(B400U20-1) 16 © WJEC CBAC Ltd. 3. Tomato plants, Solanum lycopersicum, are grown commercially for their edible fruit. They produce bright yellow flowers from which the fruit develop. Sucrose is a disaccharide transported in phloem tissue and used by tomato plants as a source of energy. Image 3.1A shows a light micrograph of Image 3.1B shows an electron phloem tissue. micrograph of a sieve plate. (a) The mechanism of transport in phloem is described as the mass flow hypothesis. Increasing hydrostatic pressure in the sieve tubes at the source in the leaves drives the transport of sucrose to sinks elsewhere in the plant. (i) Name two regions of a tomato plant that are considered ‘sinks’ for sucrose. [1] ................................................................. ................................................................. sieve plate sieve tube 16 Examiner only (B400U20-1) Turn over. B 40 0 U201 17 17 Examiner only © WJEC CBAC Ltd. 17 (ii) The rate of flow of solutes in the phloem sieve tubes of tomato plants may be up to ten times faster than the flow of solutes through other cells. Using Images 3.1A and 3.1B and your knowledge of sieve tube structure, describe and explain one structural feature that allows a relatively fast flow rate to be achieved in the sieve tubes. [2] (iii) Sucrose is loaded into the phloem in the leaves where it is formed. Explain why loading sucrose at the source results in increased hydrostatic pressure in the sieve tubes at that point. [2] (iv) The rate of uptake and translocation of sucrose in phloem was found to be inhibited by: • low temperature • phosphate deficiency • cyanide. Explain why each of these observations appear to contradict the mass flow hypothesis, which describes a passive process. [3] 18 (B400U20-1) © WJEC CBAC Ltd. (b) An experiment to investigate the rate of transport of organic molecules in the phloem of tomato plants used aphids to sample phloem contents. Aphids are insects that insert a tubular mouthpart called a stylet into the phloem, through which it feeds on phloem solution. Plants may respond to phloem damage by blocking some pores in the sieve plates and restricting flow. Aphids can suppress this response. Image 3.2 shows a diagram of an aphid feeding on phloem. Image 3.2 The experiment was set up as shown in Image 3.3 Image 3.3 • Aphid colonies were placed at 25 cm intervals from a start point (0) near the leaves on a branch and allowed to feed. • The aphids were detached from their stylets which were left in place penetrating the phloem. • Leaves were covered by a sealed glass chamber and provided with 14CO2 containing the radioisotope 14C. Organic compounds became radioactive as 14C was incorporated during and after photosynthesis. • Phloem solution, collected from the stylet at regular intervals, was tested for radioactivity. The time taken for radioactivity to reach each colony position was recorded. epidermis mesophyll stylet companion cell sieve tube 0 1 2 3 position of aphid colonies glass chamber containing radioactive carbon dioxide (14CO2) 18 (B400U20-1) Turn over. 19 © WJEC CBAC Ltd. 19 Table 3.4 shows the time taken for the radioactivity to travel between positions on the branch. Table 3.4 (i) Calculate the mean rate of flow of the solution in the phloem between position 0 and position 3. Give your answer to 3 significant figures. [3] Rate = ............................................... cm hr–1 (ii) Suggest one advantage and one disadvantage of using several aphids at each position. [2] (iii) For ethical reasons, the use of micro-injection needles inserted into the phloem was suggested as an alternative to using aphids for sampling the phloem. Suggest one reason why the use of micro-injection needles may provide less precise results. [1] Direction of movement of radioactivity between positions. (25 cm intervals) Time taken for radioactivity to travel between positions / hours 0 to 1 1.2 1 to 2 1.4 2 to 3 1.5 14 Examiner only 20 (B400U20-1) Examiner only © WJEC CBAC Ltd. 4. (a) Rabbits and foxes are mammals that consume different proportions of macronutrients. Image 4.1 shows the digestive systems of a rabbit (a herbivore) and a fox (a carnivore). Image 4.1 (i) Most products of digestion are absorbed in the ileum which is the longest section of the small intestine. With reference to its diet, explain why a fox has a proportionally shorter ileum than a rabbit. [2] (ii) The caecum and large intestine are proportionally larger in a rabbit than in a fox and contain micro-organisms similar to those found in the rumen of cattle. State one function carried out by these micro-organisms. [1] Rabbit small intestine large intestine caecum Fox 20 (B400U20-1) Turn over. 21 Examiner only © WJEC CBAC Ltd. 21 (b) Image 4.2 is a photomicrograph of part of the ileum. An epithelial cell is labelled. Image 4.2 (i) The specimen in Image 4.2 has been viewed using a light microscope. Suggest the magnification of the objective lens that was used to provide the visual detail seen in this photograph. [1] Magnification = ×............................................................................................................. (ii) Describe the process of glucose transport from the lumen of the small intestine into and then out of epithelial cells as it is absorbed along the pathway from X to Y, indicated by the arrow in Image 4.2. [3] lumen epithelial cell X Y 22 (B400U20-1) Examiner only © WJEC CBAC Ltd. (iii) The lumen of the small intestine contains water from food and digestive secretions. Describe how the transport of glucose into the epithelial cells affects the absorption of water from the lumen. [2] 9 22 (B400U20-1) Turn over. 23 © WJEC CBAC Ltd. 23 BLANK PAGE PLEASE DO NOT WRITE ON THIS PAGE