PHOTOSYNTHESIS
PHOTOSYNTHESIS
Photosynthesis – synthesis using light
General Equation : 6CO2 + 6H2O C6H12O6 + 6O2
Mesophyll – most active photosynthetic tissue
Photosynthetic Reactions:1. Thylakoid reactions2. Carbon fixation reactions
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
Absorption Spectrum
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
Absorption Spectrum of Chlorophyll
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 photon2. Direct convertion to heat ; no emission of
photon3. Energy transfer4. Photochemistry – energy causes occurence of
chemical reactions
Action Spectrum
Photosynthetic Overview
Energy Transfer during Photosynthesis
Resonance transfer – excitation energy is conveyed from the chlorophyll that absorbs the light to the reaction center
Antenna Complexes
• Eukaryotes – within the chloroplast• Prokaryotes – plasma membrane
Light Reactions : Concepts
Quantum Yield – number of photochemical products per total number of quanta absorbed Hill reactions : Robert HillIn the light, isolated chloroplast thylakoids reduce a variety of compounds, eg. Iron salts
Enhancement effect : Robert EmersonThe rate of photosynthesis was greater when red and far-red light were given together than the sum of their individual rates
Z-scheme
Photosystems I and II : Differences1. 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
Chloroplast Structure
Electron Transfer in the Thylakoid Membrane: 4 Protein complexes
Photochemical Event1. Transfer of an electron from the chlorophyll
to an acceptor molecule: chlorophyll is in oxidized state – electron deficient Acceptor is in reduced state – electron rich2. 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
3. Pheophytin and 2 Quinones accept electrons4. Electrons flow through Cytochromes b6f complex5. Plastoquinone and Plastocyanin carry electrons between PS ll and l6. PS l Reaction Center Reduces NADP
Interference in Photosynthetic Electron Flow: Herbicides : DCM (dichlorophenyl-dimethylurea) Paraquat
Carbon Reactions
1. Calvin Cycle / Reductive Pentose Phosphate Cycle / C3 Cycle
2. C4 Photosynthetic Carbon assimilation Cycle
3. Photorespiratory Carbon Oxidation Cycle
Calvin Cycle : Stages
1. Carboxylation: CO2 + RuBP 3- Phosphoglycerate 2. Reduction of 3- Phosphoglycerate to form Glyceraldehyde-3-phosphate3. Regeneration of the CO2 acceptor , RuBP
Carbon Reactions
Rubisco- Ribulose bisphosphate carboxylase/oxygenase enzyme Competition:O2and CO2 for the substrate Ribulose
bisphosphate Effect : Limits net CO2 fixation
Autocatalytic- regeneration of biochemical intermediatesStoichiometry : 1/6 – for sucrose or starch production 5/6 – for regeneration of ribulose-1,5-bisphosphate
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 kinase2. Increases in Rubisco activity due to light3. Light-dependent ion movements4. Light-dependent membrane transport
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
C3 and C4 Leaf Anatomy
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
C4 Cycle
Advantage of C4 pathway1. Concentrates CO2 in the bundle sheath cells
C4 Plants : Grasses, sugarcane, maize2. Reduces photorespiration
Crassulacean Acid Metabolism
-enables plants to improve water use efficiently1 g CO2: 400 to 500 g water loss forC3 and C4 : 50 to 100 g water los for CAM plantsTemporal and spatial separation : formation of C4 acids
Physiological and Ecological Considerationsof Photosynthesis
• Important Metabolic Steps for Optimum Photosynthesis:
1. Rubisco activity = low CO2; High light Intensity2. Regeneration of RuBP = High CO2 ; Low Light3. Metabolism of Triose PhosphatesLight Parameters:1. Spectral quality 3. Direction of Light2. Amount of Light