5.1 Photosynthesis (A Level only)

In the light-dependent stage of photosynthesis, explain the role of protons in supplying intermediate products to the light-dependent stage. 

Figure 1 shows the experimental set-up of a study of the effect of ammonium hydroxide on chloroplasts extracted from spinach leaves.  All five tubes were then placed in a water bath at 20°C and illuminated from all sides during the subsequent reaction. 

Figure 1

For each tube in Figure 1, explain the rationale for setting the tubes’ contents in the way shown. 

Referring to the experiment depicted in Figure 1, state the role of the compound DCPIP and outline the molecule that it replaces in this experiment. 

State the exact location within a palisade cell that the light-independent stages of photosynthesis takes place.

A team of researchers investigated the effect of changing the carbon dioxide concentration on the levels of glycerate-3-phosphate (GP) and ribulose bisphosphate (RuBP) in photosynthesising cells.

Table 1 shows the results obtained when the carbon dioxide concentration was reduced.

Table 1

Explain the decrease in the level of GP at the lower carbon dioxide concentration.

The researchers carried out a similar experiment but increased the carbon dioxide concentration from 1% to 2%. The relative levels of GP and RuBP remained the same both before and after the experiment.

Suggest three reasons why. 

Some primitive species of bacteria that live on the ocean floor do not have access to light for photosynthesis. Instead, they use the process of chemosynthesis to make food (glucose) using the energy stored in chemicals such as methane, hydrogen sulfide (H₂S) and carbon dioxide. A simplified equation of one of these reactions is shown below:

carbon dioxide + water + hydrogen sulfide → glucose + sulfur + sulfur compounds

Use your knowledge of photosynthesis to suggest what the hydrogen sulfide is used for in chemosynthesis in these bacteria.

There are two types of chlorophyll in chloroplasts, chlorophyll A and chlorophyll B. Researchers created a genetically-modified (GM) vine plant with an allele that caused them to synthesise higher levels of chlorophyll B than wild-type vine plants. They investigated the effect of this new allele on the rate of plant growth.

The researchers grew wild-type and GM vines. They grew some of each in low light intensity and grew others in high light intensity. They extracted chloroplasts from mature plants of both types. Finally, they measured oxygen production at different light intensities by the chloroplasts they had extracted from the plants.

Figure 1 shows the researchers’ results.

Figure 1

Explain the purpose of measuring oxygen production.

Calculate the percentage improvement in oxygen production caused by the genetic modification for vines grown at high light intensity at an experimental light intensity of 20 mmol photons m^-2 min^-1. Give your answer to 3 significant figures. 

The researchers suggested that GM plants producing more chlorophyll B would grow faster than wild-type plants in all light intensities.

Explain how these data support this suggestion.

In a woodland, the concentrations of carbon dioxide in the air at different heights above ground change during a 24-hour period. 

Use your knowledge of photosynthesis to describe and explain these changes. Assume that there is no air movement caused by wind throughout the 24-hour period. 

A research student investigated the uptake of radioactively labelled carbon dioxide in chloroplasts. He used three tubes, each containing different components of chloroplasts.

He illuminated all three tubes and after 20 minutes, measured the uptake of carbon dioxide in each of these tubes.

His results are shown in Table 1.

Table 1

Name the substance which combines with carbon dioxide in a chloroplast.

Use the information in Table 1 to predict the level of absorption of radioactively labelled carbon dioxide in each tube if they were placed in the dark for 20 minutes. Explain your answer.

Certain weed killers act by inhibiting electron transfer during photosynthesis. The addition of such a weedkiller to tube X decreased the uptake of carbon dioxide.

Explain why.

Describe the exact role of ribulose bisphosphate (RuBP) in the Calvin cycle.

Lab technicians wanted to determine the effects of light intensity and temperature on the rate of photosynthesis in Rhododendrons. They recorded the effect of different temperatures on the net rate of photosynthesis at various light intensities. Their experiment also recorded the rate of respiration at the different temperatures. Figure 1 below shows the results from their experiment.

         Figure 1

Name the limiting factor of the rate photosynthesis between A and B. Justify your answer.

Use information in Figure 1 to calculate the gross rate of photosynthesis at 25°C and low light intensity.

Rhododendrons have been found in several alpine regions. Ecologists noted that in one of the plant's natural habitats the average summer temperature rose from 21°C to 24°C, while the cloud cover increased. 

Describe and explain how these changes would impactthe growth of Rhododendrons.

Name two other variables that could act as limiting factors for the rate of photosynthesis. 

Plants possess a collection of photosynthetic pigments that allow for the absorption of light energy. This group of pigments includes two types of chlorophyll and multiple carotenoids. Different species of plants possess different quantities of each pigment. The combination and quantity of each pigment that each plant species possess is an adaptation to their habitat and behaviour. Figure 1 demonstrates how different wavelengths of light are absorbed by chlorophyll a, chlorophyll b and carotenoids from a particular plant species.

Figure 1

Describe the absorption of light at different wavelengths by pigment chlorophyll b.

Suggest why it is beneficial for plants to possess multiple photosynthetic pigments such as chlorophyll a, chlorophyll b and carotenoids.

A researcher wanted to investigate the energy of different wavelengths of light that hit the ground in a woodland area. He measured the energy from direct sunlight and sunlight that had passed through the tree canopy. Figure 2 shows his results.

Figure 2

Only a small number of plant species are found on the ground below tall trees in woodland areas. Use Figure 1 and Figure 2 to suggest why this is.

Suggest a practical method that could be used to determine the different photosynthetic pigments present in a plant species.

Herbicides are chemicals used in agriculture to reduce the growth of competing weeds. Research was carried out to determine how the use of a new herbicide “Weed Wonder” would affect the yield of corn. Some fields in different farms were treated with “Weed Wonder” and the other fields were not. Figure 1 below shows the results from the study. 

   Figure 1

Calculate the percentage increase in yield caused by the use of “Weed Wonder” in farm Q.

A scientist reviewing the data stated that no definite conclusions could be made when comparing the mean values between farms in Figure 1.

Suggest why the scientist made this statement.

“Weed Wonder” works in a very specific way. It inhibits particular proteins within the electron transport chain in the chloroplasts of weeds. 

Explain how this would affect the growth of weeds.

DCPIP is sometimes used in experiments to compare the effectiveness of different weed killers. DCPIP solution is blue when oxidised and colourless when reduced. The weed killers are measured by their ability to prevent the decolourisation of DCPIP in chloroplast suspensions.

Different concentrations of three different weed killers were added to illuminated chloroplast suspensions containing DCPIP. The  IC₅₀ was then calculated for each weedkiller. The IC₅₀ is the concentration of a chemical which prevents the decolourisation of DCPIP by 50%.

Table 1 below shows the results for the three different weed killers. Suggest and explain which weed killer would be the best choice for a farmer.

Table 1 

When plants are exposed to extreme high or cold temperatures for a continued period of time they are put under a lot of stress. This stress greatly impacts the rate of photosynthesis, in particular the light-dependent reaction of photosynthesis.

Explain why extreme cold leads to a decrease in the light-independent reaction.

State where the light-independent reactions take place in photosynthetic plants.

Extreme cold can also cause a decrease in rubisco activity that can limit the rate of photosynthesis.

Explain why.

Lab technicians isolated two enzymes from the leaf cells of tomato plants: rubisco and rubisco activase. Rubisco activase is an enzyme that activates rubisco. They investigated the effect of temperature on the two enzymes. Figure 1 shows their results.

Figure 1

The lab technicians concluded that extreme heat reduces the activity of rubisco in plant leaves by affecting rubisco activase. Use the information given in Figure 1 to evaluate this conclusion.