plant phys-quiz 3

What is the summary reaction for all of photosynthesis? What are the three major products of the light reactions?

6 CO2 + 6 H2O + energy (light)→C6H12O6 + 6 O2
Three major: NADPH, ATP and O2

Describe each of the chloroplast features:
Thylakoids:
Granum/Grana:
Lumen:
Stroma:
Chlorophyll:

Thylakoids: individual discs with light rxn machinery in the membranes (lumen inside)
Granum: Single stack of thylakoids
Grana: all the stacks of thylakoids together
Lumen: inside the thylakoid where H+ concentrate
Stroma: solution outside of thylakoid where H+ are depleted, and where carbon rxns take place
Chlorophyll: pigment within thylakoid membranes, responsible for O2 production, rings allowing for better e- sharing after a trasnfer and hydrocarbon tails to help insertion into thylakoids membrane

How do wavelength, frequency, and energy of a photon relate to each other?

wavelength and frequency (inversely proportional) are two properties of a photon (single particle of light), and light carries energy for photosynthesis. photons are impacted by how much energy it absorbs (frequency and energy directly proportional), the combination of these factors relate to the speed of light

Why is the visible spectrum ideal for photosynthetic reactions?

it is the “goldilocks” zone in terms of energy. shorter wavelengths will cause damage, while longer wavelengths will not have enough energy. Medium energy light is just right for e- to transfer or decay to its original orbital.

Briefly describe the structure/composition of the antenna complexes and reaction center. How do they support the efficiency of electron excitation?

antenna complex: clusters of accessory pigments that feed energy/e-’s to chlorophyll
reaction center: contains chlorophyll and receives e-/energy from antenna complex
energy of a photon is transferred b/w lots of intermediate pigments, resulting in more than 95% of energy form photon used to excite e- in chlorophyll. accessory pigments allow for a broad range of wavelengths to be absorbed to gain energy and transfer e-’s efficiently to chlorophyll (high rate of e-’s being excited by capturing any relevant energy level)

List the sequence of events for (non-cyclic/linear) electron transport in the light rxn?

1. Light causes excitation of an e- from PSII, which makes plastiquinone (PQ) to PQH2 (PSII oxidizes H2O to O2 in lumen-replaces e-’s transferred to PQ; H+’s released in lumen and depleted from stroma to reduce PQH2)

2. PQ is a mobile e- carrier within the thylakoid membrane

3. Cytochrome b6f oxidizes PQH2, then reduces mobile carrier plastocyanin (PC) -more H+’s released into lumen

4. PC oxidized by PSI, replacing e- about to be excited and releases more H+’s into lumen

5. light causes excitation of an e- in PSI, which reduces mobile carrier Ferredoxin (Fd) in the stroma

6. Fd reduces NADP+ to NADPH through FNR enzyme (Fd-NADP+ reducatase) - reduction of NADP+ also removes H+ from stroma

PSII → PQ → cyt b6f → PSI → Fd → FNR → NADPH

What is the role of the proton gradient in the light reactions? Describe the points which contribute to formation of the gradient?

allows for protons to be depleted from stroma and concentrated in lumen to diffuse through ATP synthase, to drive the synthesis of ATP
1) PSII oxidizes water to oxygen and protons
1.5) protons depleted from stroma and into lumen when PQ reduced to PQH2
3) cytochrome b6f oxidizes PQH2 and releases more protons into lumen
6) reduction of NADP+ to NADPH removes proton from stroma

How was the chemiosmotic model for ATP synthesis demonstrated using isolated thylakoids?

isolated intact thylakoids cultured in absence of ADP + Pi(makes ATP), stroma and lumen equilibriated at pH 4 (no net H+ movement), transferred to pH 8 and quickly remove from light, add ADP + Pi and see ATP produce.
ATP synthase and H+ gradient are necessary and sufficient to make ATP (need both), dont necessarily need light

How does cyclic electron flow differ from non-cyclic/linear electron flow?

cyclic: Fd reduces cytochrome b6f instead of FNR (no NADPH or O2, but get ATP), can occur to prioritize making ATP to focus on specific anabolic functions. does not require PSII

??Explain the dependence of the Calvin-Benson-Bassham (CBB) cycle on the light reactions.??

ancient pathway used to fix CO2 and H2O into sugars, uses ATP and NADPH from light reactions required for carbon reactions (to convert 3PG into G3P)

and light for certain enzyme activation (ferredoxin-thioredoxin system)

What are the three phases of the CBB cycle? Briefly summarize (one sentence each) what happens during each phase.

start with Ribulose 1,5-bisphosphate and carbon input (CO2 + H2O)
carboxylation/fixation: CO2 fixed into 3PGA
reduction: use ATP + NADPH (from light rxns) to convert 3PG to G3P
regenerate: G3P is used to remake ribulose 1,5-bisphosphate

??How does Rubisco Activase regulate the activity of Rubisco?

Rubisco activase will become activated during the day and will induce a conformational change in rubisco, removing the inhibitory RuBP, allowing CO2 to bind

How does light regulate the activity of Rubisco Activase?

light starts series of redox rxns involving Fd, thioredoxin, and various enzymes. rubisco activase and other enzymes active in reduced state.

during the night RuBP binds to rubiscos active site, making it inactive and unable to bind CO2;during the day rubisco becomes active and undergoes conformational change to remove inhibitory RuBP, allowing CO2 to bind

??Summarize the process of photorespiration.

RuBP + O2 → 3PGA + 2PG
rxn of RuBP with O2 instead of CO2 to get products listed above, 2PG neutralized through complicated steps depleting valuable resources (ATP, CO2 and NADPH, inefficient) but produces another 3PGA for CBB cycle

-2PG inhibits metabolic enzymes

2 RuBP (ribulose 1,5-bisphosphate) + 3 O2 → 2PG (glycerate) + 2 3PGA (phosphoglycerate)

What are three factors that determine carboxylase vs oxygenase activity in Rubisco?  Briefly discuss how each factor affects the activity.

specificity/preference of rubisco for CO2 vs O2: specificity factor estimates preference for CO2 vs O2, diff organisms have diff specificity factor values (low value higher pref for O2, higher value higher pref for CO2)
ratio of CO2 to O2 in atmosphere: determines ratio of each that will diffuse into plant, and therefore likelihood of each interacting w/ rubisco
temperature (variable effects): at high temps-increase oxygenase activity of rubisco, lower CO2 solubility more than O2 in atmosphere, prevent transpiration by closing stomata and less baility to absorb CO2 from atmosphere

??Describe the carbon fixation mechanism and carbon reactions for C4 plants, emphasizing the points of difference compared to C3 plants.

C4: fix CO2 into 4 carbon molecule instead of 3, anatomical difference (mesophyll and bundle sheaths) so less dependent on atmospheric CO2 and well suited to warm environments
-CO2 + H2O→ HCO3- → fixed onto PEP making oxaloacetate (4C) → turned into malate and moved to bundle sheath into pyruvate and CO2 → CO2 into bundle sheaths and pyruvate back to mesophyll, now do CO2 fixation w/ rubisco in bundle sheaths
-creates concentrated region of CO2 (10x higher than atmosphere) and rubisco is only expressed in the bundle sheaths (less O2)
C3: only mesophyll, dependent on atmospheric CO2 levels and temperate conditions

??Describe the carbon fixation mechanism and carbon reactions for CAM plants, emphasizing the points of difference compared to C3 plants.

CAM: no anatomy difference, CO2 uptake occurs at night instead of during day, stores CO2 as malate in vacuole to be used in CBB during day
-night: CO2 taken in and forms HCO3- into PEP into oxaloacetate into malate (stored in vacuole, no CBB)
-day: malate released from vacuole, releasing CO2 from malate into CBB cycle
-temporal separation of uptake vs assimilation of CO2
C3: not able to store malate anywhere, no help with concentrating CO2