CAIE IGCSE Biology 8.3.5 Transpiration Pull & Cohesion

Practise explaining how evaporation from leaves creates a pull that raises a cohesive water column through xylem.

Exam points

link water loss from leaves to tension or transpiration pull in xylem
explain that the pull draws a continuous column of water molecules upwards
identify cohesion as the attractive force holding water molecules together in the column

[Maximum number: 6]

A student investigated the effect of temperature on transpiration.

The student collected four small plants and placed them in beakers filled with water.
Fig. 3.1 shows the apparatus used.

Fig. 3.1

For each plant, the student measured the initial masses of the equipment, water and plant.
Each plant was placed in a different temperature-controlled room for three hours.
After three hours, the student measured the final masses of the equipment, water and plants.

The results are shown in Table 3.1.

Table 3.1

Complete the sentences about water movement through the xylem.

The xylem transports water and .

Xylem cells form a long continuous with thick walls containing cellulose and .

Water moves upwards in the xylem because of transpiration .

This draws up a of water molecules, held together by between water molecules.

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8.3.5—Mechanism by which water moves upwards question 1