Reading Comprehension

PT122 · S3 · P4 · Q23 Maize

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Every culture that has adopted the cultivation of maize—also known as corn—has been radically changed by it.

Topic

How maize (corn) and some similar crops became so important and productive due to a unique photosynthesis process.

Framework

Answer / Explain

Main Point

The incredible productivity of maize and some other crops, which has had major impacts on societies, is due to a special adaptation in their photosynthesis process called C-4 photosynthesis, which prevents oxygen from interfering with sugar production. Most Valuable Sentence: The first sentence of paragraph 3: The last paragraph as a whole reveals the explanation for maize's productivity.

P1: The Transformative Impact of Maize

Maize (corn) has radically changed every society that adopted it, dramatically boosting food production and even supporting population growth. The reason for its influence is how productive it is compared to other crops, but the passage sets up a question: why is it so much more productive?

P2: The Biological Challenge of Photosynthesis

This paragraph explains some basics of plant photosynthesis and introduces a problem: in most plants, a key enzyme called rubisco can be interfered with by oxygen, which messes up how efficiently plants make food (sugar) from sunlight and carbon dioxide.

P3: The C-4 Photosynthesis Solution

Certain plants, like maize, have evolved a special way to avoid this problem. In these plants, the process is split up in a way that keeps oxygen away from rubisco. Carbon dioxide gets turned into a special molecule called C-4, which is then used in sugar production. This process—C-4 photosynthesis—explains why crops like maize, sugar cane, and rice can produce so much food.

23.

Assuming that all other relevant factors remained the same, which one of the following, if it developed in a species of plant that does not have C-4 photosynthesis, would most likely give that species an advantage similar to that which the author attributes to C-4 plants?

  1. Water is split into its

    Not a Solution

    This plant would have hydrogen and oxygen initially separated into their own little caves. At first this is somewhat tempting. C-4 plants keep the rubisco in a bundle sheath cave to protect it from oxygen, so maybe putting oxygen in a cave could also protect rubisco from oxygen? The problem is that once plants separate the H from the O, they use the H and release the O into the atmosphere. The Oxygen that interferes with rubisco is actually just atmospheric oxygen (the regular ol' oxygen in the air everywhere, not the specific oxygen atoms that were taken from the water the plant took in). So initially having oxygen in a bundle sheath cave doesn't do anything. If rubisco is still out and about (as it is in non C-4 plants), then it's still going to interact with the gases in the air, which include oxygen.

    6% picked this

  2. Correct

    An enzyme with which oxygen

    Why this is right

    Since the original problem was rubisco binding with oxygen, if an enzyme can do what rubisco does for photosynthesis while not being threatened by oxygen, then we've solved the same problem. We don't need to hide this replacement-rubisco because it isn't interested in binding with oxygen.

    Skill tested: Application · how this choice captures the passage's function is the move to repeat next time.

    72% picked this

  3. The vascular structures of the

    Unclear Impact

    If the vascular structures are impermeable to oxygen gas, then that would be a safe place for rubisco and carbon to intermingle, but do we know if rubisco is in these vascular structures? The beginning of the final paragraph makes it sound like in non C-4 plants, rubisco is not separated from the places where water is being split into hydrogen and oxygen. Rubisco, carbon, hydrogen, and oxygen are all working in the same area, in chlorophyll-containing structures in the green leaf cells. So even though vascular structures are impermeable to oxygen gas, this answer isn't telling us that where rubisco is will be impermeable to oxygen gas.

    11% picked this

  4. The specialized chlorophyll-containing structures in

    Not a Solution

    The last paragraph's beginning informs us that in non C-4 plants, the rubisco is doing its thing in the same area where water is being split: within specialized chlorophyll-containing structures. It doesn't really matter where we move these structures, because if rubisco and oxygen are both involved, then the rubisco binding with oxygen is still unresolved.

    5% picked this

  5. An enzyme that does not

    Not a Solution

    We love that rubisco readily reacts with carbon dioxide. That wasn't our problem. We were mad that rubisco gets distracted and reacts with oxygen. This answer choice seems to describe a change that would make plants even worse at photosynthesis. These plants would struggle to make the sugars that keep the plant going.

    6% picked this

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