Material cycles

In tropical rainforests, there is a high biodiversity of animal species (lines 1–3).

Explain why.

Nutrient recycling occurs rapidly on the ground of tropical rainforests (lines 4–5).

Explain how two abiotic factors in these rainforests enable the rapid rate of recycling.

The recycling of nitrogen-containing compounds from dead organic matter sustains the growth of plants (lines 5–7).

Describe the roles of bacteria in this recycling.

Natural regeneration of rainforests occurs by secondary succession (lines 8–10).

Describe the process of secondary succession.

Farming practices have prevented the natural regeneration of rainforests by secondary succession (lines 12–14).

Suggest two farming practices that prevent the natural regeneration of rainforests by secondary succession.

Write an essay on one of the topics below.

Either

Phosphorus-containing substances and their importance in biological systems.

Or

The mechanisms and importance of transport within organisms.

Figure 1 shows part of a phosphorus cycle, including the mass of phosphorus in two stores.

One teragram (Tg) is 1 × 10¹² grams.

The supply of phosphorus to the soil is shown as a percentage of the total phosphorus mass of each store.

Calculate in teragrams the difference in the supply of phosphorus to the soil from the two stores shown in Figure 1.

Show your working.

Water shortage can inhibit crop production. Scientists investigated the effect of the mycorrhizal species Glomus intraradices on the growth of tomato plants under conditions of water shortage and no water shortage.

• The scientists planted tomato seeds into a large number of pots containing sterilised soil.

• They added a culture of G. intraradices to 50% of the pots and 50% were left untreated.

• After the seeds had developed into seedlings (young plants), the seedlings from the untreated and treated pots were planted into four separate large greenhouses (glasshouses).

• A very limited supply of water (water shortage) was provided to two of the greenhouses. A sufficient supply of water (no water shortage) was provided to the other two greenhouses.

• After 60 days, the scientists determined the mean mass of tomatoes (kg m^–2) from each greenhouse.

Greenhouse P – untreated seedlings with water shortage Greenhouse Q – treated seedlings with water shortage Greenhouse R – untreated seedlings with no water shortage Greenhouse S – treated seedlings with no water shortage

Treated seedlings are plants grown with G. intraradices.

Figure 2 shows the scientists’ results.

The error bars represent ± 2 standard deviations from the mean, which includes over 95% of the data.

Using all the information, evaluate the effectiveness of using mycorrhizae to increase crop production under conditions of water shortage and no water shortage.

The tomato seeds were planted into sterilised soil.

Suggest two reasons why the soil was sterilised.

The soil used in this investigation had the recommended concentration of fertiliser.

Suggest two reasons why the soil contained the recommended concentration of fertiliser.

Nitrogen-fixing bacteria such as Azotobacter chroococcum use the enzyme nitrogenase to produce ammonia from nitrogen gas in the air. A. chroococcum can use ammonium chloride as a direct source of ammonia. When a source of ammonia is not available this bacterium uses nitrogen fixation.

A scientist investigated the effect of an increase in the concentration of ammonium chloride on the activity of nitrogenase in this bacterium. He prepared several liquid medium cultures of the bacterium. Each liquid culture had the same volume. He grew each culture in a different concentration of ammonium chloride.

In each culture:

• he recorded the nitrogenase activity in arbitrary units

• he removed the bacteria and then recorded the concentration of ammonium chloride remaining in each liquid medium.

Table 2 shows the scientist’s results.

Apart from temperature and pH, give two variables the scientist would have controlled when preparing the liquid medium cultures.

A student concluded that this investigation showed that ammonia inhibits nitrogenase activity in nitrogen-fixing bacteria. Use all the information to evaluate the student’s conclusion.

Nitrogenase catalyses the reduction of nitrogen during nitrogen fixation. The reaction requires 16 molecules of ATP for each molecule of nitrogen that is reduced. When ammonia inhibits nitrogenase activity, nitrogen-fixing bacteria may benefit. Explain how.

Write an essay on one of the topics below.

Either

The uses and importance of ATP in organisms.

Or

The importance of cycles in biology.

Read the following passage.

Lake Malawi in East Africa has more species of fish than any other lake in the world. Many of these species have evolved from a common ancestor. Lake Malawi is one of the largest lakes in the world and was formed several million years ago. Since then, the water level has fluctuated greatly. As a result, what is now a large lake was at one time many smaller, separate lakes. 5

The country of Malawi has a total area of 118 000 km². The actual land area is only 94 080 km², because approximately one-fifth of the country is Lake Malawi.

In December 1990, forests covered 41.4% of the actual land area of Malawi. In December 2016, forests covered 26.4% of the actual land area of Malawi. 10

Deforestation and farming along the shores of Lake Malawi have caused increased soil erosion and loss of nutrients into the lake. This has resulted in a decrease in some fish populations. The mark-release-recapture method can be used to estimate the size of a fish population. However, this method can produce unreliable results in very large lakes. 15

Use the information in the passage and your own knowledge to answer the following questions.

Lake Malawi in East Africa has more species of fish than any other lake in the world (line 1).

Suggest and explain how this speciation may have occurred.

The percentage of forest cover in Malawi decreased between December 1990 and December 2016 (lines 9–10).

Calculate the mean loss of forest cover in km² per week during this time period.

Loss of nutrients into Lake Malawi has resulted in a decrease in some fish populations (lines 12–13).

Explain why.

The mark-release-recapture method can be used to estimate the size of a fish population (lines 13–14).

Explain how.

Suggest why the mark-release-recapture method can produce unreliable results in very large lakes (lines 14–15).

Freshwater marshes have one of the highest rates of gross primary production (GPP and net primary production (NPP) of all ecosystems.

Carbon use efficiency (CUE) is the ratio of NPP:GPP. Freshwater marshes have a high CUE.

Use your knowledge of NPP to explain why freshwater marshes have a high CUE and the advantage of this.

Do not refer to abiotic factors in your answer.

Freshwater marsh soils are normally waterlogged. This creates anaerobic conditions.

Use your knowledge of the nitrogen cycle to suggest why these soils contain relatively high concentrations of ammonium compounds and low concentrations of nitrite ions and nitrate ions.

A student investigated the growth rate of a freshwater marsh plant.

The growth rate (R) of a plant can be determined using this equation.

Where ln = natural logarithm t = duration of the investigation in days W₁ = plant biomass at the start of the investigation W₂ = plant biomass at the end of the investigation

The student used the equation above; however, she substituted height for biomass. This was because she did not want to destroy the plants to measure their biomass.

State the assumption the student has made and suggest why this assumption might not be valid.

At the end of the investigation, the student noted the freshwater marsh plant had grown 268 mm in height, and now measured 387 mm. She calculated the rate of

Use this information and, substituting height for biomass, use the equation to calculate the duration of the student’s investigation.

Give your answer to the nearest full day. Show your working.

A scientist investigated the effects of different fertilisers on the growth of spinach plants. The scientist:

• set up a large sample of identical pots of soil

• added different masses of different fertilisers to selected pots

• did not add fertiliser to the control pots

• planted the same number of young spinach plants in each pot

after 20 days, determined the biomass of spinach plants in each pot.

The results the scientist obtained after 20 days are shown in Figure 4.

Calculate how many times greater the mean growth rate per day was using 37.5 g potassium nitrate than using 37.5 g ammonium sulfate.

Assume the mean biomass of the spinach plants at the start of the investigation was 0.5 g per pot.

Using all the information, evaluate the effect on plant growth of adding the different fertilisers to the soil.

The scientist determined the dry mass of the spinach plants. First, he heated each sample at 80 C for 2 hours.

Suggest what the scientist should do to ensure that he has removed all the water from the sample.

Describe the role of saprobionts in the nitrogen cycle.

One environmental issue arising from the use of fertilisers is eutrophication. Eutrophication can cause water to become cloudy.

You are given samples of water from three different rivers.

Describe how you would obtain a quantitative measurement of their cloudiness.

Ammonia in soil is oxidised to nitrites and nitrates by species of nitrifying bacteria.

Scientists investigated whether two soils with a different pH contained different communities of nitrifying bacteria. These communities consist of all the nitrifying bacteria of different species in each soil. They took samples of soil from two sites, A and B.

They measured the pH of the samples and found that

• the soil from site A had a pH of 6.9

• the soil from site B had a pH of 4.3

The scientists measured the concentration of ammonia in soil samples over 20 days. Each sample contained the same concentration of ammonia at the start and had the same mass. They recorded the concentration of ammonia in

• soil A with a pH of 6.9

• soil B with a pH of 4.3

• a mixture of equal masses of soils A and B with its pH adjusted to 6.9

Their results are shown in Figure 6.

The scientists used units of µg g¹ for the concentration of ammonia in soil.

Suggest why, in this investigation, the scientists used these units.

g¹

Calculate the difference in the rate of breakdown of ammonia per day between day 0 and day 2 in soil A and soil B.

Show your working and the units for your answer.

The scientists concluded that the soil mixture experiment showed there were different communities of bacteria in soils A and B.

What evidence from Figure 6 supports their conclusions? Give reasons for your

answer.

The oxidation of ammonia by nitrifying bacteria involves the enzyme ammonia monooxygenase. Each species of nitrifying bacteria has its own specific amoA gene that codes for production of ammonia monooxygenase.

In a second investigation, the scientists determined the expression of the amoA gene in two species of bacteria, S and T. Species S was from acid soil and species T was from soil with a neutral pH.

The scientists grew cultures of each species separately in soils of different pH. They determined the amount of mRNA from the amoA gene in each culture.

Their results are shown in Figure 7.

In which species was the number of copies of mRNA more affected by changes in soil pH from 4.9 to 7.5? Use a calculation to support your answer.

This method allowed the scientists to estimate the expression of the amoA gene in each culture but not the growth of the bacterial population in each culture.

Explain why.

The scientists set up their cultures in sterile glass bottles.

Suggest one suitable method for sterilising the bottles and explain why it was necessary to sterilise them.

Turn over for Section B

Section B

one question.

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

Arbuscular mycorrhiza fungi (AMF) are fungi which grow on, and into, the roots of plants. AMF can increase the uptake of inorganic ions such as phosphate.

Suggest one way in which an increase in the uptake of phosphate could increase plant growth.

Suggest one way in which AMF may benefit from their association with plants.

Scientists investigated the effects of different AMF species on the productivity of the plant community of a prairie grassland ecosystem when growing in/on soil containing different phosphate concentrations.

The scientists set up identical plots of prairie grassland soil containing seeds of the plant species found in the ecosystem. The scientists added different AMF species and different concentrations of phosphate to particular plots. Control plots without AMF species were also set up. After 20 weeks the scientists determined the shoot biomass for each plot.

The results the scientists obtained are shown in Figure 5.

Explain why an increase in shoot biomass can be taken as a measurement of net primary productivity.

Using the data from Figure 5, evaluate the effect on plant productivity of adding AMF species and adding phosphate to the soil.

Using the e^x button on your calculator, determine the rate of shoot biomass production in grams per day for the control plot in soil with normal phosphate concentration.

Farmers use artificial fertilisers to maintain or increase yield from grain-producing crop plants such as wheat.

Artificial fertiliser is used to replace mineral ions removed from the land when crops are harvested. One of the mineral ions is nitrate.

Give two examples of biological molecules containing nitrogen that would be removed when a crop is harvested.

Scientists investigated changes in the use of artificial fertiliser in India between 1970 and 2005. They also investigated changes in the fertiliser response ratio. This ratio shows how many kg of grain are produced for each kg of fertiliser used.

Figure 4 shows their results in the form the scientists presented them. (A hectare is a unit of area commonly used in agriculture)

Use these data to calculate the difference in the mass of grain produced per hectare in 1970 compared with 2005.

Show your working.

Use the data in Figure 4 to evaluate the use of artificial fertilisers on grain-producing crops in India.