4 Fig. 4.1 shows a diagram of a plant with arrows indicating the movement of water and other materials. Fig. 4.1 (a) Name the two types of transport tissue ...

Labelled answer diagram:

(a) The two types of transport tissue in vascular bundles: [2]

• Xylem [1]
• Phloem [1]

(b) Substances transported by each tissue: [2]

• Xylem transports water and dissolved mineral salts (ions) upward from the roots to the leaves. [1]
• Phloem transports dissolved sugars (mainly sucrose) and other products of photosynthesis from the leaves to all other parts of the plant (translocation). [1]

(c) How water is absorbed from the soil by the roots: [3]

1. Root hair cells have a large surface area in contact with the film of water surrounding soil particles. [1]
2. Water enters the root hair cells by osmosis because the cell sap has a lower water potential (higher solute concentration) than the dilute soil solution. [1]
3. Water then passes from cell to cell across the root cortex by osmosis, moving along a water potential gradient towards the xylem vessels in the centre of the root. [1]

(d) How the transpiration stream helps move water from roots to leaves: [4]

1. Water evaporates from the mesophyll cell surfaces inside the leaf and diffuses out through the stomata as water vapour. This process is transpiration. [1]
2. The loss of water from the leaf cells creates a tension (negative pressure / suction) in the xylem vessels of the leaf. [1]
3. This tension pulls a continuous column of water upward through the xylem from the roots, through the stem and into the leaves. [1]
4. The cohesion between water molecules (they attract each other through hydrogen bonds) helps maintain this unbroken column throughout the plant, even in tall trees. [1]

(e)(i) Two reasons why sheltered crops lose less water than exposed crops: [2]

1. The row of trees acts as a windbreak, reducing wind speed around the crop. Slower air movement means that the humid layer of air around each leaf is not blown away as quickly, so the diffusion gradient for water vapour is less steep and transpiration is slower. [1]
2. The trees may also provide partial shade, reducing the temperature and light intensity reaching the crop leaves. Lower temperature reduces evaporation, and reduced light causes partial stomatal closure, both of which slow transpiration. [1]

(e)(ii) One other method to reduce water loss from the soil around crops: [2]

• Apply a layer of mulch (such as straw, dried grass or plastic sheeting) to the soil surface around the crops. [1] The mulch acts as a physical barrier between the soil and the atmosphere, reducing direct evaporation of soil moisture. It also helps maintain a more even soil temperature. [1]

Other acceptable methods: use drip irrigation to deliver water directly to the root zone rather than spraying it over the soil surface.