Describe the process of transport of water and mineral salts in flowering plants and explain the factors that influence the rate of transpiration (20marks)
Model Answer
(a) Transport of Water and Mineral Salts in Flowering Plants
Water and mineral salts are essential for photosynthesis, respiration, maintenance of cell turgidity, transport of manufactured food, and numerous metabolic activities within the plant. Their movement from the soil to the leaves occurs through the xylem and involves several physiological processes that ensure a continuous supply of water and dissolved minerals to all parts of the plant.
The process begins in the root hairs, where water is absorbed from the soil by osmosis. The cell sap within the root hair cells possesses a lower water potential than the surrounding soil solution, allowing water to move across the selectively permeable plasma membrane into the cells. Mineral salts dissolved in the soil solution are absorbed mainly by active transport because their concentration within the root hair cells is often higher than that in the surrounding soil. This process requires energy released during respiration.
Water and dissolved mineral salts pass from the root hairs through the epidermis and cortex by both the apoplastic and symplastic pathways until they reach the endodermis. The Casparian strip within the endodermis prevents unrestricted movement through the cell walls and compels the solution to pass through the cytoplasm of the endodermal cells. This selective barrier regulates the entry of water and mineral ions into the xylem.
After entering the xylem vessels, water and mineral salts move upwards through the stem towards the leaves. The principal force responsible for this movement is transpiration pull, which develops when water evaporates from the surfaces of the mesophyll cells within the leaf. The loss of water creates tension within the xylem, drawing a continuous column of water upwards from the roots. Cohesion between water molecules, resulting from hydrogen bonding, maintains the continuity of the water column, while adhesion between water molecules and the walls of the xylem vessels prevents the column from collapsing under gravity. Capillary action within the narrow xylem vessels further assists the upward movement of water, whereas root pressure provides an additional force that contributes to the ascent of sap, particularly during periods of low transpiration.
Upon reaching the mesophyll cells of the leaves, water is utilized in photosynthesis, serves as a solvent for biochemical reactions, maintains cell turgidity, and facilitates the transport of dissolved substances. Excess water evaporates from the moist cell surfaces into the intercellular air spaces before diffusing out of the leaf through the stomata, thereby sustaining the continuous movement of water from the soil to the atmosphere.
(b) Factors Influencing the Rate of Transpiration
The rate of transpiration is influenced by environmental conditions as well as the structural characteristics of the plant. Light intensity promotes the opening of the stomata to facilitate carbon dioxide uptake for photosynthesis. As the stomata open, the diffusion of water vapour from the leaf increases, resulting in a higher rate of transpiration.
Temperature affects the rate at which water evaporates from the mesophyll cells. High temperatures increase the kinetic energy of water molecules, accelerating evaporation and diffusion of water vapour through the stomata. Lower temperatures reduce evaporation and consequently decrease the rate of transpiration.
Atmospheric humidity determines the concentration gradient between the moist air spaces within the leaf and the surrounding atmosphere. High humidity reduces this gradient and slows the diffusion of water vapour, whereas low humidity increases the concentration gradient and enhances transpiration.
Wind influences transpiration by removing the layer of moist air surrounding the leaf surface. Continuous air movement maintains a steep concentration gradient that favours rapid diffusion of water vapour from the stomata. In still air, the accumulation of water vapour around the leaf reduces the rate of transpiration.
The availability of water in the soil also affects transpiration. Adequate soil moisture supports continuous absorption of water by the root hairs, enabling normal transpiration. During periods of water shortage, the guard cells lose turgidity, causing the stomata to close and reducing water loss from the plant.
Structural adaptations of plants contribute significantly to the regulation of transpiration. Leaves with numerous or widely opened stomata lose water more rapidly than those with fewer stomata. A thick waxy cuticle minimizes evaporation from the leaf surface, while sunken stomata and leaf hairs trap moist air around the stomata, reducing the rate of water loss. Plants with broad leaves generally transpire more rapidly than those with narrow or needle-shaped leaves because of their larger surface area exposed to the atmosphere.
Conclusion
The transport of water and mineral salts in flowering plants is a continuous process that ensures all parts of the plant receive the substances required for growth, photosynthesis, respiration, and other metabolic activities. This movement depends on the coordinated action of the root hairs, xylem, transpiration pull, cohesion, adhesion, capillary action, and root pressure. The rate of transpiration is influenced by environmental factors such as light intensity, temperature, humidity, wind, and soil water availability, as well as the structural characteristics of the plant. Understanding these processes enables students to appreciate how plants maintain water balance and sustain essential life processes.
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