Transport in Plants: Question 3
Syllabus 8.1, 8.3
A student examines a longitudinal section of stem tissue under a microscope and identifies a group of long, hollow, tube-like cells joined end to end. Her teacher explains that these are xylem vessels, and that the water they carry is pulled upward mainly by a process happening in the leaves.
(a) Describe three structural features of xylem vessels that can be seen or inferred from this description. [3]
(b) Explain how two of the features you gave in (a) help the xylem to carry a continuous stream of water from the root to the leaf. [2]
(c) Explain the mechanism, called transpiration pull, by which water is drawn upward through the xylem, referring to the loss of water vapour from the leaves and to the properties of water molecules. [3]
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Worked solution
Part (a): Structural features of xylem vessels
From the description of long, hollow, tube-like cells joined end to end, three structural features can be identified:
- The cell walls are thick and contain lignin, which strengthens them.
- The cells have no cell contents. Once mature, they contain no cytoplasm, nucleus or other living contents.
- The cells are joined end to end with no cross walls between them, so a whole file of cells forms one long, continuous tube.
Part (b): Linking structure to function
Because the cells have no cell contents and no cross walls, the inside of the vessel is completely open and unobstructed along its whole length. This lets water flow through freely, as if travelling down a single long pipe rather than having to cross from cell to cell.
The thick, lignified walls give the vessel the strength to keep its shape. Water inside the xylem is often under tension, being pulled from above, and rigid, strengthened walls stop the vessel from collapsing inwards or being squeezed shut under this pulling force, so the tube stays open and water can keep flowing.
Part (c): The transpiration pull mechanism
Inside the leaf, water constantly evaporates from the surfaces of the mesophyll cells into the air spaces between them, and this water vapour then diffuses out of the leaf through the stomata, the process of transpiration.
As this water is lost from the very top of the xylem, it leaves a “gap” that needs to be filled, creating a pulling force that draws more water up through the vessel from below to replace what has evaporated.
This pull is passed all the way down the xylem because water molecules are attracted to one another (cohesion). As the topmost water is pulled upward, it drags the water immediately below it along too, and so on down the whole vessel. This attraction keeps the water moving as a single, unbroken, continuous column, stretching from the root right up to the leaf, rather than the column breaking apart.
Final answers
- (a) Thick, lignified walls; no cell contents; joined end to end with no cross walls.
- (b) No cell contents/cross walls give an open, unobstructed tube; thick lignified walls resist collapse under tension.
- (c) Water lost from the leaf by transpiration creates a pull that draws water up the xylem; attraction between water molecules keeps this as one continuous column.