The carbon cycle

Read the passage below. Use the information in the passage and your own knowledge to answer the questions that follow. Plants to the rescue Since the early 20th century, the concentration of carbon dioxide in the atmosphere has increased rapidly. This has further increased in recent years due to more cars and the increased demand for electricity for homes and industry. Scientists have found that plants play a critical part in removing this excess carbon dioxide from the atmosphere. Using computer models, the scientists concluded that photosynthesis has increased by 30 per cent. The scientists measured carbonyl sulfide found in ice cores and air samples. In addition to carbon dioxide, plants take in carbonyl sulfide gas during their natural carbon cycle, and this is frequently used as a measure of photosynthesis on a global scale. Terrestrial plants are removing about 29 per cent of carbon dioxide emissions that would otherwise contribute to an increase in the atmospheric carbon dioxide concentration. A carbon sink is an ecosystem, such as a forest, that absorbs more carbon dioxide than it releases. The size of the carbon sink depends on the rate of photosynthesis but also on the levels of deforestation and respiration. The model the scientists used showed that the role of photosynthesis in producing a carbon sink in land plants is larger than estimated in most other models. Other scientists are less confident about using carbonyl sulfide as a measure of photosynthesis. Plant absorption of carbonyl sulfide can vary depending upon the amount of light the plants receive. Therefore, the measure of global photosynthesis could be overestimated. Regardless of the rate at which photosynthesis has increased, scientists agree that excess carbon dioxide is boosting the growth of plants. Trees are becoming leafier, and there is more wood. The wood is where most of the carbon is stored in 25 the plant. In experimental research, scientists exposed plants to double the normal concentration of carbon dioxide found in the atmosphere. Under these increased carbon dioxide conditions, the composition of the leaf tissues changed. This made the leaves tougher for herbivores to eat and made it harder for insect larvae 30 to grow. Scientists have also observed that when plants are exposed to increasing levels of carbon dioxide, the size of the stomatal pores on a leaf increases.

Explain why more cars would result in an increase in atmospheric carbon dioxide. (Lines 1-3)

Explain how increased carbon dioxide can cause climate change.

Explain why increasing carbon dioxide concentration can increase photosynthesis in plants.

Explain why the carbon sink depends upon respiration and deforestation as well as photosynthesis. (Lines 13 to 15)

Light affects gas exchange in leaves. (Lines 19 and 20) Describe an experiment that shows how the net gas exchange from a leaf varies with light intensity.

The photograph shows stomata on a small square of the lower surface of a leaf.

(Source: © Kallayanee Naloka/Shutterstock) The total area of the lower surface of the leaf is 150 cm². Using the photograph and the total lower surface area of the leaf, estimate the number of stomata on the lower surface of the leaf.

number of stomata =_........

Stomata also have a role in water transport in the plant. Explain the role of stomata in water transport.

The diagram shows the carbon cycle.

Which letter in the diagram represents respiration?

Give the name of the process labelled W.

Carbon dioxide, methane, and nitrous oxide are three greenhouse gases. The table shows a comparison of these greenhouse gases. The Global Warming Potential (GWP) is the ratio of the heat absorbed by a greenhouse gas in the atmosphere relative to the heat absorbed by the same mass of carbon dioxide gas.

Name one other greenhouse gas.

Evaluate the information in the table to identify which of these gases is likely to contribute most to global warming.

Read the passage below. Use the information in the passage and your own knowledge to answer the questions that follow.

Supercharging plants to reduce global warming

The proportion of carbon dioxide in the atmosphere has increased in the last 100 years. In 2020, a mass of 727 gigatonnes of carbon dioxide was released into the atmosphere from natural processes, along with a mass of 37 gigatonnes from human activities. Scientists have estimated that plants naturally remove a mass of 746 gigatonnes of carbon dioxide from the atmosphere every year. The difference between what is removed and what is released causes atmospheric carbon dioxide to rise every year. Carbon dioxide is a greenhouse gas, and a significant rise will cause global warming.

To help solve the problem of rising concentrations of atmospheric carbon dioxide, scientists are planning to produce transgenic, supercharged plants that can remove atmospheric carbon dioxide and store it in their roots. The scientists estimate that if these plants can be developed, the plants could remove a mass of carbon dioxide equivalent to 50% of the emissions from human activities.

Coastal plants that have their roots in seawater contain a substance called suberin in the cell walls of the outer layer of the roots. Suberin is a waterproof substance that contains a high proportion of carbon atoms. Suberin is decomposed very slowly so remains in the soil for a long time.

The photograph shows a coastal plant called a mangrove.

To produce the supercharged plants, scientists intend to take the gene that codes for high suberin production from a coastal plant and insert it into crop plants. The crop plants used are perennial plants. Perennial plants live for many years rather than dying each winter. The transgenic crops would take in large amounts of carbon dioxide and use the carbon atoms to make suberin. The carbon would then be locked up and stored as suberin in the roots. After successfully producing one plant, they will use micropropagation rather than pollination to produce others.

These supercharged crop plants may have other uses. Suberin in roots helps to make them tolerant to soil with a high salt concentration, helping to produce higher crop yields in areas that have difficult growing conditions.

Carbon dioxide is a greenhouse gas.

State the name of another greenhouse gas (line 7).

Calculate the increase in mass, in kg, of atmospheric carbon dioxide in 2020 (lines 2 to 7).

Give your answer in standard form.

[1 gigatonne = 1 000 000 000 000 kg]

State two consequences of global warming for the environment.

Explain why producing genetically engineered plants with additional suberin in their roots could reduce atmospheric carbon dioxide (lines 14 to 17 and lines 19 to 24).

Which enzyme is used to remove a gene from a section of DNA?

Explain why the additional suberin in the cell walls of the roots will make the transgenic plants tolerant to soil with a high salt concentration (lines 26 to 28).

Give three reasons why the scientists use micropropagation to reproduce the transgenic crop plants (lines 24 to 25).

The diagram shows a cycle found in ecosystems.

Give the name of the cycle.

Which process is represented by the letter W?

Which process is represented by the letter X?

Name a group of organisms that is responsible for decomposition.

Carbon dioxide is released into the atmosphere by the decomposition of organic material. The rate of this decomposition depends on a number of factors. Design an investigation to find out if changing the pH of organic material affects the rate of decomposition. Include experimental details in your answer and write in full sentences.

Deforestation is the cutting down of trees.

After deforestation many dead leaves are left on the forest floor.

A student investigates the decomposition of these leaves.

This is her method.

● collect four samples of dead leaves each with a mass of 6.0 kg

● label the samples P, Q, R and S

● cut sample P into small pieces and keep at 10 °C

● cut sample Q into small pieces and keep at 20 °C

● do not cut sample R and keep at 10 °C

● do not cut sample S and keep at 20 °C

● measure the mass of each sample after three months

The graph shows her results.

Explain the results obtained by the student.

Calculate the difference between the rate of decomposition in sample P and the rate of decomposition in sample Q.

Give your answer in kg per month.

difference =_........ kg per month

The student needs to control biotic variables in her investigation.

Give two biotic variables she should control.

The diagram shows some energy transfers along a food chain.

The numbers give the energy transfers in kJ per m² per year.

Give the name of process X.

Determine the energy transferred from the primary consumers to the secondary consumers.

energy transferred =_........ kJ per m² per year

Decomposers can be described as saprotrophs.

Describe how saprotrophs cause decomposition.

Forty per cent of the energy in the producers transfers to decomposers.

Only twenty per cent of the energy in the primary consumers transfers to decomposers.

Explain this difference in energy transfer.

The diagram shows a cycle that is important for the recycling of biological molecules. Processes in the cycle are labelled A, B, C and D.

Give the name of this cycle.

Which process reduces CO₂ in the atmosphere?

Name the two groups of microorganisms that carry out process B.

The breakdown of dead plant material by microorganisms is affected by many factors.

Design an investigation to determine the temperature at which plant material is broken down most effectively.

Include experimental details in your answer and write in full sentences.

Suggest what is meant by the term sustainable (line 3).

Explain why having a developed root system makes regrowth quicker (lines 4 to 5).

Coppiced wood can be used as a biofuel (lines 8 to 9).

Give similarities and differences in the use of biofuel compared with the use of fossil fuels.

New growth in woodland is often destroyed by species such as deer (lines 5 to 6).

Suggest one way to protect woodland from this damage.

Coppicing increases biodiversity in woodland ecosystems (line 17).

Describe a method to investigate the effect of coppicing on the biodiversity of plants in a woodland.

Coppicing increases the number of insects in woodland.

A farmer owns a field next to a coppiced woodland.

Give one reason why more insects in the woodland might be an advantage for the farmer.

Give one reason why more insects in the woodland might be a disadvantage for the farmer.

Explain why a range of different light levels in a woodland leads to an increase in plant biodiversity (lines 23 to 24).