Bio Exam 2 Study Guide

Signaling Pathways

  • Signal Reception

    • Receptors bind to specific molecules (hormones/environmental stimuli)

    • Initiates the signaling process

  • Signal Transduction

    • Messengers spread the signal within the cell

    • Activates proteins and enzymes that further relay the signal

  • Response

    • Includes processes like:

      • Phototropism (growth towards light)

      • Stress responses (e.g. closing stomata during drought)

      • Immune defense activation to combat pathogens

Light Perception in Plants

  • Perception Mechanism

    • Light is perceived by phytochromes, which absorb light

    • Conformational change in phytochromes activates plant response

  • Responses

    • De-etiolation (Greening): Stops elongation, develops chlorophyll, prepares for photosynthesis

    • Phototropism: Plant bends towards light as auxin accumulates on the shaded side, promoting cell elongation

Discovery of Auxins

  • Key Experiments

    • Darwin's observation (1880): Plant shoots bend toward light indicating a tip-sensed signal

    • Boysen-Jensen (1913): Showed a chemical signal from the tip was involved in bending using barriers (agar/mica)

  • New Experimental Setup

    • Plant with light from one side bends towards the light due to auxin accumulation on the shaded side.

    • Plant with light from all directions does not bend as auxin distribution is even.

Hormonal Sources and Effects

  • Auxin:

    • Produced in apical meristem, young leaves, seeds

    • Promotes elongation, root formation, phototropism, gravitropism, maintains apical dominance

  • Cytokinins:

    • Produced in roots, seeds, fruits, growing tissues

    • Stimulate cell division, promote lateral bud growth, delay leaf aging

  • Gibberellin (GA):

    • Produced in seeds, young leaves, roots

    • Stimulates stem elongation, seed germination, flowering, fruit development

  • Abscisic Acid (ABA):

    • Produced in leaves, roots, seeds (under stress)

    • Inhibits growth, promotes seed dormancy, induces stomatal closure during drought

  • Ethylene:

    • Produced in ripening fruit, wounded tissues, aging leaves

    • Regulates fruit ripening, leaf abscission, stress responses

Plant Responses to Light

  • Phototropism:

    • Growth towards light controlled by auxin, causing cell elongation

  • Seed Germination:

    • Light can trigger or inhibit germination; acts as a signal to break dormancy, regulated by phytochromes

  • Shade Avoidance:

    • Plants elongate towards light when shaded, competing for sunlight to maximize photosynthesis

Types of Tropisms

  • Phototropism

    • Positive: Growth towards light for photosynthesis

    • Negative: Growth away from light to absorb nutrients

  • Gravitropism

    • Positive: Growth towards gravity for anchorage and nutrient access

    • Negative: Growth against gravity to reach light and air

  • Thigmotropism:

    • Not detailed, but generally refers to plant responses to touch/environmental mechanical stimuli

Nutrient Discovery in Plants

  • Early agricultural practices indicated plants need specific soil substances

  • Jan van Helmont's Experiment (1600s): Highlighted factors beyond water for plant growth

Cation Acquisition and Soil Nutrients

  • Obtaining Cations from Soil:

    • Ion exchange process: Roots release hydrogen ions to capture cations

    • Acidic soils' high proton concentration reduces nutrient availability and solubility of toxic metals

  • 19th Century Discoveries:

    • Justus Von Liebig identified macronutrients (N, O, P, K) essential for plant growth

    • 1930s hydroponic experiments revealed the need for micronutrients (Fe, Cl, Zn), made through controlled experiments and observations

Acid Rain

  • Causes:

    • Results from atmospheric pollution, primarily from coal combustion

  • Effects:

    • Damages plant tissues, affects human health, causes soil nutrient deficiencies, leads to leaching of nutrients from soils.

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