2. Photosynthesis – synthesis using light
General Equation :
6CO2 + 6H2O C6H12O6 + 6O2
Mesophyll – most active photosynthetic tissue
Photosynthetic Reactions:
1. Thylakoid reactions
2. Carbon fixation reactions
3. Nature of Light
1. Light is both a particle and wave.
photon – particle
quantum – amount of energy of light
wavelength – distance between crests
frequency – no. of wave crests per unit time
5. Absorption spectrum – a display of the amount
of light energy taken up by a molecule as a
function of the wavelength of light
Visible region –what our eyes are sensitive to
Short wavelength – high frequency, high energy
Long wavelength – low frequency , low energy
8. Change in Electronic State
Upon Absorption of Light Energy:
Chl + hv Chl*
Pathways for Excited Chlorophyll to dispose its
energy :
1. Fluorescence – re-emit a photon
2. Direct convertion to heat ; no emission of
photon
3. Energy transfer
4. Photochemistry – energy causes occurence
of chemical reactions
12. Energy Transfer during Photosynthesis
Resonance transfer – excitation energy is conveyed from
the chlorophyll that absorbs the light to the reaction center
14. Light Reactions : Concepts
Quantum Yield – number of photochemical
products per total number of
quanta absorbed
Hill reactions : Robert Hill
In the light, isolated chloroplast thylakoids reduce a
variety of compounds, eg. Iron salts
Enhancement effect : Robert Emerson
The rate of photosynthesis was greater when red
and far-red light were given together than the sum
of their individual rates
16. Photosystems I and II : Differences
1. PS l produces a strong reductant, capable of
reducing NADP, and a weak oxidant.
2. PS ll produces a very strong oxidant, capable of
oxidizing water, and a weaker reductant than he
one produced by PS l
3. PS l : found in the stroma lamella and edges of
grana lamella
PS ll : predominantly located in the grana
lamella
Oxygenic organisms – Oxygen-evolving organisms
20. Photochemical Event
1. Transfer of an electron from the chlorophyll
to an acceptor molecule:
chlorophyll is in oxidized state – electron
deficient
Acceptor is in reduced state – electron rich
2. Water is oxidized to Oxygen by PS ll
2 H2O O2 + 4H+ + 4 e-
protons – released into lumen of thylakoid,
to stroma by ATP synthase
21. 3. Pheophytin and 2 Quinones accept electrons
4. Electrons flow through Cytochromes b6f
complex
5. Plastoquinone and Plastocyanin carry
electrons between PS ll and l
6. PS l Reaction Center Reduces NADP
Interference in Photosynthetic Electron Flow:
Herbicides : DCM (dichlorophenyl-dimethylurea)
Paraquat
24. Calvin Cycle : Stages
1. Carboxylation:
CO2 + RuBP 3- Phosphoglycerate
2. Reduction of 3- Phosphoglycerate to form
Glyceraldehyde-3-phosphate
3. Regeneration of the CO2 acceptor , RuBP
26. Rubisco- Ribulose bisphosphate carboxylase/oxygenase
enzyme
Competition:O2and CO2 for the substrate Ribulose
bisphosphate
Effect : Limits net CO2 fixation
Autocatalytic- regeneration of biochemical
intermediates
Stoichiometry :
1/6 – for sucrose or starch production
5/6 – for regeneration of ribulose-1,5-bisphosphate
27. Regulation of the Calvin Cycle:
1. Light-dependent enzyme activation
Rubisco, NADP:glyceraldehyde-3-phosphate
dehydrogenase;fructose-1,6-bisphosphatase,
Sedoheptulose-1,7—bisphosphatase,
ribulose-5-phosphate kinase
2. Increases in Rubisco activity due to light
3. Light-dependent ion movements
4. Light-dependent membrane transport
28. Photorespiration
Oxygenation – combination of Rubisco with
Oxygen instead of CO2.
- results to CO2 loss
Rise in temperature effect:
decrease in CO2 relative to O2
enhances the kinetic properties of Rubisco
32. C4 Metabolism
1. CO2 fixation by PEP in mesophylly to form a
C4 acid ( malate or aspartate)
2. Transport of C4 acids to bundle sheath cells
3. Decarboxylation of C4 acids within bundle
sheath cells and generation of CO2 which is
brought to Calvin cycle.
4. Transport of the C3 acid back to the
mesophyll
34. Advantage of C4 pathway
1. Concentrates CO2 in the bundle sheath cells
C4 Plants : Grasses, sugarcane, maize
2. Reduces photorespiration
35. Crassulacean Acid Metabolism
-enables plants to improve water use efficiently
1 g CO2: 400 to 500 g water loss forC3 and C4
: 50 to 100 g water los for CAM plants
Temporal and spatial separation : formation of
C4 acids
36. Physiological and Ecological Considerations
of Photosynthesis
• Important Metabolic Steps for Optimum
Photosynthesis:
1. Rubisco activity = low CO2; High light Intensity
2. Regeneration of RuBP = High CO2 ; Low Light
3. Metabolism of Triose Phosphates
Light Parameters:
1. Spectral quality 3. Direction of Light
2. Amount of Light
