1. Photosynthesis

2. Photosynthesis in Nature
Autotrophs:
biotic producers
photoautotrophs/ chemoautotrophs
obtains organic food without eating other organisms
Heterotrophs:
biotic consumers
obtains organic food by eating other organisms or their by-products (includes decomposers)

3. Chloroplasts Sites of photosynthesis In the mesophyll of leaves
Gas exchange: stomata (small pores on leaf surface)
Structure:
Double membrane
Thylakoids- contain pigments (chlorophyll)
Grana- stacks of thylakoids
Stroma- surrounds thylakoid

4. 6CO2 + 12H20 + light NRG  C6H12O6 + 6O2 + 6H2O
The Reaction
6CO2 + 12H20 + light NRG  C6H12O6 + 6O2 + 6H2O
Redox process
H2O is split
- e- (along w/ H+) are transferred to CO2, reducing it to sugar

5. Photosynthesis Overview
2 major steps:
Light reactions (“photo”)
NADP+ (e- acceptor) to NADPH
Photophosphorylation: ADP  ATP
Calvin cycle (“synthesis”)
Carbon fixation: carbon into organic compounds

6. Light Light= electromagnetic NRG Light particles= photons
Visible light broken up into colors
The colors we see are the ones which are reflected. All others are absorbed
Chlorophyll REFLECTS green light and absorbs others

7. Absorption of Light
Absorption can be determined by a spectrophotometer
Determines the amount of light transmitted at a certain wavelength
Different wavelengths = different colors

8. Pigments in Photosynthesis
Chlorophyll A
Absorbs violet blue and red light
Appear blue-green
Chlorophyll B (Accessory pigment)
Absorbs violet blue and orange light
Appear yellow-green
Carotenoids (Accessory pigment)
Absorbs violet blue and blue green light
Appear yellow or orange

9. Exciting Chlorophyll Chlorophyll absorbs photons of light
Electrons move to a higher NRG level (outer orbital
Molecule is unstable
As electrons drop back down, they release heat and photons of light (fluorescence)

10. Photosystems Light harvesting units of the thylakoid membrane
Pigment molecules make up an “antenna complex” which attracts photons
Energy is passed to the reaction center (made up of 2 special chlorophyll a molecules)
Excited e- from chlorophyll is trapped by a primary e- acceptor

11. Two Photosystems
PS II and PS I… Named in order of discovery, but PS II happens first
PS II
Reaction center = P680
Most effective at 680 NM
PS I
Reaction center = P700
Most effective at 700 NM

12. Noncyclic electron flow
PS II:
photons excite chlorophyll e- to an acceptor
e- replaced by splitting of H2O (release of O2)
e-’s travel to PS I down an ETC
as e- fall, ADP  ATP (noncyclic photophosphorylation)
PS I:
‘fallen’ e- replace excited e- to primary e- acceptor
2nd ETC transfers e- to NADP+  NADPH (...to Calvin cycle…)
These photosystems produce equal amounts of ATP and NADPH

13. Cyclic electron flow Alternative cycle when ATP is deficient
Photosystem I used but not II
Produces ATP but no NADPH or O2
Why? The Calvin cycle consumes more ATP than NADPH…
Cyclic photophosphorylation of ATP keeps Calvin cycle going w/o extra NADPH

14. Chemiosmosis Movement of e-
PS II  plastoquinone (Pq) cytochrome  plastocyanin (Pc)  PS I  ferredoxin (Fd)  NADP+
NRG trapped in thylakoid membrane used to pump H+ into membrane
ATP synthase uses H+ flow to generate ATP

15. Haven’t we seen this before?
ETC in photosynthesis is very similar to the one in cellular respiration
H+ pumped into thylakoid
H+ pumped into intermembrane space

16. The Calvin Cycle Carbon fixation Reduction Regeneration
3 molecules of CO2 are ‘fixed’ into glyceraldehyde 3-phosphate (G3P)
Carbon fixation
each CO2 is attached to RuBP (rubisco enzyme)
Reduction
e- from NADPH reduces to G3P; ATP used used
Regeneration
G3P rearranged to RuBP; ATP used; cycle continues

17. Net Synthesis For each G3P (and for 3 CO2)…
Consumption of 9 ATP’s & 6 NADPH (light reactions regenerate these molecules)
G3P can then be used by the plant to make glucose and other organic compounds

18. C3 Plants Fix Carbon in to 3-phosphoglycerate Rice, wheat, soybeans
Must have stomata open to take in CO2. Causes loss of water as well
Photorespiration: on hot/dry days, stomata close, CO2 decrease, O2 increase in leaves
O2 added to rubisco instead of CO2
no ATP or food generated

19. C4 Pathway C4 Plants 2 photosynthetic cells, bundle-sheath & mesophyll
PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll
New 4-C molecule releases CO2 into the Calvin cycle
corn, sugarcane, grasses

20. Alternative Pathway II
CAM
Crassulacean Acid Metabolism
Open stomata during night, take up CO2 and store it as organic acids
Close stomata during the day; CO2 released from acids and released into the Calvin cycle
Cacti, pineapples, etc.

21. Review of Photosynthesis