CBSE Class 11 Biology Chapter 11 Transport in Plants NCERT Solutions

Reverse osmosis is a process used for water purification. In this technique, a pressure that is greater than the osmotic pressure is applied to the system. This forces the solvent (water) to move from a region of higher solute concentration to a region of lower solute concentration, effectively separating impurities. Although external pressure is applied to drive the process in the reverse direction of natural osmosis, it is considered a passive process because it does not directly consume metabolic energy in the form of ATP. Active processes, by definition, require energy input from ATP hydrolysis.

The rate of transpiration, which is the loss of water vapor from plants, is directly influenced by several environmental factors. Increasing temperature generally increases transpiration by increasing the rate of evaporation. Light intensity affects the opening and closing of stomata; higher light intensity usually leads to wider stomatal opening and thus increased transpiration. Wind speed also increases transpiration by removing water vapor from the leaf surface, maintaining a steep water potential gradient. However, the chlorophyll content of leaves, while essential for photosynthesis, does not directly regulate the rate at which water evaporates from the plant.

In a dorsiventral leaf, which is typically found in dicotyledonous plants, the upper and lower surfaces have distinct anatomical differences. The lower epidermis usually contains a significantly higher number of stomata compared to the upper epidermis. This arrangement helps in efficient gas exchange for photosynthesis and respiration while minimizing water loss from the upper surface, which is more exposed to direct sunlight. In contrast, an isobilateral leaf, common in monocots, has stomata distributed more or less equally on both the upper and lower surfaces.

Photosynthesis produces glucose, which is a simple sugar and a primary source of energy for plant cells. However, glucose is a reactive molecule. For transport through the phloem to various parts of the plant, plants convert glucose into a more stable and less reactive disaccharide called sucrose. Sucrose is composed of one glucose unit and one fructose unit linked together. This form is ideal for translocation because it does not readily participate in metabolic reactions during its journey. Fructose is also a simple sugar, and ribose is a component of nucleic acids; neither is the primary sugar transported in the phloem.

Guttation is the process by which water is lost from plants in the liquid form, typically from specialized pores called hydathodes, usually located at the margins of leaves. This phenomenon occurs when the rate of water absorption by the roots exceeds the rate of transpiration. This condition often arises when root pressure is high (e.g., at night or in high humidity conditions when stomata are closed) and the rate of transpiration is low. The accumulation of water in the xylem, driven by root pressure, forces the water out through the hydathodes.

Imbibition is a special type of diffusion where water is absorbed by solid particles of a substance (like seeds, wood, or cell walls) due to the presence of hydrophilic colloids, causing an increase in volume. When a dry seed is placed in soil, it absorbs water through imbibition. This absorbed water activates the enzymes within the seed, leading to the breakdown of stored food and initiating the process of germination. The uptake of water by root hairs is primarily absorption driven by osmosis and active transport. The exchange of gases in stomata and the opening/closing of stomata are related to diffusion and turgor pressure changes, respectively, not imbibition.

Senescence is the biological process of aging, leading to the deterioration and eventual death of plant parts or the entire organism. During this phase, plants often reallocate valuable resources. Essential nutrients that are mobile within the plant, such as nitrogen and phosphorus, are typically withdrawn from senescing tissues (like older leaves) and translocated to younger, actively growing parts of the plant, such as developing fruits, seeds, or new leaves. This ensures that these vital elements are conserved and utilized for future growth and reproduction.