Practise how light, carbon dioxide and temperature limit photosynthesis, then investigate its rate with chloroplast suspensions and whole plants.
Environmental conditions such as light intensity affect plant physiology.
Use the letter X to identify a point on the sketch graph in Fig. 8.1 where light intensity is acting as a limiting factor on the rate of photosynthesis.
Fig. 8.1
An experiment investigated how light intensity affected gene expression in kale, Brassica oleracea sabellica.
- Two groups of kale plants were grown, with one group in high light intensity and one group in low light intensity. All other conditions were standardised.
- After the same period of time, messenger RNA (mRNA) was extracted from each group of kale plants.
- The mRNA was used to produce cDNA.
- The cDNA was hybridised with probes for 89621 kale genes on a microarray.
- The microarrays showed which genes were switched on (expressed) in each set of conditions.
The results showed that expression of 18% of the genes was affected by light intensity.
- 14 % of the genes were switched on only in high light intensity.
- 4 % were switched on only in low light intensity.
Suggest why more genes were switched on only in plants growing in high light intensity compared to fewer genes that were switched on only in plants growing in low light intensity.
A student investigated the effect of temperature on the rate of photosynthesis at two different light intensities. The rate of photosynthesis was determined by measuring the rate of oxygen production of the plant.
Fig. 7.1 shows the result of the investigation.
rate of oxygen production
Fig. 7.1
With reference to Fig. 7.1, describe and explain the results of the investigation for the plant in low light intensity.
With reference to Fig. 7.1, describe and explain the results of the investigation for the plant in high light intensity above .
The light-dependent stage of photosynthesis occurs within chloroplasts. In this stage, electrons are emitted from the chlorophyll a molecules and passed to electron acceptors.
If a redox indicator, such as DCPIP, is added to a suspension of illuminated chloroplasts, electrons will be transferred to DCPIP, causing the colour of the DCPIP to change from blue to colourless.
A student investigated the effect of the wavelength of light (colour of light) on the rate of photosynthesis.
- DCPIP was added to three colorimeter tubes, each containing a suspension of chloroplasts. The chloroplast suspensions were kept in the dark until required.
- The colorimeter tubes were each exposed to light of a different colour: red, blue or green. The intensity of light was the same for all tubes, and each was exposed to light for four minutes. All other conditions were kept the same.
- The absorbance of each chloroplast suspension was measured at one-minute intervals using a colorimeter.
The results are shown in Fig. 7.1.
Fig. 7.1
(i) Explain why the chloroplast suspensions were kept in the dark until required.
(ii) Describe the results shown in Fig. 7.1.
(iii) With reference to the light-dependent stage of photosynthesis, explain the differences between the results shown in Fig. 7.1 for red light and for green light.
(b) Changes in the atmospheric carbon dioxide concentration, light intensity and temperature can affect the rate of photosynthesis. These three factors directly affect different processes of photosynthesis.
Complete Table 7.1 using a tick to identify the processes that can be directly affected by each factor or a cross ( X ) to identify the processes that are not directly affected by each factor.
Indirect effects where a change in the rate of one process affects the rate of a different process should not be considered.
A tick or a cross must be placed in the final column of every row.
Table 7.1
Environmental conditions such as light intensity affect plant physiology.
Use the letter X to identify a point on the sketch graph in Fig. 8.1 where light intensity is acting as a limiting factor on the rate of photosynthesis.
Fig. 8.1
The rate of photosynthesis is affected by many environmental factors.
A student carried out an experiment to investigate the relationship between the concentration of carbon dioxide available to an aquatic plant, Cabomba caroliniana, and its rate of photosynthesis.
Fig. 1.1 shows the experimental set-up for this investigation.
- Sodium hydrogencarbonate solution was used as a source of carbon dioxide.
- The concentration of carbon dioxide was varied using six different concentrations of sodium hydrogencarbonate solution.
- All C. caroliniana plants were kept in the dark before the light was switched on at the start of the experiment.
- Five replicates were carried out at each concentration.
- The rate of photosynthesis was obtained by calculating the percentage change in dissolved oxygen concentration in the solution over five minutes.
Fig. 1.2 shows the results of the investigation.
Fig. 1.2
The percentage change in dissolved oxygen for C. caroliniana at of sodium hydrogencarbonate solution is negative.
Suggest reasons for this negative value.
To minimise temperature changes, the student decided to use an LED lamp as a light source. LED lamps release very little heat energy.
Explain the importance of minimising temperature changes in this experiment.