Xylem, Transpiration and Cohesion–Tension

Water evaporates from moist mesophyll cell surfaces into internal air spaces and water vapour diffuses through stomata to the atmosphere. Loss of water lowers the water potential of those cells, encouraging water movement from neighbouring cells and ultimately from the xylem.

A water-potential gradient can extend from soil through roots and stem to leaf and atmosphere. Root cell membranes regulate uptake of ions and water pathways, but transpiration-driven bulk flow is a different stage of transport. Explain the full route instead of saying water rises merely because “the plant needs it”.

Cohesion between water molecules, associated with hydrogen bonding, helps maintain a continuous column. Transpiration creates tension transmitted down that column, pulling water up xylem. Adhesion to vessel walls can also contribute to maintaining the column. A pressure gradient drives mass flow; individual water molecules are not passed upwards by a row of active carrier proteins.

Mature xylem vessel elements are dead and form hollow pathways with reduced end-wall obstruction. Lignified walls strengthen them against collapse under tension and provide support. Pits permit lateral movement between vessels and surrounding tissue. Air entry can interrupt a water column, so continuity is a functional requirement, not a decorative feature.

Higher temperature, lower external humidity or air movement can increase water loss by affecting evaporation or the vapour gradient, provided other factors and stomatal responses do not override the effect. Light can promote stomatal opening, but real plants regulate pores and do not respond through one uncontrolled factor alone.

A potometer measures water uptake, which is often a proxy for transpiration rather than a direct measurement of water vapour lost. Some absorbed water is used or stored. Keep the apparatus airtight and watertight, control leaf area and allow acclimation. A moving bubble gives volume only when capillary cross-sectional area is known.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.