7. Transport in plants
A section of Biology, 9700
Listing 10 of 457 questions
Explain how water moves up xylem vessels in the trunks of trees. Cavitation is the formation of air-filled spaces inside the columns of water in xylem vessels in trees. These air-filled spaces form more often when there are high rates of movement of water in xylem vessels. When an air-filled space forms in a xylem vessel, a noise is made that can be detected as a ‘click’ by a sensor placed close to the xylem vessels in the trunk of a tree. Students investigated the relationship between two environmental factors and the rate of cavitation in a Scots pine tree, Pinus sylvestris, over a 50-hour period. The two factors they investigated were: • photosynthetically active radiation (PAR), which is the light energy available to plants that they absorb and use in photosynthesis • wind speed. The cavitation rate was estimated by recording the number of clicks detected by the sensor. The results are shown in . Suggest the conclusions that can be made with reference to the data in .
9700_w22_qp_23
THEORY
2022
Paper 2, Variant 3
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Plants have specialised cells for the efficient transport of assimilates. Table 3.1 shows some of the features of two different types of cell found in plant tissue, which are adapted for the efficient transport of assimilates. Table 3.1 feature of cell cell type A cell type B cytoplasm ✓ ✓ nucleus ✗ ✓ mitochondria few many cellulose in the cell wall ✓ ✓ ribosomes ✗ ✓ Key ✓ present ✗ absent Many plasmodesmata connect type A cells with type B cells. Identify cell type A and explain why the plasmodesmata are important. cell type A explanation The polysaccharide callose is found in the cell walls of cells close to plasmodesmata. is a simplified diagram showing the structure of callose. Not all hydrogen atoms are shown. O O 2C 4C 4C OH 4C 3C 1C 3C 5C 6CH2OH OH OH OH O O H H H H H H O 2C OH 1C 5C 6CH2OH O O 2C OH 1C 5C 3C 6CH2OH Complete Table 3.2 to compare the structure of a callose molecule with a cellulose molecule. Table 3.2 feature of polysaccharide callose molecule cellulose molecule monosaccharide used to synthesise the polysaccharide β-glucose bond connecting the monomers 1,4 glycosidic bond shape of molecule helix orientation of monosaccharides in the molecule alternate glucose molecules are rotated by 180 ° Hydrogen bonding is important for movement of water through xylem vessels in a plant. Describe the roles of hydrogen bonding in the movement of water through xylem vessels. Water that has travelled through xylem vessels reaches the leaves. Cooling of the leaf occurs as a result of the evaporation of water during transpiration. Water has a high latent heat of vaporisation because water molecules form hydrogen bonds. With reference to hydrogen bonding, suggest why cooling of the leaf occurs as a result of evaporation of water during transpiration.
9700_w23_qp_21
THEORY
2023
Paper 2, Variant 1
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Questions Discovered
457