1. 6H2O + 6CO2 ----------> C6H12O6+ 6O2
Photosynthesis
The process by which plants, some bacteria, and some protists use the energy from sunlight to produce sugar.
6H2O + 6CO > C6H12O6+ 6O2

2. Photo = Light
Synthesis = to make

3. Photosynthesis in nature
Autotrophs:
Biotic (living) producers
Obtains organic food without eating other organisms
Photoautotrophs
Chemoautotrophs
Heterotrophs:
Biotic consumers
Obtains organic food by eating other organisms or their by-products (includes decomposers)

4. Leaves and Leaf Structure
A solar collector
Plant cell: mesophyll
Palisades: mostly photosynthetic
Spongy: mostly gas exchange
Gas exchange: stomata
Controlled by guard cells

5. The chloroplast Sites of photosynthesis Pigment: chlorophyll
Double membrane
Thylakoids = discs
Grana = stacks of discs
Stroma = fluid surrounding discs

6. Nature of Sunlight Light behaves both as a wave and a particle
Particle properties (photons) seen by the photoelectric effect.
Zinc exposed to ultraviolet light becomes + charged because light energy forces electrons away from the zinc.
Light separates into different colors (wavelengths) by passing through a prism.
E is inversely proportional to wavelength.
Longer wavelengths E < shorter ones

7. Chlorophyll and Antenna Pigments
Absorb light energy to provide energy for photosynthesis
Pigment absorbs all colors it doesn’t reflect.
Wavelength reflected is color seen
Chlorophyll a and b: energy from Violet-Blue and Orange-Red.
Antenna pigments absorb energy that chlorophyll a and b don’t.
xanthophylls (yellow)
Carotenoids (orange,red)

8. Photosynthesis: an overview
Redox process
H2O is split
e- (along w/ H+) are transferred to CO2 reducing it to sugar
2 major steps:
Light reactions (“photo”)
NADP+ (electron acceptor) to NADPH
Photophosphorylation ADP ---> ATP
Calvin cycle (“synthesis”)
Carbon fixation: carbon into organics

9. Photosystems Light harvesting units found in thylakoid membranes
Protein and antenna pigment complexes
Two Photosystems:
PSII (p680)
PSI (p700)
PSII comes before PSI
Photophosphorylation: converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule

10. Light Reaction Step 1 Photosystem II (P680)
photons excite chlorophyll e- to an acceptor
e- replaced by splitting of H2O (photolysis) release of O2
e-’s travel (redox) to Photosystem I down an electron transport chain (ETC) (Pq~cytochromes~Pc)
as e- fall H+ pass to inside thylakoid creating a gradient which drives ADP ---> ATP

11. Light Reaction Step 2 Photosystem I (P700)
‘fallen’ e- replace excited e- to primary e- acceptor
2nd ETC ( Fd~NADP+ reductase) transfers e- to NADP+ ---> NADPH
...to Calvin cycle…
These photosystems produce equal amounts of ATP and NADPH

12. The Calvin cycle This process occurs in the stroma.
ATP and NADPH produced by the light reactions are used in the Calvin cycle to reduce CO2 to sugar.
ATP = energy
NADPH = reducing agent
Carbon enters as CO2 and leaves as 3 carbon sugar glyceraldehyde 3-phosphate (PGAL).
3 phases:
Carbon Fixation
Reduction
Regeneration

13. Carbon Fixation Each CO2 is attached to RuBP
rubisco enzyme
Produces an unstable 6C compound that immediately splits into 2 molecules of
3-phosphoglycerate (PGA)

14. Reduction
PGA gets a phosphate from the ATP producing a 1-3-bisphosphoglycerate.
Electrons from NADPH reduces it to PGAL.

15. Regeneration PGAL rearranged to RuBP ATP used cycle continues

16. Calvin Cycle, net synthesis
For every 3 CO2 there is one PGAL removed from the cycle
PGAL can then be used by the plant to make glucose and other organic compounds

17. Cyclic electron flow Alternative cycle when ATP is deficient
Photosystem I used but not II
produces ATP but no NADPH
Why? The Calvin cycle consumes more ATP than NADPH…….
Cyclic photophosphorylation

18. Photorespiration: Hot/dry days stomata close
CO2 decrease, O2 increase in leaves
O2 added to rubisco results in the breakdown of the Calvin cycle (no bueno)
no ATP or food generated

19. Alternative carbon fixation methods
C4 plants:
2 photosynthetic cells
bundle-sheath & mesophyll
PEP carboxylase (higher affinity than rubisco) fixes CO2 in mesophyll
new 4C molecule releases CO2
grasses, corn, sugarcane, etc…
CAM plants:
open stomata during night, close during day (crassulacean acid metabolism)
cacti, pineapples, etc.

20. A review of photosynthesis