Reading Comprehension

PT138 · S1 · P2 · Q12 Plant Evolution

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The characteristic smell or taste of a plant, to insects as well as to humans, depends on its chemical composition.

Topic

How plants’ distinctive smells and tastes (from secondary chemical substances) have evolved, especially through interactions with insects.

Framework

Old / New and Answer / Explain

Main Point

Plants contain unique secondary chemical substances—mainly for attracting or defending against insects—and this chemical arms race has driven the evolutionary specialization between plants and plant-eating insects. The "most valuable sentence" for the main point is near the end of P2:

P1: Basic Plant Chemistry—Primary vs. Secondary Substances

Plants all have “primary substances” (things like proteins and carbs) they need for survival, but the interesting “secondary substances”—unique to different plants and responsible for their taste and smell—don’t impact their basic function.

P2: Why Plants Have Secondary Substances—Insects and Natural Selection

These secondary substances evolved because of genetic mutations, but only stuck around if they provided an evolutionary advantage, mostly by either attracting helpful insects (like pollinators with appealing scents) or defending against harmful ones (with bad tastes, toxins, or warning smells).

P3: The Ongoing Evolutionary Battle between Plants and Insects

Plants and insects have been locked in a chemical arms race for ages: as plants develop new defenses, insects are forced to adapt or find new food. Over time, this has led to insect species specializing in eating just a few types of plants—basically, both sides have become more and more choosy!

12.

Which one of the following describes a set of relationships that is most closely analogous to the relationships between plants and their primary and secondary substances?

  1. Electrical power for the operation

    Bad Match

    Generators are a back-up plan for achieving the core functioning of supplying electrical power to a hospital. The hospital usually gets its power from the local power grid (presumably), which would play the role of Primary. The generators, the Secondary substances, are used when the grid is down. In plants, are secondary substances things that take over the normal functioning when primary substances are unavailable? No, secondary substances are never handling the core functioning of growth and metabolism.

    1% picked this

  2. Correct

    Mechanical components such as engines

    Why this is right

    This gives us the innards vs. the outer stuff, the core components vs. the bells and whistles. Engines/transmissions are the Primary substances that deal with core functioning. Paint/taillights are the Secondary substances (the bells and whistles) that give the car its signature appearance. But they have nothing to do with the core functioning of the car.

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

    76% picked this

  3. Mechanical components such as gears

    Bad Match

    The contrast here between gears/rotors and wires/transformers is one of being directly related to the core functioning vs. being indirectly related to core functioning. The wires/transformers (the Secondary substances) provide the raw material that powers the gears/rotors (the Primary substances). While this partially works by making one thing seem less connected to core functioning than the other, Secondary substances in plants aren't at all connected to the core functioning of growth and metabolism. In this answer, without wires and transformers those gears and rotors wouldn't work. Meanwhile, without Secondary substances the Primary substances would be totally fine in terms of making the plant 'work'.

    19% picked this

  4. Some type of braking system

    Bad Match

    In this answer, the things potentially playing the role of Primary (friction components) and Secondary (pneumatic components) are both cooperatively involved in the core functioning of the braking system. But in plants, secondary substances are not combining with primary substances to accomplish the same task. They are assigned to completely different roles: PRIMARY: make plant grow / photosynthesize SECONDARY: give plant a certain smell / taste

    3% picked this

  5. Specially designed word processing programs

    Bad Match

    The distinction presented here is whether a useful thing will be stored internally (saved on the computer's internal hard drive) or stored externally (on disks that can be inserted or removed). That doesn't apply at all to Primary and Secondary, both of which are internal substances to the plant but have different roles. And whether a word processing program is saved on the hard drive or on a disk, it's still playing the same role (allowing the user to use their computer like a word processor).

    1% picked this

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