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.