Chapter 7 - Photosynthesis Flashcards
What is the most important basis for energy used by mankind? Give examples.
The transformation of sunlight into biomass.
- Fossil fuels
- Burning of wood
What is the average content of dry biomass?
About 20 MJ/kg
How much energy is the total biomass production?
About 4*10^21 J / year.
What is the efficiency of photosynthesis, when comparing annual production of biomass to the total irradiation of the sun.
About 0.1%.
What are the two different forms of photosynthesis?
Oxygen-generating PS: H2O + CO2 –light-> + O2
Non-oxygen generating PS: H2S + CO2 –light-> + SO
Give the reaction for the photosynthesis of glucose. What is the enthalpy?
6H2O + 6CO2 –light-> C6H12O6 + 6O2, ∆H = 2870 kJ/mol
What are the two subcycles for PS? Draw a schematic.
The light cycle (uses photons) and the dark cycle (without photons, also called Calvin-Benson cycle).
Schematic on page 2.
What is the fundamental efficiency of photosynthesis? Calculate this.
30%.
For fixation of one carbon atom, need to split two water molecules:
2H2O –8 photons-> O2 + 4H+ + 4e-
For the generation of one free electron, need two photons of wavelength ≈700nm ≈ 1.8 eV.
Per caron atom about 4eV of chemical energy is stored. This gives:
eta = 4 eV / 8 * 1.8 eV ≈ 30%.
Draw the Z-scheme
See slides.
What is the difference between C3 and C4 plants?
During the carbon fixation, C3 plants will make intermediaries with 3 C-atoms, while C4 plants will make intermediaries with 4 C-atoms. This means that C4-plants have a higher carbon dioxide absorption per leaf area and hour than C3.
Name four things the efficiency of photosynthesis depend on.
i) Wavelength of light. Chl a and b mainly absorb in blue (400-480nm) and red (600-700 nm). Experiences a “red drop” for wavelengths above 690, that is it suddenly stops.
ii) Light intensity: photosynthesis starts at ≈ 10 W / m^2. Optimal at ≈ 230 W / m^2.
iii) Temperature: optimum at 35C
iv) CO2 content: optimum at 1 vol% (atmospheric 0.04%).
What happens at photosystem I?
PS1 produces reduced ferrodoxin, which is necessary for the generation of NADPH.
i) 2e- + 2Fd_ox —> 2Fd_red
ii) 2Fd_red + NADH+ + H+ —> 2Fd_ox + NADPH (catalyzed by ferrodoxin-NADP-reductase)
PS1 contains an antenna protein (called LHC I, Light harvesting complex I). This consists of about 200 chlorophyll a and b molecules, pus the reaction center P700.
What happens at photosystem II?
This transfers electrons and protons from water to plastoquinon (PQ).
2PQ + 2H2O —photon + 2e- –> O2 + 2PQH2
PS2 contains a light harvesting complex of about 250 Chl a and b, P680 reaction center and a Mn-O-oxygen evolving complex (OEC))
What is the role of Cytochrome bf?
It produces a proton gradient across the Thylakoid membrane powered by the electron flux from PSII to PSI, plus oxidation of PQH2.
PQH2 + 2PCy(Cu2+) — e —> PQ + 2PCy(Cu+) + 2H+
Describe the process that produces ATP in the photosynthesis.
ATP is produced via the enzyme ATP synthase which catalyzes the reaction ADP + P -> ATP. The enzyme is powered by concentration gradient of H+ inside vs outside of the Thylakoid membrane.
How is the spatial arrangement of the different transmembrane proteins in Thylakoid, to make everything run optimal?
Thylakoid membrane consists of stacked regions containing mainly PSII and Cyt bf (production of H+ gradient) and non-stacked regions with all four proteins (production of NADPH and ATP).
In which molecules does the absorption of light occur, and where are the absorption centers?
The absorption occurs in chlorophyl molecules. The absorption centers are the pyrrole rings which has a conjugated electron system. The absorption of red light occurs via transitions from π to π* (bonding-antibonding) transitions of the ppπ-bonds. The absorption of UV/blue light occurs via transitions from sigma to sigma* of the pp-sigma bonds.
What does the splitting of the HOMO-LUMO-levels depend on in the pyrrole rings?
It depends on the ligands bound to the pyrrole rings.
At full sunlight, how many photons are absorbed per chlorophyl molecule per second?
About 10.
Which purpose does the antenna complexes serve?
Amplification of absorption through quick energy transport between the Chl-molecules by resonant dipole-dipole interactions called Förster-transfer.
How is the rate of transfer in the antenna complex given?
k_transfer = k_luminesence * (R_0/r)^6,
where R_0 is the Förster-radius.
Which main forms does biomass exist in?
1) Strach, sugar (sugar cane, corn)
2) Cellulose (grass, wood)
3) Plant oil (soja, palms)
4) Algae
How can biomass be used?
Physical (burning), chemical (gasification, pyrolysis, carbonization) or biological (fermantation, fowling, animals)
What kind of efficiency has direct burning of biomass?
About 5% in a simple fire place, and up to 50% in modern bio power plant.
About 75% of the energy is lost in fumes and gases, which has to be captured and burned to be more efficient.
What is pyrolysis?
Processing of biomass without air at 300-500C. Creates charcoal
What is gasification?
Process of reacting biomass with water steam and air at temperatures between 300 and 1000C at pressures between 1 to 30 bar. Creates a mixture of CO, CO2 and H2, can also be used for production of most carbohydrates.
What is fermentation?
Creating bioethanol from sugar-containing biomass.
What is fowling?
Creating biogas from bacteria in the presence of air.
