Video Transcript
Where in a plant are the highest
concentrations of auxins found? (A) In the wall of the xylem, (B)
in the tip of the stem, (C) in the guard cells of the stomata, or (D) in the pollen
of the flowers.
To answer this question, we need to
work out where the highest concentrations of auxins are found.
Auxins are a group of chemical
messengers called hormones that control cell elongation in plants, among their many
other functions. Auxins are produced in and released
from cells at the tip of plant stems and roots, as these are often the areas of the
plant where most cell elongation, and therefore growth, is required. From there, auxins can diffuse to
the rest of the plant from cell to cell.
The action of auxins in the stems
of plants is especially important for various reasons. For example, the stems and any
leaves that may be attached to them usually contain the majority of a plant’s
photosynthesizing cells. Remember, photosynthesis is the
process by which most plants use light energy, carbon dioxide, and water to
synthesize sugars like glucose, providing the organism with a food source.
As this process requires light, it
is imperative that the stems of a plant are able to grow in the appropriate
direction to respond to a light stimulus. This occurs through a process
called phototropism, which involves a plant stem growing toward a light source and
therefore increasing the surface area of the photosynthesizing parts of the plant
that are exposed to light.
Phototropism is controlled by
auxins. So, high concentrations of auxins
must be continually produced in the tip of the plant stem to allow these directional
growth movements in response to light. This means that the correct answer
to this question is (B). The highest concentrations of
auxins are found in the tip of the stem.
