Limiting factors in photosynthesis

The diagram shows the process of carbon dioxide fixation in the light independent stage of photosynthesis (Calvin cycle).

Use the diagram to answer questions 14 and 15.

Which of the statements about the effect of reducing light intensity is/are correct?

1 The amount of glucose produced will be reduced. 2 The concentration of GP will increase. 3 More ATP and reduced NADP will be available from the light dependent reaction.

A 1, 2 and 3 are correct

B Only 1 and 2 are correct

C Only 2 and 3 are correct

D Only 1 is correct

Your answer

The diagram shows the process of carbon dioxide fixation in the light independent stage of photosynthesis (Calvin cycle).

Use the diagram to answer questions 14 and 15.

Which of the statements about the effect of reducing carbon dioxide concentration is/are correct?

1 The amount of glucose produced will be increased. 2 The concentration of RuBP will increase. 3 The concentrations of GP and TP will both decrease.

A 1, 2 and 3 are correct

B Only 1 and 2 are correct

C Only 2 and 3 are correct

D Only 1 is correct

Your answer

A student investigates the effect of light intensity on the rate of photosynthesis in algae that had been immobilised in calcium alginate to make algal beads.

The student places the algal beads in small screw-cap bottles containing hydrogencarbonate indicator.

The indicator turns from orange to red and then purple as CO₂ is removed from the solution.

The student uses the time taken for the indicator to turn purple as a measure of the rate of photosynthesis.

State two variables that the student should keep constant during this experiment.

The student places the bottles at different distances from a light source.

There are four replicates at each distance.

The student’s results are shown in the table.

The student calculates relative light intensity using the following formula:

Calculate the relative light intensity and mean for the 375 cm distance.

Outline an improvement to the method that the student could use to get a more accurate measure of the rate of photosynthesis.

The student wants to use the algal beads method to investigate the effect of CO₂ concentration on the rate of photosynthesis.

Explain why it would not be possible to use the hydrogencarbonate indicator method to investigate the effect of CO₂ concentration.

Suggest how the student could modify the method so the effect of CO₂ concentration on the rate of photosynthesis could be measured.

The graph shows the effect of light intensity on the rate of photosynthesis at two temperatures

Explain the evidence from the graph that shows that CO₂ concentration is a limiting factor in these experiments.

A student looked at the graph and concluded that CO₂ concentration has a greater effect on the rate of photosynthesis than temperature.

Use your knowledge of Q₁₀ and a calculation to support the student’s conclusion.

Genetic modification and selective breeding can be used to improve the characteristics of crop plants, such as maize.

In high light intensity, photosystem II absorbs excess light energy.

A process called nonphotochemical quenching (NPQ) converts the excess light to heat energy.

NPQ can continue when light intensity is no longer in excess. This makes photosynthesis inefficient.

Scientists genetically modified (GM) crop plants to limit NPQ.

The scientists exposed unmodified plants and GM plants to a period of high light intensity followed by lower light intensity. The scientists then compared the rate of NPQ and the rate of carbon dioxide (CO₂) fixation in unmodified plants and GM plants.

Some of the results are shown in the table.

State one dependent variable in this investigation.

Explain the effect of genetic modification on the rate of CO₂ fixation after 150 seconds of low light intensity.

Often a single company holds the patent for a GM crop plant.

Suggest a potential ethical issue that may exist if one company holds the patent for a GM crop plant.

Humans have been selectively breeding maize as a crop plant for thousands of years, which has resulted in many different varieties of maize.

Different varieties of maize have different genomes.

Describe how bioinformatics and computational biology can be used to compare the genomes of different varieties of maize.

Two characteristics that farmers have selectively bred in maize are colour and kernel (fruit) shape.

Colour is determined by a gene with two alleles:

• Allele A is dominant and results in a purple colour.

• Allele a is recessive and results in a yellow colour.

Kernel shape is determined by a gene with two alleles:

• Allele B is dominant and results in a smooth shape.

• Allele b is recessive and results in a wrinkled shape.

The two genes are found on different chromosomes.

Two maize plants are crossed.

• One parent plant is purple and smooth and heterozygous for both genes.

• One parent plant is yellow and smooth and heterozygous for gene B/b.

Complete the answer lines below to show this genetic cross.

Parental genotypes:........ ........

Expected offspring phenotypes:........

Expected phenotypic ratio:........

Cyanobacteria are photosynthetic prokaryotes.

A scientist exposed cyanobacteria to light of different colours and intensities and made the following observations:

• Most cyanobacteria are blue in colour.

• At low light intensities, glucose production in cyanobacteria is low.

• When light intensity reaches a certain level the rate of glucose production in cyanobacteria stops increasing.

Which of the following statements, A to D, correctly explains these observations?

A The pigments in cyanobacteria absorb blue light and light intensity is a limiting factor for the rate of photosynthesis.

B The pigments in cyanobacteria absorb red light and light intensity is not a limiting factor for the rate of photosynthesis.

C The pigments in cyanobacteria absorb blue light and light intensity is not a limiting factor for the rate of photosynthesis.

D The pigments in cyanobacteria absorb red light and light intensity is a limiting factor for the rate of photosynthesis.

Your answer

Plants are capable of synthesising a variety of molecules from the products of the light-independent stage of photosynthesis.

Fig 22.1 summarises these processes.

Identify the molecules represented by the letters A, B, C and D in Fig. 22.1

Identify the following variables from the scientist’s method:

independent variable

dependent variable

one control variable

Identify one variable that was not controlled in the scientist’s method.

Fig. 22.2 is a graph of the scientist’s results.

Describe and explain what these results show about photosynthesis in P. pusillus.

The light-independent stage of photosynthesis used to be referred to as the ‘dark reaction’.

Explain why this is both an accurate and an inaccurate way to describe the light-independent stage.

Name the enzyme responsible for fixing CO₂ in the light-independent stage.

The scientist then investigated the effect of auxin on P. pusillus stems.

The growing tips of stems were removed and the stems were placed in solutions containing different concentrations of auxin.

The scientist analysed the results and determined the following relationship:

The higher the concentration of auxin in the solution, the fewer side shoots grew on the P. pusillus stems.

Explain why this relationship occurs in P. pusillus stems.

Give two examples of the commercial uses of auxin.

A scientist used a respirometer to investigate the rate of respiration and photosynthesis of maize in different light intensities.

• The scientist placed ten maize seedlings in a respirometer and kept it in the dark for three hours.

• The respirometer contained soda-lime to remove any CO₂ produced by the seedlings.

• The scientist placed ten maize seedlings in a separate respirometer without soda-lime and placed it in different light intensities for three hours at a time.

The diameter of the capillary tubing was 0.1 mm.

The volume of a cylinder can be calculated using the following formula:

Calculate the rate of oxygen uptake by the seedlings in the dark. Give your answer to two significant figures. Show your working.

Answer =........ mm³ h^–1

1700 lux is a typical light intensity on a cloudy day in the UK. Calculate the percentage increase in gas production between 1700 and 2000 lux. Show your working.

Suggest why soda-lime was not placed in the respirometer with the seedlings grown in the light.

The scientist made the following claim:

These results suggest that, in maize seedlings, the rate of photosynthesis only exceeds the rate of respiration when the light intensity is above 1510 lux.

Use the data in Table 22.1 to explain why the scientist made this claim.

Temperature and light intensity are two factors that affect the rate of photosynthesis.

A student investigated how temperature and light intensity affected the rate of photosynthesis in the aquatic plant Elodea canadensis. The rate of photosynthesis was measured by counting the number of bubbles produced by the plant per minute.

The student’s results are shown in Table 3.

Identify the anomalous result in Table 3 and explain how this result could be confirmed as an anomaly.

* Describe how the student could improve their experimental method and the presentation of their data.

Photosynthesis occurs in two stages: the light-dependent stage and the light-independent stage. The light-independent stage is affected by temperature more than the light-dependent stage.

Explain why temperature has a greater effect on the rate of the light-independent stage.

Scientists are able to clone desirable plants that show a high rate of photosynthesis. The following passage describes how plants are cloned.

Complete the passage using the most appropriate words or phrases.

Cells are removed from the meristem tissue in axial buds or........

tips. The tissue sample that is removed is called the........ Ethanol can

be used to........ the plant tissue. Hormones are used to stimulate mitosis,

which produces a mass of cells called a........