Plant Physiology Questions

Plant Physiology Questions Question 1 Which one of the following DOES NOT contribute to water flow through xylem? A. Cavitation B. Root pressure C. Cohesion-tension D. Capillary force Answer: A. Cavitation Explanation: Cavitation refers to the formation of air bubbles in the xylem, which can disrupt the continuous flow of water by breaking the column of water within the xylem vessels. In contrast, root pressure, cohesion-tension, and capillary force all contribute to the upward movement of water through xylem. Question 2 The vessels and tracheids in xylem can withstand negative pressures (tension) of -25 to -30 MPa. This is possible because: A. Tension is counterbalanced by high cohesiveness between water molecules. B. Tracheid and vessel cell walls are fortified with lignin. C. Cavitation acts as a pressure release valve within the xylem. D. Tracheid and vessel cells remain live at maturity. Answer: B. Tracheid and vessel cell walls are fortified with lignin. Explanation: The cell walls of tracheids and vessels are reinforced with lignin, which provides the structural strength needed to withstand the high negative pressures (tension) that occur during water transport in the xylem. While cohesion between water molecules aids in maintaining a continuous water column, it is the lignified walls that enable these cells to resist collapse under such tension. Question 3 Which of the following factors contributes the most to ensure plants take water from the soil via root hairs and root tips? A. Root hairs only develop on the oldest parts of roots. 1 B. The water potential of older root cells is too high to take water from the soil. C. Roots grow toward available water, which is encountered first by root tips. D. Suberin deposition in older root tissue blocks uptake of water. Answer: D. Suberin deposition in older root tissue blocks uptake of water. Explanation: Suberin, a waxy substance deposited in the cell walls of older root tissues, forms the Casparian strip, which prevents water from passing through these regions and redirects it to younger root regions where water uptake can occur more efficiently. This structural adaptation ensures that water absorption occurs primarily through the root hairs and root tips, which lack suberin deposits and are actively engaged in water and nutrient uptake. Question 4 Which of the following statements about bulk flow through phloem sieve elements are TRUE? i. Phloem loading increases the solute concentration in sieve elements causing an influx of water. ii. High solute concentration in the sieve elements results in increased pressure potential and the bulk flow of water toward sieve elements with lower pressure potential. iii. Increased pressure potential in one sieve element results in bulk flow toward another sieve element because sieve elements form a continuous column. iv. Bulk flow in phloem tissue is unidirectional between leaf and root tissues. v. Phloem loading increases the solute concentration in sieve elements causing an efflux of water. A. i, ii, iii B. ii, iii, iv C. i, iii, v D. iii, iv, v Answer: A. i, ii, iii Explanation: • Statement i is true because phloem loading increases the solute concentration in sieve elements, leading to an influx of water due to osmosis. • Statement ii is also true, as the high solute concentration raises the pressure potential, causing water to flow from areas of higher pressure potential to areas of lower pressure potential in the sieve elements. • Statement iii is correct since sieve elements form a continuous column, allowing bulk flow to occur from one sieve element to another due to pressure differentials. Statement iv is incorrect because bulk flow in phloem can be bidirectional in different parts of the plant, depending on source and sink relationships. Statement v is incorrect because increased solute concentration causes an influx, not an efflux, of water into the sieve elements. 2 Question 5 Which of the following statements about phloem is FALSE? A. Phloem tissue transports waste material from sink tissue to source tissues for disposal. B. Sieve elements in angiosperms are joined end-to-end to form tubes connected through sieve plates. C. Phloem loading from companion cells into sieve elements is primarily a secondary active transport process. D. Sieve elements and associated companion or Strasburger cells are alive at maturity. Answer: A. Phloem tissue transports waste material from sink tissue to source tissues for disposal. Explanation: This statement is false because phloem primarily transports sugars, hormones, amino acids, and other nutrients from source tissues (like leaves) to sink tissues (like roots, fruits, and growing tissues) where they are used or stored. Phloem does not play a role in transporting waste materials for disposal; waste is typically handled by other cellular mechanisms. 3