Plants bend toward a light source as a result of:
A. the necessity of light for transpiration.
B. an inability to synthesize chemical regulators.
C. an unequal auxin distribution in their stems.
D. the increased amount of food synthesized by their leaves.
Plant hormones, auxins, are produced in the meristematic regions of the plant, one of which is the tips of the stems. These auxins initiate growth in the plant. If the sunlight is normally found directly above the plant, the auxins are distributed evenly and the plant grows upwards. If, however, the light is found more on one side of the plant, the auxins are produced unevenly. More auxin will be produced on the side away from the light. Then the plant on that side will grow more, causing it to bend, or grow, towards the light.
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While these questions do not fit the MCAT format (they have no related reading passages) they cover concepts one should understand in order to do well in the biological sciences portion of the MCAT.

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