bio midterm plants

What do shoots exhibit with respect to gravitropism, and why?

Shoots exhibit negative gravitropism because they grow upward against the direction of gravity.

What do roots exhibit with respect to gravitropism, and why?

Roots exhibit positive gravitropism because they grow downward in the same direction as gravity.

In shoots, where does auxin accumulate when the stem is placed horizontally?

In shoots placed horizontally, auxin accumulates on the lower side of the stem.

In shoots, what effect does auxin have on the cells where it accumulates?

In shoots, auxin increases cell elongation on the side where it accumulates.

How does the accumulation of auxin on the lower side of a shoot cause it to bend?

Cells on the lower side elongate faster than cells on the upper side, causing the shoot to curve upward against gravity.

In roots, where does auxin accumulate when the root is placed horizontally?

In roots placed horizontally, auxin also accumulates on the lower side.

In roots, what effect does auxin have on the cells where it accumulates?

In roots, auxin inhibits cell elongation (cells grow more slowly where auxin accumulates).

How does the accumulation of auxin on the lower side of a root cause it to bend?

Cells on the lower side grow more slowly, while cells on the upper side grow faster, causing the root to bend downward with gravity.

What is the key similarity in auxin distribution in roots and shoots during gravitropism?

In both roots and shoots, auxin accumulates on the lower side where gravity pulls.

What is the key difference in auxin’s effect between roots and shoots?

In shoots, auxin stimulates cell elongation; in roots, auxin inhibits cell elongation.

What cellular structures directly sense gravity in plants during gravitropism?

Amyloplasts (statoliths) in specialized cells directly sense gravity.

What is the role of amyloplasts in gravitropism?

Amyloplasts settle to the lower side of the cell under gravity and trigger a signaling pathway that redistributes auxin to the lower side of the organ.

Conceptually, how do amyloplasts and auxin work together to produce gravitropism?

Amyloplasts sense gravity and initiate signaling that moves auxin to the lower side; auxin then alters cell elongation differently in roots vs shoots, causing curvature.

Shoots growing upward against gravity show which type of gravitropism, and what is the auxin pattern?

They show negative gravitropism, with auxin accumulating on the lower side of the shoot, promoting elongation there.

Roots growing downward with gravity show which type of gravitropism, and what is the auxin pattern?

They show positive gravitropism, with auxin accumulating on the lower side of the root, inhibiting elongation there.

What is a tropism in plants?

A tropism is directional growth of a plant toward or away from a stimulus.

What is thigmotropism?

Thigmotropism is directional growth in response to touch or contact with an object.

Give an example of a plant structure that shows thigmotropism.

Tendrils of climbing plants that coil around supports show thigmotropism.

What is chemotropism?

Chemotropism is directional growth in response to a chemical stimulus (toward or away from chemicals).

What is traumatropism?

Traumatropism is directional growth in response to wounding or injury.

Why is gravitropism emphasized more than other tropisms in this lecture?

Because gravitropism involves detailed mechanisms of auxin distribution and differential growth that are heavily tested, while the others are mainly definitional.

How does phototropism differ from gravitropism?

Phototropism is directional growth in response to light, while gravitropism is directional growth in response to gravity.

What is dormancy in plants?

Dormancy is a state of greatly reduced growth and metabolism that allows plants or seeds to survive unfavorable conditions, like extreme temperatures or drought.

How does dormancy provide a survival advantage?

By pausing growth during harsh conditions, plants avoid damage and can resume growth when conditions improve.

What are some environmental signals that can induce dormancy?

Cold temperatures, drought, and other tough environmental conditions can induce dormancy.

How can dormancy occur in seeds?

Seeds can remain dormant until specific cues (such as fire, mechanical damage, or passage through an animal gut) break dormancy and trigger germination.

In cold temperatures, what problem do plant cell membranes face?

Cold temperatures make cell membranes more rigid, which can prevent them from functioning properly.

What enzyme helps plants acclimate to cold by changing membrane properties?

The enzyme desaturase helps plants acclimate to cold.

What does desaturase do to membrane fatty acids?

Desaturase converts saturated fatty acids into unsaturated fatty acids.

How does increasing unsaturated fatty acids affect membrane fluidity at low temperatures?

More unsaturated fatty acids introduce kinks, making membranes more fluid and preventing them from becoming too rigid in the cold.

What is the difference between saturated and unsaturated fatty acids?

Saturated fatty acids have no double bonds and pack tightly (more rigid); unsaturated fatty acids have double bonds that create kinks, preventing tight packing (more fluid).

Why is membrane fluidity important for plant cells in cold conditions?

Membrane fluidity is necessary for proper membrane transport, signaling, and enzyme function; if membranes are too rigid, these processes fail.

How do high temperatures damage plant proteins?

High temperatures can denature proteins, disrupting their three-dimensional structure and causing loss of function.

What are heat shock proteins (HSPs)?

Heat shock proteins are chaperone proteins that help stabilize and refold other proteins, protecting them from heat-induced denaturation.

How do heat shock proteins help plants tolerate high temperatures?

They bind to partially unfolded proteins, prevent aggregation, and help them maintain or regain their functional shape.

Conceptually, how do plants acclimate to cold vs heat at the molecular level?

In cold, they increase desaturase activity to make membranes more unsaturated and fluid; in heat, they increase heat shock protein production to protect proteins from denaturation.

What is a hormone (in the context of plants)?

A hormone is a chemical signal produced in small quantities, transported to specific target tissues, where it triggers a physiological response.

How are plant hormones similar to animal hormones?

Both are produced in one region, travel (e.g., through fluids), and act only on target cells that can respond, triggering specific signaling pathways.

What can plant hormones do to physiological processes?

Plant hormones can stimulate or inhibit processes such as growth, division, differentiation, and responses to stress or environment.

Which three plant hormones are the main focus of this lecture?

Au xin, cytokinin, and ethylene (plus briefly gibberellins earlier in the course).

What is the primary effect of auxin on shoot cells?

In shoots, auxin promotes cell elongation by increasing cell wall looseness (making them more extensible).

How does auxin affect the flexibility of cell walls?

Auxin increases cell wall plasticity, allowing cells to elongate more easily.

Besides gravitropism, what are two other roles of auxin mentioned in the lecture?

Promoting lateral root growth and maintaining apical dominance.

What is apical dominance?

Apical dominance is the phenomenon where the main central stem (apical bud) grows more strongly than lateral buds, keeping the plant taller rather than bushy.

How does auxin contribute to apical dominance?

Apical buds produce auxin, which moves downward and suppresses the growth of lateral buds, favoring upward growth.

What is the vascular cambium, and how does auxin affect it?

The vascular cambium is a meristem that produces secondary xylem and phloem; auxin stimulates its activity, promoting secondary growth.

What classic experiment showed auxin’s role in shoot bending?

An experiment where the tip of an oat seedling was cut off, auxin diffused into an agar block, and the block was placed on one side of a decapitated shoot, causing it to bend.

In the classic agar block auxin experiment, why did the shoot bend toward the side with the agar block?

Because auxin in the block caused cells on that side to elongate more, making the shoot curve toward the auxin source.

Why was the classic auxin experiment done in the dark?

To remove light as a variable, ensuring that bending was due to auxin and not phototropism.

In the context of the lecture, what is the main thing you need to remember about auxin’s action in shoots vs roots?

In shoots, auxin promotes elongation; in roots, auxin inhibits elongation.

Where are cytokinins primarily produced in plants?

Cytokinins are primarily produced in the root apical meristems.

What is the general role of cytokinins?

Cytokinins stimulate cell division and differentiation, often working in combination with auxin.

How does the ratio of cytokinin to auxin affect plant development?

Different cytokinin:auxin ratios promote different developmental outcomes, such as more root vs more shoot growth.

What developmental outcome is associated with high auxin and low cytokinin in culture?

High auxin and low cytokinin promote root formation.

What developmental outcome is associated with high cytokinin and low auxin in culture?

Shoot formation.

What happens to development when both auxin and cytokinin are at intermediate levels?

Intermediate levels of both can lead to little or no organized development.

How do auxin and cytokinin interact in controlling lateral bud growth?

When auxin is present from the apical bud, it suppresses cytokinin-driven growth of lateral buds; when auxin is removed, cytokinins can promote lateral bud growth into branches.

In the experiment with lateral buds, what happens when the apical tip (auxin source) is removed but cytokinin is present?

Without auxin, cytokinin promotes the growth of lateral buds into branches.

In the same experiment, what happens if auxin is added back to the cut stem while cytokinin is present?

Adding auxin back suppresses lateral bud growth again, preventing them from becoming full branches.

Conceptually, why does adding auxin back to a decapitated shoot stop lateral bud growth?

Because auxin restores apical dominance by suppressing the cytokinin-induced growth of lateral buds.

What is the main role of ethylene in plants discussed in this lecture?

Ethylene plays a key role in fruit development and ripening.

How does ethylene level change in response to plant stress?

Ethylene production increases when plants experience stress such as toxic chemicals, temperature extremes, drought, or pathogens.

Why might it be adaptive for plants to increase ethylene under stress?

Increasing ethylene can accelerate fruit development and seed maturation, allowing reproduction before the plant is severely damaged.

How is ethylene involved in ripening fruits like bananas and avocados?

Ethylene gas promotes ripening; storing fruits together (e.g., avocado with banana) allows ethylene from one fruit to speed ripening of the other.

What is a practical use of ethylene manipulation in agriculture (as described in the lecture)?

Tomatoes can be engineered or treated so they produce less ethylene during shipping (staying firm), then exposed to ethylene later to ripen before sale.

In the transgenic tomato example, what is the purpose of using antisense to the ethylene gene?

Antisense RNA pairs with the normal ethylene mRNA, preventing its translation and reducing ethylene production, which slows ripening.

What are the three major categories of plant defenses discussed in this lecture?

(1) Physical defenses (dermal tissue, cuticle, trichomes, bark, thorns); (2) Chemical defenses (secondary metabolites, allelopathy, toxins); (3) Mutualistic/animal-assisted defenses (ants, parasitoid wasps).

Why do plants rely heavily on structural and chemical defenses?

Because they are immobile and cannot escape threats, so they must protect themselves where they stand.

What is the first line of defense in plants?

The dermal tissue, which forms the outer protective layer of the plant.

Name three physical structures that are part of dermal defenses.

Waxy cuticle, trichomes, and bark (plus thorns as a physical deterrent).

What is the main defensive role of the waxy cuticle?

It reduces water loss and forms a barrier that helps protect against pathogens and some herbivores.

What are trichomes and why are they important in defense?

Trichomes are hair-like outgrowths that can reduce water loss and secrete sticky or toxic substances to deter herbivores.

Why are thorns considered a plant defense?

They physically deter animals by poking or injuring them when they try to eat the plant.

Which of these is NOT a dermal protection structure: trichomes, bark, thorns, stomata?

Stomata; they are mainly for gas exchange and water transpiration, not defense.

What is the primary function of stomata?

To regulate gas exchange (CO₂ in, O₂ and water vapor out) and water loss, rather than directly defending the plant.

What are secondary metabolites?

Secondary metabolites are plant-produced chemicals not directly needed for growth or reproduction, but used for defense (e.g., toxins that harm or kill herbivores).

Give three classic examples of plant secondary metabolites that humans use as drugs.

Caffeine, nicotine, and cocaine.

Why are many pharmaceuticals derived from secondary metabolites?

Because the same potent biological effects that deter herbivores also strongly affect human physiology, making them useful as medicines or psychoactive drugs.

What is allelopathy?

when a plant secretes chemicals (often from roots) that inhibit seed germination or growth of neighboring plants.

How does allelopathy benefit the plant that produces the chemicals?

It reduces competition for water, nutrients, and light, giving the allelopathic plant a competitive advantage.

What is the classic allelopathic example given in this lecture?

The black walnut tree, which releases toxins that inhibit germination and growth of many nearby plants.

From which plant is ricin derived, and what type of molecule is it?

Ricin is a highly toxic protein derived from the castor bean plant (castor bean endosperm).

Why is ricin not toxic to the plant that produces it?

In the plant, ricin is stored as an inactive heterodimer (ricin A + ricin B together), so it does not disrupt the plant’s own translation.

What happens to ricin when a human ingests castor beans?

Stomach acid and digestion separate ricin A from ricin B, activating ricin A.

What is the effect of activated ricin A on human cells?

Ricin A inhibits ribosomal RNA (rRNA) production, halting translation and leading to cell death.

Why does blocking rRNA synthesis kill cells?

Without rRNA, functional ribosomes can’t form, proteins can’t be synthesized, and the cell dies.

How does the toxicity of ricin compare to cyanide and cobra venom?

Ricin is about six times more lethal than cyanide and twice as lethal as cobra venom.

What are cyanogenic glycosides (like maniotoxin) and where are they found?

Cyanogenic glycosides, such as maniotoxin, are compounds found in cassava and yucca that can release cyanide when metabolized.

What is the danger of improperly prepared cassava?

Cyanogenic glycosides may convert to cyanide when eaten, inhibiting the electron transport chain and potentially killing the consumer.

What are phytoestrogens and what is a common dietary source?

Phytoestrogens are plant compounds that mimic estrogen; a common source is soybeans and soy products.

Why are phytoestrogens biologically significant in humans?

They can influence hormonal balance and may affect conditions linked to estrogen levels (e.g., menopause symptoms).

What is Paclitaxel (Taxol) and what is its medical role?

Paclitaxel (Taxol) is a chemotherapy drug that stabilizes microtubules and prevents cell division, used in cancers such as breast cancer.

From which plant was Paclitaxel originally isolated?

From the Pacific yew tree.

What conservation issue did Paclitaxel originally cause, and how was it solved?

Overharvesting of Pacific yews for Paclitaxel nearly drove them extinct; synthetic/semisynthetic production methods were developed to avoid cutting the trees.

What is quinine and what disease is it used to treat?

an alkaloid from bark used as an anti-malarial drug.

How does quinine affect malaria parasites?

It interferes with the parasite’s DNA replication and survival.

How was quinine historically used before its mechanism was fully understood?

People drank bark preparations that empirically reduced malaria symptoms, and the practice continued long before the active compound and mechanism were known.

What is morphine and from what plant is it derived?

a powerful painkiller derived from the opium poppy.

Why does morphine need strict medical control?

Because it is highly addictive and can cause dependence and abuse.

What type of symbiotic relationship allows animals to help defend plants?

Mutualism, where both the plant and the animal benefit.