata close
Problem: CO2 , O2 
Rubisco can bind either CO2 OR O2 to RuBP
When O2 binds, no useful cellular E is produced

40. Photorespiration
When rubisco adds O2 to RuBP, RuBP splits into a 3-C piece and a 2-C piece
The 2-C fragment is exported from the chloroplast and degraded to CO2 by mitochondria and peroxisomes
Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle
Up to 50% of the C fixed by Calvin cycle can be drained away on a hot, dry day

41. C4 Plants
Mesophyll cells use PEP carboxylase to fix CO2 to phosphoenolpyruvate, forming oxaloacetate (4C)
PEP carboxylase has a very high affinity for CO2 and can fix CO2 efficiently when rubisco cannot - on hot, dry days with the stomata closed

42. C4 Plants
Oxaloacetate then dumps the extra CO2 into the Calvin cycle in bundle-sheath cells
Rubisco can then work with a high concentration of CO2, thus minimizing photorespiration
C4 plants thrive in hot regions with intense sunlight
Examples: sugar, corn

43. C4 Plants

44. CAM Plants Crassulacean Acid Metabolism
CO2 is fixed at night, but NO photosynthesis takes place at night
During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is released from the organic acids

45. CAM Plants
Allows plants to keep their stomata closed during the hot, dry hours of day and open in the cooler hours of night
Less water is lost in the process
Less photorespiration occurs
Ex: succulent plants, cacti, pineapples, and several other plant families

46. CAM Plants

47. Both C4 and CAM plants add CO2 into organic intermediates before it enters the Calvin cycle
In C4 plants, carbon fixation and the Calvin cycle are spatially separated
In CAM plants, carbon fixation and the Calvin cycle are temporally separated
Both eventually use the Calvin cycle to incorporate light energy into the production of sugar