PT 49 Section 3 Question 23

[jesuis]

Regarding: last passage of RC section (maize passage)

I find this whole passage challenging and had trouble with a lot of the questions, so any input on conceptualizing/processing/etc. the passage as a whole would be greatly appreciated.

Question 23: as I understand it, basically asks what process could replace (get same effect of) C4-photosynthesis in a plant which doesn't have the C4 thing. I found most of the answer choices confusing... can someone please explain this one?

[JJDancer]

similar advantage to C4...think of what C4 does in a plant.
Line 30-35 : oxygen relative to CO2 can rise under normal conditions, oxygen begins to bind competitively with rubisco, interfering with photosynthesis.
you need to stop oxygen from impairing photosynthesis (line 36..) READ ON FOR HOW TO DO THIS:
Line 45 (key to the process is that gases are separated from rubisco)
Line 50 - CO2 has to go through reactions to become nongas C4 - enters sheath cells and there undergoes reactions that fuel the production of carbs (sugars).

So..B says you have something else instead of rubisco, but it is something that oxygen can't bind with - so the problem described in lines 30-35 wouldn't happen.

Passage as a whole:
- intro: maize/corn changes cultures - urbanization. corn+american potatoes = euro population explosion. plant is awesome -but why? because of PRODUCTIVITY (it grows a lot). but WHY does it grow a lot? "physical mechanism"
- parag 2: How all plants do photosynthesis + an issue that can arise.
Detail: I drew a little diagram that shows H2O with one arrow (H) going to energy and another (O) to atmosphere.
. . During photosynth plant takes in CO2 from air and uses it to build sugars inside plant. Enzyme RUBISCO (very imp) assists. if oxygen > CO2, problem comes that O binds to rubisco.

Parag 3: Some plants (the productive ones like corn) eVOLVED w some MECHANISM (what is it!!?!?! i want to know! lol) that prevents O from being an issue.
They compartmentalize where they split H2O and where they build sugar so that there isn't any binding/interference from the O. Water split is same as all plants but rubisco is airtight in center of leaf (special to the productive plants). Cells containing rubisco = bundle sheath cells. CO2 has to become nongas to enter it and create carbs/sugars there (because otherwise O would get inside too.. if all gases could).

Sorry if the explanation is sort of weird. I simplify things and think pretty elementarily so that's how I presented it. When reading I skip over stuff that seems too detailed like lines 41-43 abt water splitting or stuff that seems weird like lines 49-50 ( i was worried there would be some question abt the human blood vessel analogy in which case i would refer to those lines and try to understand but on the first read I just read for "Where can i find which info" and "What are the main things the author agrees/disagrees with" or in this case "corn is awesome because it grows a lot but why does it grow more than other plants, it has some cool physical mechanism but i dont know what that is until paragraph 3 and paragraph 2 is about normal plants photosynthesis + the problem that the mechanism solves.)

HTH

[jesuis]

Beautiful! Thank you so much, your way of simplifying that mess of a passage makes it much more clear and now when I read it it makes sense to me since I'm approaching it in that light.

I was also able to realize that I missed a major point--Line 34: problem = oxygen binds to RUBISCO--passage says "to the enzyme" but now I see they mean it binds to rubisco, so now B really makes sense to me

Thanks again!