Unit 13 Flashcards
Distinguish between the light-dependent and the carbon-assimilation reactions of photosynthesis.
Light dependent requires photon energy to drive electron transport. Net reaction consumes H2O and light energy to produce reduced electron carriers, an H+ gradient, and evolved O2.
Carbon assimilation occurs independent of light but requires the products of the light reaction. Net reaction consumes CO2 and reduces it to trioses.
Name and describe the molecules that absorb light.
Chromophores: molecules contain electrons that are excited to a higher energy state by a particular photon energy or quantum. Light energy is absorbed and stored momentarily as increased energy state of the electron.
Examples: pigments such as chlorophylls, carotenoids
Explain how chloroplasts maximize the efficiency of absorption of visible light.
Contain multiple pigments that absorb maximally at different wavelengths. Combination of absorptions allows chloroplasts to harvest maximum amount of available light energy. Can have chlorophylls (absorb red and purple) as well as accessory pigments, carotenoids (absorb in-between wavelengths).
What is a photosystem?
An array of molecules that harvest light (pigments) associated with a reaction center that can convert the light energy to electron transfer.
What is a reaction center?
A complex of special pigments (chlorophylls) that associate with an electron acceptor and a donor. The complex translates exciton energy to electromotive force. When the chlorophylls are excited, they donate the excited electron to the acceptor (becomes negative) and become positive. The donor replaces the chlorophyll electron.
What is the mechanism by which light drives electron transfer?
1) Light photon excites an antennae pigment.
2) energy transferred between antennae pigments via exciton transfer until it reaches the reaction center.
3) reaction center chlorophyll has excited electron
4) excited electron donated to acceptor
5) electron donor is oxidized and replaces the removed chlorophyll electron.
Why is the diagram called the Z scheme?
When you plot decreasing reduction potential of molecules holding the electron against progression of the electron through the pathway, there is a rise in energy associated with PSII, a cascade through cytochrome b6f, and then another energy rise with PSI that appears like a “Z.”
What steps in the Z-scheme require energy input? Which steps occur passively?
Energy input required to excite electrons held by the reaction center chlorophylls, P680 and P700. Energy is transferred from light harvesting antennae to reaction center.
Steps associated with decrease in transfer potential or transfer to a more positive reduction potential carrier occur passively. Less free energy is available after the transfer in these cases.
Compare the cytochrome b6f complex to the cytochrome bc1 complex in the mitochondrion.
Similar in structure and function.
Structure: both are dimeric with a cavern that arranges the Q and QH2 sites. Each uses 2 b-type hemes, a Rieske Fe-S protein, and an extra heme (f-heme or c-heme) for electron carriers.
Function: both conduct Q cycles to move pairs of electrons one by one from a reduced quinone (lipid soluble, hydrogen carrying) to two cytochromes (water soluble, no hydrogen) to move electrons on to either the cytochrome c oxidase complex or the P700 reaction center.
What molecules in the photosynthetic electron transport chain are equivalent to ubiquinone and cytochrome c in the mitochondrial chain?
Plastoquinones and plastocyanin
Name the process that occurs in the cytochrome b6f complex and explain why it is important.
Cyclic electron flow or cyclic photophosphorylation. This allows the cell to create more ATP and less NADPH as needed.
Why is cyclic electron flow (Fd to cytochrome b6f, rather than to the oxidoreductase) sometimes advantageous to the plant?
This helps the plant adjust to changes in wavelength exposure to maintain the ratio of ATP:NADPH produced at a level ideal for carbon assimilation reactions to proceed.
What are the functions of the oxygen-evolving complex?
Splits water to give oxygen and protons to:
1) Restore P680+ to P680 after P680* donates its electron to Pheo
2) Pump generated protons into lumen
Discuss the arrangement of the photoelectron transfer chain in the thylakoid membrane.
PSII, carriers, PSI, and ATP synthase all arranged in thylakoid membrane.
Where do the protons accumulate in the photoelectron transport chain? Which reactions contribute to this accumulation?
Accumulate in lumen. Contributed to by oxygen evolving complex and cytochrome b6f pumping action.
