Study Notes on Transport in Plants: Xylem, Phloem, Transpiration, and Translocation

Detailed Comparison of Xylem and Phloem

• Feature / Basis of Comparison:     * Type of Transport:         * Xylem: Transpiration.         * Phloem: Translocation.     * Direction of Flow:         * Xylem: Unidirectional (upwards).         * Phloem: Multidirectional.     * Material Transported:         * Xylem: Water and minerals ($H_2O$, $K^+$, $Na^+$).         * Phloem: Sucrose (solutes), amino acids ($a.a.$).     * Type of Cells:         * Xylem: Xylem vessels, xylem parenchyma, xylem fibres, and tracheids.         * Phloem: Sieve-tubes / elements, phloem parenchyma, and companion cells.     * Cell Wall:         * Xylem: Lignified (contains lignin in the cell wall).         * Phloem: Non-lignified.     * Living/Dead Cells:         * Xylem: Dead cells (mostly).         * Phloem: Living cells.     * Presence of Cytoplasm:         * Xylem: Absent.         * Phloem: Present.     * Cell Wall Thickness:         * Xylem: More / Thick.         * Phloem: Less / Thin.     * Main Components (Elements):         * Xylem: Derived from the type of cells.         * Phloem: Derived from the type of cells.     * Energy Requirement ($ATP$):         * Xylem: No ($X ATP$).         * Phloem: Yes ($\checkmark ATP$).     * Location in Vascular Bundle:         * Xylem: Inner side.         * Phloem: Outer side.     * Role in Plant:         * Xylem: Transport water.         * Phloem: Transport food.

Structural Characteristics and Components of Xylem

• Function: Transports water and dissolved minerals against gravity.
• Cellular Composition: Xylem is a complex tissue consisting of both lignified dead cells and living cells (parenchyma).
• Water-Conducting Cells:     * Tracheids: Long, thin tube-like structures without perforations at the ends. They possess lignified cell walls and pits.     * Vessel Elements: Short, wide tubes that are perforated at the ends. When joined together, they form a continuous pipe called a vessel. They have no end walls between individual cells.     * Pits: Both tracheids and vessel elements have pits, which are thin sections on the cell walls.
• Structural Features:     * The vessels have thick walls stiffened with lignin to provide support.     * Flow in xylem is strictly one-way (from roots upwards).

Structural Characteristics and Components of Phloem

• Definition: Phloem is a complex permanent tissue meant for the conduction of food within the plant. It is also referred to as "bast" or "laptone."
• Cellular Composition: Phloem is comprised of four types of cells, three of which are living and one of which is dead.     * Sieve-Tubes/Elements: Living cells with end walls that have perforations.     * Companion Cells: Support the sieve-tube elements.     * Phloem Parenchyma: Living cells involved in storage and lateral transport.     * Phloem Fibres: Dead cells at maturity. They are made of sclerenchymatous cells.
• Phloem Fibres (Bast Fibres):     * Absent in the primary phloem (found in newborn plants).     * Present in the secondary phloem.     * Features: Elongated, unbranched, and bear pointed apices.     * At maturity, they lose their protoplasm and become dead.
• Structural Features:     * Cells have end walls with perforations.     * Flow is two-way (multidirectional).

Anatomy: Position of Vascular Tissues

• In Sections of Roots:     * Xylem and phloem are arranged centrally.     * Xylem typically forms a central star-like shape (in many species) while phloem is located between the arms of the xylem.
• In Sections of Stems:     * Xylem and phloem are organized into vascular bundles arranged in a ring.     * Xylem is located on the inner side of the bundle.     * Phloem is located on the outer side of the bundle.

The Pathway of Water and Water Uptake

• Root Hair Cells:     * Specialized cells found in plant roots adapted to absorb water and minerals from the soil.     * They feature root hairs, which are long extensions or outgrowths of epidermal cells.     * Function: Specifically designed to increase the surface area for the absorption of water and minerals.
• Sequential Pathway of Water:     1. Soil: Source of water and minerals.     2. Root Hair Cells: Water enters via osmosis.     3. Root Cortex Cells: Water moves through the cortex layer.     4. Xylem: Water enters the vascular tissue to be transported upwards.     5. Mesophyll Cells: Water moves from the xylem vessels to the mesophyll cells in the leaf by osmosis.
• Evaporation Process in the Leaf:     * Water evaporates from the surface of the mesophyll cell walls into air spaces.     * The air spaces contain water vapour.     * Water vapour diffuses out of the air spaces and exits the leaf through the stomata.

Transpiration and the Transpiration Stream

• Definition of Transpiration: The process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems, and flowers.
• The Transpiration Stream Steps:     1. Evaporation: Water evaporates from internal leaf cells through stomata.     2. Osmosis in Leaf: Water passes from xylem vessels in the stem to leaf cells by osmosis to replace evaporated water.     3. Suction/Pull: This movement 'pulls' the water up through the xylem.     4. Stem Transport: Water enters xylem vessels in the stem from root tissue.     5. Root Absorption: Water enters root hair cells by osmosis from the soil to maintain the flow.
• Transpiration Pull and Cohesion:     * Cohesion: Water molecules are held together by forces of attraction between them.     * The Pull: Transpiration pull draws up a long, continuous column of water molecules up the xylem vessels. As water diffuses out of stomata, more water is drawn from the roots.

Investigations and Experiments

• Pathway of Water (Celery Experiment):     * Celery stalks are placed in different solutions: Water (Control), Red Food Dye, and Blue Food Dye.     * Results: The control shows no stain. In the dyed solutions, the xylem vessels are highlighted/stained by the dye, proving they are the conduits for water.
• Factors Affecting Transpiration Rate (Potometer Method):     * Apparatus: Cut shoot, rubber tubing, reservoir with tap/inlet, air bubble, ruler, capillary tube, beaker of water, and timer.     * Methodology:         1. A single air bubble is introduced into the capillary tubing.         2. The reservoir tap is used to add water and reset the bubble to zero on the scale.         3. A timer measures a specific duration.         4. The distance the air bubble travels along the scale is recorded.         5. The experiment is repeated under different environmental conditions.

Factors Affecting the Rate of Transpiration

• Temperature:     * Effect: Rate increases.     * Explanation: The kinetic energy of water molecules increases, causing them to evaporate and diffuse faster from the mesophyll cells.
• Wind Speed:     * Effect: Rate increases.     * Explanation: Wind removes water vapour surrounding the leaf faster, maintaining a steep concentration gradient.
• Humidity:     * Effect: Rate decreases.     * Explanation: If the surrounding air has high water vapour, the concentration gradient for diffusion is weak.
• Stomatal Response:     * When conditions favor transpiration (temperature/wind), stomata open up.     * In unfavorable conditions (high humidity or dehydration), stomata close.

Translocation

• Definition: The movement of sucrose and amino acids in phloem from sources to sinks.
• Key Terms:     * Sources: Parts of the plant that release sucrose or amino acids (e.g., leaves during summer).     * Sinks: Parts of the plant that use or store sucrose or amino acids (e.g., roots, fruits, or growing tips).
• Seasonal Variability: Plants can switch which parts act as source or sink.     * In Summer: The leaves act as the source (producing glucose/sucrose via photosynthesis), and the roots/stems act as the sink (storing sucrose as starch).     * In Winter and Spring: Stored starch in the roots/stems is converted back to sucrose and transported to the new buds/leaves to support growth; here, the storage organs become the source and the new growth becomes the sink.

Questions and Discussion

• Question (a): Explain why transpiration is faster in high wind.     * Answer: High wind speed removes the layer of water vapour that accumulates around the surface of the leaf. This maintains a steep concentration gradient between the internal air spaces of the leaf and the external environment, allowing water vapour to diffuse out more rapidly.
• Question (b): Explain why transpiration is slower in high humidity.     * Answer: High humidity means the air outside the leaf already contains a high concentration of water vapour. This reduces the concentration gradient between the inside of the leaf and the outside air, which slows down the rate of diffusion through the stomata.