BIO 193 Exam II Terms and Concepts

What is the purpose of signal transduction pathways in plants?

Signal transduction pathways link signal reception to a cellular response, allowing plants to detect and respond to environmental stimuli.

Why is the idea that plants are passive organisms incorrect?

Plants actively sense and integrate environmental information and respond by altering growth and development.

How do plant responses to stimuli generally differ from animal responses?

Animals often respond through movement, while plants primarily respond through changes in growth and development.

What is etiolation?

Etiolation is a set of physical adaptations that allow a plant to grow in darkness.

What is de-etiolation?

De-etiolation (greening) is the set of changes that occur when a dark-grown plant is exposed to light.

What major changes occur during de-etiolation?

Stem elongation slows, leaves expand, roots elongate, and chlorophyll production begins.

What triggers de-etiolation in plants?

Exposure to light detected by phytochrome receptors.

Define signal reception

Signal reception is the detection of a stimulus by receptor proteins that change shape in response to the signal.

Define signal transduction.

Signal transduction is the process by which a signal is relayed and amplified inside the cell through molecular pathways.

Define cellular response.

The cellular response is the final action triggered by signal transduction, such as gene expression or growth changes.

What is a phytochrome?

A phytochrome is a light-sensitive receptor pigment involved in detecting light signals in plants.

Where is the phytochrome receptor involved in de-etiation located?

In the cytoplasm.

What are second messengers?

Small molecules or ions that amplify and relay signals inside the cell during signal transduction.

What is cG?

Cyclic GMP is a second messenger produced by guanly cyclase during de-etiolation.

What is gu early cyclase?

Gu early cyclase is an enzyme activated by phytochrome that produces cG

What are caroteno noise?

yellow and orange accessory pigments visible when chlorophyll is absent.

What is a protein kinase?

An enzyme that modifies other proteins by adding phosphate groups during signal transduction.

Why are expanded leaves disadvantageous for plants growing underground?

They interfere with soil penetration and can be damaged while pushing through soil.

Why do dark-grown plants have short roots?

Because little water is lost through transpiration, reducing the need for extensive water absorption.

Why don’t dark-grown plants produce much chlorophyll?

Producing chlorophyll without light would waste energy because photosynthesis cannot occur.

What advantage does stem elongation provide during etiolation?

It helps the shoot reach the soil surface before nutrient reserves are exhausted.

Which tomato mutant helped researchers understand phytochrome function?

The aurea mutant

What characteristic does the aurea mutant show?

Reduced levels of phytochrome and reduced greening when exposed to light.

What experiment confirmed phytochrome’s role in de-etiations?

Injecting phytochrome from other plants into aurea cells restored normal de-etiolation when exposed to light.

Why can very weak light trigger de-eti actions responses?

Because signal transduction pathways amplify the signal through second messengers.

What happens to cytosolic calcium levels after phytochrome activation?

Calcium channels open and cytosolic calcium increases about 100-fold.

What shape change occurs during phytochrome activation?

The phytochrome undergoes a conformational (shape) change.

What enzyme is activated by phytochrome during de-etiation?

Gu early cycl lease.

Why are both cG and calcium required for full de-etiolation?

Both signaling pathways are necessary for a complete cellular response.

What are the three major stages of signal processing in plants?

1. reception

2. transduction

3. response

What occurs during the reception stage of de-etiation?

Light is detected by the phytochrome receptor.

What are the two signal transduction pathways activated by phytochrome?

One pathway uses cG and the other increases cytosolic calcium.

What does the cGMP pathway activate?

A protein kinase.

What does the calcium pathway activate?

A different protein kinase.

What is the final result of both signaling pathways?

Expression of genes involved in the de-etiation response.

A plant has pale stems, unexpanded leaves, and little chlorophyll. What condition is it likely growing under?

Darkness; the plant is showing etiolation.

A mutation prevents calcium channels from opening after phytochrome activation. How would de-etiolation be affected?

The de-etiolation response would be incomplete or impaired.

Why would a plant exposed to light develop broad green leaves?

Light activates phytochrons, triggering gene expression involved in chlorophyll production and leaf expansion.

If a plant could not produce cG after light exposure, what would likely happen?

The plant would show only partial de-etiion responses.

Which of the following best describes etiolation?
A. Rapid leaf drop in winter
B. Growth adaptations to darkness
C. Root expansion in dry conditions
D. Flower production in spring

B. Growth adaptations to darkness

Which receptor is responsible for detecting light during de-etiation?
A. Insulin receptor
B. Phytochrome
C. Aquaporin
D. Glycoprotein

B, Phytochrome

Where is the phytochrome receptor involved in de-etiion located?
A. Cell wall
B. Vacuole
C. Cytoplasm
D. Nucleus

C, cytoplasm

Which molecules act as second messengers during de-etiation?
A. DNA and RNA
B. ATP and lipids
C. Calcium ions and cG
D. Glucose and proteins

C, Calcium ions and cG

What directly activates protein kindness in the cGMP pathway?
A. Chlorophyll
B. Calcium ions
C. A proton kinase
D. Water pressure

C, A protein kinase

Explain why etiolation is considered an adaptation rather than a defect.

Etiolation increases the likelihood that shoots will successfully reach sunlight by prioritizing stem elongation and conserving energy underground.

How does de-etiion demonstrate the relationship between environmental signals and gene expression?

Light activates signal transduction pathways through phytochrons, leading to gene expression changes that alter plant growth and development.

Compare the role of second messengers in plants with their role in animal cells.

In both plants and animals, second messengers amplify signals and relay information inside the cell to trigger responses.

What is a protein kinase>

A protein kinase is an enzyme that activates proteins by adding phosphate groups to them (phosphorylation). This can change the protein’s activity, shape, or function during signal transduction.

What role do protein kinases play in de-etioring?

Protein kindness are activated by second messengers like c and cGMP and Ca2+, helping transmit the light signal and ultimately activating genes involved in de-etioring

What are second messengers in signal transduction pathways?

Second messengers are small molecules or ions inside the cell that relay and amplify signals from receptors to target proteins, helping produce a cellular response.

Why are second messages important in cell signaling?

They amplify weak external signals, allowing a small stimulus to trigger a large cellular response inside the cell.

How do calcium ions function as second messengers during de-etioring?

Light activation opens calcium channels, causing a temporary increase in cytosolic calcium levels, which helps activate protein kindness involved in the response.

Why is etiolation considered adaptive for underground growth?

It conserves energy and helps shoots reach sunlight by prioritizing stem elongation over leaf expansion, chlorophyll production, and root growth.

Why are leaves unexpanded during etiolation?

Expanded leaves would hinder soil penetration and could be damaged underground.

Why is water loss low in etiolateded plants?

The leaves are unexpanded, so transpiration is minimal.

What are the two main mechanisms by which signaling pathways enhance enzymatic steps in biochemical pathways?

Transcriptional regulation and post-translational modification.

What is transcriptional regulation?

Regulation that increases or decreases the synthesis of mRNA encoding a specific enzyme.

What is post-translational modification?

The activation or modification of preexisting proteins after they are synthesized.

What type of post-translational modification is most common in signal transduction pathways?

Phosphorylation of specific amino acids in proteins.

How does phosphorylation affect proteins?

It alters the protein’s hydrophobicity and activity.

What molecules commonly activate protein kinases?

Second messengers such as cGMP and Ca²⁺.

What is a kinase cascade?

A sequence in which one protein kinase phosphorylates another kinase, amplifying the signal.

Why are kinase cascades important?

They amplify signals and can connect external stimuli to changes in gene expression.

How can signal transduction pathways affect gene expression?

Through phosphorylation of transcription factors that regulate specific genes.

Why must signal transduction pathways have “switch-off” mechanisms?

To stop the response when the original signal is no longer present.

What enzymes are important in turning off signal transduction pathways?

Protein phosphatases.

What do protein phosphatases do?

They remove phosphate groups from proteins (dephosphorylation).

What determines a cell’s functioning at a given moment?

The balance between protein kinase activity and protein phosphatase activity.

What are specific transcription factors?

Proteins that bind to specific DNA regions and regulate transcription of specific genes.

How are transcription factors activated during phytochrome-induced de-etiolation?

By phosphorylation in response to light conditions.

What activates the protein kinases that phosphorylate these transcription factors?

Second messengers such as cGMP and Ca²⁺.

What is the difference between activator and repressor transcription factors?

Activators increase transcription of specific genes, while repressors decrease transcription.

What unusual trait do some Arabidopsis mutants show when grown in darkness?

They appear light-grown, with expanded leaves and short sturdy stems, despite being pale.

Why are these Arabidopsis mutants pale even though they look light-grown?

The final step of chlorophyll production requires light directly.

What defect do these Arabidopsis mutants have?

A defective repressor that normally inhibits light-activated genes.

What happens when the repressor is eliminated by mutation?

The normally blocked pathway proceeds even in darkness.

What types of proteins are produced or activated during de-etiolation?

Enzymes involved in photosynthesis, chlorophyll precursor synthesis, and hormone regulation.

Which plant hormones decrease after phytochrome activation?

Auxin and brassinosteroids.

What is the effect of decreased auxin and brassinosteroid levels during de-etiolation?

Stem elongation slows.

Why is stem elongation reduced during de-etiolation?

Because hormones that promote elongation decrease after phytochrome activation

How do plants communicate internally and with their environment?

Through chemical signaling molecules.

What structures allow macromolecules to move directly between plant cells?

Plasmodesmata

How are plasmodesmata different from animal gap junctions?

Plasmodesmata can dilate enough to allow large macromolecules such as proteins to pass between cells.

What is a hormone?

A signaling molecule produced in low concentrations that triggers responses in target cells.

Why is the traditional hormone concept considered limiting for plants?

Plants lack circulatory systems, some signals act locally, and some signaling molecules occur at much higher concentrations than typical hormones.

What is a plant growth regulator?

An organic compound, natural or synthetic, that modifies or controls physiological processes in plants

Why does the textbook continue using the term “plant hormone”?

For historical continuity and because these molecules act at very low concentrations.

Why can tiny amounts of plant hormones have large effects?

Because they activate signal transduction pathways that strongly influence growth and development

Do plant hormones usually work independently?

No, plant responses often depend on interactions between multiple hormones.

What factors determine a hormone’s effect in plants?

Site of action, concentration, developmental stage, and interactions with other hormones.

Where is auxin primarily produced?

Shoot apical meristems and young leaves.

What are the major functions of auxin?

Stimulates stem elongation, promotes root formation, regulates fruit development, enhances apical dominance, functions in phototropism and gravitropism, promotes vascular differentiation, and delays leaf abscission.

What concentration effect is important for auxin?

Low concentrations stimulate stem elongation.

Where are cytokinins primarily synthesized?

Roots

What are major functions of cytokinins?

Regulate cell division, promote lateral bud growth, stimulate seed germination, move nutrients into sink tissues, and delay leaf senescence.

Where are gibberellins primarily produced?

Apical buds, roots, young leaves, and developing seeds.

What are major functions of gibberellins?

Stimulate stem elongation, pollen development, fruit growth, seed development and germination, and regulate developmental transitions.

What is the primary role of abscisic acid?

Growth inhibition and stress responses

What happens to stomata during drought stress due to ABA?

They close

What processes does ABA promote?

Seed dormancy, desiccation tolerance, and leaf senescence.