In the mesophyll tissue of leaves, products of photosynthesis can be used to synthesise organic compounds, such as the polysaccharide cellulose and some amino acids. A source of nitrogen for amino acid synthesis can be provided by nitrate ions that have been taken up in the roots and transported to the leaves.
(a) Describe the structure of a cellulose molecule.
(b) Studies of nitrate uptake and nitrate metabolism help to provide information to scientists who are investigating ways to increase the yield of crop plants. The first step of nitrate metabolism in leaf cells is the reduction of nitrate to nitrite, catalysed by the enzyme nitrate reductase. The activity of the enzyme can be studied by detecting the presence of nitrite formed.
(i) Researchers have found that adding nitrate to leaf tissue results in an increase in messenger RNA (mRNA) molecules of the gene NR, which codes for nitrate reductase. State one benefit to leaf cells of an increase in mRNA molecules of gene NR after the addition of nitrate.
(ii) One method used to detect the presence of nitrite formed from the reduction of nitrate in leaf tissue involves using an inhibitor to prevent nitrite from taking part in further reactions in the leaf tissue and immersing the leaf tissue in a solution containing a colourless test reagent. The nitrite from the leaf tissue enters the surrounding solution, changing the colour of the solution to magenta (red-purple). Suggest why using a colorimeter can improve this method to detect the presence of nitrite.
(c) In the leaf, transport of amino acids from mesophyll cells to companion cells involves using a number of different membrane transport proteins called amino acid transporters. There is evidence that amino acids can move from the apoplast into the cytoplasm of a companion cell using the same transport mechanism that is used for sucrose transport.
(i) Outline and explain the sequence of events that occurs, which allows amino acids to be transported from the apoplast into the cytoplasm of a companion cell.
(ii) Suggest why amino acid transporters are not needed to move amino acids from the companion cell into a phloem sieve tube element.
To investigate nitrate uptake, roots can be cut and removed (excised) and placed in a buffered solution containing nitrate ions. The root tissue can be analysed to determine the quantity of nitrate taken up over a set time period.
(d) Excised roots of the crop plant maize, Zea mays, were placed in three different concentrations of nitrate solution: 0.2 mmoldm–3, 1.0 mmoldm–3, and 5.0 mmoldm–3. The solutions were maintained at 30°C and were aerated to provide a continuous supply of oxygen to the root tissue. Nitrate (NO3–) uptake by the root tissue was determined each hour for five hours. The results are shown in Fig. 4.1.
Fig. 4.1 shows that the rate of nitrate uptake is very low initially and then increases for all three concentrations of nitrate solution tested. Describe the differences in the rates of nitrate uptake for 5.0 mmoldm–3 nitrate solution compared with 0.2 mmoldm–3 nitrate solution, between 2h and 5h.
(e) The nitrate uptake of excised maize roots was investigated under different conditions. Table 4.1 shows details and results for a control experiment and four modified experiments, 1, 2, 3, and 4. The same concentration of nitrate solution was used throughout for all the experiments. All the results were taken after a set time period.
The results for experiments 1, 2, 3, and 4 in Table 4.1 can be compared to the results for the control experiment. Discuss how comparing each of the results with the control provides information about how nitrate ions are taken up by the root cells and the factors affecting the uptake of nitrate ions.
