Ch. 12 Flashcards
What is photosynthesis?
Production of energy from sunlight
What is photooxidation?
Transfer of electrons from chlorophyll to electron acceptor
What is photooxidation important in?
Redox-based energy conversion
What happens in the photosynthetic electron transport system?
Light-absorbing pigment is photoexcited and transfers electron to another molecule as a result of photooxidation
What are the components of the photosynthetic electron transport system?
- PSI
- PSII
- Cytochrome b6f
What is the Calvin cycle?
Metabolic pathway of carbon fixation
- Uses chemical energy (ATP and NADPH) to convert CO2 to triose phosphates and other carbs
Under what conditions is the Calvin cycle most active?
Most active under light (high ATP and NADPH levels)
What does the Calvin cycle produce?
Triose phosphates
What are triose phosphates used to make?
Hexose sugars
What are the steps of photosynthetic electron transport?
- 4 photons are absorbed by chlorophyll molecules in PSII
- Electron transport via carrier molecules in photosynthetic electron transport system; creates proton gradient across membrane
- 4 photons are absorbed by PSI
- Chloroplast ATP synthase produces ATP
- ATP and NADPH are used in enzymes in the Calvin cycle to drive carbon fixation
What are the membranes of a chloroplast?
- Inner membrane (permeable)
- Outer membrane (impermeable)
- Thylakoid membrane (impermeable)
What is the thylakoid membrane the site of?
Photosynthetic electron transport system and ATP synthase complex
What the pH in the lumen?
pH = 5 (more acidic than stroma)
What is the pH in the stroma?
pH = 8 (less acidic than lumen)
Label the parts of a chloroplast.
- Thylakoid membrane
- Thylakoids
- Lumen
- Lamella
- Strong
- Granum
- Outer membrane
- Inner membrane
What does the change in pH from the stroma to the lumen cause?
Results in a proton gradient and ATP synthesis
What are chromophores?
Light-absorbing molecules found as part of photosensitive biological proteins/complexes
What is chlorophyll?
Primary chromophore in chloroplasts and cyanobacteria
How many electrons are required to oxidize 2 H2O into O2 + NADPH?
4 electrons
What is a photon?
Quantum of light energy
What is an exciton?
Quantum of energy passed from an excited molecule to another molecule
What is free chlorophyll?
Upon excitation, absorbed energy is released as fluorescence and heat
What is LHC-bound chlorophyll?
Upon excitation, exciton transfer occurs
What are the 3 possible outcomes of photon absorption by cholorphyll?
- Resonance energy transfer
- Fluorescence
- Photooxidation
What is resonance energy transfer?
Excited electron returns to ground state by transferring energy of excitation to a nearby molecule, causing that molecule to get excited
What is resonance energy transfer important in?
“Harvesting” light energy
What is a reaction center?
Specialized pair of chlorophyll molecules in PS that can convert photon energy to electrochemical energy via photooxidation
What is the structure of a chlorophyll?
- Mg2+ in the middle of the 4 nitrogen atoms of the pyrrole
- Heterocyclic 5-ring system
- Have a long tail (phytol)
Why are chlorophylls highly effective photoreceptors?
- Polyene: alternating single and double bonds
- Strong absorption in visible light
- Unusually high extinction coefficient
- Good for absorbing light during photosynthesis
What are photosystems?
Functional arrays of light-absorbing pigments
What are light-harvesting complexes (LHCs)?
“Solar panels” that capture light for photooxidation
Are there more LHCI or LHCII?
LHCII
What is the PSII reaction center?
P680
What is the PSI reaction center?
P700
What is the Z scheme?
Describes the complete route of electron flow from H2O to NADP+
Calvin cycle equation
2H2O + 2NADP+ + 8 photons –> O2 + NADPH +2H+
What does phosphorylation generate?
ATP
Where does CO2 assimilation occur?
In the stroma via the Calvin cycle
What is the key intermediate in CO2 assimilation?
Ribulose-1,5-BP
What is RUBISCO?
Enzyme involved in the first major step of CO2 fixation
What does CO2 assimilation produce?
- 3-phosphoglycerate
- G3P and DHAP (in equilibrium)
What is the net result of CO2 assimilation?
Reduction of CO2 with NADPH
What are the metabolites of CO2 assimilation (3-phosphoglycerate, G3P, DHAP) used to make?
- Fructose-1,6-BP
- Fructose-6-P
What are the 3 stages of the Calvin cycle?
- Stage 1: fixation of CO2 into 3-phosphoglycerate by RUBISCO
- Stage 2: reduction of 3-phosphoglycerate to form hexose sugars
- Stage 3: regeneration of ribulose-1,5-bisphosphate (carbon shuffle)
What 3 mechanisms control the activity of Calvin cycle enzymes?
- Enzyme inhibitor (CA1P) blocks RUBISCO activity
- RUBISCO and F-1,6-bisphosphate are activated by increased pH and increaed [Mg2+] which occurs in light
- Some enzymes are reduced by a small protein called thioredoxin (redox protein)
What is photorespiration?
Costly side reaction of photosynthesis due to the lack of RUBISCO specificity
What are the building blocks for the 3 primary carbohydrates?
Hexose phosphates
Why is RUBISCO not a good enzyme?
Its kcat = 3 while other enzymes are generally around kcat = 1000
- Only 3 molecules of CO2 fixed per second per molecule of RUBISCO at 25ºC
What does photosynthesis accomplish for the cell?
Converts light to redox energy –> generates ATP –> reduces NADP+ to NADPH
What are the overall net reactions of photosynthesis?
- Photosynthetic electron transport system
2H2O + 8 photons + 2NADP+ + ~3ADP + ~2Pi –> O2 +2NADPH + ~3ATP - Calvin cycle
3CO2 + 6NADPH + 9ATP + 6H2O –> G3P + 3RuBP + 6NADP+ + 9ADP + 9Pi
What are the key enzymes in photosynthesis?
Photosynthetic electron transport system
- Cytochrome b6f
Calvin cycle
- RUBISCO
