Plant Nutrition and Transport of Resources

Macronutrients Required by Plants

Plants require several essential macronutrients for growth and development. These nutrients are needed in large quantities and play critical roles in physiological processes.

• Nitrogen (N): Important for amino acids, proteins, nucleic acids, and chlorophyll.

• Phosphorus (P): Component of nucleic acids, ATP, and phospholipids.

• Potassium (K): Regulates osmotic balance, enzyme activation, and stomatal function.

• Calcium (Ca): Structural component of cell walls, signaling molecule.

• Magnesium (Mg): Central atom in chlorophyll, enzyme cofactor.

• Sulfur (S): Component of some amino acids and vitamins.

Soil Profile and Its Components

The soil profile consists of distinct layers, each with unique properties affecting plant growth.

• Topsoil (A horizon): Contains organic matter (humus), minerals, and living organisms. Humus improves water retention and nutrient availability.

• Subsoil (B horizon): Accumulates leached materials from above; less organic matter.

• Parent Material (C horizon): Weathered rock and minerals.

• Large particles (sand): Provide aeration but poor water retention.

• Small particles (clay): Retain water and nutrients but may limit aeration.

• Organic matter: Includes decomposed plant and animal material; humus is the most stable form.

Cation Exchange and Leaching

Cation exchange is the process by which plant roots exchange hydrogen ions for nutrient cations (e.g., K+, Ca2+) bound to soil particles. Leaching is the loss of soluble nutrients from the soil, often affecting anions like nitrate (NO3-).

• Cation exchange: Roots release H+ ions, displacing nutrient cations for uptake.

• Leaching: Water movement removes nutrients, especially anions, from the root zone.

• Problem: Leaching reduces soil fertility and can lead to nutrient deficiencies.

Nitrogen-Fixing Bacteria and Plant Nitrogen Utilization

Plants rely on bacteria to convert atmospheric nitrogen (N2) into usable forms.

• Nitrogen-fixing bacteria: Convert N2 to ammonia (NH3).

• Nitrifying bacteria: Convert NH3 to nitrate (NO3-).

• Denitrifying bacteria: Convert nitrate back to N2, returning it to the atmosphere.

• Usable forms: Plants absorb nitrate (NO3-) and ammonium (NH4+).

• Reason: Plants cannot use atmospheric N2 due to its triple bond and chemical inertness.

Mycorrhizal Fungi and Plant Nutrition

Mycorrhizal fungi form symbiotic relationships with plant roots, enhancing nutrient uptake.

• Ectomycorrhizae: Fungi form a sheath around roots; do not penetrate cells.

• Endomycorrhizae (arbuscular): Fungi penetrate root cells, forming arbuscules.

• Benefit: Increase surface area for nutrient and water absorption.

• Surface area:volume ratio: Higher ratios facilitate exchange across biological systems.

Pathways of Water and Mineral Transport

Water and minerals move from soil into roots, past the Casparian strip, and into the xylem, then travel upward to leaves.

• Apoplastic pathway: Through cell walls and intercellular spaces.

• Symplastic pathway: Through cytoplasm via plasmodesmata.

• Casparian strip: Blocks apoplastic flow, forcing entry into symplast before xylem.

• Xylem: Conducts water and minerals upward.

• Stomata: Water exits via transpiration.

Transpiration, Root Pressure, and Guard Cells

Transpiration and root pressure drive water movement; guard cells regulate stomatal opening.

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

• Root pressure: Positive pressure from roots pushes water upward, mainly at night.

• Guard cells: Control stomatal opening/closing, regulating water loss and gas exchange.

Phloem Transport: Source to Sink

Sugars produced in photosynthetic tissues (sources) are transported to non-photosynthetic tissues (sinks) via phloem.

• Source: Mature leaves, storage organs during breakdown.

• Sink: Growing tissues, roots, fruits.

• Phloem loading: Sugars actively transported into phloem sieve tubes.

• Water movement: Water from xylem enters phloem, facilitating sugar transport by pressure flow.

Plant Structure and Growth

Plant Organs and Their Functions

Plants are composed of roots, stems, and leaves, each with specialized functions.

• Roots: Anchor plant, absorb water and minerals, store nutrients.

• Stems: Support leaves and flowers, transport fluids, store nutrients.

• Leaves: Main site of photosynthesis, gas exchange.

Plant Tissues and Their Arrangement

Three main tissue types are arranged differently in monocots and dicots.

• Dermal tissue: Protective outer layer.

• Ground tissue: Photosynthesis, storage, support.

• Vascular tissue: Transport of water (xylem) and nutrients (phloem).

• Monocots: Vascular bundles scattered in stem.

• Dicots (eudicots): Vascular bundles arranged in a ring.

Vascular Tissue Cells and Their Functions

• Xylem: Tracheids and vessel elements; transport water/minerals.

• Phloem: Sieve tube elements and companion cells; transport sugars.

Primary vs Secondary Growth

Primary growth increases length; secondary growth increases girth.

• Primary growth: Occurs at apical meristems; cell division and elongation.

• Secondary growth: Occurs at lateral meristems (vascular and cork cambium); produces wood and bark.

Meristems, Cambium, and Stem Cells

Meristems are regions of undifferentiated cells; cambium is a lateral meristem responsible for secondary growth.

• Meristem: Site of active cell division.

• Cambium: Produces secondary xylem (wood) and phloem.

• Stem cells (initials): Maintain meristem activity; differentiate into specialized cells.

Cell Division, Elongation, and Auxin

Cell division and elongation drive primary growth; auxin regulates cell elongation.

• Cell division: Increases cell number.

• Cell elongation: Increases cell size; auxin promotes elongation by loosening cell walls.

Organization of Stem and Root Tissues

• Eudicots: Root has central vascular cylinder; stem has ring of vascular bundles.

• Monocots: Root has scattered vascular bundles; stem has scattered bundles.

Primary and Secondary Xylem and Phloem

Secondary growth produces new layers of xylem and phloem each year.

• Primary xylem/phloem: Formed during primary growth.

• Secondary xylem/phloem: Formed by vascular cambium during secondary growth.

Plant Hormones and Responses to External Stimuli

Role of Plant Hormones

Plant hormones regulate growth, development, and responses to environmental stimuli.

• Auxin: Promotes cell elongation, apical dominance, phototropism.

• Gibberellins: Stimulate stem elongation, seed germination.

• Cytokinins: Promote cell division, delay senescence.

• Abscisic acid (ABA): Induces dormancy, closes stomata during drought.

• Ethylene: Promotes fruit ripening, leaf abscission, response to mechanical stress.

Hormone Interactions and Applications

• Growth: Auxin and gibberellins promote elongation; cytokinins promote division.

• Branching: Auxin inhibits, cytokinins promote.

• Dormancy: ABA maintains, gibberellins break dormancy.

• Drought tolerance: ABA closes stomata.

• Triple response: Ethylene mediates stem thickening, reduced elongation, horizontal growth.

• Leaf abscission: Ethylene promotes, auxin delays.

• Fruit ripening: Ethylene accelerates.

Plant Responses to Light

Plants use light receptors to regulate growth and development.

• Phototropism: Growth toward light; mediated by auxin.

• Blue-light receptors: Control stomatal opening, phototropism.

• Phytochromes: Red/far-red light receptors; regulate seed germination, shade avoidance.

• Red light: Promotes germination, leaf expansion.

• Far-red light: Inhibits germination, promotes stem elongation in shade.

Gravitropism and Other Stimuli

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

• Flooding/drought: Adaptations include aerenchyma formation, stomatal closure.

• Heat-shock proteins: Protect cells from extreme heat by stabilizing proteins.

• Herbivore defense: Mechanical barriers, chemical deterrents, and signaling for predatory attraction.

Plant Reproduction

Flower Structure and Types

• Complete flower: Has all four whorls (sepals, petals, stamens, carpels).

• Incomplete flower: Missing one or more whorls.

Alternation of Generations in Angiosperms

Angiosperms alternate between diploid sporophyte and haploid gametophyte generations.

• Microspore: Develops into male gametophyte (pollen).

• Megaspore: Develops into female gametophyte (embryo sac).

Gametophyte Structure

• Female gametophyte: Contains egg cell and two central nuclei.

• Male gametophyte: Contains tube cell and two sperm cells.

Double Fertilization

Double fertilization is unique to angiosperms.

• One sperm fertilizes the egg (forms zygote).

• Other sperm fuses with two central nuclei (forms endosperm).

Seed Development and Structure

• Embryo: Develops from zygote.

• Endosperm: Nutrient tissue for embryo.

• Cotyledons: Seed leaves.

• Seed coat: Protective outer layer.

Seed Dormancy and Germination

• Dormancy adaptations: Physical barriers, chemical inhibitors, environmental cues (e.g., temperature, light).

• Germination: Initiated by water uptake, temperature, and light.

• Monocots: One cotyledon; eudicots: two cotyledons.

Fruit Types and Seed Dispersal

Fruits develop from flowers and aid in seed dispersal.

• Fleshy fruits: Apples, berries.

• Dry fruits: Nuts, grains.

• Seed dispersal adaptations: Wind, water, animals, mechanical ejection.

Additional info: These notes expand on brief study guide points to provide academic context and explanations suitable for exam preparation in General Biology.