Paper 2 · Geotropism
Why is a root positively geotropic?
AAuxin accumulates on the lower side of the root, causing that side to elongate faster, so the root curves upward, against gravity
BAuxin accumulates on the lower side of the shoot, not the root, causing that side to elongate faster, so the shoot curves downward towards gravity; this describes shoot behaviour, not a root's response
CAuxin is evenly distributed on both sides of the root, so both sides elongate at the same rate and the root does not curve
DAuxin accumulates on the upper side of the root, causing that side to elongate faster than the lower side, so the root curves downward
Explanation: Auxin builds up on the upper side of the root, making that side elongate faster than the lower side; the faster-growing upper side pushes the root tip to curve downward, towards gravity.
Derived from ZIMSEC Biology 4025/2 Paper 2 Theory, November 2019, Q2
