13.2 Investigation of Limiting Factors

A group of scientists used radioactive carbon to investigate the Calvin Cycle in both light and dark conditions. Multiple leaves of a plant were enclosed within a sealed bag containing radioactive carbon dioxide for different lengths of time. The leaves were then removed and analysed for radioactive compounds. Their results are shown in Fig. 1.

Fig. 1

Describe how radioactively labelled carbon dioxide is fixed inside the leaves of the plant while it is exposed to light in Fig..1.

Explain the results shown in Fig. 1 when the plant is moved into the dark.

At high temperatures, the enzyme that is involved with fixing carbon for the plant increasingly combines with oxygen instead of carbon dioxide in a reaction known as photorespiration. A simplified version of the photorespiration reactions can be seen in Fig. 2.

Fig. 2

Photorespiration reduces the efficiency of photosynthesis.

Use Fig. 2 to suggest why this is the case.

Many scientists are interested in finding ways of improving the action of the enzyme that fixes carbon to reduce the rate at which it carries out photorespiration.

Suggest how the action of this enzyme could be improved.

Scientists investigated the effect of elevated carbon dioxide (CO₂) levels (measured in microatmospheres, μatm) and temperature on the growth and photosynthetic rates of Ulva pertusa, a common bloom-forming green algae.

U. pertusa was grown under four temperature and CO₂ conditions:

• Ambient CO₂ levels (400 μatm) and temperature (16°C), modelling present day conditions
• Elevated temperature only (19°C), modelling warming conditions
• Elevated CO₂  levels only (1,000 μatm), modelling acidification conditions
• Elevated temperature and CO_2, modelling the greenhouse effect

The rate of photosynthesis was measured for each condition.

Fig. 1 shows the results of the investigation.

Fig. 1

Name the limiting factors between points X and Y.

Use the information from Fig. 1 to explain your answer to part (a).

Describe the results of the investigation shown in Fig. 1.

Calculate the percentage change in the rate of photosynthesis at 100 Lux between present day and acidification. Give your answer to 2 significant figures. 

A student investigated the effect of changing light wavelength on the rate of photosynthesis. They set up four beakers of sodium hydrogen carbonate solution, each containing 20 spinach leaf discs, and each under a lamp with a different coloured bulb. They measured the time taken for 50 % of the leaf discs to rise to the surface of the beaker; this time is notated as ET₅₀. The student's results are shown in table 1 below.

Table 1

Calculate the rate of photosynthesis for leaf discs under white, red, and blue bulbs.

Explain why sodium hydrogen carbonate (NaHCO₃) solution was used in the experiment described in part a).

Explain the results shown in Table 1.

The student suggested that the relatively low rate of photosynthesis could be due to light being primarily absorbed by pigments such as chlorophyll b and carotene, known as accessory pigments.

Describe the role of accessory pigments in the light dependent reactions.

A group of scientists carried out a study into the effects of temperature and light intensity on photosynthesis. The results of the study are shown in Fig. 1

Fig. 1

State the limiting factor between the points marked A and B in Fig.1.

[1]

Explain your answer to part i).

[1]

Describe the effect of increasing light intensity on the rate of photosynthesis in Fig. 1

Explain the effect of increasing temperature above 30 °C on the rate of photosynthesis in Fig.1

The study shown in Fig.1 uses carbon dioxide uptake as a measure of the rate of photosynthesis.

Explain why carbon dioxide uptake does not give a true measure for the rate of photosynthesis.

Explain what is meant by the term limiting factor and explain how knowledge of limiting factors is used to increase crop yields in glasshouses.

Describe how you would carry out an investigation into the effect of wavelength of light on the rate of photosynthesis of a plant, using a redox indicator such as DCPIP.