Transport of Food and Minerals in Plants

1. Conducting Function: Xylem transports water and dissolved inorganic minerals (sap); Phloem translocates synthesized organic food (sucrose and amino acids).
   2. Direction of Flow: Xylem flow is strictly unidirectional (upward) from roots to leaves; Phloem flow is bidirectional / multidirectional from source leaves to storage sinks.
   3. Living/Dead Composition: Xylem consists predominantly of dead, lignified cells (tracheids, vessels, fibres) with only xylem parenchyma being living; Phloem consists predominantly of living cells (sieve tubes, companion cells, parenchyma) with only phloem fibres being dead.
   4. Mechanical Support: Xylem provides immense mechanical structural support due to thick lignin deposits; Phloem has soft cellulosic walls with perforated sieve plates.

• The term was coined by botanist Curtis in 1926.
• Why "Evil"?
Over $98\%$ of all water absorbed by the root system with immense energy expenditure is lost uselessly as water vapor into the atmosphere. Under hot, dry drought conditions, excessive transpiration outpaces root absorption, leading to flaccidity, wilting, leaf desiccation, and plant death.
• Why "Necessary"?
1. It generates the monumental Transpiration Pull (suction force) required for the ascent of sap in tall trees.
2. It facilitates the continuous absorption and upward transport of essential dissolved mineral nutrients from the soil.
3. It produces an evaporative cooling effect, dissipating intense solar thermal energy and protecting delicate photosynthetic enzymes from being heat-denatured.

• Osmosis:
1. The movement of water (solvent) molecules from a region of higher water concentration (dilute solution) to lower water concentration (concentrated solution) across a semi-permeable membrane.
2. It is a passive physical process along the concentration gradient.
3. Requires NO metabolic energy (zero ATP).
• Active Transport:
1. The movement of mineral ions (solutes) from a region of lower concentration in soil into higher concentration inside root hair cells.
2. It occurs against the concentration gradient.
3. Requires active metabolic energy in the form of ATP provided by root cellular respiration.

• Plasmolysis: The phenomenon of contraction or shrinkage of the protoplasm (cytoplasm and cell membrane) away from the rigid cellulosic cell wall when a plant cell is placed in a hypertonic solution.
• Mechanism:
1. The external salt solution has a higher solute concentration (lower water potential) than the cell sap inside the vacuole.
2. Water moves rapidly out of the central vacuole into the surrounding solution by Exosmosis.
3. The vacuole shrinks, and the plasma membrane pulls away from the cell wall.
4. The space between the shrunken protoplast and the cell wall becomes filled with the external hypertonic solution.
5. If transferred back to pure water, Deplasmolysis occurs as water re-enters by endosmosis.

1. Enormous Surface Area: Millions of microscopic root hairs extend out from the root epidermal cells, creating a colossal total surface area for contact with soil capillary water.
   2. Concentrated Cell Sap (Hypertonic): The large central vacuole contains a sap rich in dissolved sugars and organic salts, maintaining a higher osmotic concentration than the surrounding soil water, ensuring continuous inward endosmosis.
   3. Dual Permeability Barriers: The outer cellulosic cell wall is freely permeable, allowing rapid entry of soil solution, while the inner cell membrane is semi-permeable, regulating selective entry of essential ions.

• Guttation: The loss of water in the form of liquid droplets along the uninjured margins and tips of leaves through specialized water pores called Hydathodes.
• Cause: High Root Pressure developed in roots when soil water absorption is vigorous but transpiration is virtually absent (e.g., humid, cool mornings).
• Differences from Transpiration:
1. Transpiration loses water as invisible water vapor; Guttation loses water as liquid droplets.
2. Transpiration occurs through stomata; Guttation occurs through hydathodes.
3. Transpiration water is pure distilled water; Guttation liquid contains dissolved mineral salts and organic acids.

• Formulated by Dixon and Joly (1894).
1. Cohesion of Water: Water molecules have extraordinary mutual attractive forces (cohesion) due to hydrogen bonding, creating a continuous, unbroken liquid column inside the microscopic xylem vessels that resists tensile breaking under high suction.
2. Adhesion: Water molecules adhere firmly to the cellulose and lignin of the xylem vessel walls, preventing the column from slipping down.
3. Transpiration Pull: Evaporation of water from leaf mesophyll cells creates a powerful negative suction tension that pulls the entire cohesive water thread upward through the xylem vessels from root tips to tree canopy.

• (a) Increase in Temperature: Increases transpiration rate (warm air holds more moisture and accelerates the kinetic evaporation of water from mesophyll cells).
• (b) Increase in Humidity: Decreases transpiration rate (saturated humid air reduces the diffusion gradient between the leaf interior and surrounding atmosphere).
• (c) Increase in Wind Velocity: Increases transpiration rate (wind sweeps away the humid vapor boundary layer over the leaf surface, maintaining a steep concentration gradient for rapid diffusion).