Video Transcript
The diagram provided shows the effect of IAA, a type of auxin, on the growth of a
plant shoot. What effect does IAA have on the cells in the shoot? (A) IAA stimulates cell differentiation. (B) IAA prevents cell growth. (C) IAA helps break down dead cells in the stem. Or (D) IAA encourages cell elongation.
Let’s start by investigating some key terms. Auxins are plant hormones that are usually produced in cells at the tip of plant
shoots and roots. Once produced, auxins typically diffuse from cell to cell to access different parts
of the plant. IAA or indole-3-acetic acid is one such auxin, produced predominantly by cells in
developing leaves and the top bud of the plant, often called the apical bud.
IAA is responsible for inducing cell division and controlling cell elongation, which
occurs when a plant needs to grow, for example, in response to a stimulus, such as
light. IAA produced by cells in the apical bud diffuses downward, binding to receptors on
the cells below. The binding of IAA stimulates a decrease in the pH of the cell, so it becomes more
acidic.
The increased acidity of the cell leads to the cell wall loosening. More water enters the cell, stored in the cell’s vacuoles that become larger and more
numerous. The vacuoles merge, and the increased water volume in the cell increases the turgor
pressure on the loosened cell wall, causing the cell to expand and elongate.
As we can see in the diagram provided by the question, IAA moves to and accumulates
on the shaded side of the plant when a light stimulus is coming from one side. This causes cell elongation on the shaded side causing the shoot to bend and grow
towards the light source.
Now, we can answer the question correctly. The effect that IAA has on the cells of the shoot is (D). IAA encourages cell elongation.
