Chapter 7 - Photosynthesis

Question 1

What is the most important basis for energy used by mankind? Give examples.

Answer 1

The transformation of sunlight into biomass.

• Fossil fuels
• Burning of wood

Question 2

What is the average content of dry biomass?

Answer 2

About 20 MJ/kg

Question 3

How much energy is the total biomass production?

Answer 3

About 4*10^21 J / year.

Question 4

What is the efficiency of photosynthesis, when comparing annual production of biomass to the total irradiation of the sun.

Answer 4

About 0.1%.

Question 5

What are the two different forms of photosynthesis?

Answer 5

Oxygen-generating PS: H2O + CO2 –light-> + O2

Non-oxygen generating PS: H2S + CO2 –light-> + SO

Question 6

Give the reaction for the photosynthesis of glucose. What is the enthalpy?

Answer 6

6H2O + 6CO2 –light-> C6H12O6 + 6O2, ∆H = 2870 kJ/mol

Question 7

What are the two subcycles for PS? Draw a schematic.

Answer 7

The light cycle (uses photons) and the dark cycle (without photons, also called Calvin-Benson cycle).

Schematic on page 2.

Question 8

What is the fundamental efficiency of photosynthesis? Calculate this.

Answer 8

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%.

Question 9

Draw the Z-scheme

Answer 9

See slides.

Question 10

What is the difference between C3 and C4 plants?

Answer 10

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.

Question 11

Name four things the efficiency of photosynthesis depend on.

Answer 11

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%).

Question 12

What happens at photosystem I?

Answer 12

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.

Question 13

What happens at photosystem II?

Answer 13

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))

Question 14

What is the role of Cytochrome bf?

Answer 14

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+

Question 15

Describe the process that produces ATP in the photosynthesis.

Answer 15

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.

Question 16

How is the spatial arrangement of the different transmembrane proteins in Thylakoid, to make everything run optimal?

Answer 16

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).

Question 17

In which molecules does the absorption of light occur, and where are the absorption centers?

Answer 17

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.

Question 18

What does the splitting of the HOMO-LUMO-levels depend on in the pyrrole rings?

Answer 18

It depends on the ligands bound to the pyrrole rings.

Question 19

At full sunlight, how many photons are absorbed per chlorophyl molecule per second?

Answer 19

About 10.

Question 20

Which purpose does the antenna complexes serve?

Answer 20

Amplification of absorption through quick energy transport between the Chl-molecules by resonant dipole-dipole interactions called Förster-transfer.

Question 21

How is the rate of transfer in the antenna complex given?

Answer 21

k_transfer = k_luminesence * (R_0/r)^6,

where R_0 is the Förster-radius.

Question 22

Which main forms does biomass exist in?

Answer 22

1) Strach, sugar (sugar cane, corn)
2) Cellulose (grass, wood)
3) Plant oil (soja, palms)
4) Algae

Question 23

How can biomass be used?

Answer 23

Physical (burning), chemical (gasification, pyrolysis, carbonization) or biological (fermantation, fowling, animals)

Question 24

What kind of efficiency has direct burning of biomass?

Answer 24

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.

Question 25

What is pyrolysis?

Answer 25

Processing of biomass without air at 300-500C. Creates charcoal

Question 26

What is gasification?

Answer 26

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.

Question 27

What is fermentation?

Answer 27

Creating bioethanol from sugar-containing biomass.

Question 28

What is fowling?

Answer 28

Creating biogas from bacteria in the presence of air.