Unit 2 Problems Sets Flashcards
Which photons (wavelengths) “waste” the most energy when operating photosynthesis?
Only wavelengths in the visible spectrum are absorbed by the plant. Green light in the 500-600 nm range– these wavelengths are extremely high energy but are reflected or weakly absorbed by chloroplasts. Blue and violet light in the 400 nm range also technically “waste” energy. These wavelengths are the shortest used by the plant and thus the highest in energy, but also among the least efficient. Excitation of carotenoids and chloroplasts by blue light generates an enormous amount of energy and that energy needs to be transferred through chloroplasts to reach the photosystems, releasing some energy as heat with each step.
Why do leaves fluoresce? Under what conditions would leaf fluorescence be high?
Light energy excites chlorophyll molecules. That energy is transferred to photosystems and transformed into chemical reactions for photosynthesis and chloroplasts molecules must return to their unexcited state to begin the cycle again. To return to the unexcited state, excess energy (not used in photosynthetic reactions) must be either released as heat or re-emitted at a lower wavelength via fluorescence. Fluorescence is highest in high light and when photosynthetic rates are highest.
Explain the contributions of Robert Emerson to the photosynthetic field. Hint: Report two separate findings.
Emmerson mapped changes in quantum yield across different wavelengths of light and first explained the “red drop” wherein quantum yield drops off at the high end of the visible spectrum before absorption by chlorophyll drops off. He discovered that both red and far-red light were needed for photosynthesis to run on red light. He further discovered an “enhancement effect” wherein quantum yield with both red and far-red light together is greater than the sum of red and far-red light one at a time. Together this led Emmerson to the conclusion that photosynthesis occurs via two separate photosystems optimized for different wavelengths.
Explain the Plastoquinone cycle. Explain why this is important for cyclic and non-cyclic electron transport.
Electrons generated by the splitting of water by PSII get passed to plastoquinone forming plastohydroquinone. Cyctochrome is a multisubunit protein that accepts electrons from Plastohydraquinone. One electron is passed to PSI via plastocyanin, the other is cycled back into PQH. This generates a cyclic electron transfer happening in cytochrome that turns plastohydroquinone back into plastoquinone. cytochrome oxidizes plastohydroquinone and passes electrons to plastocyanin
Why are alternate e-and H+carriers necessary in photosynthetic electron transport?
e- electrons are used to power transfer of chemical energy via electron transport chain across photosystems ultimately generating NAPH. H+ are used to generate a proton motive force that powers ATP synthase, generating ATP, to maintain the Q cycle, and to transform NAP+ to NAPH. Electron carriers operate inside the thylakoid membrane, while H+ carriers operate in the lumen and stroma. These ions must operate independently to maintain chemical electrochemical potential gradients.
What are the major anatomical differences between C3 and C4 leaves?
C4 leaves have specialized Kranz anatomy. This includes exaggerated bundle sheath used for carbon concentration and to physically separate the light and carbon reactions of photosynthesis. Airspace in the mesophyll is limited. Radial symmetry surrounding parallel vasculature and stomata in rows are common.
What is (are) the biochemical difference(s) between C3 and C4 photosynthesis? Do C4 plants have any enzymes that C3 plants lack? Do both use the Calvin cycle? Do both C3 and C4 operate the PCO cycle?
C4 plants use PEP-carboxylase to generate either malate or aspartate, a four carbon (rather than three carbon) intermediate. The malate/aspartate is transferred to the bundle sheath, where it is decarboxylated by an NAP-malic enzyme in mitochondria to produce pyruvate and re-release CO2 into the bundle sheath. Bundle sheath chloroplasts use the re-released CO2 to run the normal C3 Calvin Cycle to generate starch and sucrose. The pyruvate is moved back to the mesophyll where pyruvate phosphate dikinase reduces it to the 3 carbon sugar phosphoenol-pyruvate. Both use the Calvin Cycle. Rubisco oxidation/photorespiration (PCO cycle) is minimized in C4 relative to C3. PEP-carboxylase, NAP-malic enzyme, and pyruvate phosphate dikinase are unique to C4.
In a single phrase, what does the C4 cycle ultimately accomplish? What is the cost?
This criterion is linked to a Learning Outcome3
The purpose of the C4 photosynthetic pathway is to concentrate carbon to minimize photorespiration by mitigating oxidation of rubisco
Where should C3 species perform best? Where should C4 species perform better than C3?
C3 plants will outperform C4 plants in moist shady environments with high CO2 . C4 plants outperform C3 plants in hot, dry environments or when atmospheric CO2 concentration is low
How does CAM photosynthesis differ from C3 and C4 photosynthesis?
Just like C4 plants, CAM plants use PEP carboxylase to fix HCO3- to malate and then releases CO2 to the run the normal C3 Calvin cycle, but unlike C4 plants, CAM plants generate malate at night and stores it in the vacuole as malic acid. Then, during, the day, CAM plants use light to break down malic acid to pyruvate and CO2 and run normal C3 light reactions and Calvin cycle. The extra steps to store and release malate is metabolically costly, but still results in 10x greater water use efficiency than c3 because stomata are only open at night when temperature is lower.
To celebrate re-charge day, you adopt a plant. You want to know what kind of light environment that plant needs. What leaf characteristics would you use to determine if the plant is adapted to high or low light (sun or shade adapted)?
Shade adapted plants have large, thin leaves that don’t overlap. Sun adapted leaves are smaller and thicker, sometimes with red tips or white hairs and thick cuticles.
To test the hypothesis you generated in question 3, you run an experiment and plot a light response curve. What clues from the curve can you use to confirm that your plant is sun or shade adapted?
Sun adapted leaves have higher saturation irradiance, higher Amax, higher dark respiration rates, and higher light compensation points that shade leaves (but similar quantum yield)
Explain the Munch pressure-flow model of phloem translocation. How do bulk flow and osmosis relate to phloem function? How are sugars loaded and unloaded?
Companion cells load solutes into sieve elements causing a decrease in sieve element water potential. Water flows into the sieve element following the more negative water potential via osmosis. This generates a positive pressure potential in the sieve element causing phloem sap to move through the sieve tube via mechanical pressure driven bulk flow. Phloem loading can be apoplastic or symplastic. Apoplastic loading requires ATP – H+-ATPase transports H+ into the apoplast and a sucrose-H+ symporter transports sucrose from the apoplast into the SE-CC complex. Symplastic loading happens via polymer loading, wherein raffinose and stachyose are synthesized from the sucrose in intermediary cells. These larger molecules cannot reverse their path back through the small plasmodesmata to the bundle sheath, so they diffuse through the larger plasmodesmata leading to the sieve cell due to a chemical potential diffusion gradient. Unloading is just loading in reverse at the sink.
What is a P-protein? How does it work?
P-proteins are special tubular, fibrillar, granular, or crystalline proteins that seal damaged sieve elements by plugging up sieve plate pores. Because phloem cells are under positive pressure, the contents of the cells (including the P-proteins) surge towards a cut or puncture and the P-proteins are trapped in the sieve plate pores sealing off the damaged element
How is flooding damaging to plant roots (be specific)? List an adaptation to flooding and explain how it alleviates flooding stress.
Normal respiration is aerobic, meaning it requires lots of oxygen. Diffusion of oxygen across a liquid is too slow to support the required rate of aerobic respiration. If roots are flooded there isn’t enough, the products of glycolysis go to the fermentation pathway, rather than enter the citric acid cycle and oxidative phosphohorylation. Fermentation reduces pyruvate to convert NADH produced by glycolysis into NAD+, converting pyruvate into ethanol and CO2, or into lactic acid. This is very inefficient leading to production of only 4 ATP per sucrose and generates a semi-toxic byproduct, so root cells starve. Aerenchyma and pneumatiphores both produce internal air channels that increase oxygen flow to respiring root tissues.
