MBIO 2710 / Topic 7: Dark Reactions
Studied by 0 people
Learn
Practice Test
Spaced Repetition
Match
Flashcards1/7
There's no tags or description
Looks like no tags are added yet.
Name | Mastery | Learn | Test | Matching | Spaced |
|---|
No study sessions yet.
8 Terms
What can rubisco also function as an E?
Rubisox (ribulose bisphosphate oxygenase).
Why is rubixox’s second side reaction metabolically wasteful to the plant?
O2 used instead of CO2 → C2 product.
> Waste carbon: C2 product cannot enter Calvin cycle and thus cannot help fix carbon to make Glc → must enter a recycling pathway but expensive.
> Waste energy: In already expensive recycling pathway, carbon lost as CO2 + ATP/NADPH wasted.
> Reduces sugar yield: Some 3-PGA recovered, but less than if rubisco fixed CO2 directly.
> For rubisco, What is the approx KM of CO2 and O2?
> For CO2, 9uM (higher affinity).
> For O2, 350um (lower affinity).
> At 20ºC, what is the usual [CO2] and [O2] around us?
> What does this mean for the rubisco’s oxygenase vs carboxylase activity?
> For CO2, 10uM.
> For O2, 250uM.
> Reaction rate for rubisco is higher than ½ Vmax.
> Reaction rate for rubisox is lower than ½ Vmax.
> However, there is more O2 than CO2 at 20ºC.
> O2 has quantity advantage, while CO2 has affinity advantage.
> Every 1:3 to 1:4 rubisco reactions will be of the oxygenase type.
What is the state of the ZS, CC, and thus rubisco under light conditions and under no light conditions?
Under light conditions
> ZS active → NADPH + ATP produced.
> CC runs → needs CO2.
> 3‑PGA products constantly consumed → low product levels pull rubisco forward in carboxylase mode.
Under no light conditions
> ZS inactive → no NADPH + ATP produced.
> No CO2 demand for CC → CC slows.
> 3PGA products not being used b/c CC slows → products accumulate → no product pull → oxygenase activity prevails.
How does temp affect rubisco activity as carboxylase/oxygenase?
↑ Temp
Gas physics
↓ Gas solubility (gas escape solution more easily)
↓ CO2 solubility than O2 b/c CO2 less abundant
Enzymology
↑ Rubisco rxn rate → rubisco AS gets “looser”/less selective
Overall
↑ O2 affinity of rubisco relative to CO2
> Rubisox predominates.
How do bundle sheath cells regulate photorespiration?
> Located under mesophyll cells, tightly packed w/ thick cell walls → less O2 exp & diffusion from outside (farther from stomata).
> Have high CO2 affinity PEP carboxylase too.
> No O2-utilizing side reaction → reduces rubisox activity.
Why is the C4 pathway more energy‑intensive but beneficial?
> +2 ATP per CO₂ required (extra energy cost)
> But avoids wasteful rubisox rxn by concentrating CO2 in bundle sheath cells.