Edexcel iGCSE Biology 4BI1 - Paper 2B -Cycles within ecosystems- Exam Style Questions- New Syllabus

Question

A group of students investigate the biodiversity in two fields, A and B. Field A is grazed by cattle and field B has no cattle.

(a) Explain what is meant by the term biodiversity.

(b) The students investigate the plant species in each field. They counted the number of each species in five 0.5 m × 0.5 m quadrats. They repeated this for each field.

Table 1 shows their results.

(i) Describe how the quadrats should be placed to obtain unbiased species numbers for each field.

(ii) For the most frequently occurring plant species in field B, calculate the number of plants in one square metre.

(iii) Name the species with the greatest variation in number of plants recorded in quadrats for field B.

(iv) The students conclude that grazing affects the species in field A. Using information in Table 1 and your own knowledge, discuss the students’ conclusion.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 4(a): The organism in the environment — parts (a), (b)(i), (b)(iv)
• 4(b): Feeding relationships — part (b)(iv)
• 4(c): Cycles within ecosystems — part (b)(iv)
• 4(d): Human influences on the environment — part (b)(iv)
• 5.3B: Biodiversity — part (a)
• 5.4B: Practical: investigate distribution and measure biodiversity — parts (b)(i), (b)(ii), (b)(iii)

▶️ Answer/Explanation

Solution

(a) An explanation that makes reference to two of the following:

(number / amount / how many / range) the different species / eq (1)
number / abundance / how many of each species / eq (1)
variation / variety (of organisms) (in an ecosystem) / eq (1)

(b)(i) A description that makes reference to the following:

place at random / eq (1)
(place quadrats at) co-ordinates / eq (1)

Use random coordinates = 2 marks

(b)(ii) Calculation:

Most frequently occurring species in field B: common sorrel

Total plants counted: \(20 + 5 + 10 = 35\)

Total area sampled: \(5 \times (0.5 \times 0.5) = 5 \times 0.25 = 1.25 \text{ m}^2\)

Number per square metre: \(35 \div 1.25 = 28 \text{ per m}^2\)

Answer: 28

(b)(iii) meadow buttercup (1)

(b)(iv) An answer description that makes reference to five of the following:

more species present / eq (1)
more even distribution / similar numbers of each / eq (1)
common sorrel / stinging nettles absent / eq (1)
more biodiversity / grazing increases biodiversity / eq (1)
reliable / repeated / more than one quadrat done in each field / eq (1)
no information on water / sunlight / temperature / fertilisers / minerals / seasons / eq (1)
only done on one field / repeat in other fields / eq (1)
no information about amount of animals / age of animals / duration / eq (1)
grazing reduces competition allows different species to grow / prevents succession / eq (1)
grazing provides increased nitrates / minerals / manure / eq (1)

Question

Loss of trees is known as deforestation.

From 2001 to 2021, Russia, Brazil and Canada had the most deforestation.

Russia lost 76.0 Mha of tree cover. This loss was 10% of forests in Russia. The loss accounted for 17% of the total tree cover lost in the entire world.
Brazil lost 62.8 Mha of tree cover. This loss was 12% of forests in Brazil. The loss accounted for 14% of the total tree cover loss in the entire world.
Canada lost 49.3 Mha of tree cover. This loss was 11% of forests in Canada. This loss accounted for 11% of the total tree cover loss in the entire world.

(a) (i) Use this data to determine the total tree cover loss by countries other than Russia, Brazil and Canada from 2001 to 2021.
(ii) Calculate the loss of tree cover in Brazil per month from 2001 to 2021. Give your answer in standard form.
(iii) Explain why Russia contributed 17% of the total tree cover loss in the entire world even though the loss of tree cover in Russia was only 10%.

(b) Describe the effects of deforestation on an ecosystem.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• Appendix 3: Mathematical skills — parts (a)(i), (a)(ii)
• 4(d): Human influences on the environment — part (a)(iii), (b)
• 4(b): Feeding relationships — part (b)
• 4(c): Cycles within ecosystems — part (b)

▶️ Answer/Explanation

Solution

(a) (i)
Global loss from Russia, Brazil and Canada = \(76.0 + 62.8 + 49.3 = 188.1\) Mha.
Their combined percentage of global loss = \(17\% + 14\% + 11\% = 42\%\).
Therefore, \(42\% = 188.1\) Mha.
\(1\% = \frac{188.1}{42} \approx 4.4786\) Mha.
Total global loss (\(100\%\)) = \(4.4786 \times 100 \approx 447.86\) Mha.
Loss by other countries = Total global loss − Loss by Russia, Brazil, Canada = \(447.86 – 188.1 \approx 259.8\) Mha.
Allow answer in range 258 to 260 Mha.

(a) (ii)
Loss in Brazil from 2001 to 2021 = 62.8 Mha.
Number of months = \(20 \text{ years} \times 12 = 240\) months.
Loss per month = \(\frac{62.8}{240} \approx 0.2617\) Mha.
In standard form = \(2.617 \times 10^{-1}\) Mha per month.
Allow \(2.6 \times 10^{-1}\) or \(2.62 \times 10^{-1}\).

(a) (iii)
An explanation that makes reference to two of the following:
• Russia has the most forest / larger area / bigger country / most tree cover / eq (1)
• Russia makes up a large % of the world’s total tree cover / has a high proportion of the world’s trees / eq (1)
• so even a low percentage change (10%) represents a large absolute amount / eq (1)

(b)
A description that makes reference to four of the following points:
1. reduces biodiversity / loss of species / habitat loss / eq (1)
2. increases CO\(_2\) (in atmosphere) / less CO\(_2\) absorbed / eq (1)
3. less photosynthesis / eq (1)
4. reduced soil quality / less minerals returned to soil / less fertile / leaching / eq (1)
5. soil erosion / soil washed away / eq (1)
6. flooding / eq (1)
7. disturbs water cycle / less transpiration / less rainfall / eq (1)

Question

(a) The graph shows the change in area of rainforest on the Earth from 1990 to 2015.

(i) Determine the mean rate of decrease of rainforest area, in km² per year, from 1990 to 2015.

(ii) Loss of rainforests causes an increase in atmospheric carbon dioxide gas. Explain the negative effects of an increase in atmospheric carbon dioxide gas.

(b) Scientists investigated the effect of deforestation on a river. The river flows through a forest, part of which has been deforested.

The graphs show the changes in nitrate concentration and the biodiversity of animals in the river as it flows through the forest.

(i) Describe the role of microorganisms in the conversion of organic waste into nitrate ions in the river.

(ii) The number of different animal species living in the river was recorded as a measure of the biodiversity of animals. Give a reason why this may not be a complete measure of biodiversity.

(iii) Explain the change in the biodiversity of animals along the river. Use information from the graphs in your answer.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 4(d): Human influences on the environment — parts (a)(ii), (b)(iii)
• 4(a): The organism in the environment — part (b)(ii)
• 4(c): Cycles within ecosystems — part (b)(i)
• 4(b): Feeding relationships — part (b)(iii)
• Appendix 4: Mathematical skills — part (a)(i)

▶️ Answer/Explanation

Solution

5 (a)(i)

Answer: 48 km² per year

Detailed Explanation:

To calculate the mean rate of decrease, we need to find the total decrease in rainforest area over the 25-year period (1990 to 2015) and then divide by the number of years.

From the graph, we can see that in 1990 the rainforest area was approximately 41,200 km², and by 2015 it had decreased to approximately 40,000 km².

The total decrease is: 41,200 km² – 40,000 km² = 1,200 km²

The time period is: 2015 – 1990 = 25 years

Therefore, the mean rate of decrease per year is: 1,200 km² ÷ 25 years = 48 km² per year

This means that on average, the Earth lost 48 square kilometers of rainforest each year during this period.

5 (a)(ii)

Detailed Explanation:

An increase in atmospheric carbon dioxide gas has several negative effects on the environment:

First, carbon dioxide is a greenhouse gas that contributes significantly to the greenhouse effect. This means it traps heat in the Earth’s atmosphere, leading to global warming and an overall increase in Earth’s temperature.

This temperature rise causes polar ice caps and glaciers to melt, which in turn leads to rising sea levels. Higher sea levels can cause flooding in coastal areas, resulting in habitat loss for both human and animal populations.

Additionally, increased CO₂ levels contribute to climate change, which alters weather patterns worldwide. This can result in more frequent and severe extreme weather events such as storms, droughts, and desertification, making some regions less habitable and disrupting agricultural systems.

The changing climate also affects ecosystems, leading to extinctions as species struggle to adapt to new conditions. It can change the distribution of organisms, force migrations, spread pests, and disrupt food chains.

Another significant effect is ocean acidification. When CO₂ dissolves in seawater, it forms carbonic acid, lowering the ocean’s pH. This acidification harms marine life, particularly organisms with calcium carbonate shells or skeletons, and can lead to coral reef bleaching.

5 (b)(i)

Detailed Explanation:

Microorganisms play essential roles in converting organic waste into nitrate ions through the process of decomposition and the nitrogen cycle.

Decomposers, primarily bacteria and fungi, break down organic waste material containing nitrogen compounds like proteins. Through the process of ammonification, these decomposers convert the nitrogen in organic matter into ammonia (NH₃) or ammonium ions (NH₄⁺).

Next, specific nitrifying bacteria convert ammonium into nitrite ions (NO₂⁻). Another group of nitrifying bacteria then oxidize these nitrite ions into nitrate ions (NO₃⁻), which is the form most readily absorbed by plants.

This entire process, known as nitrification, is crucial for making nitrogen available to plants and maintaining the nutrient cycle in aquatic ecosystems like rivers.

5 (b)(ii)

Detailed Explanation:

Recording only the number of different animal species does not provide a complete measure of biodiversity because it ignores several important factors:

This measure doesn’t account for the population sizes or abundance of each species. An ecosystem might have many species but be dominated by just one or two of them, which isn’t reflected in a simple species count.

It also fails to consider other kingdoms of organisms such as plants, bacteria, fungi, and protoctists, which are all important components of ecosystem biodiversity.

Additionally, some species may be seasonal or migratory, meaning they’re only present at certain times of year. A survey conducted at one time might miss these species, giving an incomplete picture of the true biodiversity.

5 (b)(iii)

Detailed Explanation:

The graphs show that as nitrate concentration increases (particularly after the deforested area), biodiversity of animals in the river decreases. This relationship can be explained through a process called eutrophication.

Deforestation leads to soil erosion and runoff, which carries minerals and organic waste into the river. This includes nitrates from decomposed organic matter and possibly fertilizers from agricultural activities following deforestation.

The increased nitrate levels act as nutrients, causing excessive growth of algae and aquatic plants in a process called eutrophication. The algal growth forms blooms on the water surface that block light penetration to deeper water.

With reduced light, submerged plants cannot photosynthesize effectively and eventually die. The dead plant material, along with the algae (which also eventually die), provides more organic matter for decomposers.

As bacteria decompose this increased organic matter, they respire, consuming oxygen from the water. This leads to oxygen depletion in the river.

The low oxygen levels cause fish and other animal species to die because they cannot respire adequately. This results in decreased biodiversity as more sensitive species disappear from the ecosystem.

Additionally, deforestation may directly cause habitat loss and disruption of food chains, further contributing to the decline in biodiversity observed in the graph.

Question

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

Antitranspirants

Antitranspirants are substances applied to plants to reduce water loss. There are three types of antitranspirant, film-forming compounds, stomatal regulators and reflective compounds.

The first type of antitranspirant contains film-forming compounds. These compounds are oils or waxes that are mixed with water to form an emulsion. This emulsion is then sprayed onto the leaves of plants. The emulsion forms a physical barrier that covers the stomata. The film is designed to prevent water loss but allow gas exchange. Scientists have found that that the film-forming antitranspirant is effective in increasing yield and increasing photosynthesis in drought conditions. The increased leaf turgor in plants sprayed with film, compared to control plants, shows that the film decreases water loss and maintains turgor in drought conditions.

Film-forming antitranspirants are used in horticulture to reduce water stress. This can occur during storage, transport and planting of container grown plants. Application of the antitranspirant before transplanting, helps the plant survive until the root system is established. The film-forming compounds are used to reduce drying out in rolled turf and in transporting flowering plants and trees with root balls.

The second type of antitranspirants are stomatal regulators that reduce the size of the stomatal pore. Stomatal pores affect diffusion as well as transpiration. Therefore, photosynthesis and growth may change when stomatal regulators are applied to leaf surfaces. If the stomatal pores close completely this will stop photosynthesis. However, if stomatal openings are only reduced in size, transpiration will be reduced but some photosynthesis will continue.

The third type of antitranspirants are reflective compounds. These work by adding a white coating to leaves. This changes the amount of light that is reflected by the leaf surface. The coating is only applied to the upper surface of leaves so does not block the stomatal pores. Coating a leaf with a reflective chemical can reduce leaf temperature, transpiration rate and also increase the efficiency of water use. Experiments show that transpiration rate can be reduced by up to 30% and leaf temperature reduced from 40°C to 30°C after a coating of reflective compound is applied to leaf surfaces.

Crops depend on photosynthesis for growth and final yield. Therefore, it is unlikely that currently available antitranspirants would increase the yield of a crop unless the crop suffers from a low water supply or very high rates of evaporation. Spraying film-forming or stomata regulating antitranspirants on a field-grown cereal crop under drought conditions, produced a grain yield increase of 17%. Experiments also show that any effect that antitranspirants have on the minerals supplied to the leaves has little effect on plant growth.

(a) Explain why plants that have been dug up and transported are at most risk of drying out.

(b) The photograph shows some stomata on a leaf surface.

Use the photograph to determine the number of stomata per mm² on this leaf surface. [1 mm = 1000 μm]

(c) Explain why stomatal regulators reduce photosynthesis.

(d) (i) Explain why reflective compounds should only be applied to the upper surface of a leaf.

(ii) Explain why reducing leaf temperature reduces transpiration rate.

(e) Describe the role of one named mineral ion in plant growth.

(f) Describe how water is transported from the soil to the leaves in plants.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 2(e): Nutrition — Flowering plants — parts (e), (f)
• 2(g): Gas exchange — Flowering plants — parts (c), (d)(i), (d)(ii)
• 2(h): Transport — Flowering plants — parts (a), (f)
• 4(a): The organism in the environment — part (b)
• 4(c): Cycles within ecosystems — part (e)

▶️ Answer/Explanation

Solution

(a) Plants that have been dug up and transported are at most risk of drying out because their roots (and root hair cells) are damaged or not in soil/exposed. This means water cannot be absorbed or taken up effectively. Additionally, water continues to be lost through transpiration or evaporation from the leaves, creating a water deficit that the damaged root system cannot replenish.

(b) number of stomata per mm² = 300

Explanation: The photograph shows an area of 100 μm × 100 μm, which contains 3 stomata. To find the number per mm²:
1 mm = 1000 μm, so 1 mm² = 1000 μm × 1000 μm = 1,000,000 μm².
The area of the photograph is 100 μm × 100 μm = 10,000 μm².
So, the number of stomata per mm² = (number in photo ÷ area of photo) × area of 1 mm² = (3 ÷ 10,000) × 1,000,000 = 300.

(c) Stomatal regulators reduce photosynthesis because they cause the stomatal pores to close or become smaller. This reduces the amount of carbon dioxide that can diffuse into the leaf. Since carbon dioxide is a key reactant in photosynthesis, a reduced supply limits the rate at which photosynthesis can occur.

(d)(i) Reflective compounds should only be applied to the upper surface of a leaf because the stomata are mainly or only located on the lower surface in most plants. Applying the coating only to the upper surface ensures the stomatal pores are not blocked, allowing gas exchange (carbon dioxide absorption and oxygen release) to continue uninterrupted. Additionally, the upper surface receives the most direct sunlight, so applying the reflective coating there is most effective at reducing heat absorption.

(d)(ii) Reducing leaf temperature reduces the transpiration rate because lower temperatures decrease the kinetic energy of water molecules. With less energy, water molecules move more slowly and are less likely to evaporate from the leaf surface (especially from the stomata). This reduces the rate of diffusion of water vapor out of the leaf, thereby lowering the transpiration rate.

(e) Nitrate ions (NO₃⁻) play a crucial role in plant growth. They are absorbed from the soil and are used by the plant to synthesize amino acids. These amino acids are then built up into proteins, which are essential for growth (e.g., enzymes for metabolic reactions, structural proteins for cell walls) and development.

Alternatively, magnesium ions (Mg²⁺) are a key component of chlorophyll, the pigment that absorbs light energy for photosynthesis. Without sufficient magnesium, chlorophyll production is impaired, leading to reduced photosynthesis and stunted growth.

(f) Water is transported from the soil to the leaves through the following process:
1. Water is absorbed from the soil by root hair cells through osmosis. Root hairs increase the surface area for absorption.
2. Osmosis occurs because the soil water is a dilute solution (higher water potential) compared to the concentrated cell sap inside the root hair cells (lower water potential).
3. Once inside the root, water moves across the cortex and into the xylem vessels.
4. Water is then transported upwards through the xylem to the leaves due to transpiration pull. This is a suction force created by the evaporation of water from the surfaces of mesophyll cells in the leaves and its subsequent diffusion out of the stomata.
5. The cohesion (water molecules sticking together) and adhesion (water molecules sticking to the xylem walls) properties of water help maintain a continuous column of water from the roots to the leaves.

Question

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

When the Oceans Glow

In some areas of the world the oceans around the coast sometimes glow with a blue light. The photograph shows an area of coast that is glowing. This blue light is produced by the presence of thousands of microscopic, living organisms called dinoflagellates. These dinoflagellates are protocysts. The production of light by living organisms is called bioluminescence, a process that has evolved many times in different species of organism.

To generate light, dinoflagellates use special proteins and the ATP produced within their cells. Many species of dinoflagellate contain chlorophyll and are able to photosynthesize. The appearance of glowing dinoflagellates in the sea used to be a rare event but this now occurs much more frequently. Many of the events occur in the sea around river estuaries and scientists think that intensive farming and deforestation could be to blame. Due to overpopulation of dinoflagellates in these areas, other species of animal are often harmed. After a series of glowing events, large numbers of dinoflagellates die causing oxygen levels in the water to decrease.

People have often wondered why dinoflagellates glow. They only glow in areas where the water moves around, such as when waves hit a beach. Scientists now think that the production of light is a type of warning to stop predators eating the dinoflagellates. If an animal eats dinoflagellates, the dinoflagellates in the area glow making the animal obvious to its own predators. To test this, scientists placed dinoflagellates into a tank along with 15 copepods, which are predators of dinoflagellates. When the dinoflagellates glowed, the copepods ate 1200 dinoflagellates in two hours. When the dinoflagellates did not glow, the copepods ate 2100 dinoflagellates in two hours.

Some scientists think that we could make use of the dinoflagellates to provide sustainable street lighting. Tanks of dinoflagellates could be placed on top of lamp posts. The dinoflagellates would photosynthesize during the day when it is light. A stirrer powered by a small battery would then move them at night so that they would glow. These sustainable lamps could be carbon neutral and help to reduce pollution.

(a) Which of these processes produces ATP? (line 6)

A. active transport
B. diffusion
C. respiration
D. transpiration

(b) (i) Explain why intensive farming and deforestation would cause an increase in populations of dinoflagellates. (lines 9 to 11)

(ii) Explain why oxygen levels decrease after a series of glowing events. (lines 10 to 13)

(c) Which type of bacteria converts ammonia into nitrates?

A. decomposer
B. denitrifying
C. nitrifying
D. nitrogen fixing

(d) (i) Each of the 15 copepods ate glowing dinoflagellates at a mean rate of 40 dinoflagellates per hour. Calculate the mean rate at which each copepod ate dinoflagellates that were not glowing. (lines 18 to 21)

(ii) Explain how natural selection could have resulted in the evolution of dinoflagellates that glow. (lines 15 to 18)

(e) Explain why using dinoflagellates for street lighting would help to reduce pollution. (lines 22 to 26)

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — identification of dinoflagellates as protocysts
• 2(f): Respiration — part (a) ATP production
• 2(e): Nutrition (Photosynthesis) — reference to chlorophyll and photosynthesis in dinoflagellates
• 4(d): Human influences on the environment — parts (b)(i), (e) pollution, eutrophication, deforestation
• 4(c): Cycles within ecosystems (Nitrogen cycle) — part (c) role of bacteria
• 3(b): Inheritance and evolution — part (d)(ii) natural selection
• 2(e): Nutrition (Diet and energy) — part (d)(i) calculation of feeding rate
• 5(a): Use of biological resources (Food production) — part (e) sustainable use of organisms

▶️ Answer/Explanation

Solution

(a) C (respiration)

Explanation: Respiration is the metabolic process that breaks down glucose to release energy, which is stored in ATP molecules. Active transport uses ATP but does not produce it. Diffusion is a passive process and does not require or produce ATP. Transpiration is the loss of water vapor from plants and is not directly involved in ATP production.

(b)(i)

Explanation: Intensive farming often involves the heavy use of fertilizers. Deforestation removes trees whose roots help bind the soil. The combination of these factors leads to soil erosion. When it rains, eroded soil and excess fertilizers (rich in minerals like nitrates and phosphates) are washed into rivers and eventually into estuaries and the sea. These minerals act as nutrients for dinoflagellates, allowing their populations to grow rapidly, a process known as eutrophication.

(b)(ii)

Explanation: After the glowing events, large numbers of dinoflagellates die. Their bodies are decomposed by bacteria and other microorganisms. These decomposers respire as they break down the organic matter, a process that consumes oxygen. The large algal bloom may also block light, reducing photosynthesis and oxygen production by other organisms. The high rate of oxygen consumption by decomposers leads to a decrease in dissolved oxygen levels.

(c) C (nitrifying)

Explanation: Nitrifying bacteria are specifically responsible for converting ammonia into nitrites and then into nitrates in the nitrogen cycle. Decomposer bacteria break down organic matter into ammonia. Denitrifying bacteria convert nitrates back into nitrogen gas. Nitrogen-fixing bacteria convert atmospheric nitrogen gas into ammonia.

(d)(i) 70 dinoflagellates per hour

Explanation: The total number of non-glowing dinoflagellates eaten in 2 hours was 2100. The total eaten per hour is \( 2100 \div 2 = 1050 \) dinoflagellates per hour. This is the rate for all 15 copepods. The mean rate per copepod is \( 1050 \div 15 = 70 \) dinoflagellates per hour per copepod.

(d)(ii)

Explanation: A random mutation gave some dinoflagellates the allele to glow. This created variation. When predators were present, dinoflagellates that glowed were less likely to be eaten (as the glow attracted the predators’ own predators). These dinoflagellates had a higher survival rate and were more likely to reproduce, passing the advantageous allele for glowing to their offspring. Over many generations, the frequency of the glowing allele increased in the population, leading to the evolution of this trait.

(e)

Explanation: It would reduce reliance on electricity generated from burning fossil fuels. The dinoflagellates photosynthesize during the day, taking in carbon dioxide (\(CO_2\)) from the atmosphere. At night, they produce light through bioluminescence without burning fuels. Therefore, this method produces no direct air pollutants and contributes less to the greenhouse effect, making it a more sustainable and carbon-neutral alternative.

Question

The diagram shows the nitrogen cycle. Some of the stages have been labelled.

(a) (i) Which stage shows nitrogen fixation?

A. P
B. Q
C. T
D. V

(a) (ii) Which stage shows nitrification?

A. Q
B. U
C. T
D. W

(a) (iii) Which stage shows denitrification?

A. P
B. T
C. V
D. W

(b) Farmers sometimes add chemical fertiliser to the soil.

(i) Describe how pollution by fertiliser can affect aquatic ecosystems.

(ii) Give an alternative to chemical fertiliser that a farmer could use.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 4(c): Cycles within ecosystems — parts (a)(i), (a)(ii), (a)(iii)
• 4(d): Human influences on the environment — parts (b)(i), (b)(ii)

▶️ Answer/Explanation

Solution

(a)(i) Answer: A (P)

Explanation: Nitrogen fixation is the process where atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related compounds by nitrogen-fixing bacteria. In the diagram, this process is represented by stage P, where nitrogen from the air enters the soil ecosystem.

(a)(ii) Answer: C (T)

Explanation: Nitrification is the biological oxidation of ammonia to nitrite (NO₂⁻) followed by the oxidation of nitrite to nitrate (NO₃⁻). This process is carried out by specific nitrifying bacteria. In the diagram, stage T represents this conversion process within the nitrogen cycle.

(a)(iii) Answer: D (W)

Explanation: Denitrification is the microbial process where nitrate (NO₃⁻) is reduced to nitrogen gases (N₂ or N₂O), which are then released back into the atmosphere. This completes the nitrogen cycle. In the diagram, stage W shows this return of nitrogen to the atmosphere.

(b)(i) Answer: Fertilizer pollution can severely impact aquatic ecosystems through a process called eutrophication.

Explanation: When excess fertilizer containing nitrates and phosphates is washed into water bodies from agricultural fields, it acts as a nutrient source for algae and aquatic plants. This leads to rapid algal growth known as an algal bloom. The dense algal growth covers the water surface, blocking sunlight from reaching deeper aquatic plants. Without sufficient light, these plants cannot photosynthesize effectively and eventually die. As the algae and plants die, they sink to the bottom where they are decomposed by bacteria. These decomposing bacteria respire, consuming large amounts of oxygen in the process. This leads to oxygen depletion in the water, creating hypoxic (low oxygen) conditions. The lack of oxygen causes fish and other aquatic organisms to suffocate and die, disrupting the entire aquatic ecosystem.

(b)(ii) Answer: Animal manure / dung / faeces / animal waste

Explanation: Instead of chemical fertilizers, farmers could use organic alternatives like animal manure. Animal waste contains essential nutrients like nitrogen, phosphorus, and potassium that plants need for growth. When properly composted and applied, manure slowly releases these nutrients into the soil, improving soil structure and fertility while reducing the risk of water pollution compared to synthetic fertilizers. Other alternatives include compost, green manure (growing and plowing under plants specifically for soil improvement), and bone meal.

Question

The diagram shows the nitrogen cycle with some processes labelled.

(a) (i) Explain why plants need nitrate ions.
(ii) Name the processes \( V \), \( X \) and \( Y \).

(b) Scientists measure the dissolved nitrate levels in a river that passes through farmland.

They also measure the nitrogen compounds being used on the farmland either as fertiliser or as manure from livestock.

The graph shows how the nitrate levels in the river and the nitrogen compounds applied to the farmland changed from 1950 to 2005.

(i) Comment on the relationship between nitrate levels in the river and the nitrogen compounds applied to the farmland.

(ii) Describe the changes within the river that would have occurred between 1950 and 2000.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 4(c): Cycles within ecosystems — part (a)
• 5.11B: Describe the stages in the nitrogen cycle — part (a)(ii)
• 2.22: Understand that plants require mineral ions for growth — part (a)(i)
• 4(d): Human influences on the environment — parts (b)(i), (b)(ii)
• 5.17: Understand the biological consequences of eutrophication — part (b)(ii)
• 5.15: Understand how human activities contribute to greenhouse gases / pollution — part (b)(i)

▶️ Answer/Explanation

Solution

(a)(i)
Explanation: Plants need nitrate ions (\( NO_3^- \)) because:
1. Nitrates are a source of nitrogen, which is a key element needed to synthesize amino acids.
2. Amino acids are the building blocks for proteins, which are essential for plant growth, enzyme function, and cell structure.
Additional detail: Nitrogen is also required for making other important compounds like chlorophyll, DNA, and RNA.

(a)(ii)
Process Names:
• \( V \): Nitrogen fixation (carried out by nitrogen-fixing bacteria).
• \( X \): Nitrification (carried out by nitrifying bacteria, converting ammonium to nitrite then nitrate).
• \( Y \): Denitrification (carried out by denitrifying bacteria, converting nitrates back to nitrogen gas).

(b)(i)
Comment on the relationship:
1. There is a general positive correlation between the two variables from 1950 to 1970; as nitrogen application increased, river nitrate levels also increased.
2. From 1980 onwards, the relationship becomes less clear/more variable.
3. The amount of nitrogen applied to fields fluctuates more (shows greater variability) than the nitrate levels in the river, which remain relatively more stable.
4. This can be explained by factors such as:
   – Leaching of excess nitrate from fields into the river.
   – Variations in fertilizer application rates by farmers.
   – The river receiving nitrates from other sources (e.g., sewage, natural decay).

(b)(ii)
Describe changes in the river (eutrophication process):
1. Increased nitrate levels led to excessive algal growth / algal blooms (eutrophication).
2. The increase in algae and later their death led to a rise in decomposer bacteria that break down the organic matter.
3. These decomposers respire, using up oxygen, leading to reduced oxygen levels (anoxia) in the water.
4. This resulted in increased turbidity (cloudiness) and reduced light penetration, harming aquatic plants.
5. Ultimately, there was a loss of biodiversity, including death of fish and other oxygen-dependent organisms.

Question

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 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 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.

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

(b) Explain how increased carbon dioxide can cause climate change.

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

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

(e) 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.

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

The total area of the lower surface of the leaf is \(150 \text{ cm}^2\).
Using the photograph and the total lower surface area of the leaf, estimate the number of stomata on the lower surface of the leaf.

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

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 2(e): Nutrition (Photosynthesis) — parts (c), (e)
• 4(d): Human influences on the environment — parts (a), (b)
• 4(c): Cycles within ecosystems (Carbon cycle) — part (d)
• 2(g): Gas exchange — parts (e), (f), (g)
• 2(h): Transport in plants — part (g)

▶️ Answer/Explanation

Solution

(a)
An explanation that makes reference to:
• Burning / combustion of petrol / diesel / fuel in car engines. (1 mark)
• This combustion reaction releases carbon dioxide (\( \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \)). (1 mark)
More cars mean more fuel burned, directly increasing \( \text{CO}_2 \) emissions.

(b)
An explanation that makes reference to two of the following:
• Carbon dioxide is a greenhouse gas. (1 mark)
• It traps / absorbs infrared (IR) radiation (heat) from the Earth, preventing its escape into space. (1 mark)
• This leads to an enhanced greenhouse effect, causing global warming / climate change. (1 mark)
(Maximum 2 marks)

(c)
An explanation that makes reference to:
• Carbon dioxide is a reactant / raw material needed for photosynthesis. (1 mark)
• At lower concentrations, \( \text{CO}_2 \) can be a limiting factor for photosynthesis; increasing its concentration can increase the rate up to a point. (1 mark)
The photosynthesis equation is: \( 6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \).

(d)
An explanation that makes reference to two of the following:
• A carbon sink absorbs more \( \text{CO}_2 \) than it releases. (1 mark)
• Respiration (by plants, animals, decomposers) releases \( \text{CO}_2 \) back into the atmosphere, reducing net absorption. (1 mark)
• Deforestation (cutting down trees) reduces the number of plants for photosynthesis and often involves burning/decay, releasing stored carbon as \( \text{CO}_2 \). (1 mark)
(Maximum 2 marks)

(e)
A description that makes reference to three of the following:
• Place leaves (or aquatic plants like pondweed) in test tubes containing hydrogen-carbonate indicator. (1 mark)
• Expose one setup to bright light and another to darkness (or vary light intensity using a lamp at different distances). (1 mark)
• In bright light, the indicator turns purple/dark red (due to net \( \text{CO}_2 \) uptake in photosynthesis). In darkness, it turns yellow (due to net \( \text{CO}_2 \) release from respiration). (1 mark)
• Control other variables: use leaves of the same species, size, age; same volume and concentration of indicator; same temperature; same time period. (1 mark)
(Maximum 3 marks)

(f)
Step-by-step calculation:
1. Area in photograph: side = \( 400 \mu\text{m} = 0.4 \text{ mm} = 0.04 \text{ cm} \).
2. Area of square = \( (0.04 \text{ cm})^2 = 0.0016 \text{ cm}^2 \).
3. Number of stomata in this area = 2.
4. Stomatal density = \( \frac{2}{0.0016} = 1250 \) stomata per \( \text{cm}^2 \).
5. Total number on leaf = \( 1250 \times 150 = 187500 \).
Answer: \( \mathbf{187500} \) stomata. (3 marks)

(g)
An explanation that makes reference to:
• Stomata allow water vapour to evaporate / be lost from the leaf in a process called transpiration. (1 mark)
• This transpiration pull creates a tension / negative pressure in the xylem, drawing a continuous column of water up from the roots, through the stem, and into the leaves. (1 mark)
This is known as the transpiration stream, and it relies on stomatal opening for water movement against gravity.

Question

Fish farming is often used to produce protein rich food.

(a) Selective breeding is often used to produce fish that grow rapidly and do not waste much food.

Explain how selective breeding can be used to produce fish that grow rapidly.

(b) Fish farming systems can often release ammonia into the water. The ammonia is converted into nitrates.

Describe how ammonia is converted into nitrates.

(c) Multi-trophic level aquaculture is a method of fish farming that has been developed to reduce environmental pollution and increase profits.

The diagram shows a multi-trophic level aquaculture system.

Explain how the multi-trophic level aquaculture system reduces environmental pollution, and increases the profits of fish farming.

Use information from the diagram and your own knowledge to support your answer.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 5(a): Food production — part (a), (c) (fish farming context)
• 5(b): Selective breeding — part (a) (selective breeding in animals)
• 4(c): Cycles within ecosystems — part (b) (nitrogen cycle processes)
• 4(d): Human influences on the environment — part (c) (pollution reduction, sustainable practices)

▶️ Answer/Explanation

Solution

(a)

Explanation: Selective breeding involves a deliberate process to enhance desirable traits. To produce fish that grow rapidly, fish farmers would first identify and select parent fish that already exhibit fast growth rates. These selected fish are then bred together. From their offspring, the fastest-growing individuals are again selected to be the parents for the next generation. By repeating this process over many generations, the alleles (gene variants) responsible for rapid growth become more common in the population, leading to a stock of fish that consistently grows quickly.

(b)

Explanation: The conversion of ammonia to nitrates is a natural biological process called nitrification. This process is carried out by specific types of bacteria known as nitrifying bacteria. These bacteria are autotrophs that obtain energy by oxidizing nitrogen compounds. The process occurs in two main stages. First, bacteria such as Nitrosomonas convert ammonia (NH₃) into nitrites (NO₂⁻). Then, other bacteria, like Nitrobacter, convert these nitrites into nitrates (NO₃⁻). Nitrates are a form of nitrogen that can be more easily utilized by plants and algae.

(c)

Explanation: The multi-trophic level aquaculture system is designed to create a more balanced and efficient ecosystem, which directly addresses pollution and profitability.

Reducing Pollution:

The system significantly reduces solid waste pollution because lobsters and crabs consume the waste food and faeces produced by the main fish stock. This means less organic matter accumulates on the seabed.
With less waste material available, there is a reduction in the population of decomposing bacteria. This is beneficial because these bacteria consume large amounts of oxygen during respiration; therefore, lower bacterial numbers help to maintain higher oxygen levels in the water, preventing oxygen depletion that can kill aquatic life.
Furthermore, the seaweed plays a crucial role in reducing dissolved nutrient pollution. It absorbs nitrates, phosphates, and other minerals from the water that would otherwise act as fertilizers, potentially causing excessive algal growth (algal blooms) and eutrophication. The seaweed also releases oxygen through photosynthesis, further improving water quality.

Increasing Profits:

This system creates additional saleable products. The farmer can now harvest and sell not only the main fish but also the lobsters, crabs, and seaweed, diversifying their income streams.
It also reduces costs. There is no need to purchase extra feed for the lobsters and crabs, as they consume the waste from the fish. Similarly, the seaweed obtains its minerals naturally from the water, eliminating the need for artificial fertilizers.
Healthier fish, resulting from the improved water quality (higher oxygen, lower disease risk), are likely to grow better and have lower mortality rates, leading to higher yields and reduced losses.

Question

The diagram shows some parts of the nitrogen cycle.

(a) Name the processes labelled A, B and C.

(b) The mass of nitrogen that moves within the nitrogen cycle has been estimated and some of the masses are given in the table.

(i) Some of these processes remove nitrogen from the atmosphere. Calculate the total mass of nitrogen removed from the atmosphere by these processes. Give your answer in standard form.

(ii) Calculate the percentage of the nitrogen released into the atmosphere that comes from burning of biomass.

(iii) Explain how burning biomass returns nitrogen to the atmosphere.

(c) Explain the effect of nitrous oxide on global warming.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 4(c): Cycles within ecosystems — part (a)
• 4(c): Nitrogen cycle — part (a)
• 4(d): Human influences on the environment — parts (b)(iii), (c)
• Appendix 4: Mathematical skills — parts (b)(i), (b)(ii)

▶️ Answer/Explanation

Solution

(a)

A: Denitrification

B: Nitrification

C: Nitrogen Fixation

Explanation: In the nitrogen cycle, process A shows nitrogen gas returning to the atmosphere from nitrates, which is denitrification carried out by denitrifying bacteria. Process B shows the conversion of ammonium ions to nitrites and then to nitrates, which is nitrification performed by nitrifying bacteria. Process C shows atmospheric nitrogen being converted to ammonia/ammonium ions, which is nitrogen fixation by nitrogen-fixing bacteria.

(b)(i)

Mass = \(2.53 \times 10^{14}\) g

Explanation: To calculate the total nitrogen removed from the atmosphere, we add all the processes that remove nitrogen: fertilizer manufacture (120), nitrogen fixation (58), lightning (5), and deposition of nitrogen oxides (70). Adding these gives 120 + 58 + 5 + 70 = 253. Since the units are \(g \times 10^{12}\), we convert to standard form: \(253 \times 10^{12} = 2.53 \times 10^{14}\) g.

(b)(ii)

Percentage = 20% (or 19.5%)

Explanation: First, calculate the total nitrogen released into the atmosphere: burning biomass (40) + denitrification (100) + ammonia release (60) + nitrogen oxides release (5) = 205. The percentage from burning biomass is (40 ÷ 205) × 100 = 19.51%, which rounds to 20%.

(b)(iii)

Explanation: Burning biomass returns nitrogen to the atmosphere because biomass contains nitrogen compounds such as proteins, amino acids, and nucleic acids. When biomass burns, these nitrogen-containing compounds break down and release nitrogen in the form of nitrogen oxides (NOₓ) and other gases into the atmosphere.

(c)

Explanation: Nitrous oxide (N₂O) is a potent greenhouse gas that contributes significantly to global warming. It absorbs infrared radiation (heat) emitted by the Earth’s surface and prevents it from escaping into space, effectively trapping heat in the atmosphere. This enhanced greenhouse effect leads to an increase in global temperatures and climate change. Nitrous oxide is particularly concerning because it has a much greater warming potential per molecule than carbon dioxide.

Question

A student uses this method to investigate the nitrogen cycle.

take two samples of soil, each of mass 100 g
sterilise one sample of soil by heating at 100 °C for one hour
place the sterilised and unsterilised samples into separate filter funnels
pour 25 cm³ of water through each soil sample and collect the filtrate in a test tube
test each filtrate for nitrates
pour water through each soil sample for 5 minutes
pour another 25 cm³ of water through each soil sample and collect the filtrate in a test tube
test each filtrate for nitrates
add 1 cm³ of a solution of ammonium salts to each soil and leave for three days
pour 25 cm³ of water through each soil sample again and collect the filtrate in a test tube
test each filtrate for nitrates

The diagram shows the student’s apparatus.

The table shows the student’s results.

(a) Give the independent variable in the investigation.

(b) (i) Suggest why the student poured water through the soil samples for five minutes before adding the ammonium salts.

(ii) Comment on the results of the nitrate tests on the two soil samples three days after adding ammonium salts.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 4(c): Cycles within ecosystems — whole question, focus on nitrogen cycle
• 4(a): The organism in the environment — experimental design, variables
• 1(b): Variety of living organisms — role of bacteria (nitrifying bacteria)
• 2(e): Nutrition — plant mineral requirements (nitrate ions)

▶️ Answer/Explanation

Solution

(a) Whether the soil sample was sterilised or unsterilised / The presence or absence of bacteria.

Explanation: The independent variable is the factor that the investigator deliberately changes. In this experiment, the student intentionally sterilized one soil sample (by heating it) while leaving the other unsterilized. This manipulation directly alters the presence of living microorganisms, particularly bacteria, in the soil, which is the key factor being tested for its effect on the nitrogen cycle (specifically, the conversion of ammonium to nitrate).

(b)(i) To remove or wash away any nitrates that were initially present in the soil. This ensures that any nitrate detected after adding the ammonium salts must have been produced from the ammonium during the investigation, making the test fair and valid.

Explanation: The initial tests showed that nitrate was present in both soil samples at the start. By thoroughly rinsing the soil with water for five minutes, the student aimed to leach out these pre-existing nitrates. This step is crucial for the validity of the experiment. If nitrates were not removed, it would be impossible to tell if nitrates found later came from the original soil or were newly produced from the added ammonium salts. This washing step creates a “clean slate,” ensuring that any nitrate detected after the ammonium addition is indeed a product of the processes being studied.

(b)(ii)

Nitrates are present in the unsterilised soil but absent in the sterilised soil three days after adding ammonium salts.
This shows that nitrates were produced in the unsterilised soil from the added ammonium salts.
The conversion of ammonium to nitrate is called nitrification and is carried out by nitrifying bacteria.
The sterilised soil, which was heated, had its bacteria killed, so no nitrification could occur, and thus no nitrates were produced.

Explanation: The contrasting results for the two soil samples three days after adding ammonium salts are very revealing. The unsterilised soil tested positive for nitrates, indicating that the ammonium ions \( (NH_4^+) \) were converted into nitrate ions \( (NO_3^-) \). This biological process, known as nitrification, is performed by specific types of bacteria called nitrifying bacteria (e.g., Nitrosomonas and Nitrobacter). In contrast, the sterilised soil, which was heated to 100°C, tested negative for nitrates. The heating process killed all living bacteria. The absence of nitrate production in this sample provides strong evidence that living bacteria are essential for the nitrification process to occur. The experiment effectively demonstrates the role of microorganisms in this crucial stage of the nitrogen cycle.

Question

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.

(a) (i) Carbon dioxide is a greenhouse gas. State the name of another greenhouse gas (line 7).

(ii) 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]

(iii) State two consequences of global warming for the environment.

(b) 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).

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

A. amylase
B. ligase
C. lipase
D. restriction

(d) 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).

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

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 4(d): Human influences on the environment — part (a)(i), (a)(iii)
• 4(c): Cycles within ecosystems — part (a)(ii), (b)
• 5(c): Genetic modification (genetic engineering) — part (c)
• 2(d): Movement of substances into and out of cells — part (d)
• 5(d): Cloning — part (e)
• Appendix 4: Mathematical skills — part (a)(ii)

▶️ Answer/Explanation

Solution

(a)(i) Methane / nitrous oxides / CFCs / water vapour

Explanation: Greenhouse gases are those that trap heat in the Earth’s atmosphere, contributing to the greenhouse effect. While carbon dioxide is the most commonly discussed, other significant greenhouse gases include methane (released from livestock and landfills), nitrous oxides (from agriculture and industrial processes), chlorofluorocarbons or CFCs (from refrigerants and aerosols, though now largely phased out), and water vapour. Carbon monoxide is not a significant greenhouse gas and is therefore rejected.

(a)(ii) \( 1.8 \times 10^{13} \) kg

Explanation: To find the net increase in atmospheric carbon dioxide, we calculate the total released minus the total removed. The total released is 727 (natural) + 37 (human) = 764 gigatonnes. The amount removed by plants is 746 gigatonnes. The net increase is therefore 764 – 746 = 18 gigatonnes. Since 1 gigatonne = \( 1 \times 10^{12} \) kg, we convert 18 gigatonnes to kg: 18 × \( 10^{12} \) kg = \( 1.8 \times 10^{1} \) × \( 10^{12} \) kg = \( 1.8 \times 10^{13} \) kg.

(a)(iii) Any two from: ice caps/glaciers melt, sea level rise/flooding, loss of habitat/desertification/droughts, extinctions/disrupted food chains, destruction of coral reefs/coral bleaching, spread of disease/pests, extreme weather/changes in weather patterns.

Explanation: Global warming, driven by an enhanced greenhouse effect, has wide-ranging environmental consequences. Two major effects are the melting of polar ice caps and glaciers, which contributes to rising sea levels and subsequent coastal flooding. Another significant impact is the disruption of ecosystems, leading to habitat loss, species extinction as animals and plants cannot adapt quickly enough, and phenomena like coral bleaching where warmer ocean temperatures cause corals to expel the algae living in their tissues, turning them white and threatening the entire reef ecosystem.

(b) An explanation that makes reference to four of the following points:

Plants take in/absorb carbon dioxide.
This is for the process of photosynthesis.
The carbon (from CO₂) is converted into/stored as suberin/locked up in suberin.
Suberin does not decay for long periods/decomposes slowly/remains for a long time.
Perennial plants remain for long periods/don’t die off each year.
Slower/less carbon dioxide is released from decomposition/decay.

Explanation: Genetically engineered plants with high suberin content act as enhanced carbon sinks. They absorb carbon dioxide from the atmosphere during photosynthesis. Instead of this carbon being used solely for immediate growth or being released back quickly, a significant portion is incorporated into suberin in their root cell walls. Suberin is a very stable, waterproof compound that decomposes extremely slowly, meaning the carbon is effectively “locked away” in the soil for a very long time. Furthermore, because these are perennial plants, they live for many years, continuously performing this carbon sequestration without the need for annual replanting, which could disturb the soil and release stored carbon. This long-term storage reduces the net amount of carbon dioxide in the atmosphere.

(c) D (restriction enzyme)

Explanation: In genetic engineering, specific enzymes are used to cut DNA at precise locations. Restriction enzymes (also called restriction endonucleases) are the enzymes responsible for cutting a gene out of a section of DNA. Amylase digests starch, ligase joins DNA fragments together, and lipase digests lipids (fats).

(d) An explanation that makes reference to two of the following points:

Prevents water loss from the plant roots.
Due to osmosis.
Prevents plant cells from becoming flaccid/wilting; helps them stay turgid by preventing water from moving out to the higher salt concentration in the soil.

Explanation: Soil with a high salt concentration has a low water potential (a high solute concentration). Water naturally moves by osmosis from areas of high water potential (inside the root cells) to areas of low water potential (the salty soil). This can cause the plant to lose water and wilt. Suberin, being a waterproof substance in the cell walls of the roots, acts as a barrier. It reduces the movement of water out of the root cells into the salty soil, thereby helping the plant to retain water and maintain turgor pressure, which is essential for support and function.

(e) Any three from: produces large numbers/large scale, fast/quick process, all crops produce suberin/are genetically identical/clones, less risk of cross-pollinating with wild plants/spreading the transgene, can be done at any time of year/all year.

Explanation: Micropropagation (tissue culture) is used for several advantages over traditional pollination. Firstly, it allows for the rapid production of a very large number of plants from a single, successfully modified individual. Secondly, the process is much faster than waiting for seeds to develop and grow. Thirdly, all the plants produced are genetically identical clones, guaranteeing that every single plant will have the desired high-suberin trait. Fourthly, since micropropagation is asexual and doesn’t involve pollen, there is no risk of the transgene escaping via cross-pollination and spreading into wild plant populations. Finally, it is not season-dependent and can be carried out in a lab throughout the year.

Question

(a) The diagram shows the nitrogen cycle with four processes labelled A, B, C and D.

(i) Which process is nitrogen fixation?

(ii) Which process is decomposition?

(iii) Which process is nitrification?

(iv) Name a type of organism that carries out process C.

(b) A student does an investigation to determine if nitrate ions are required for plant growth.

He uses this method.

Step 1: Use a measuring cylinder to add \(10 \, \text{cm}^3\) of the control solution containing all the minerals a plant requires to a test tube
Step 2: Cover the top of the tube with foil
Step 3: Make a small hole in the foil
Step 4: Push the root of a germinated bean seedling through the hole so it is in the solution
Step 5: Rinse the measuring cylinder with distilled water
Step 6: Repeat Steps 1 to 4 using a test solution containing all the mineral a plant requires apart from nitrate ions
Step 7: Wrap both tubes in black paper
Step 8: Place both tubes in a test tube rack in bright sunlight

(i) Explain why the control solution contains all the mineral ions the plant requires but the test solution contains all the mineral ions the plant requires except nitrate.

(ii) State the purpose of Step 5.

(iii) Explain the purpose of Step 7.

(iv) Explain the measurements that the student could make to determine if nitrate ions are required for plant growth.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 4(c): Cycles within ecosystems — part (a)(i), (a)(ii), (a)(iii), (a)(iv)
• 2(e): Nutrition (Flowering plants) — parts (b)(i), (b)(iv)
• Appendix 6: Suggested practical investigations — parts (b)(ii), (b)(iii), (b)(iv)

▶️ Answer/Explanation

Solution

(a)(i) A

Explanation: Nitrogen fixation is the process where atmospheric nitrogen (\(N_2\)) is converted into ammonia (\(NH_3\)) or related compounds by certain bacteria. In the diagram, process A shows atmospheric nitrogen being converted into a form that can be used by plants, which matches the definition of nitrogen fixation.

(a)(ii) D

Explanation: Decomposition is the breakdown of organic matter, such as waste and dead organisms, into simpler substances like ammonium. Process D in the diagram shows the conversion of waste and remains into soil ammonium, which is the result of decomposition carried out by decomposers like bacteria and fungi.

(a)(iii) C

Explanation: Nitrification is the biological oxidation of ammonia (\(NH_3\)) or ammonium (\(NH_4^+\)) to nitrite (\(NO_2^-\)) and then to nitrate (\(NO_3^-\)). Process C shows the conversion of soil ammonium into soil nitrates, which is precisely what happens during nitrification, performed by nitrifying bacteria.

(a)(iv) (Nitrifying) bacteria

Explanation: Process C is nitrification, which is specifically carried out by nitrifying bacteria. These bacteria, such as those from the genera Nitrosomonas and Nitrobacter, convert ammonium into nitrites and then into nitrates, making nitrogen available in a form that plants can absorb and use.

(b)(i) The control solution shows normal growth with all minerals present, allowing comparison with the test solution (without nitrate) to see if nitrate is essential. If the control lacked other minerals, it wouldn’t show normal growth, making it impossible to isolate the effect of nitrate absence.

Explanation: In a scientific experiment, a control is used as a benchmark to compare the results of the test. The control solution contains all necessary minerals, including nitrate, so the plant in it should grow normally. The test solution is identical except it lacks nitrate. By comparing the growth in both solutions, any difference (like stunted growth in the test) can be attributed to the missing nitrate ions. If the control also missed other minerals, we wouldn’t know which missing mineral caused poor growth.

(b)(ii) To remove any residue of nitrate ions or other minerals from the measuring cylinder, preventing contamination of the test solution.

Explanation: Step 5 ensures that the measuring cylinder is clean and free from any traces of the control solution before it is used to measure the test solution. If nitrate ions from the control were left in the cylinder, they could contaminate the test solution, making it no longer “nitrate-free.” This would ruin the experiment because the test plant might get some nitrate, making the results unreliable.

(b)(iii) To block light from reaching the roots and the solution, preventing photosynthesis and algal growth.

Explanation: Wrapping the tubes in black paper (Step 7) serves two main purposes. First, it stops light from reaching the roots. Plant roots don’t perform photosynthesis, and exposure to light could be unnatural or stressful. More importantly, it prevents algae from growing in the nutrient solution. Algae need light for photosynthesis, and if they grow, they would compete with the bean seedling for minerals and carbon dioxide, confusing the results of the experiment.

(b)(iv) The student could measure the height/length, leaf area, or mass of the plants from both solutions using a ruler or balance, and compare the growth.

Explanation: To determine if nitrate is required, the student needs quantitative data on plant growth. After a set period, they could carefully remove the seedlings and measure their height or stem length with a ruler. They could also count the number of leaves and measure leaf area, or even better, find the dry mass by drying the plants in an oven and weighing them. By comparing these measurements between the plant grown in the complete control solution and the one grown in the nitrate-deficient test solution, a significant difference (e.g., the test plant being smaller and lighter) would indicate that nitrate ions are essential for healthy plant growth.

Question

Decomposer bacteria are involved in the nitrogen cycle.

The bacteria release an enzyme called urease.

(a) The diagram shows part of one strand of DNA used to make urease.

Complete the diagram by giving the missing bases on the other strand of DNA.

(b) Urease acts on urine to produce ammonia.

The graph shows how pH affects the activity of urease.

(i) Which of these is the optimum pH for urease?

A. 2.5
B. 4.5
C. 7.5
D. 8.5

(ii) Explain the activity of urease at pH 8.5

(c) Describe the role of the other bacteria involved in the nitrogen cycle.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 3(b): Inheritance — DNA structure and base pairing (part a)
• 2(c): Biological molecules — Enzymes and factors affecting enzyme activity (parts b(i), b(ii))
• 4(c): Cycles within ecosystems — The nitrogen cycle and roles of bacteria (part c)

▶️ Answer/Explanation

Solution

(a)

Explanation: DNA strands are complementary. Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). Therefore, to complete the complementary strand, we match A with T, T with A, G with C, G with C, C with G, and T with A.

(b)(i) C 7.5

Explanation: The optimum pH is the point at which the enzyme shows its highest activity. From the graph (which would show this visually), urease activity peaks at around pH 7.5, making it the optimum pH for this enzyme’s function.

(b)(ii) At pH 8.5, the activity of urease is lower than its optimum. This happens because the higher pH (alkaline conditions) can cause the enzyme to denature. Denaturation means the enzyme’s active site changes shape, so it can no longer bind effectively to its substrate (urea in this case), resulting in fewer enzyme-substrate complexes and reduced activity.

Explanation: Enzymes are proteins that are sensitive to pH changes. Each enzyme has an optimum pH where its structure is ideal for catalysis. When the pH moves away from this optimum, particularly to extremes like pH 8.5 for urease, the ionic bonds and hydrogen bonds that maintain the enzyme’s specific three-dimensional shape can be disrupted. This alteration in shape, especially of the active site, prevents the substrate from fitting properly, drastically reducing the rate of reaction.

(c) Other bacteria in the nitrogen cycle have crucial roles:

Nitrogen-fixing bacteria: These convert atmospheric nitrogen gas (\(N_2\)) into ammonia (\(NH_3\)), making nitrogen available to plants in a usable form.
Nitrifying bacteria: This is a two-step process. First, some bacteria convert ammonia (\(NH_3\)) into nitrites (\(NO_2^-\)). Then, other bacteria convert these nitrites (\(NO_2^-\)) into nitrates (\(NO_3^-\)), which is the form most easily absorbed by plant roots.
Denitrifying bacteria: These bacteria perform denitrification, which is the conversion of nitrates (\(NO_3^-\)) back into nitrogen gas (\(N_2\)). This process releases nitrogen back into the atmosphere, completing the cycle.

Explanation: The nitrogen cycle is essential for life, as nitrogen is a key component of amino acids and nucleic acids. Different groups of bacteria drive this cycle. Nitrogen-fixing bacteria, often found in root nodules of legumes, “fix” inert atmospheric nitrogen into reactive ammonia. Nitrifying bacteria in the soil then oxidize ammonia to nitrites and then to nitrates, which are soluble and can be taken up by plants. Finally, denitrifying bacteria, typically active in waterlogged, anaerobic soils, reduce nitrates back to nitrogen gas, returning it to the atmosphere and balancing the cycle. Without these bacterial processes, the vast reservoir of nitrogen in the air would be largely inaccessible to living organisms.

Question

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 \(10^\circ C\)
cut sample Q into small pieces and keep at \(20^\circ C\)
do not cut sample R and keep at \(10^\circ C\)
do not cut sample S and keep at \(20^\circ C\)
measure the mass of each sample after three months

The graph shows her results.

(a) Explain the results obtained by the student.

(b) 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.

(c) The student needs to control biotic variables in her investigation.

Give two biotic variables she should control.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 4(c): Cycles within ecosystems — part (a) Decomposition
• 2(c): Biological molecules — part (a) Role of enzymes in decomposition
• Appendix 3: Mathematical skills — part (b) Calculation
• 4(a): The organism in the environment — part (c) Biotic variables

▶️ Answer/Explanation

Solution

(a) An explanation that makes reference to four of the following points:

more (decomposition) / faster with warmer temperatures / eq (1)
(allow mass remains high in low temp)
enzymes (1)
more (decomposition) / faster with cut material / eq (1)
(allow mass remains high in uncut)
more surface area (1)
fungi / bacteria (1)
(allow converse)

Example answer: Decomposition is faster at \(20^\circ C\) (samples Q and S) than at \(10^\circ C\) (samples P and R) because enzymes in decomposers like fungi and bacteria work more efficiently at higher temperatures. Cutting the leaves into small pieces (samples P and Q) increases the surface area available for decomposers and their enzymes to act on, leading to faster decomposition compared to uncut leaves (samples R and S) at the same temperature.

(b) Calculation:

Mass loss for P: \(6.0 – 3.6 = 2.4 \text{ kg}\)
Rate for P: \(2.4 \div 3 = 0.8 \text{ kg per month}\)
Mass loss for Q: \(6.0 – 2.0 = 4.0 \text{ kg}\)
Rate for Q: \(4.0 \div 3 = 1.333… \text{ kg per month}\)
Difference: \(1.333… – 0.8 = 0.533… \text{ kg per month}\)

Answer: \(0.53\) (allow \(0.5\), \(0.53\), \(0.533\), etc.) kg per month

Award full marks for correct numerical answer without working.

(c) An answer that makes reference to two of the following points:

species / type of leaves / plant (1)
age of plant / leaves (1)
(ignore volume of leaves)
same (number of) / type of decomposers / eq (1)
insects or organisms that might consume leaf / eq (1)

Example answer: 1. The species/type of leaf used. 2. The presence/absence of specific decomposers (e.g., fungi, bacteria).

Edexcel iGCSE Biology 4BI1 - Paper 2B -Cycles within ecosystems- Exam Style Questions- New Syllabus

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