Transport in plants (xylem, phloem, transpiration) Knowledge-Based Practice

Questions

1. 1.

   During the radial transport of water and dissolved mineral ions from the root epidermis to the vascular cylinder, which structure blocks the apoplastic pathway and forces water to cross a plasma membrane into the symplast?

   • A) Pericycle
   • B) Casparian strip of the endodermis
   • C) Sieve plate of the phloem
   • D) Cuticle of the epidermis
2. 2.

   Which of the following cellular events directly triggers the opening of stomata to facilitate transpiration in plant leaves?

   • A) Active uptake of potassium ions into guard cells, which lowers their water potential and drives osmotic water entry.
   • B) Active export of potassium ions from guard cells, which raises their water potential and drives osmotic water exit.
   • C) Rapid loss of water from guard cells into subsidiary cells, causing the guard cells to become flaccid and bow outward.
   • D) Enzymatic breakdown of cellulose microfibrils in guard cell walls, causing the pore to expand passively.
3. 3.

   Which of the following statements correctly compares the mature conducting cells of xylem with those of phloem in angiosperms?

   • A) Vessel elements are living cells with thin primary walls, whereas sieve-tube elements are dead cells reinforced by lignified secondary walls.
   • B) Both vessel elements and sieve-tube elements are dead at functional maturity and rely on adjacent cells for metabolic maintenance.
   • C) Vessel elements are dead at functional maturity with lignified cell walls, whereas sieve-tube elements remain alive but lack nuclei and certain organelles.
   • D) Both vessel elements and sieve-tube elements retain complete nuclei and standard organelle complements to actively transport sap.
4. 4.

   In vascular plants, organic nutrients such as sucrose are transported through the phloem from sources to sinks. Which of the following statements correctly describes the mechanism of phloem translocation according to the pressure-flow hypothesis?

   • A) Evaporative tension generated at the leaf surface creates negative pressure that pulls phloem sap upward.
   • B) Dissolved sugars diffuse passively through non-living sieve elements along an electrical gradient without fluid flow.
   • C) Gravitational pull drives the downward movement of sap through companion cells without requiring metabolic energy.
   • D) Active loading of sucrose into sieve-tube elements draws water in by osmosis, generating a high hydrostatic pressure that drives bulk flow toward sinks.
5. 5.

   Which of the following statements correctly describes the primary mechanism responsible for the long-distance upward movement of water through the xylem in vascular plants?

   • A) Transpiration of water from leaves generates negative pressure that pulls a continuous column of water upward due to cohesion and adhesion.
   • B) Active transport pumps in the root endodermis continuously generate positive pressure that pushes water upward to the shoot apex.
   • C) Sieve-tube elements generate high osmotic pressure gradients by actively loading sucrose to drive bulk water flow toward the canopy.
   • D) Companion cells supply metabolic energy to drive continuous cytoplasmic streaming of water through tracheids and vessel elements.

Answers and step-by-step solutions appear after you finish the test.