Calvin's experiments. Students read a summary of the experiments Calvin did to work out the detail of the light-independent stage of photosynthesis. They then ...
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Activity 5.6 How Calvin won the Nobel prize Purpose
To reinforce knowledge and understanding of the Calvin cycle.
Calvin’s experiments Calvin did a series of experiments to work out the detail of the light-independent stage of photosynthesis. Read the following summary and then answer the questions. Melvin Calvin and his colleagues worked at the University of California in the 1940s and 1950s. They wanted to know how plants fix carbon dioxide into organic substances. At the time this was poorly understood. Calvin filled a lollipop-shaped flask with a solution containing the single-celled green protoctist, Chlorella (Figure 1). At the bottom of the flask was a tap which allowed samples to be drained from the flask. The flask was lit from both sides.
Figure 1 Calvin’s lollipop apparatus consisted of a hollow disc-shaped flask with strong lights on each side.
Calvin used the following procedure to identify the compounds made by Chlorella.
Calvin injected the radioactive carbon dioxide into the flask. Two seconds later he took a sample from the flask. The sample was released through the tap into a flask of hot methanol to stop the reaction. Over a short period of time he took further samples. The substances in each sample were separated using two-way paper chromatography. The chromatography papers, known as chromatograms, were covered with photographic film, then left in the dark for several days. Radioactivity from the 14C in the samples produced spots on the film, showing where the substances containing 14C were on the paper. The underlying pieces of chromatography paper were then cut out, and the substances in them washed out and analysed.
In this way, the substance making each spot on the photographic film (known as an autoradiogram) could be identified. A simplified version of Calvin’s results is shown in Figure 2.
Figure 2 Calvin’s results, showing the products of photosynthesis after three different lengths of time.
By taking samples over a range of time intervals, Calvin was able to follow the sequence of reactions. Using these methods Calvin worked out all the intermediate stages in what has become known as the Calvin cycle. In 1961 he received the Nobel Prize for Chemistry for this work.
Look at Figure 1. Suggest why glass plates were placed between the lights and the flask. Explain why placing the sample in hot methanol stopped all photosynthetic reactions.
Q2 Give one advantage and one disadvantage of using Chlorella in the investigation. Q3 a
Describe how the quantities of GP differ between the three sampling times shown on Figure 2 (assuming a larger spot means more GP).
Describe how the quantities of hexose differ between the three sampling times.
Use your knowledge and understanding of the Calvin cycle to suggest an explanation for the differences you describe in part a and b.
Describe and explain the occurrence of amino acids in the three samples.
Q4 At first only half the GP molecules formed contained a radioactive carbon atom. Suggest an explanation for this observation. Q5 Suggest what would happen to the quantity of GP and ribulose bisphosphate (RuBP) a few seconds after the light source was switched off.