Bio 106 Unit 2

Carbon (C), Hydrogen (H), Oxygen (O)

Major components of plant organic compounds

Nitrogen (N)

Component of proteins, nucleic acids, chlorophyll, and coenzymes

Potassium (K)

Regulates stomatal opening/closing, osmoregulation, and enzyme activation

Calcium (Ca)

Cell wall structure (middle lamella stabilization) and signal transduction

Magnesium (Mg)

Central atom of chlorophyll molecule; enzyme cofactor

Phosphorus (P)

Component of nucleic acids (DNA/RNA), phospholipids, and ATP

Sulfur (S)

Component of key amino acids (methionine, cysteine) and proteins

Organic Layer

Surface litter, partially decomposed leaves, organic debris, and humus layer

Topsoil

Rich in organic matter (humus) combined with mineral particles. Main zone of root activity and nutrient absorption

Subsoil

Accumulation of leached minerals (iron, clay, aluminum) washed down from topsoil

Parent Material

Partially weathered bedrock or original soil-forming deposits

Large Particles (Sand)

High aeration and drainage, but poor water retention and rapid nutrient leaching

Small Particles (Clay)

Excellent water and ion retention due to large surface area and negative charges, but poor aeration and prone to waterlogging (hypoxia)

Organic Matter & Humus

Humus is fully decomposed organic material. It prevents soil compaction, increases cation exchange capacity (CEC), retains soil moisture, and slowly releases essential nutrients to roots

Leaching

Anions (NO₃^-, H₂PO₄^-, SO₄^2-) are negatively charged and do not bind to negative soil particles. They stay dissolved in soil water and easily wash away (leach) during heavy rain or irrigation, depriving plants of vital nitrogen and phosphorus

Cation Exchange Process

1. Plant roots pump protons (H⁺) into the soil solution via H⁺-ATPase membrane pumps.

2. Respiration in root cells produces CO₂, which reacts with soil water to form carbonic acid (H₂CO₃), dissociating into H⁺ and HCO₃^-.

3. The higher concentration of H⁺ ions displaces bound mineral cations from negatively charged soil particles into the soil solution.

4. Displaced cations are absorbed into root hair membranes through ion channels or transporters.

Nitrogen-Fixing Bacteria

Convert atmospheric N₂ gas into ammonia (NH₃/NH₄⁺) using the enzyme nitrogenase. Rhizobium forms symbiotic root nodules in legumes

Nitrifying Bacteria

Two-step conversion: Nitrosomonas convert NH₄⁺ into Nitrite (NO₂^-); Nitrobacter convert NO₂^- into Nitrate (NO₃^-), the primary form plants absorb.

Ammonifying Bacteria

Break down organic nitrogen from dead organisms and plant waste back into ammonium (NH₄⁺)

Mutualistic Role

Fungal hyphae increase the effective root surface area-to-volume ratio by orders of magnitude, optimizing the absorption of water and immobile minerals (especially Phosphate, PO₄^3-). In return, the plant supplies photosynthetic sugars to the fungus.

Ectomycorrhizae

Hyphae form a dense mantle over the root surface and grow into the extracellular spaces between outer root cortical cells (forming a Hartig net) without penetrating cell walls.

Endomycorrhizae

Fungal hyphae penetrate through the root cell wall and invaginate the plasma membrane, forming highly branched tree-like structures (arbuscles) for exchange

Path from Soil to Xylem

Soil Solution → Root Hair → Epidermis → Cortex → Endodermis (Casparian Strip) → Pericycle → Xylem Vessel

Apoplastic Route

Movement through continuous cell walls and intercellular spaces. Blocked at the endodermis by the Casparian strip (waxy suberin band), forcing water/minerals into the symplast for cell membrane screening

Symplastic Route

Movement through the continuous cytoplasm connected via plasmodesmata

Transpiration

Evaporative loss of water vapor through stomata creates negative pressure (tension/pull) at the air-water interface in leaves

Cohesion

Water molecules stick to each other via hydrogen bonding, pulling a continuous column of water up the xylem

Adhesion

Water molecules bind to hydrophilic cellulose cell walls of xylem vessels, resisting gravity

Root Pressure & Guttation

At night (when stomata close), roots continue active transport of ions into the xylem, lowering water potential. Water flows in, building positive root pressure, which can force water droplets out of leaf tips (guttation)

Guard Cell Regulation

• Stomatal Opening: Light or low CO₂ triggers active H⁺ pumping out of guard cells → K⁺ and Cl^- enter cell → hypertonic interior draws water in by osmosis → guard cells turgidly bow outward opening the pore.

• Stomatal Closing: Abscisic acid (ABA) or water stress causes K⁺ exit → water exits by osmosis → guard cells become flaccid and close.

Sugar Movement

Photosynthates move from Sources (net producers like mature leaves) to Sinks (net consumers/consumers like growing roots, tubers, developing fruits, and shoot tips)

Phloem Loading Mechanism

1. Sucrose is actively loaded from mesophyll cells into sieve-tube elements via H⁺/sucrose symporters.

2. High sucrose concentration dramatically lowers solute potential in phloem sieve tubes.

3. Water moves by osmosis from adjacent xylem vessels into phloem, generating high hydrostatic pressure (bulk flow).

4. Pressure pushes phloem sap along the sieve tube toward the sink area.

5. At the sink, sucrose is unloaded (used or stored as starch); water potential rises, and water flows back into xylem.

Roots

Anchor plant, absorb water/minerals, store carbohydrates

Stems

Structural support, orient leaves toward light, transport fluids between roots and shoots

Leaves

Primary photosynthetic organs; gas exchange (CO₂/O₂) and transpiration regulation

Dermal Tissue

Epidermis: Protects plant; secretes waxy cuticle to prevent water loss.

Guard Cells & Trichomes: Regulate gas exchange and defense.

Ground Tissue

Parenchyma: Metabolic, photosynthetic, and storage cells; thin primary walls.

Collenchyma: Flexible structural support in young growing shoots; unevenly thickened walls.

Sclerenchyma: Rigid support; thick lignified secondary walls; dead at functional maturity (sclerids & fibers).

Vascular Tissue

Xylem: Conducts water/minerals; dead at maturity. Consists of Tracheids (tapered, pitted) and Vessel Elements (wider, continuous tube elements with perforation plates).

Phloem: Translocates sugars; living at maturity. Consists of Sieve-Tube Elements (lack nucleus/organelles) connected to Companion Cells via plasmodesmata.

Monocots

Vascular Bundles in Stem

Scattered randomly throughout ground tissue.

Root Vascular Anatomy

Pith in center ringed by alternating xylem/phloem bundles.

Leaf Venation

Parallel veins.

Eudicots (Dicots)

Vascular Bundles in Stem

Arranged in a distinct ring surrounding a central pith.

Root Vascular Anatomy

Central Xylem "X" or star shape with phloem arms.

Leaf Venation

Netted/Reticulate (branching) veins.

Meristems & Stem Cells

Meristems contain initials (undifferentiated stem cells that divide to self-renew) and derivatives (cells that divide further and differentiate into specialized plant tissues).

Primary Growth (Length/Height)

Driven by Apical Meristems at root tips and shoot tips.

Root Zones: Zone of Cell Division (mitosis) → Zone of Elongation (cells expand driven by auxin-induced cell wall loosening) → Zone of Differentiation/Maturation (cells gain specialized structures like root hairs).

Secondary Growth (Girth/Thickness)

Driven by Lateral Meristems in woody eudicots:

Vascular Cambium: Cylinder of meristematic cells between primary xylem and phloem. Divides inwardly to produce Secondary Xylem (Wood) and outwardly to produce Secondary Phloem.

Cork Cambium

Produces phelloderm (inward) and cork cells (outward, suberin-rich dead cells). Cork cambium + cork layer = Periderm.

Bark

All tissues external to the vascular cambium (Secondary Phloem + Periderm).

Auxin (IAA)

Stem cell elongation (acid growth hypothesis); apical dominance (inhibits lateral buds); phototropism and gravitropism; lateral root formation.

Cytokinins

Promote cell division (mitosis) and differentiation; delay leaf senescence; promote lateral bud outgrowth (antagonistic to auxin).

Gibberellins (GA)

Stem elongation (bolting); seed germination trigger (breaks dormancy by signaling $\alpha$-amylase production to break down endosperm starch); fruit development.

Abscisic Acid (ABA)

Maintains seed dormancy (inhibits germination); initiates rapid stomatal closure during drought stress; acts as stress signal.

Ethylene

Gas hormone. Promotes fruit ripening (positive feedback); leaf abscission (drop); Triple Response to mechanical stress.

Apical Dominance & Branching

High ratio of Auxin (from terminal tip) to Cytokinin suppresses axillary bud growth. Removing the apical bud removes the auxin source, allowing cytokinins to trigger lateral branching.

Seed Germination vs. Dormancy

Regulated by the balance between ABA (maintains dormancy) and GA (breaks dormancy upon imbibition of water).

Triple Response to Mechanical Barriers

Stem encounters obstacle → ethylene released → 1) Slows stem elongation, 2) Thickens stem, 3) Induces horizontal growth to navigate around obstruction.

Leaf Abscission

Aging leaf produces less Auxin; elevated Ethylene activates enzymes in the abscission zone at the petiole base to shed the leaf cleanly.

Phototropism

Bending toward unidirectional light. Blue-light receptors (phototropins) detect light at shoot tip → Auxin translocates to the shaded side → lower pH activates expansins (acid growth) → shaded side cells elongate faster, bending shoot toward light.

Phytochromes (Red / Far-Red Light Switches)

Pr (Red-absorbing form, 660 nm): Inactive state.

Pfr (Far-red-absorbing form, 730 nm): Active state triggering biological responses.

Gravitropism

Shoots show negative gravitropism (grow up); roots show positive gravitropism (grow down). Heavy starch-filled plastids called statoliths settle to the bottom of cap cells in roots, directing auxin flow to lower side. In roots, high auxin inhibits cell elongation, bending root downward.

Drought Stress

ABA causes stomatal closure. Leaves roll to reduce exposed surface area.

Flooding Stress

Ethylene production in roots triggers programmed cell death (apoptosis) to form air channels (aerenchyma) for oxygen delivery.

Heat Shock

High temperatures produce heat-shock proteins (HSPs) that act as chaperones to prevent protein denaturation and misfolding.

Herbivore Defense Mechanisms

1. Physical: Thorns, trichomes, sclereids, thick cuticles.

2. Chemical: Secondary metabolites (alkaloids, tannins, terpenes, poisons).

3. Systemic/Indirect Recruitment: Damaged leaves release volatile organic compounds (VOCs) that attract predatory insects (e.g., parasitoid wasps) to attack herbivores.

Whorls

Sepals (calyx), Petals (corolla), Stamens (androecium), Carpels/Pistils (gynoecium)

Stamen (Male)

Anther (contains microsporangia) + Filament

Carpel/Pistil (Female)

Stigma (sticky landing pad) + Style (neck) + Ovary (contains ovules)

Complete vs. Incomplete

Complete flowers possess all 4 organs; Incomplete flowers lack one or more whorls

Male Gametophyte (Pollen Grain)

Microsporocyte ($2n$) undergoes meiosis → 4 Microspores ($n$) → mitosis produces pollen grain containing:

• 1 Tube Cell (builds pollen tube down style).

• 1 Generative Cell (divides via mitosis to form 2 Sperm Cells).

Female Gametophyte (Embryo Sac)

Megasporocyte ($2n$) in ovule undergoes meiosis → 4 Megaspores ($n$) (3 degenerate, 1 survives) → surviving megaspore divides by mitosis 3 times without cytokinesis to form 7-celled, 8-nucleate Embryo Sac:

• 1 Egg Cell + 2 Synergids (at micropyle end).

• 2 Central Polar Nuclei (in 1 large central cell).

• 3 Antipodal Cells.

Process of Double Fertilization

1. Pollen grain lands on stigma and germinates a pollen tube down the style into the micropyle of the ovule.

2. Two sperm cells are discharged into the embryo sac:

3. First Fertilization: One sperm ($n$) fertilizes the Egg cell ($n$) → forms Diploid Zygote ($2n$).

4. Second Fertilization: Second sperm ($n$) fuses with the Two Central Polar Nuclei ($2n$) → forms Triploid Endosperm ($3n$), which provides nutrient supply for developing embryo.

Seed Components

Mature Embryo + Triploid Endosperm reserve + Protective Seed Coat (derived from integuments)

Monocot vs. Eudicot Embryo Structure

• Eudicot: Two cotyledons (seed leaves), epicotyl, hypocotyl, radicle (embryonic root).

• Monocot: Single cotyledon (scutellum), coleoptile (protective sheath over shoot), coleorhiza (sheath over root).

Adaptations for Breaking Seed Dormancy

Physical scarification (abrasion/digestion), cold stratification (winter chill), heavy water leaching (washing away ABA), fire/smoke chemical signals

Seed Germination Steps

1. Imbibition: Uptake of water expands seed coat and activates metabolic enzymes.

2. GA triggers synthesis of hydrolytic enzymes ($\alpha$-amylase) to break stored starch into sugars.

3. Radicle Emergence: Embryonic root breaches seed coat first to establish water access.

Classification of Main Fruit Types

• Simple Fruit: Develops from a single carpel or fused carpels of one flower (e.g., pea pod, tomato, cherry).

• Aggregate Fruit: Develops from a single flower with multiple separate carpels (e.g., raspberry, blackberry).

• Multiple Fruit: Develops from an inflorescence (cluster of separate flowers fused together, e.g., pineapple, fig).

• Pome: Accessory fruit where tissue surrounding ovary derives from receptacle stem tissue (e.g., apple, pear).