Photosynthesis (SL IB Biology)

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. The graph below shows the results from their experiment.

Identify, with a reason, the factor which is limiting the rate of photosynthesis between A and B.

Add labels to the axis below and sketch an action spectrum for photosynthesis of the rhododendron plant exposed to high 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 across a 20 year period rose from 21°C to 24°C, while the cloud cover increased.

Describe and explain how these changes would impact the growth of Rhododendrons. 

Name two variables, other than temperature and light intensity, that could act as limiting factors for the rate of photosynthesis in rhododendrons. 

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 possesses is an adaptation to their habitat and behaviour. The graph below demonstrates how different wavelengths of light are absorbed by chlorophyll a, chlorophyll b and carotenoids from a particular plant species.

Explain the absorption of light at different wavelengths by pigment chlorophyll a.

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

A researcher investigating the wavelengths of light that hit the ground in a woodland area found that the majority of the light energy reaching the ground sat within the 500-600 nm wavelength part of the spectrum. They also noted that a reduced number of species were found on the ground below the tall trees. 

Suggest, with reference to the light energy spectrum, why few species of plant would be found below tall trees in woodland areas.

Some students used paper chromatography to separate out the photosynthetic pigments in a leaf. The resulting chromatogram can be seen below.

Using the measurements made by the students, calculate the R_f values for carotene and chlorophyll B and complete the table below.

Explain what the results in part (a) show about the properties of the photosynthetic pigments.

The method used to prepare the photosynthetic pigments for chromatography requires the use of propanone. As a health and safety precaution, a student using this method decided to use water instead of propanone.

State what the student’s results would have shown and explain why..

Triticum aestivum is a species of wheat which has been genetically modified to synthesize more chlorophyll B as a way of increasing yield for farmers. 

Explain how an increase in chlorophyll B may lead to an increased yield from this genetically modified crop.

A student set up the following investigation to establish the relationship between light intensity and photosynthesis.

Identify the dependent variable and one control variable for this investigation.

The graph below shows the results obtained by the student. 

A second student suggested that they should position a sheet of blue transparent perspex in between the lamp and the pondweed.

Sketch and annotate a graph to describe the effect of increasing carbon dioxide levels on the rate of photosynthesis.

Plants can contain more than one type of chlorophyll; a and b.  Scientists grew plants that contained a mutated version of chlorophyll a. They investigated the effect of this mutation on the rate of photosynthesis.

 The scientists

• Grew the mutant and normal plants in a range of light intensities
• Isolated the chloroplasts from both types of plants
• Measured the oxygen produced by the chloroplasts over a period of 20 minutes

Their results are shown in the graph below:

Explain why the scientists used the oxygen produced as a measure of the rate of photosynthesis.

Use the graph to calculate the difference in oxygen produced by the chloroplasts from normal plants compared to the mutant plants at a light intensity of 400 Watts/m². 

Paper chromatography can be used to separate photosynthetic pigments obtained from chloroplasts. The chromatography strip below shows distinct pigment bands.

The teacher explains that the four bands represent four pigments: chlorophyll a, chlorophyll b, xanthophylls and carotenes (but not necessarily in that order). The teacher asks a student to suggest which bands most likely represent the two chlorophyll pigments. The student suggests bands 3 and 4.

Explain why this is the correct choice.

Describe how someone could accurately identify the pigments in the chromatogram shown in part a).

The photosynthetic pigments from two aquatic algae, green alga genus Oedogonium and red alga genus Palmaria, were separated by thin layer chromatography. The chromatograms are shown below.

Palmaria also contain a red pigment known as phycoerythrin. The pigment appears red because it absorbs blue light and reflects red light. The pigment phycoerythrin is absent from the chromatogram above.

Suggest why this might be.

Light of shorter wavelengths penetrates water to greater depths than light of longer wavelengths.

Using information here and from part c), suggest why red algae such as Palmaria can live at greater depths than many other aquatic algae.

The graph below shows the effect of light intensity, carbon dioxide, and temperature on the rate of photosynthesis.

 A - 0.04 % carbon dioxide at 15 °C

B - 0.04 % carbon dioxide at 25 °C

C - 0.04 % carbon dioxide at 35 °C

D - 0.1 % carbon dioxide at 25 °C

State three factors that could be measured to allow an estimate of the rate of photosynthesis in this experiment.

A commercial farmer growing peppers keeps her greenhouses at 25 °C and circulates air around the greenhouses. Air has a CO₂ concentration of 0.04 % and the ambient light intensity is 3 000 lux.

Use the graph in part a) to state whether the farmer would be more likely to achieve higher pepper yields by raising the carbon dioxide concentration to 0.1 % or by raising the temperature to 35 °C.

[1]

Explain your answer to part i).

[2]

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.

Oxygen production here is used as a measure of the rate of photosynthesis.

State why this is possible.

[1]

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.

[2]

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

Explain how the data in part c) support this suggestion.

Lab technicians wanted to determine the effects of light intensity and temperature on the rate of photosynthesis in rhododendron plants growing in parks and gardens. 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. The graph below shows the results from their experiment.

Explain the increase in the net rate of photosynthesis between points A and B on the graph.

In another experiment a student looked at the effect of light intensity on the rate of photosynthesis in pond weed. They set up the investigation as shown below and altered the light intensity by changing the distance between the lamp and the pondweed.

Identify four variables that need to be controlled in this investigation.

The graph below shows the student’s results from their investigation in part b).

Calculate the percentage decrease in the rate of photosynthesis that takes place when the distance from the lamp is 10 cm compared to when it is 90 cm. Show your working.

Identify, with a reason, one challenge that would be faced by the technicians in a) that would not be faced by the student in part b) when carrying out their experiments.

Suggest why photosynthesis was inhibited for 12 hours.

From Fig. 3 in part a) identify the following:

The two areas of the leaf from A-D that the student could use to show that light is required for photosynthesis.

[1]

The two areas of the leaf from A-D that the student could use to show that chlorophyll is required for photosynthesis.

[1]

The student concluded that the detection of starch was proof that photosynthesis had occurred in the leaf during the experiment. 

Suggest why this conclusion may not be correct.

Explain how researchers could prepare an initial concentrated solution containing photosynthetic pigments for separation during thin layer chromatography.

In a woodland the concentration of carbon dioxide gas in the air changes during a 24-hour period. It can also vary depending on the height above the ground at which a gas measurement is taken.

Explain the variation in carbon dioxide concentration in a woodland over time and at different heights. Assume that there is no air movement caused by wind throughout the 24-hour period.

The scientists also measured the rate of the light-dependent reactions in the seaweed at each of the different wavelengths.

Suggest how the scientists measured the rate of the light-dependent reactions.

The scientists repeated the investigation twice more under the following conditions

1. They increased light intensity from lamps X and Y
2.  They increased the temperature of the investigation from 15 °C to 20 °C.

Discuss how these changes may affect the amount of oxygen produced by the seaweed

The scientists concluded that both species of seaweed showed increased rates of photosynthesis under lamp X compared to lamp Y.

Evaluate this conclusion.