The light-independent reaction

The diagram shows some of the reactions that take place in a leaf cell in a flowering plant.

Which of the following statements about triose phosphate is/are correct?

1 Six turns of the Calvin cycle produces 6 molecules of triose phosphate. 2 Triose phosphate from the Calvin cycle can be recycled to ribulose bisphosphate. 3 Triose phosphate from the Calvin cycle can be used to make cellulose, lipids and proteins.

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

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:........

The image is a transmission electron micrograph (TEM) of a chloroplast.

Complete the table using the letters from the TEM which identifies the structure or function. Each letter may be used once, more than once or not at all.

Complete the sentences using the most appropriate terms.

Carbon fixation occurs in the Calvin cycle when CO₂ combines with the five-carbon

molecule........ This forms an unstable six-carbon

molecule that immediately breaks down into two molecules of........

These are reduced to triose phosphate using........ and

........ that are formed in the light-dependent stage of

photosynthesis. Triose phosphate is used to form hexose phosphates which can be

converted to........ for transport elsewhere in the plant.

A student investigated photosynthetic pigments in spinach leaves using thin layer chromatography (TLC).

Fig. 17.1 shows the student’s plate at the end of the investigation.

The table shows colours and R_f values for several photosynthetic pigments.

Calculate the R_f value for spot 3 and use this to identify the pigment that spot 3 represents.

Predict the colour of spot 4.

The solvent used for the separation was non-polar.

Identify the spot corresponding to the least polar pigment. Give a reason for your choice.

The student used the following method for the investigation:

Step 1: Extraction of pigments

• Take 0.5 g of fresh spinach and add 1 g of sand.

• Grind the mixture until it becomes a fine, light green powder.

• Transfer the powder to a test tube and add 2 cm³ of propanone.

• Stir for about 1 minute then allow to stand for another minute.

• Transfer the dark green upper layer with a pipette to a clean test tube and seal with film when not in use.

Step 2: TLC analysis

• Hold the TLC plate carefully by the edges and avoid damaging the surface of the plate.

• Draw a pencil line across the width of the TLC plate 1 cm from the bottom edge.

• Spot the extract on the pencil line using a pipette, one drop at a time, allowing the spot to dry before adding the next drop.

• Place chromatography solvent in a jar so that it is no more than 0.5 cm deep.

• Lower the TLC plate into the jar and lean the top against the side of the jar. Make sure the plate does not touch the sides of the jar anywhere else.

• Place a cap on the jar and allow the solvent to soak up the plate.

• When the solvent has reached a few mm from the top of the plate, remove the plate from the jar and mark the position of the solvent front with a pencil.

Explain why the method included the following precautions:

Hold the TLC plate carefully by the edges and avoid damaging the surface of the plate.

Make sure the plate does not touch the sides of the jar anywhere else.

Suggest an advantage of working as quickly as possible in Step 1.

Algae are single-celled photosynthetic organisms.

An investigation was carried out into the effect of CO₂ concentration on the concentrations of ribulose bisphosphate (RuBP) and glycerate 3-phosphate (GP) present in algae.

Algae were grown in a solution containing a high concentration of CO₂. The concentration of CO₂ was then reduced.

RuBP and GP concentration were measured for 2 minutes before and 3 minutes after the reduction in CO₂ concentration.

The results are shown in Fig. 17.2.

concentration

time (min)

Fig. 17.2

Describe and explain the effect of reducing the CO₂ concentration on the concentration of RuBP and GP in the algae.

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.

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........