Video Transcript
In an experiment investigating
hormonal control in plants, two young plant shoots are taken and the tips of both
are removed. In experiment one, the cut section
of a shoot is covered with a block of permeable gelatin. In experiment two, the cut section
of a shoot is covered with a layer of impermeable mica. Then, the tips are placed back on
the shoots and they are left to grow for a week in the presence of a light
source. In which experiment would you
expect the shoot to bend toward the light?
Hormones are chemical messengers
that travel throughout an organism’s body to cause a particular effect. Auxins are a group of plant
hormones that can control cell elongation among their many other functions. Auxins are influenced by light,
which allows them to stimulate directional growth in response to a light stimulus
through a process called phototropism.
Let’s look more closely at how
auxins work. Auxins are usually produced in
cells at the tip of a plant shoot and diffuse down the plant from cell to cell. When a light source is arriving
from one side, auxins usually accumulate in the cells on the shaded side of the
shoot, causing these cells to elongate comparatively more than the cells on the
illuminated side of the shoot. The asymmetrical cell elongation
results in the plant bending, or curving, toward the light source. This allows the photosynthetic
cells, which are mostly found in the leaves and shoots of plants, to access more
light for photosynthesis so that they are able to produce sugars at a faster
rate.
Now that we understand how plants
grow in response to light, let’s think about the proposed experiment in the
question. We know that auxins are made in the
tip of the shoot and diffuse downward through the plant. The question tells us that the
gelatin placed between the tip and the rest of the shoot in experiment one is
permeable. If permeable gelatin separates the
shoot tip from the rest of the plant, the auxins that are produced in the shoot tip
will still diffuse down through the gelatin and influence directional growth in the
same manner as without the gelatin present.
However, we are told that the mica
which is placed between the tip and the rest of the shoot in experiment two is
impermeable. If the impermeable mica is placed
between the tip of the shoot and the rest of the plant, the auxins produced in the
shoot tip cannot diffuse through it and therefore cannot access the cells lower down
in the shoot. This means that they cannot affect
directional growth. So phototropism does not occur and
the plant will likely not grow at all. This means that we can deduce the
experiment in which the shoot would be expected to bend toward light and therefore
the correct answer to this question. We would only expect the shoot to
bend toward the light source in experiment one.
