Question

Compare and contrast the forces that move phloem sap and xylem sap over long distances.

Short answer

Long-distance transport is a bulk flow caused by a pressure differential at opposite ends of tubes in both phloem and xylem sap movement.

The loading of sugar and the ensuing osmotic flow of water into the phloem generate positive pressure at the source end of a sieve tube, and this pressure forces sap from the source end to the sink end of the tube.

On the other hand, transpiration generates a negative pressure potential as a push that drives xylem sap upward.

Step-by-step solution

Role of bulk flow in plants

Diffusion is a robust transport process that operates at spatial sizes similar to those observed at the cellular level. On the other hand, diffusion is far too sluggish to be used for long-distance movement within a plant.

While long-distance transport is accomplished through bulk flow or the movement of liquid in response to a pressure gradient, bulk material flow generally moves from higher to lower pressure. Bulk flow, unlike osmosis, is not affected by solute concentration.

Long-distance bulk flow occurs within vascular tissue specialized cells, particularly the tracheid and vessel elements of the xylem and the sieve-tube elements of the phloem.

Transport of water and minerals in plants

The unidirectional flow of water and minerals via the xylem from soil to roots to leaves is upward.

In contrast, photosynthesis frequently goes in the reverse direction, transferring sugars from mature leaves to lower plant areas, such as root tips, which require vast amounts of sugars for energy and growth.

The movement of photosynthetic products, known as translocation, is conducted by another tissue, the phloem.

Difference between the forces that move phloem sap and xylem sap

Phloem sap flows at speeds of up to 1 m/hr from source to sink, significantly quicker than diffusion or cytoplasmic streaming.

Researchers have concluded that phloem sap travels through angiosperm sieve tubes via bulk flow driven by positive pressure, also known as pressure flow. Sap flows from source to sink due to pressure buildup and pressure reduction at the sink.

In contrast, as more evaporation occurs from the cell wall, the curvature of the air-water contact rises, and the water pressure decreases.

Water molecules from the leaf's highly hydrated areas are then drawn toward the cell wall, lowering tension so that each water molecule is cohesively connected to the next by hydrogen bonds.

Hence, the negative pressure potential (tension) reduces water potential, and its participation in transpiration is compatible with the water potential equation.

So, water travels from locations with higher water potential to areas with lower water potential. The increased negative pressure potential at the air-water interface causes the water in xylem cells to be "tugged" into mesophyll cells, which lose water to the air spaces via stomata.