Problem : Define a hormone and list the five different classes of plant hormones.

A hormone is a chemical that affects the ways in which an organism functions. It is produced in one part of the plant body but, by traveling to target cells throughout the body, affects many other parts as well. The five classes of plant hormones are auxins, cytokinins, gibberellins, inhibitors, and ethylene.

Problem : What is phototropism, and which hormone is responsible for it?

Phototropism is the growth of a plant toward a light source, resulting from the rapid elongation of cells on the dark side of the plant. The hormone that causes phototropism is auxin.

Problem : One of the plant hormones delays senescence (aging), and another stimulates it. Which hormone is responsible for each task?

Cytokinins delay senescence, especially in leaves. Ethylene, on the other hand, encourages senescence, especially in terms of fruit development and leaf abscission.

Problem : Why is the balance between auxins and cytokinins important to the plant?

The balance between these two hormones regulates the growth of roots and shoots so that the two portions of the plant remain balanced themselves.

Problem : Explain the acid growth hypothesis. What phenomenon in plants does this hypothesis account for?

The acid growth hypothesis explains the occurrence of phototropism. According to the hypothesis, auxins trigger proton pumps in cell membranes, lowering the pH in the cell wall to such an extent that the hydrogen bonds holding its cellulose fibers together break apart. These broken bonds give the cell wall greater flexibility and expandability, so that more water can enter the cell by diffusion, causing the cell itself to elongate.
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