AP BIO: CH. 39 - Plant Responses to Internal and External Signals (39.1 - 39.2)

Definition: Signal transduction pathways are mechanisms through which cells respond to external signals.

Definition: Ideolation refers to the growth responses of plants in the absence of light, characterized by tall, thin, and pale appearances, as they search for light.
Example: Plants grown in darkness tend to stretch upwards, exhibiting long stems as they allocate energy from their endosperm in search of sunlight.
Observation: Common household plants (e.g., potatoes, onions) that are stored in darkness may sprout and show undesirable growth patterns.

Definition: The process that occurs when a plant is exposed to light, leading to normal growth characterized by the production of chlorophyll and initiation of photosynthesis.
Role: Enzymes produced during deeddulation facilitate photosynthesis and assist in the regulation of plant hormones.

Overview: Hormones are chemical signals in plants controlling various processes and are typically present in low concentrations. Similar to second messengers in animals, they exert significant influence over plant behavior.
Comparison with animal hormones: Reference to human hormones like epinephrine as significant in physiological processes.

Xylem: Responsible for the upward transport of water and dissolved minerals from roots to stems and leaves.
- Function: Facilitates growth by supplying vital resources, primarily water.
Phloem: Functions in transporting food (sugars, amino acids) produced by photosynthesis from leaves to other parts of the plant.
- Storage sites include roots and tubers (potatoes, onions, carrots).
Analogy: Xylem and phloem can be likened to arteries and veins in animals, facilitating nutrient and water distribution.
Hormonal movement: Hormones utilize xylem and phloem for distribution across plant structures.

Overview: Discussion focused on key plant hormones like auxin and ethylene.
- Excluded hormones: Jasmonates and tricholactones, not covered in detail.

Definition: A critical hormone for growth and development, influencing the phototropic responses in plants (movement towards or away from light).
Tropism: A directional growth response; specifically, phototropism refers to responses to light direction.
- Historical experiment by Darwin: Studied phototropism in grass seedlings, concluding that the presence of the coleoptile tip was necessary for directional bending.
- Experimental findings: Removing or covering the coleoptile tip prevented bending, indicating that growth response is regulated by substances produced in the tip.
Acid Growth Hypothesis: Auxin promotes cell elongation by activating proton pumps, increasing hydronium ion concentration, and allowing cell walls to loosen through expansin enzymes, which leads to cell elongation.
- Geographic specificity: The acid growth strictly occurs on the apical and basal sides of the cell wall to influence upward plant growth.
Polar Transport of Auxin: Involves the movement from meristematic regions downwards to influence growth patterns, leaf arrangements, and branching development.
- Influence on leaf arrangement (phyllotaxy): Leaf position is optimized to maximize sunlight exposure and avoid overlapping.
Practical Uses of Auxin:
- Rooting in plant cuttings (e.g., Plumeria), stimulating root formation.
- Herbicides that overdose plants can inhibit normal growth leading to plant death.

Definition: Hormones that stimulate cell division, often found alongside auxins during growth.
Locations of production: Actively growing tissues such as roots, embryos, and fruits.
Interaction: Operate in tandem with auxins to promote cell division and differentiation.

Overview: Key hormone influencing various growth effects, including stem elongation, fruit growth, and seed germination.
- Distinction from auxins and cytokinins is based on specific growth processes influenced (e.g., fruit enlargement).
- Gibberellins are commonly applied in agriculture to enhance the size of seedless grapes.
Seed Germination: Initiation occurs via hydration signaling gibberellin production, breaking dormancy and leading to mitosis.

Definition: A growth inhibitor that regulates dormancy and drought tolerance in plants.
Seed dormancy: ABA coats seeds to maintain dormancy; removal or breakdown can trigger germination.
Drought response: ABA is released during dry conditions, leading to stomatal closure and water retention.

Characteristics: Unique gaseous hormone involved in various stress responses, including aging, injury, and fruit ripening.
Triple Response Mechanism: Assists plants in navigating obstacles by decreasing stem elongation, thickening the stem, and promoting horizontal growth.
Senescence: Involves programmed cell death, which can target specific cells, leaves, or organs to prevent the spread of damage.
Fruit Ripening: Ethylene enhances ripening through a positive feedback loop, with increased levels of ethylene triggering further production.
- Agricultural practices may involve controlling ethylene to manage fruit ripening, e.g., green bananas ripening in transit.

Homework: Complete section one concept check and summary, focusing on key plant processes discussed.
Reminder: Ensure plants receive adequate water as part of their care regimen.