Reading Comprehension

PT122 · S3 · P4 · Q21 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.

21.

Which one of the following most accurately states the main point of the passage?

  1. Correct

    The greater productivity of maize,

    Why this is right

    The main clause is "the greater productivity of maize is due to its C-4 photosynthetic process". That seems on point. It's about our central topic and it's giving us the main payoff idea of the Answer a Question framework. Do the rest of the details check out? In C-4 photosynthesis, is it true that the reactions that build sugars are protected from the effects of excess oxygen. We were hoping it would say "rubisco is prevented from competitively binding with oxygen". Well, in the 2nd paragraph we're told that "an enzyme, rubisco, assists in the sugar-forming chemical reaction", so it looks like "the reactions that build sugars" is just a buzzphrase for "rubisco". Can we live with "the effects of excess oxygen" as a match for "oxygen binds competitively with rubisco"? The end of the 2nd paragraph says, "when the concentration of oxygen relative to carbon dioxide in a leaf rises to a certain level ... oxygen begins to bind competitively to the enzyme, thus interfering with the photosynthetic reaction". If oxygen concentration rising to a certain level is what's causing the problem then I guess we can accept "the effects of excess oxygen" as a buzzphrase for this idea.

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

    72% picked this

  2. Because of their ability to

    Wrong Emphasis Wrong Causal Relationship

    The main clause here is "Those plants that use a C-4 photosynthetic process have helped to shape the development of many human cultures". That seems off for two reasons: 1) it doesn't mention maize 2) the main point was about how these C-4 plants have such impressive productivity, not about how much they've shaped the development of many cultures. This is basically First Sentence Bait. It's emphasizing an idea from the very first sentence because students are often hoping the first or last sentence of the passage is the main point. This also misconstrues the causal relationship between C-4 and cultures who have cultivated it. These plants aren't special because of "higher quality and quantity of nutrients". They're special because of their sheer productivity. They just grow more and more easily than other plants, because they have a more efficient photosynthetic process that doesn't get derailed by oxygen.

    10% picked this

  3. C-4 photosynthesis, which occurs in

    Wrong Causal Relationship Wrong Emphasis

    This answer's central topic is C-4 photosynthesis, which is not a total dealbreaker but not centering maize the way we expected. This says the magic sauce of C-4 is that the plant makes more efficient use of atmospheric hydrogen (rather than it prevents rubisco from competitively binding with oxygen). It doesn't seem like atmospheric hydrogen is used at all in photosynthesis. Plants get hydrogen from breaking about liquid water into hydrogen and oxygen. And then plants use atmospheric CO2.

    2% picked this

  4. The presence of the enzyme

    Wrong Causal Relationship

    Like (B) and (C), this makes maize an aside rather than putting it in the main clause. And like (B) and (C), it misidentifies what makes C-4 plants so awesome. They're awesome because rubisco is sequestered in airtight pockets. This answer is saying they're special because rubisco is present. Rubisco is present in all photosynthetic plants (or at least the vast majority). Its presence isn't the key factor that allows C-4 to thrive more than other plants. Its isolated presence within the airtight bundle sheath cells is the key factor. This answer is also a little weird for insinuating that the obstacle normal plants face is "the negative effects of gases such as oxygen". There wasn't a plurality of gases messing things up, of which oxygen is just one example. It was oxygen alone that was messing things up.

    2% picked this

  5. Some of the world’s most

    Too Strong: prevent gases from leaves

    This is pretty close. Again, maize is not center stage, which doesn't feel quite right, but it's still in there. The problem here is that C-4 plants do not prevent atmospheric gases from entering the plants leaves. They just prevent atmospheric gases from entering the bundle sheath cells, which are the protected cave that rubisco lives in. We know from the 2nd paragraph that "During photosynthesis, carbon dioxide that the plant takes in from the atmosphere is used to build sugars within the plant". All plants are taking in the atmospheric gas of CO2 into their leaves. What makes C-4 special is that these atmospheric gases are excluded from the cells containing rubisco, but they're not excluded from entering the plants' leaves entirely.

    14% picked this

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