AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Xylem as the tissue that transports water in the stem and leaves of…
Full specification wording
Mass transport in plants: Xylem as the tissue that transports water in the stem and leaves of plants. The cohesion-tension theory of water transport in the xylem.
Xylem vessels are dead, hollow cells stacked end to end with their end walls broken down, and with walls thickened by lignin, so they form a continuous open tube from root to leaf.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 1 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Xylem vessels are dead, hollow cells stacked end to end with their end walls broken down, and with walls thickened by lignin, so they form a continuous open tube from root to leaf. Cohesion-tension theory explains how water rises through them without any cell doing work. Water evaporates from the surfaces of mesophyll cells and diffuses out through the stomata, which is transpiration. This lowers the water potential of those cells, so water is drawn out of the xylem in the leaf, and that creates tension, a negative pressure, at the top of the column. Water molecules are polar and form hydrogen bonds with one another, so they stick together; this cohesion means the entire column is pulled up as a single unit. Water also adheres to the lignified walls. The column must be unbroken, which is why lignin holding the vessel open matters and why an air bubble stops the flow entirely.
What earns the marks
- An unbroken continuous column of water, with no barrier to its movement.
- Cohesion between water molecules caused by hydrogen bonding between polar molecules.
- Evaporation or transpiration from the leaf creating tension in the column.
- Water being pulled up the xylem by that tension rather than pushed up from below.
- A correct reference to cohesion-tension causing the movement of water, credited as a principle mark where the detailed points are not gained.
Common mistakes and how to put them right
Writing chain of water molecules, which mark schemes ignore, instead of column of water.
Describing water being pushed up from the roots when the driving force is a pull from the leaves.
Confusing cohesion between water molecules with adhesion to the vessel wall.
Saying hydrogen bonds between water and lignin are what hold the column up.
Forgetting that xylem vessels are dead, and describing them using energy to move water.
Exam tips
- Build the answer on three keywords: continuous column, cohesion by hydrogen bonding, tension from transpiration.
- Start the explanation at the leaf, because that is where the driving force is generated.
- If the detail deserts you, stating clearly that cohesion-tension moves the water can still earn a mark.
Try the lesson
The start of the Mass transport in plants: Xylem as the tissue that transports water in the stem and leaves of plants. The cohesion-tension theory of water transport in the xylem. lesson, made from checked teaching notes for this specification point.
Have a go first
Which tissue transports water through the stem and leaves of a plant?
Listen
Xylem vessels are dead, hollow cells stacked end to end with their end walls broken down, and with walls thickened by lignin, so they form a continuous open tube from root to leaf. Cohesion-tension theory explains how water rises through them without any cell doing work. Water evaporates from the surfaces of mesophyll cells and diffuses out through the stomata, which is transpiration. This lowers the water potential of those cells, so water is drawn out of the xylem in the leaf, and that creates tension, a negative pressure, at the top of the column. Water molecules are polar and form hydrogen bonds with one another, so they stick together; this cohesion means the entire column is pulled up as a single unit. Water also adheres to the lignified walls. The column must be unbroken, which is why lignin holding the vessel open matters and why an air bubble stops the flow entirely. A strong answer earns marks for these points. an unbroken continuous column of water, with no barrier to its movement. cohesion between water molecules caused by hydrogen bonding between polar molecules. evaporation or transpiration from the leaf creating tension in the column. water being pulled up the xylem by that tension rather than pushed up from below. Exam tip: Build the answer on three keywords: continuous column, cohesion by hydrogen bonding, tension from transpiration.
Key words
Say what each word means before you turn the card.
transpiration
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tension
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cohesion
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adhesion to the wall
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See every definition
- transpiration
- Water evaporates from the surfaces of mesophyll cells and diffuses out through the stomata.
- tension
- A negative pressure created at the top of the column when water is drawn out of the xylem in the leaf.
- cohesion
- Water molecules are polar and form hydrogen bonds with one another, so they stick together and the entire column is pulled up as a single unit.
- adhesion to the wall
- Water also adheres to the lignified walls of the xylem vessels.
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