Plant Structure, Growth, and Responses: Study Notes for Biology 106

Plant Structure and Types of Roots

Root Types and Functions

Plants have evolved different root systems to support their structure and absorb water and nutrients efficiently.

• Taproots: A single, large root with smaller lateral roots branching off. Common in dicots; provides strong anchorage and stores nutrients.

• Lateral roots: Branch from the taproot, increasing surface area for absorption.

• Root hairs: Extensions of individual cells that increase the surface area for absorption.

Example: Carrots and dandelions have prominent taproots, while grasses have fibrous root systems.

Plant Organs and Tissues

Major Plant Organs

• Leaves: Main site of photosynthesis; appear green due to chlorophyll reflecting green light.

• Stems: Support leaves and transport fluids between roots and shoots.

• Roots: Anchor the plant and absorb water/nutrients from the soil.

Potatoes are modified stems (tubers), not roots.

Plant Tissues

• Dermal tissue: Outer protective layer (epidermis, cuticle).

• Vascular tissue: Xylem (transports water/minerals) and phloem (transports sugars).

• Ground tissue: Fills the interior; functions in photosynthesis, storage, and support.

Cell Types in Plants

Ground Tissue Cell Types

• Parenchyma: Thin-walled, living cells; function in photosynthesis and storage.

• Collenchyma: Elongated, flexible cells; provide support in young stems.

• Sclerenchyma: Thick, lignified walls; provide rigid support, often dead at maturity.

Vascular Tissue

• Xylem: Transports water and minerals from roots to shoots.

• Phloem: Transports sugars and nutrients throughout the plant.

Transport in Plants

Water and Mineral Transport

Water and minerals move from the soil into roots, then are transported upward through the xylem by:

• Osmosis: Water moves from areas of high to low water potential.

• Transpiration: Evaporation of water from leaves creates negative pressure, pulling water upward.

• Cohesion and Adhesion: Water molecules stick to each other (cohesion) and to xylem walls (adhesion).

Equation: Water potential ($\Psi$) is determined by solute concentration and pressure:

$\Psi = \Psi_s + \Psi_p$

where $\Psi_s$ is solute potential and $\Psi_p$ is pressure potential.

Phloem Transport

• Translocation: Movement of sugars from sources (leaves) to sinks (roots, fruits) via phloem.

• Bulk flow: Driven by pressure differences between source and sink.

Membrane Transport Mechanisms

• Selective permeability: Cell membranes control movement of substances in and out via transport proteins.

• Diffusion: Passive movement of molecules down a concentration gradient.

• Active transport: Movement against a gradient, requiring energy (ATP).

Plant Responses and Hormones

Major Plant Hormones

• Auxin: Promotes cell elongation, root formation, and phototropism. Synthetic auxins used as herbicides.

• Gibberellins (GA): Stimulate stem elongation, seed germination.

• Abscisic Acid (ABA): Promotes seed dormancy, closes stomata during drought.

• Ethylene: Promotes fruit ripening and response to stress.

Photoreceptors and Light Responses

• Phytochrome: Light receptor sensitive to red/far-red light; regulates seed germination and flowering.

• Phototropism: Growth towards light, mediated by auxin distribution.

Other Environmental Responses

• Gravitropism: Growth in response to gravity; roots show positive gravitropism, shoots negative.

• Thigmotropism: Growth in response to touch (e.g., tendrils wrapping around objects).

• Stress responses: Plants adapt to salt, drought, flooding, and temperature extremes via physiological and structural changes.

Plant Defense Mechanisms

• Physical defenses: Thorns, trichomes, thick cuticles.

• Chemical defenses: Production of toxins or deterrent compounds.

• Behavioral defenses: Attracting predators of herbivores.

Exam Practice and Key Concepts

• Difference between sclerenchyma (rigid, dead at maturity) and collenchyma (flexible, living).

• Xylem and phloem are found in all plant organs.

• Water potential is lower when solute concentration is high.

• Stomata regulate gas exchange and water loss; controlled by potassium ion movement.

• Circadian rhythms: Biological cycles of about 24 hours, persist even without external cues.

Table: Comparison of Major Plant Hormones

Hormone	Main Function	Example/Application
Auxin	Cell elongation, phototropism	Herbicide (2,4-D), root formation
Gibberellin	Stem elongation, seed germination	Promotes growth in crops
Abscisic Acid	Seed dormancy, stress response	Closes stomata during drought
Ethylene	Fruit ripening, senescence	Commercial fruit ripening

Additional info:

• Some content inferred and expanded for clarity, such as detailed definitions of tissue types and hormone functions.

• Topics correspond to General Biology chapters on plant structure, transport, and responses (Ch.28-31).