（b）Plants transport sucrose from a source to a sink． Fig． 4.2 is a scanning electron micrograph（SEM）of a transverse section through a plant tissue used to transport sucrose． Fig． 4.2 （i）Name the structure labelled $$$\mathbf{X}$$$ in Fig．4．2． ................................................................................................................................. （ii）A scientist carried out an experiment to study carbohydrate transport in the stem of a woody plant． Fig． 4.3 shows a plan diagram of a transverse section of the stem studied by the scientist． The position of the xylem tissue in the stem is shown． Fig． 4.3 The scientist carried out a set of experiments using plants of the same species． In each experiment, a ring of tissue was removed from the outer stem of the plant, but the xylem tissue was left intact. This is shown in Fig. 4.4. Fig. 4.4 The mass of carbohydrate transported to the lower part of the stem in 24 hours was recorded. In each experiment a different percentage of the outer stem tissue was removed. All other variables remained constant. Table 4.1 shows the results of this investigation. Table 4.1 Explain the results shown in Table 4.1. ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... .

Biology

IGCSE&ALevel

CAIE

Exam No：9700_w24_qp_21 Year：2024 Question No：4(b)

Answer:

Knowledge points:

7.1.1 draw plan diagrams of transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants from microscope slides and photomicrographs

7.1.2 describe the distribution of xylem and phloem in transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants

7.1.3 draw and label xylem vessel elements, phloem sieve tube elements and companion cells from microscope slides, photomicrographs and electron micrographs

7.1.4 relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their functions

7.2.1 state that some mineral ions and organic compounds can be transported within plants dissolved in water

7.2.2.1 apoplast pathway, including reference to lignin and cellulose

7.2.2.2 symplast pathway, including reference to the endodermis, Casparian strip and suberin

7.2.3 explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere

7.2.4 explain how hydrogen bonding of water molecules is involved with movement of water in the xylem by cohesion-tension in transpiration pull and by adhesion to cellulose in cell walls

7.2.5 make annotated drawings of transverse sections of leaves from xerophytic plants to explain how they are adapted to reduce water loss by transpiration

7.2.6 state that assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes

7.2.7 explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins

7.2.8 explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink

Answer:

Knowledge points:

Solution:

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