Plant Biology: Nutrition, Growth & Reproduction

Nutrient Transport

Ascent of Sap

Gross: The movement of water and minerals from the roots to the leaves via the stem's xylem. Water flows upward due to two factors:

• Transpiration Pull: Water loss during photosynthesis and transpiration creates negative pressure.
• Root Pressure: The lower osmotic concentration in the soil compared to the root interior drives water flow.

Gas Exchange

Plants have a lower respiratory rate than animals and require less oxygen. Gas exchange occurs through stomata, facilitated by the enzyme carbonic anhydrase:

H₂O + CO₂ ↔ H₂CO₃ ↔ H⁺ + HCO₃^-

Increased H⁺ concentration lowers pH, activating an enzyme that breaks down starch into glucose.

Photosynthesis

Photosynthesis occurs in chloroplasts, where chlorophyll and carotenoids capture light energy. Plants use this energy to synthesize organic molecules from water and minerals, a fundamental difference from animal nutrition.

Nutrient Distribution

Photosynthesis products are transported via the phloem. Nutrients are actively transported into phloem cells, creating a flow through sieve plates. Xylem and phloem run parallel, allowing for interaction.

Waste Removal

Plants produce minimal waste due to their low metabolic rate. Some waste is reused in anabolic processes. Strategies for dealing with waste include:

• Storing salts internally.
• Expelling waste through vesicles.
• Secreting substances through the plasma membrane.

Heterotrophic Nutrition in Plants

Some plants supplement their nutrition through heterotrophic means:

• Carnivorous Plants: Obtain nitrogen and phosphorus from insects using traps or sticky secretions.
• Parasitic Plants: Lack chlorophyll and obtain nutrients from other plants through specialized structures.

Plant Reproduction

Asexual Reproduction

Vegetative Propagation: Occurs through cell division in meristematic tissue.

• Budding: New plants develop from buds.
• Fragmentation: The parent plant divides into fragments that develop into new plants (e.g., rhizomes, tubers, aerial stems).
• Artificial Methods: Cuttings, grafting, layering.

Sporulation: Spores are produced within sporangia and develop into new plants.

Sexual Reproduction

Mosses: Rely on water for reproduction. The gametophyte produces sperm and eggs. Fertilization occurs when sperm swim to the egg.

Seed Plants (Spermatophytes): More evolved plants that reproduce through seeds.

• Gymnosperms (Conifers): Male cones produce pollen, which is transported to female cones containing ovules. Fertilization is simple.