AS 2 extended response, transport in plants · 6 marks
Explain how water moves from the roots to the leaves of a tall plant against gravity.
Marked instantly, no account needed
AS · AS 2 Organisms and Biodiversity
Transpiration and the cohesion–tension theory
The short answer
Transpiration is the evaporation of water from leaf mesophyll cells and its diffusion out through the stomata. Water lost from the leaf lowers the water potential there, and cohesion between water molecules pulls a continuous column up the xylem under tension — the cohesion–tension theory.
The pathway, root to atmosphere
- Water enters root hair cells by osmosis, down a water potential gradient.
- It crosses the cortex by the apoplast and symplast pathways; the Casparian strip forces it into the symplast at the endodermis.
- Water enters the xylem and moves up under tension as a continuous column, held together by hydrogen bonding (cohesion) and to the vessel walls (adhesion).
- It evaporates from mesophyll cell walls into air spaces and diffuses out through open stomata down a water potential gradient.
Factors affecting the rate
| Factor | Effect | Why |
|---|---|---|
| Light intensity | Increases rate | Stomata open for gas exchange |
| Temperature | Increases rate | Faster evaporation and diffusion |
| Humidity | Decreases rate | Smaller water potential gradient out of the leaf |
| Air movement | Increases rate | Removes saturated air, maintaining the gradient |
Potometer questions
A potometer measures water uptake, which is only an estimate of transpiration because some water is used in photosynthesis and to keep cells turgid. Say that when asked to evaluate the method.
Marks come from the detail: cut the shoot underwater and at a slant, assemble underwater to exclude air bubbles, ensure airtight seals with petroleum jelly, allow the plant to acclimatise, and change only one factor at a time. Rate is distance moved by the bubble ÷ time, or volume (πr² × distance) ÷ time when a scale and capillary radius are given.
Xerophytes
- Sunken stomata and hairs trap humid air, reducing the water potential gradient.
- Rolled leaves trap moist air and reduce exposed surface area.
- Thick waxy cuticle reduces evaporation through the epidermis.
- Reduced leaves or spines lower the total surface area for transpiration.
Phrases that earn the marks
- water potential gradient
- cohesion between water molecules due to hydrogen bonding
- continuous column of water under tension
- adhesion to the xylem walls
- evaporation from mesophyll cell walls, diffusion through stomata
- the potometer measures uptake, which estimates transpiration
Where marks get lost
- Describing water as being 'sucked' or 'pulled by the roots'.
- Saying humidity increases the rate.
- Giving potometer rates with no units or no time interval.
- Forgetting that not all water taken up is transpired.
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These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Exam questions on transpiration a level biology, marked
Two AS-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
AS 2 extended response, transport in plants · 6 marks
Explain how water moves from the roots to the leaves of a tall plant against gravity.
Command word: Explain — how to answer itMark scheme — 6 creditworthy points
- Water evaporates from the surfaces of mesophyll cells and diffuses out through the stomata — transpiration
- This lowers the water potential of the mesophyll cells, drawing water from the xylem
- A transpiration pull / tension is created in the xylem
- Water molecules are cohesive due to hydrogen bonding, so they form a continuous column
- Adhesion of water to the lignified walls of the xylem also helps
- Water is pulled up the xylem in a continuous column — the cohesion–tension theory; root pressure contributes at the base
Full-mark answer
Water evaporates from the surfaces of the mesophyll cells and diffuses out of the leaf through the stomata as transpiration. This lowers the water potential of the mesophyll cells, so water moves into them from the xylem down a water potential gradient. Removal of water from the xylem creates a tension, or transpiration pull. Because water molecules are polar they hydrogen bond to one another, so cohesion holds them in a continuous column, and adhesion to the lignified xylem walls supports the column. The tension therefore pulls the whole column of water up the xylem from the roots to the leaves — the cohesion–tension theory. Root pressure from active transport of ions into the xylem adds a small push at the base.
Examiner insight: Answers must name cohesion and hydrogen bonding as separate ideas and must never say water is 'sucked' up. The mark for a continuous, unbroken column is frequently missed.
AS 3 practical skills, potometer · 4 marks
A potometer was used to measure water uptake. Explain two precautions needed to obtain valid results, and why water uptake is not exactly equal to transpiration rate.
Command word: Explain — how to answer itMark scheme — 4 creditworthy points
- The shoot must be cut under water and the apparatus assembled under water to prevent air entering the xylem
- All joints must be airtight / sealed with petroleum jelly so no water is lost or air drawn in
- Allow shoot to equilibrate before readings / keep environmental conditions constant
- Some water taken up is used in photosynthesis or retained in cells for turgidity, so uptake exceeds transpiration slightly
Full-mark answer
The shoot must be cut under water and the potometer assembled under water so that no air bubbles enter the xylem, which would break the water column and stop uptake. All joints must be sealed and airtight, using petroleum jelly, so that water is not lost and air is not drawn in at the connections. Water uptake is not exactly equal to transpiration because a small proportion of the water absorbed is used in photosynthesis and retained in the cells to maintain turgidity, so uptake slightly exceeds the water lost by transpiration.
Examiner insight: The last mark asks for a reason, not a restatement. 'Some water is used by the plant' is too vague — name photosynthesis or turgidity.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemeCommon questions
- What is the cohesion–tension theory?
- Evaporation from the leaves creates tension in the xylem; hydrogen bonding between water molecules holds the column together so it is pulled up as one continuous thread.
- Why does a potometer not measure transpiration exactly?
- It measures water uptake. A small proportion of that water is used in photosynthesis and to maintain cell turgor rather than being transpired.
- How does wind affect transpiration?
- Moving air blows away water vapour from around the stomata, maintaining a steep water potential gradient and increasing the rate.
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