Book cover for Biology

Biology

Sylvia S. Mader, Michael Windelspecht

ISBN #9780078024269

12th Edition

687 Questions

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153,501 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter explores the complex roles of plant hormones and signal transduction in regulating growth, development, and movement responses. It covers the unique and overlapping functions of the five major hormones—auxins, gibberellins, cytokinins, abscisic acid, and ethylene—as well as the molecular mechanisms behind tropisms, turgor movements, and light-sensing by photoreceptors like phototropin and phytochrome. The chapter also highlights the practical applications of these processes in agriculture and biotechnology, offering insights into both basic scientific concepts and their real-world implementations.

Learning Objectives

1

Explain the role of major plant hormones (auxins, gibberellins, cytokinins, abscisic acid, and ethylene) in regulating plant growth and development.

2

Describe the cellular signal transduction mechanisms involved in hormone action.

3

Analyze how plant growth responses such as tropisms, turgor movements, and photoreceptor-mediated responses are executed.

4

Evaluate the commercial applications of plant hormone research in agriculture and biotechnology.

Key Concepts

CONCEPT

DEFINITION

Plant Hormones

Chemical messengers that regulate various aspects of plant growth, development, and response to environmental stimuli.

Auxins

A class of hormones primarily responsible for cell elongation, apical dominance, and tropic responses such as phototropism and gravitropism.

Gibberellins

Hormones that promote stem elongation, seed germination, and the transition from vegetative growth to flowering.

Cytokinins

Hormones that stimulate cell division and influence organ development and nutrient mobilization.

Abscisic Acid

A hormone that generally acts to inhibit growth, induce dormancy, and mediate responses to stress conditions.

Ethylene

A gaseous hormone involved in fruit ripening, leaf abscission, and the response to mechanical stress.

Signal Transduction

The process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, leading to a cellular response.

Tropisms

Directional growth responses that occur in response to environmental stimuli such as light (phototropism) and gravity (gravitropism).

Turgor Movements

Movements in plants resulting from changes in cell turgidity, which affect cell pressure and enable rapid movements or adjustments.

Phototropin

A blue light photoreceptor that mediates phototropic responses by triggering cell growth on one side of the plant.

Phytochrome

A red/far-red light photoreceptor that regulates various aspects of plant development, including seed germination, shade avoidance, and flowering.

Example Problems

Example 1

During which step of signal transduction is a second messenger released into the cytoplasm? $$\begin{array}{ll}{\text { a. reception }} & {\text { c. transduction }} \\ {\text { b. response }} & {\text { d. final step }}\end{array}$$

Example 2

For questions $2-8,$ match each statement with a hormone in the key. Answers can be used more than once. KEY: a. auxin b. gibberellin c. cytokinin d. ethylene e. abscisic acid It is present in a gaseous form

Example 3

For questions $2-8,$ match each statement with a hormone in the key. Answers can be used more than once. KEY: a. auxin b. gibberellin c. cytokinin d. ethylene e. abscisic acid Grapes can grow larger and exhibit stem elongation

Example 4

For questions $2-8,$ match each statement with a hormone in the key. Answers can be used more than once. KEY: a. auxin b. gibberellin c. cytokinin d. ethylene e. abscisic acid Stomata close when a plant is water-stressed.

Example 5

For questions $2-8,$ match each statement with a hormone in the key. Answers can be used more than once. KEY: a. auxin b. gibberellin c. cytokinin d. ethylene e. abscisic acid Stems bend toward the sun.
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Step-by-Step Explanations

QUESTION

How does auxin promote cell elongation in plants?

STEP-BY-STEP ANSWER:

Step 1: Synthesis and transport - Auxin is produced mainly in the apical meristems and is transported down the stem via polar transport.
Step 2: Receptor binding - Auxin binds to specific receptors in the cell, initiating a signal transduction cascade.
Step 3: Activation of gene expression - The signal leads to the activation of genes that encode cell wall-loosening enzymes.
Step 4: Cell wall modification - The enzymes cause the cell wall to loosen, allowing cells to expand as water enters.
Final Answer: Auxin promotes cell elongation by triggering a cascade that leads to cell wall loosening and expansion.

Auxin-Mediated Cell Elongation

QUESTION

What steps are involved in a phototropin-mediated phototropic response?

STEP-BY-STEP ANSWER:

Step 1: Light perception - Phototropin absorbs blue light, primarily on the side of the plant facing the light source.
Step 2: Signal initiation - Light absorption activates phototropin which then starts an intracellular signaling cascade.
Step 3: Redistribution of auxin - The signal causes an uneven distribution of auxins, with more auxin accumulating on the shaded side.
Step 4: Differential growth - The higher auxin concentration stimulates elongation on the shaded side, curving the plant towards the light.
Final Answer: Phototropin detects blue light and redistributes auxin to drive differential cell growth, resulting in phototropism.

Phototropism via Phototropin

QUESTION

How does phytochrome regulate plant developmental processes?

STEP-BY-STEP ANSWER:

Step 1: Light absorption - Phytochrome exists in two interconvertible forms that absorb red and far-red light.
Step 2: Conformational change - Absorption of light induces a conformational change, switching the phytochrome’s state.
Step 3: Nuclear signaling - The activated phytochrome moves into the nucleus where it influences gene transcription.
Step 4: Modulation of development - Changes in gene expression affect processes such as seed germination, shade avoidance, and flowering.
Final Answer: Phytochrome regulates development by switching between active and inactive forms upon light exposure, thereby modulating gene expression.

Phytochrome-Mediated Responses

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Common Mistakes

  • Mistaking the roles of different hormones, such as confusing the functions of auxins and gibberellins.
  • Overlooking the importance of signal transduction pathways in mediating hormonal effects.
  • Assuming that all plant growth responses are solely dictated by hormone levels without considering environmental cues.
  • Misinterpreting tropic responses by not recognizing the role of differential hormone distribution.
  • Ignoring the impact of photoreceptors like phytochrome and phototropin in regulating developmental processes.