1. An intact plant shoot was sealed in a glass container. The concentration of oxygen in the air surrounding the leaf inside the container was measured. The levels of oxygen measured under three different conditions are shown in the figure below. Explain the results.
Answer
In the dark: No photosynthesis is taking place, so no oxygen is being produced. Respiration is still occurring, meaning oxygen is required as the terminal electron acceptor. This oxygen is taken from the surroundings, causing a drop in oxygen levels.
At high light intensities: High rate of photosynthesis means large amounts of oxygen produced during photolysis. Not all is used in respiration, so some is released into the surrounding air.
At average light intensities: The amount of oxygen produced during photosynthesis meets the oxygen demand of respiration, so levels remain constant.
2. Triose phosphate leaves the chloroplast via a specific carrier protein. Explain why other molecules are not transported via this protein.
Answer
Proteins have specific tertiary structure. Only triose phosphate fit.
3. Phosphate ions enter the chloroplasts, yet there is no build-up of this ion inside the organelle. Explain how.
Answer
Phosphate ions are used to phosphorylate ADP. In the light-dependent reaction catalysed by ATP synthase.
4. Scientists investigated the light-independent stage of photosynthesis using the apparatus shown in the figure below. The apparatus was set up in the dark. Explain why.
Answer
To be sure that photosynthesis was not happening and any change in concentration of substances would be because of photosynthesis.
5. The apparatus is a thin, flat shape. Explain why.
Answer
To ensure that light could reach all the algae cells.
6. The algae were sampled by pouring them immediately into hot alcohol. Suggest why.
Answer
Hot alcohol kills cells quickly. In this way concentrations measured can be measured accurately for a specific time.
7. For a specific time, light was shone on to the algae in the apparatus and then the light was turned off so that the apparatus was in darkness again. Samples of algae were removed from the apparatus at various time intervals, and the concentrations of glycerate 3-phosphate (GP) and ribulose bisphosphate (RuBP) in the cells were measured. The figure below shows the results. A short time after the light was switched on, the concentration of both RuBP and GP in the algae increased and then remained constant. Explain why.
Answer
The concentrations increased because RuBP reacts with carbon dioxide to form GP. They then remain constant because GP is used to produce TP and other substances, while RuBP is regenerated at the same rate that GP is converted to TP.
8. Explain the shape of the curves for RuBP and GP after the light was switched off.
Answer
After the light is switched off, the light-dependent stage cannot occur. This means no reduced NADP or ATP is produced to reduce GP to TP or provide energy for the reaction. As a result, GP accumulates and RuBP is not regenerated.
9. Explain the advantage of converting pyruvate to lactate in anaerobic conditions, as shown in the figure below.
Answer
Converting pyruvate to lactate in anaerobic conditions allows NADH to be oxidised back to NAD+. NAD+ is essential for glycolysis to continue. Without the regeneration of NAD+, glycolysis would stop because all NAD+ would be reduced, and no ATP would be produced.