8 min readConcept sheetPart III (4 marks): root pressure and transpiration pull; Part II (2 marks): guttation or translocation

Ascent of sap and translocation of food

Water rises in the xylem, pushed by root pressure and mainly pulled by transpiration pull. Food moves through the phloem from leaves to other parts: translocation.

Ascent of sapUpward movement of water and minerals through the xylem

Cohesion-tension theoryProposed by Dixon and Joly (1894)

Evidence of root pressureGuttation; sap oozing from a cut stem

TranslocationMovement of food (sucrose) through the phloem

Root pressure: the push from below

Root cells keep taking in water and minerals. This builds up a pressure in the root, called root pressure, which pushes water a little way up the xylem. Evidence: when a stem is cut near the ground, sap oozes from the stump. In the early morning, water drops appear at the tips and edges of grass leaves; this is guttation. Limit: root pressure is weak and is absent in many tall trees. So it cannot explain how water reaches the top of a tall tree.

Transpiration pull (cohesion-tension theory)

Proposed by **Dixon and Joly (1894)**, this is the accepted explanation of how water reaches the tops of tall trees.

1
Water evaporates from leaf cells and escapes through the stomata (transpiration).
2★
Leaf cells then draw water from the xylem of the veins. This creates a pull (tension) on the water in the xylem.
3★
Water molecules stick to each other by cohesion, so the water column in the xylem does not break.
4
Water also sticks to the xylem walls by adhesion, so the column does not slip down.
5
The pull passes down the unbroken column all the way to the roots, and water is lifted up like a rope being pulled.

★ key step

Ascent of sap vs translocation

Ascent of sap (xylem)Carries water and minerals. Moves one way: upwards, from roots to leaves. Driven by transpiration pull and root pressure. Xylem vessels and tracheids are dead cells.

Translocation (phloem)Carries food, mainly sucrose. Moves both ways, up and down, from source to sink. Needs energy from living cells. Phloem sieve tubes are living.

How translocation works (source to sink)

1
Source: leaves make sugar by photosynthesis and load sucrose into the phloem sieve tubes, using ATP.
2
Water enters the sieve tubes from the nearby xylem by osmosis, building up pressure.
3
This pressure pushes the sugar solution along the sieve tubes towards the sink.
4
Sink: growing tips, roots, fruits and storage organs take the sugar out and use it or store it as starch.

In real life

The sweetness of sugarcane, the starch in potato and tapioca and the sugar in mangoes all arrive there by translocation from the leaves. Drops at the very tips of grass blades early in the morning are often guttation, not dew.

Exam angle

2 marks: “What is guttation?” The loss of water as drops from the tips and edges of leaves (as in grass) in the early morning, caused by root pressure.

2 marks: “What is translocation?” The movement of food, mainly sucrose, from the leaves to other parts of the plant through the phloem.

4 marks: “Explain the transpiration pull theory of ascent of sap.” Dixon and Joly; transpiration creates tension in the xylem; cohesion keeps the water column unbroken; adhesion holds it to the xylem walls; the pull passes down to the roots and lifts water.

Remember it

Root pressure pushes; transpiration pulls. Cohesion is water holding Close to water; Adhesion is water holding A wall.