The table below shows results from an investigation on young spinach plants. A student grew identical plants in pots containing the same mass of soil. Each ...
Question 1 Report
The table below shows results from an investigation on young spinach plants. A student grew identical plants in pots containing the same mass of soil. Each pot received the same volume of water and was kept at 22 degrees C. Different concentrations of nitrate ions were added once each week. After 28 days, the dry mass of each plant was measured. Fig. 1 shows the root hairs that absorb mineral ions from soil water.
| Nitrate ion concentration in soil water / mg dm-3 | Mean dry mass of one spinach plant after 28 days / g |
|---|---|
| 0 | 0.42 |
| 20 | 0.91 |
| 40 | 1.38 |
| 80 | 1.41 |
(a) Describe the relationship between nitrate ion concentration and plant dry mass. [2]
(b) Calculate the increase in mean dry mass between 0 and 40 mg dm-3 nitrate ions. Give your answer in g. [2]
(c) Explain why plants need nitrate ions to increase their mass. [3]
(d) Design an investigation to test whether bacteria that convert nitrogen compounds into nitrates improve the growth of spinach plants. [5]
Answer Details
(a) Dry mass increases as nitrate concentration rises from 0 to 40 mg dm-3. There is little or no further increase between 40 and 80 mg dm-3, suggesting nitrate is no longer the limiting factor. [2]
(b) \[1.38\text{ g} - 0.42\text{ g} = 0.96\text{ g}\] The increase in mean dry mass is 0.96 g. [2]
(c) Nitrate ions provide nitrogen. Nitrogen is needed to make amino acids and proteins. Proteins are required to make new cells, so plant growth and mass increase. [3]
(d) Use similar spinach seedlings in identical pots containing equal masses of the same soil. Add a measured culture of nitrifying bacteria to one group and no bacteria, or a sterilised culture, to a control group. Use several pots per group and calculate means. Keep starting nitrate concentration, water volume, light, temperature and time constant. Measure final dry mass, or increase in dry mass, and compare the mean values. [5]
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