Transport in Plants Test - 30

Holard is the total amount of water content present in the soil. Whereas chresard is the water available to plants for absorption and echard is the soil water unavailable to plants. It consists of hygroscopic water, combined water, water vapors and about 25 percent of capillary water. Only capillary water is available to plant roots. Thus, the correct answer is option B.

Herbs are defined as small plants that are totally green and do not have lignin or brown parts. Thus, wood and secondary growth are totally absent and plant remains closer to ground as compared to shrub and trees. Shrubs come in between trees and herbs and may become woody. Trees show secondary growth, have lignin and are woody. Trees also attain height and need to lift water to greater heights. Also water requirement and absorption is considerable in trees. The osmotic pressure is the main force responsible for absorption of water by plants. If other things like environmental conditions, soil type etc. remain constant the value of osmotic pressure of cell sap is maximum in tree and least in herbs because of the different water requirements and morphological and anatomical differences as outlined above.

The apoplast pathway comprises of non living components of a plant body i.e., intercellular spaces and cell wall. The movement through apoplast is unregulated and fast. Symplast comprises of living components i.e., protoplasm, cell membrane and plasmodesmata. Water potential or free energy of water depends on the number of water molecules in the system. More the number of water molecules greater will be water potential. When solutes are dissolved in water, there is a decrease in relative number of water molecules and this causes corresponding decrease in water potential. Membrane permeability depends on the nature of biological membrane and the nature of solutes. The membrane is composed of phospholipids and proteins. Consequently lipophilic substances can freely pass through the membrane, while hydrophilic substances cannot freely pass through the membrane and require special carrier or transport proteins for transport across the membrane.

Water flow in the roots occurs via the apoplast because the cortical cells are loosely packed and hence offer no resistance to water movement. The inner boundary of the cortex, the endodermis is impervious to water because of a band of suberised matrix called the casparian strip. Water molecules are unable to penetrate the layer, so they are directed to wall regions that are not suberised, into the cells proper through the membranes.

• Osmosis is the movement of water molecules along the concentration gradient across the cell membrane. Thus, water supply in the plants is due to the process of osmosis. 
• Paper is basically made up of cellulose. Cellulose microfibrils are arranged in such a manner that there are capillary spaces formed between them. When exposed to water or some other solvent, the water rises in these capillaries due to capillary action. This is a purely physical process and is responsible for the fast absorption of water by paper. 
• Water rises in the plant body due to transpiration pull. Transpiration leads to loss of water from the plant, which generates a pulling force on the continuous water column present in the xylem elements. Hence, the water rises in the plant body.
• Carrier proteins are located in the cell membrane for the transport of specific solutes. They are integral membrane proteins, which transport solutes using ATP energy. This process is called active transport.

Root pressure theory was advanced by Priestley in 1916. According to root pressure theory, water rises upwardly in the xylem due to a positive pressure developed in the root. A root pressure of 1 to 2 atmospheres has been reported in a number of cases. However, root pressure is not universal and is not found in all the seasons. A positive root pressure has not been demonstrated in gymnosperms, even as some of the gymnosperms (Sequoia) are the tallest trees. Water can rise upwards even in the absence of roots.  

Antitranspirants are substances that are applied to the leaves to reduce transpiration. They are used on trees, cut flowers and shrubs. They reduces transpiration in fruit plants like apples, pears, orange, crop plants like cotton, tea, roses, tuber crops like potato, carrot and vegetable plants like chillies, tomato, cabbage. These are also used to protect plants from drying out too quickly. Various antitranspirants include abscisic acid (ABA), aspirin, silicon emulsions, plastics, waxes.

Transpiration is the loss of water in vapour form, from the plant to the atmosphere. Transpiration is characteristic of land plants only and is not shown by submerged hydrophytic plants and lower aquatic plants like Algae. 

Transpiration leads to the establishment of water current in plants. The water current leads to bulk transport pathway for transport of water and minerals. Excessive transpiration will lead to water stress, which is visible as wilting of leaves. Bleeding is when sap leaks from a wound or pruning cut on a tree, shrub or woody climber. Sometimes, this may be a gentle seeping, other times a copious flow. Bleeding occurs due to the pressure of sap within the tissues that conduct water and sugars around the plant. Bleeding is not related to transpiration.

Hydathodes are secretory tissues involved in the process of guttation. These water glands are present on the edge of the leaves. They contain epithem which has loosely arranged cells with large intercellular spaces. During guttation, positive root pressure forces water to come out from the epithem hydrathodes in the form of drops.