Photosynthesis

A biologist investigated photosynthesis. They:

• measured the percentage of light absorbed by a plant when it was exposed to different wavelengths

• measured the rate of photosynthesis at each wavelength of light.

Figure 3 shows the results they obtained.

Using Figure 3, what can you conclude about the relationship between:

the colour of light and the rate of photosynthesis

Give three environmental factors that should be controlled when measuring the rate of photosynthesis in this investigation.

Do not include features of the plant in your answer.

Describe how the products of the light-dependent reaction are used in the light-independent reaction to produce triose phosphate.

Do not include the role of ribulose bisphosphate (RuBP) in your answer.

A student used chromatography to separate the different photosynthetic pigments in a chlorophyll solution. She had the following materials.

Chromatography paper A ruler and pencil Suitable glassware A solvent (solvent A)

Describe how she could use these materials to separate the photosynthetic pigments by chromatography.

Using solvent A, the student separated five pigments. She then repeated her method using a different solvent, B. Using solvent B, she separated six pigments.

Explain the difference between these results.

In the following passage the numbered spaces can be filled with biological terms.

carbon, such as carbohydrates, lipids and proteins. Most of the sugars synthesised

per given time. The chemical energy store in dry biomass can be estimated

Write the correct biological term beside each number below, that matches the space in the passage.

Describe the light-independent reaction of photosynthesis.

Scientists investigated the effect of full sun and shade on the rate of photosynthesis in a species of shade-tolerant tree.

To estimate the rate of photosynthesis, the scientists measured uptake of carbon dioxide by trees in a forest. They measured uptake of carbon dioxide during two parts of the day:

• 08.30 – 09.40 hours

• 11.40 – 13.15 hours.

Figure 8 shows the scientists’ results.

Figure 8

Figure 8 not reproduced here due to third-party copyright restrictions

Calculate the total uptake of carbon dioxide between 11.40 and 13.15 hours in trees exposed to full sun in a forest that is 12 000 m² in area.

Give your answer in standard form. Show your working.

Figure 8 shows there is a small difference in the mean uptake of carbon dioxide between 08.30 and 09.40 hours by trees in full sun and by trees in the shade. When the scientists performed a statistical test on these data, they calculated P > 0.5

State what this P value tells you about this difference.

Explain your answer using the words probability and chance.

In this species of tree, very high light intensities can inhibit the release of electrons from chlorophyll.

Suggest how this could explain the results shown in Figure 8 for 11.40 to 13.15 hours.

Tomato plants grow best in high light intensities. To increase the yield of tomato plants, a farmer uses LED lightbulbs to provide additional light.

The increase in dry mass (D) produced when using additional light can be calculated using this equation.

Where L = light used in photosynthesis F = GPP to NPP conversion factor for tomato plants

Table 5 shows some of these values for LED lightbulbs.

Use the equation and Table 5 to calculate the increase in dry mass produced when using LED lightbulbs.

Give your answer in standard form and give the units.

Mature leaves from slow-growing, shade-tolerant plants produce poisonous chemicals that are a defence against being eaten by herbivores.

Suggest how this benefits slow-growing, shade-tolerant plants.

Scientists measured the concentration of poisonous chemicals produced by

shade-tolerant plant species in six taxa. They compared this with the mean concentration of poisonous chemicals produced by all plants and the phylogenetic relationships between the six taxa.

Figure 9 shows the scientists’ results.

A journalist published the following summary of these results.

‘The more recently a shade-tolerant plant species evolved, the greater the concentration of poisonous chemicals it produces.’

Do the data in Figure 9 support this summary? Justify your answer.

Plants transport sucrose from leaves to other tissues for growth and storage. SUT1 is a sucrose co-transporter protein.

Scientists investigated whether the cells of tobacco plant leaves used SUT1 to transport sucrose to other tissues.

The scientists used a radioactively labelled DNA probe to show that the cells of tobacco plant leaves contained the SUT1 gene.

Describe how they would do this.

Do not include PCR in your answer.

To study the role of SUT1 in tobacco plants, scientists reduced the expression of the SUT1 gene.

When the SUT1 gene is transcribed, the SUT1 mRNA produced is called ‘sense’ SUT1 mRNA. The scientists genetically modified plants by inserting an extra gene so that this also allowed the production of ‘antisense’ SUT1 mRNA.

The scientists had two types of tobacco plants:

• type A – plants that were genetically modified

• type B – plants that were not genetically modified.

Suggest how the production of ‘antisense’ SUT1 mRNA in type A plants would reduce the expression of the SUT1 gene.

The scientists hypothesised that lower rates of sucrose transport from leaves would cause reduced growth.

To test this hypothesis, the scientists provided leaves of type A and type B plants with labelled carbon dioxide (¹⁴CO₂). To estimate sucrose transport out of leaves, they measured the percentage of¹⁴C remaining in the leaves for 16 hours.

Figure 5 shows their results.

Calculate the ratio of percentage of¹⁴C remaining in leaves of type B to type A plants 16 hours after providing¹⁴CO₂

In type B plants, the percentage of¹⁴C remaining in the leaves does not reach zero per cent, as shown in Figure 5.

Suggest two reasons why.

1

2

The scientists measured physiological differences between type A plants and

type B plants.

Table 3 shows the scientists’ results as they presented them.

Sucrose is able to inhibit the production and activity of rubisco in leaves of a plant. Type A plants have decreased dry mass compared with type B plants.

Use all the information to suggest and explain how the physiological factors in Table 3 would contribute to the decreased dry mass observed in type A plants.

Turn over for Section B

Section B

one question.

You are advised to spend no more than 45 minutes on this section.

Figure 2 shows one type of calorimeter.

A calorimeter can be used to determine the chemical energy store of biomass. A known mass of biomass is fully combusted in a calorimeter. The heat energy released from this combustion increases the temperature of the water in the calorimeter. The increase in the temperature of a known volume of water is recorded.

Other than the thermometer, explain how two features of the calorimeter shown in Figure 2 would enable a valid measurement of the total heat energy released.

A 2 g sample of biomass was fully combusted in a calorimeter.

The volume of water in the calorimeter was 100 cm³

The increase in temperature recorded was 15.7 °C

4.18 J of energy are needed to increase the temperature of 1 cm³ of water by 1 C

Use this information to calculate the heat energy released in kJ per g of biomass.

Show your working.

Plants and algae produce fuels called biofuels. Scientists have used Chlorella to produce biofuel. Chlorella is a genus of single-celled photosynthetic alga. Chlorella can be grown in open ponds and fermenters.

In natural ecosystems, most of the light falling on producers is not used in photosynthesis.

Suggest two reasons why.

The light absorbed by chlorophyll is used in the light-dependent reaction.

Name the two products of the light-dependent reaction that are required for the light-independent reaction.

Chlorella cells can divide rapidly. A culture of 2000 Chlorella cells was set up in a fermenter. The cells divided every 90 minutes.

You can assume that there were no limiting factors and that no cells died during the 24 hours.

Calculate the number of cells in the culture after 24 hours.

Give your answer in standard form.

Show your working.

In photosynthesis, which chemicals are needed for the light-dependent reaction? Tick (✓) one box.

Describe what happens during photoionisation in the light-dependent reaction.

A student obtained a solution of pigments from the leaves of a plant. Then the student used paper chromatography to separate the pigments.

Figure 5 shows the chromatogram produced.

Explain why the student marked the origin using a pencil rather than using ink.

Describe the method the student used to separate the pigments after the solution of pigments had been applied to the origin.

Calculating the R_f values of the pigments can help to identify each pigment. An R_f value compares the distance the pigment has moved from the origin with the distance the solvent front has moved from the origin.

The distance each pigment has moved is measured from the middle of each spot.

Pigment A has an R_f value of 0.95

Use Figure 5 to calculate the R_f value of pigment C.

The pigments in leaves are different colours. Suggest and explain the advantage of having different coloured pigments in leaves.

Heat stress is a condition that often occurs in plants exposed to high temperatures for a prolonged period of time. Heat stress is a major factor in limiting the rate of photosynthesis.

Heat stress decreases the light-dependent reaction of photosynthesis.

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

Another effect of heat stress is a decrease in the activity of the enzyme rubisco. A decrease in the activity of an enzyme means that the rate of the reaction it catalyses becomes slower.

A decrease in the activity of the enzyme rubisco would limit the rate of photosynthesis.

Explain why.

Where precisely is rubisco found in a cell?

Scientists investigated the effect of temperature on the activity of two enzymes isolated from the leaf cells of cotton plants.

• Rubisco

• Rubisco activase – an enzyme that activates rubisco

Figure 1 and Figure 2 show their results.

The scientists concluded that heat stress reduces the activity of rubisco in plant leaves by affecting rubisco activase.

Use all the information to evaluate their conclusion.

A student isolated chloroplasts from spinach leaves into a solution to form a chloroplast suspension. He used the chloroplast suspension and DCPIP solution to investigate the light-dependent reaction of photosynthesis. DCPIP solution is blue when oxidised and colourless when reduced.

The student set up three test tubes as follows:

Tube 1 – 1 cm³ of solution without chloroplasts and 9 cm³ of DCPIP solution in light.

Tube 2 – 1 cm³ of chloroplast suspension and 9 cm³ of DCPIP solution in darkness.

Tube 3 – 1 cm³ of chloroplast suspension and 9 cm³ of DCPIP solution in light.

The student recorded the colour of the DCPIP in each of the tubes at the start and after the tubes had been left at 20 °C for 30 minutes.

His results are shown in Table 1.

The solution that the student used to produce the chloroplast suspension had the same water potential as the chloroplasts.

Explain why it was important that these water potentials were the same.

Explain why the student set up Tube 1.

Explain the results in Tube 3.

The student evaluated the effectiveness of different chemicals as weed-killers by assessing their ability to prevent the decolourisation of DCPIP in chloroplast suspensions.

He added different concentrations of each chemical to illuminated chloroplast suspensions containing DCPIP. He then determined the IC₅₀ for each chemical. The IC₅₀ is the concentration of chemical which inhibits the decolourisation of DCPIP by 50%.

Explain the advantage of the student using the IC₅₀ in this investigation.

Explain how chemicals which inhibit the decolourisation of DCPIP could slow the growth of weeds.