Edexcel iGCSE Biology 4BI1 - Paper 2B -Variety of living organisms- Exam Style Questions- New Syllabus

Question

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

Rivers – the arteries of the world

Rivers are important supplies of water for drinking, farming, industry, and leisure activities. Like our circulatory system, rivers are essential for transport, and have a homeostatic effect on nature. The biodiversity in rivers is high as they are the habitat for many different species. In many countries rivers are under threat from human impact, particularly the release of untreated sewage.

Flooding can cause untreated sewage to run into rivers from pipelines. Scientists estimated that in England and Wales during 2022, sewage was released into rivers for a total time of 300 000 hours. This sewage came from 1200 different pipelines. Untreated sewage may contain fertilisers, pesticides, pathogenic bacteria, and pharmaceutical drugs.

The photograph shows sewage being released into a river. The sewage causes the growth of something called ‘sewage fungus’. Sewage fungus looks like fungus but is a solid collection of several types of anaerobic bacteria. Sewage fungus is common in rivers that are polluted with untreated sewage or are near to cattle and intensive crop farms. If sewage fungus is found in a river it often indicates that the biodiversity of the river will be low.

Pesticides from agriculture are also released into rivers, and these are frequently transferred through natural food chains. Even pharmaceutical drugs, such as those containing the hormones oestrogen and progesterone, have been found in polluted rivers. These drugs must have been consumed by humans, excreted and then released into the sewage flowing into rivers.

Untreated sewage can also contain pathogenic species of bacteria that then enter rivers. If humans are infected with these bacteria, the large intestine absorbs less water, causing diarrhoea. Some of the bacteria found in sewage are resistant to antibiotics.

Scientists are looking for ways to prevent river pollution. One way is to reduce the risk of pollution from fertilisers and from cattle urine and faeces. Woodchip bioreactors are being trialled in agricultural fields that are near to rivers. These bioreactors are pits filled with woodchips and denitrifying bacteria. Water drains through these bioreactors, which helps to remove nitrates before they reach the rivers. In some countries, efforts are being made to replant forests in areas upstream of rivers to reduce river flooding and the risk of untreated sewage release.

(a) Rivers have a homeostatic effect on nature. This is similar to the role of homeostasis in humans.
State what is meant by the term homeostasis (lines 2 and 3).

(b) Calculate the mean number of hours that each pipeline released sewage into rivers in 2022 (lines 7 and 8).

(c)(i) Sewage fungus is not a fungus but is made from several different species of bacteria.
Which of these structures are present both in fungi and in bacteria?

A) cell wall and cytoplasm only
B) cell wall and nucleus only
C) cell wall, cytoplasm and nucleus
D) cytoplasm and nucleus only

(ii) Explain why rivers near to farms may have low biodiversity (lines 13 to 16).

(d) Oestrogen and progesterone in polluted water lower the levels of FSH and LH in mammals.

(i)State why lower levels of FSH can reduce fertility in mammals.
(ii) State why lower levels of LH can reduce fertility in mammals.

(e)(i) Pathogenic bacteria produce poisons that prevent the active transport of salt into the blood vessels surrounding the intestines.
Explain why preventing the transport of salt into the blood vessels will cause more water to be present in faeces (lines 22 to 24).

(ii) Explain why overuse of antibiotics has led to an increase in antibiotic-resistant bacteria (lines 24 and 25).

(f)(i) Explain how woodchip bioreactors reduce pollution in rivers (lines 27 to 31).
(ii) Explain why replanting forests in areas upstream of rivers will reduce flooding (lines 31 to 33).

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 2(j): Co-ordination and response — part (a)
• 4(d): Human influences on the environment — parts (c)(ii), (e)(ii), (f)(i), (f)(ii)
• 1(b): Variety of living organisms — part (c)(i)
• 3(a): Reproduction — parts (d)(i), (d)(ii)
• 2(d): Movement of substances into and out of cells — part (e)(i)
• 3(b): Inheritance — part (e)(ii)

▶️ Answer/Explanation

Solution

(a) An answer that makes reference to one of the following points:
• maintenance of constant (internal) conditions in body / eq (1)
• control / regulating / maintaining internal conditions / eq (1)
• control / regulating / maintaining internal environment / eq (1)
• keeping conditions in body the same / within a (narrow) range / eq (1)

(b) • 250 (1)
Calculation: \( \frac{300000}{1200} = 250 \)

(c)(i) A (cell wall and cytoplasm only)
B is not the answer as bacteria do not have nuclei
C is not the answer as bacteria do not have nuclei
D is not the answer as bacteria do not have nuclei

(c)(ii) An explanation that makes reference to the following points:
1. fertilisers / animal waste / minerals / nitrates / phosphates (run off / leach) / eq (1)
2. algal growth / eutrophication / eq (1)
3. less light (penetrates) / competition for light / less photosynthesis / eq (1)
4. decomposition / decay (of dead algae / plants / organisms / organic waste / manure / faeces / sewage) eq (1)
5. less oxygen / only anaerobic species grow / eq (1)
6. bacteria respire / other species cannot respire / reduced respiration / eq (1)
7. pesticides kill organisms / eq (1)

(d)(i) An answer that makes reference to one of the following points:
• eggs / ova / follicles, do not mature / do not grow / eq (1)
• (FSH is required to) mature eggs / ova / follicles (1)
• (low FSH leads to) less oestrogen / (FSH) stimulates oestrogen release (1)

(d)(ii) An answer that makes reference to one of the following points:
• no ovulation occurs / egg not released / ova not released / eq (1)
• LH / it stimulates ovulation / LH / it stimulates egg release / eq (1)
• (lower LH leads to) less progesterone / LH / it stimulates progesterone (1)

(e)(i) An explanation that makes reference to two of the following points:
• less water absorbed / less water enters blood / less water enters capillaries / eq (1)
• (water moves by) osmosis (1)
• water potential in blood is high(er) / water potential in gut / faeces / lumen is low(er) / eq (1)

(e)(ii) An explanation that makes reference to three of the following points:
1. mutation (1)
2. (only resistant bacteria) survive / are not killed / non-resistant bacteria die / eq (1)
3. bacteria reproduce / multiply / eq (1)
4. pass on allele / gene / mutation / eq (1)

(f)(i) An explanation that makes reference to two of the following points:
• (denitrifying bacteria convert) nitrate into nitrogen (1)
• so less eutrophication / less algal growth / more oxygen in river / lower BOD / eq (1)
• wood chips have large surface area (for bacteria) / wood chips are biodegradable (so do not pollute) / eq (1)

(f)(ii) An explanation that makes reference to two of the following points:
1. trees / roots absorb water / trees intercept water / eq (1)
2. (more) transpiration (1)
3. less runoff / slows down water flow / more soil permeability / more infiltration / eq (1)
4. less soil erosion / roots hold onto soil / roots stabilise soil / eq (1)
5. rivers do not get blocked (with soil) / eq (1)

Question

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

Extinctions

During the evolution of living organisms, most species have become extinct. Evolution by natural selection means that species constantly replace each other. The photograph shows the fossilised jaws of a shark called megalodon. Megalodon was a giant, predatory shark that became extinct 2.6 million years ago.

Some scientists think megalodon was replaced by giant, predatory toothed whales. These whales were the ancestors of modern orcas. Any sharks that remained evolved to become smaller. As well as the steady loss of species over time, there have been mass extinction events. Approximately 250 million years ago 90% of all animals and plants became extinct. The cause of this mass extinction is not known, but one theory is that it was due to the eruptions of volcanoes. The volcanoes released sulfur dioxide, carbon dioxide and dust into the atmosphere. This caused populations of producer species to fall, atmospheric carbon dioxide levels to rise, and oxygen levels to fall.

Human activity is now causing another mass extinction. Some scientists estimate that approximately one million animal and plant species are at risk. To prevent species loss, cryozoos are being developed. Cryozoos are storage tanks containing samples of body cells from animals and are kept at a temperature of -170 oC. The cells are put in a salt and sugar solution and then frozen. If a species becomes extinct, the frozen cells can be used to clone new animals. The banteng is an endangered breed of cattle. Banteng body cells have been stored in a cryozoo. These cells have been used with eggs from another species to successfully produce a living banteng.

In the future we may be able to bring back extinct species such as mammoths by making clones using the remains of frozen mammoths. Scientists need to consider the advantages and disadvantages of bringing back extinct animals. Currently, the biodiversity of the world is different from the time when mammoths were alive. Many natural predators of mammoths are now extinct. Mammoths may have helped to keep methane-releasing soil frozen, reducing the release of the methane. Mammoths were also an important part of the nitrogen cycle and increased the nutrient content of low-nutrient Arctic soil.

(a) Megalodon was an animal.

Which of these features does megalodon share with the fungus yeast?

nuclei present in cells
nervous coordination
stores carbohydrate as glycogen

A) 1 and 2 only
B) 1 and 3 only
C) 2 and 3 only
D) 1, 2 and 3

(b) Scientists think that the giant, predatory toothed whales that replaced the extinct megalodon consumed the same sources of food. Explain how the evolution of giant, predatory toothed whales may have caused the extinction of megalodon. (lines 5 to 7)

(c) (i) Volcanoes release sulfur dioxide into the atmosphere. Describe the biological consequences of sulfur dioxide release. (lines 10 to 12)
(ii) The release of large amounts of dust into the atmosphere reduced light intensity. Explain why this would cause a loss of food chains. (lines 9 to 11)

(d) (i) Explain why the cells in cryozoos are put in salt and sugar solution rather than pure water. (lines 16 to 18)
(ii) Describe how scientists could clone a bantering. (lines 18 to 20)

(e) Explain why releasing cloned mammoths could have negative and positive effects on Arctic biodiversity. (lines 22 to 28)

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)
• 4(d): Human influences on the environment — part (c)(i)
• 4(a), 4(b): The organism in the environment, Feeding relationships — part (c)(ii)
• 2(d): Movement of substances into and out of cells — part (d)(i)
• 5(d): Cloning — part (d)(ii)
• 4(d), 5(d): Human influences on the environment, Cloning — part (e)

▶️ Answer/Explanation

Solution

(a) B (1 and 3 only)
A is incorrect as yeast has no nervous system
C is incorrect as yeast has no nervous system
D is incorrect as yeast has no nervous system

(b) An explanation that makes reference to three of the following:
• competition (between megalodon and whales) (1)
• whales ate more food / eq (1)
• whales better adapted / swam faster / better at feeding / better at hunting / eq (1)
• whales survived / eq (1)
• whales reproduced more / had more offspring / eq (1)
Accept less food left for megalodon / no food / megalodon starve. Accept megalodon less well adapted. Accept megalodon did not survive/died (out). Ignore extinct.

(c) (i) A description that makes reference to two of the following:
• acid rain (1)
• deforestation / death of trees / death of plants / eq (1)
• death of fish / death of animals in lakes / loss of species in lakes or rivers / eq (1)
Accept sulphuric acid. Accept damages plants/trees. Accept loss of food chains/biodiversity.

(ii) An explanation that makes reference to three of the following:
• (less) photosynthesis (1)
• so less glucose / sugar / starch / eq (1)
• fewer producers / fewer plants / less mass of producers / producers die / plants do not grow as much / eq (1)
• less food for consumers / less energy for consumers / eq (1)
Ignore ‘cannot produce food’ for mp2. Accept fewer consumers/herbivores/carnivores.

(d) (i) An answer that makes reference to two of the following:
• prevents osmosis / less osmosis (1)
• so water will not enter cells / eq (1)
• so cells do not burst / eq (1)
Accept osmosis would occur if in water. Accept cells burst if in water.

(ii) A description that makes reference to four of the following:
• place diploid nucleus into enucleated egg cell / place body cell nucleus into enucleated egg cell / eq (1)
• electric shock / eq (1)
• mitosis / cell division / eq (1)
• place embryo into uterus / womb (1)
• of surrogate (mother) (1)
Accept fuse body cell with enucleated egg. Accept empty egg cell for enucleated egg cell.

(e) A description that makes reference to three of the following:
Negative:
• mammoth population increases / over-population occurs / eq (1)
• mammoth eats too much food / less food for other species / eq (1)
Positive:
• less global warming / less greenhouse effect / eq (1)
• decomposition of faeces / decay of dead mammoths (1)
• more amino acid / protein / DNA / chlorophyll synthesis in plants / eq (1)
• (soil nutrients means) more plants / increased plant growth / eq (1)
Accept high mammoth reproduction rate. Accept overeats prey / prey could go extinct. Accept less greenhouse gas. Accept better plant growth.

Question

Rabies is a dangerous disease caused by a virus.

(a) The rabies virus is a pathogen.

Which other groups of microorganisms include pathogens?

A. bacteria and fungi only
B. bacteria, fungi, and protoctists
C. bacteria and protoctists only
D. fungi and protoctists only

(b) Rabies is transmitted to humans when they are bitten by an infected animal. Domestic dogs are dogs kept by people as pets or as working animals. Dogs and some wild animals are known to transfer rabies to humans.

In the 1940s, a rabies vaccine for animals was introduced in the United States and most domestic dogs were vaccinated.

The graph shows the number of cases of human rabies and domestic dog rabies in the United States from 1938 to 2018.

(i) The immune system of dogs works in a similar way to the immune system of humans.

Explain how the rabies vaccine prevents dogs from developing rabies.

(ii) Discuss the effects of the use of the rabies vaccine to immunise domestic dogs on the number of cases of rabies in humans.

Use information from the graph to support your answer.

(iii) A new RNA vaccine is being developed to treat rabies.

RNA that codes for parts of the rabies virus protein coat are injected into the body. Cells then take up the RNA and produce the viral proteins.

Describe how the cells use the RNA to produce the viral proteins.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)
• 2(h): Transport — part (b)(i)
• 2(i): Excretion — part (b)(ii)
• 3(b): Inheritance — part (b)(iii)

▶️ Answer/Explanation

Solution

(a) B (bacteria, fungi, and protoctists)

Explanation: Pathogens are disease-causing microorganisms. While viruses like rabies are one type, other major groups also contain pathogenic species. Bacteria include pathogens like those causing tuberculosis and cholera. Fungi include pathogens responsible for athlete’s foot and ringworm. Protoctists (protists) include pathogenic organisms like Plasmodium which causes malaria and Entamoeba which causes dysentery. Therefore, all three groups – bacteria, fungi, and protoctists – include pathogens.

(b)(i)

Explanation: The rabies vaccine contains weakened or inactivated forms of the rabies virus or its antigens. When this vaccine is administered to dogs, it stimulates their immune system without causing the actual disease. The immune system recognizes these viral antigens as foreign invaders and produces specific antibodies against them. Specialized white blood cells called lymphocytes are activated during this process. Some of these lymphocytes develop into memory cells that remain in the body long-term. If the vaccinated dog is later exposed to the actual rabies virus, these memory cells recognize the pathogen immediately and trigger a rapid, strong immune response. This secondary response produces antibodies much faster and in greater quantities, effectively neutralizing the virus before it can establish an infection and cause disease.

(b)(ii)

Explanation: The graph shows a clear correlation between the vaccination of domestic dogs and the decline in human rabies cases. Before mass vaccination began in 1947, both dog and human rabies cases were relatively high. After vaccination programs were implemented, we observe a continuous and dramatic decline in rabies cases in both dogs and humans throughout the 1950s and beyond. This strong correlation suggests that most human rabies cases were originating from infected domestic dogs rather than wild animals. The data shows that as dog vaccination reduced the reservoir of infection in the canine population, human cases consequently decreased. However, the graph also indicates that human rabies cases didn’t disappear completely but plateaued at low levels, suggesting that some transmission still occurs from wild animals or unvaccinated dogs. This demonstrates that while vaccinating domestic dogs significantly reduces human rabies risk, it doesn’t completely eliminate it due to other potential sources of infection.

(b)(iii)

Explanation: When the RNA vaccine is injected, cells take up the RNA molecules that code for parts of the rabies virus protein coat. Inside the cell, these RNA molecules move to the ribosomes, which are the cellular structures responsible for protein synthesis. The process of translation then occurs, where the genetic code on the RNA is read and converted into a sequence of amino acids. Transfer RNA (tRNA) molecules with specific anticodons bind to complementary codons on the mRNA, each bringing with it a specific amino acid. As the ribosome moves along the mRNA strand, it facilitates the formation of peptide bonds between adjacent amino acids, creating a growing polypeptide chain. This chain folds into the three-dimensional structure of the viral protein. These viral proteins then act as antigens, stimulating the immune system to produce antibodies against rabies without exposure to the actual virus, thus providing protection against future infection.

Question

Measles is a disease caused by a virus. The graph shows the number of cases of measles each year and the percentage of five year-olds vaccinated between 1950 and 2016 in the United Kingdom.

(a) Determine the difference in the percentage change in the number of cases of measles between 1950 and 1965 and the percentage change in the number of cases of measles between 1968 and 1983.

(b) Explain the relationship between the number of cases of measles and the percentage of five year-olds vaccinated. Use information from the graph in your answer.

(c) Some children are receiving treatments that reduce the activity of their immune system. Explain why vaccinations are not used on these children.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a) (knowing measles is viral)
• 2(h): Transport — part (c) (immune response, antibodies)
• 5(a): Use of biological resources — part (b) (vaccination as a biological control method)
• 4(d): Human influences on the environment — part (b) (public health, vaccination programmes)

▶️ Answer/Explanation

Solution

(a) Difference = approximately 105% (range: 94-106%)

Explanation: To calculate this difference, we need to find the percentage change for each time period separately and then find the difference between them.

For 1950-1965: Cases increased from approximately 400-420 to 510-520. Using the midpoint values (410 to 515):

Percentage change = \(\frac{515 – 410}{410} \times 100 = \frac{105}{410} \times 100 ≈ 25.6\%\)

For 1968-1983: Cases decreased from approximately 480-490 to 120-130. Using the midpoint values (485 to 125):

Percentage change = \(\frac{125 – 485}{485} \times 100 = \frac{-360}{485} \times 100 ≈ -74.2\%\)

Difference = |25.6 – (-74.2)| = 99.8% (approximately 100%)

The exact answer may vary slightly depending on the precise values read from the graph, but should fall within the range of 94-106%.

(b) The relationship shows that as vaccination rates increase, measles cases decrease significantly.

Explanation: The graph demonstrates an inverse relationship between vaccination rates and measles cases. Before widespread vaccination (pre-1968), cases were high and fluctuated significantly. After the introduction of vaccination, cases began to decline steadily as vaccination rates increased.

For example, in the early 1970s when vaccination rates were around 50-60%, cases dropped to about 100-200 thousand. By the 1990s, when vaccination rates reached 80-90%, cases fell dramatically to very low levels (less than 20 thousand). This shows that vaccination provides herd immunity, reducing the virus’s ability to spread through the population.

The relationship is not perfectly linear because other factors like vaccination campaigns, public awareness, and natural immunity from previous infections also play a role. However, the overall trend clearly shows that higher vaccination coverage leads to fewer measles cases.

(c) Vaccinations are not used on immunocompromised children because they may not develop proper immunity and could develop the disease.

Explanation: Children with compromised immune systems (due to treatments like chemotherapy or immunosuppressive drugs) have reduced ability to produce antibodies and memory cells in response to vaccines. Vaccines contain weakened or inactivated pathogens that stimulate the immune system to create protection.

In immunocompromised children, their weakened immune systems may not be able to mount an effective response to the vaccine, leaving them unprotected. Even more concerning, there’s a risk that the weakened pathogen in the vaccine could cause the actual disease in these vulnerable children.

Instead of vaccination, these children rely on herd immunity – the protection provided when enough people in the community are vaccinated to prevent the disease from spreading. This is why high vaccination rates in the general population are so important, as they protect those who cannot be vaccinated themselves.

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

Food items can often be spoiled if saprotrophic microorganisms such as mould fungi grow on them.

(a) Describe how a saprotrophic fungus such as mould obtains its food.

(b) A student uses this method to investigate ways of preventing peas from being spoiled.

place three peas in each of three test tubes as shown in the diagram
cover the peas in test tube A with water and keep at 37°C
cover the peas in test tube B with water and keep at 4°C
cover the peas in test tube C with vinegar, which is a weak acid, and keep at 37°C
leave the peas for 24 hours

The student observes the level of cloudiness of the solution to determine how spoiled the peas have become. The level of cloudiness can be used as a measure of fungal growth.

The table shows the student’s results.

(i) Suggest a problem with using the level of cloudiness of the solution to determine how spoiled the peas have become.

(ii) Explain the appearance of the peas in water at 4°C.

(iii) Explain the appearance of the peas in vinegar at 37°C.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)
• 2(c): Biological molecules and enzymes — parts (b)(ii), (b)(iii)
• 2.11–2.13: Enzyme function and factors affecting it — parts (b)(ii), (b)(iii)
• Appendix 6: Suggested practical investigations — part (b) experimental context

▶️ Answer/Explanation

Solution

(a) Saprotrophic fungi like mould obtain their food through extracellular digestion. They secrete enzymes onto the dead or decaying organic matter. These enzymes break down complex molecules like carbohydrates and proteins into simpler, soluble substances. The fungus then absorbs these digested nutrients through its hyphae.

(b)(i) The main problem is that judging cloudiness is subjective and qualitative rather than quantitative. Different people might interpret “very cloudy,” “slightly cloudy,” and “no cloudiness” differently. There’s no precise, numerical measurement, which makes it difficult to compare results accurately or for other scientists to repeat the experiment exactly.

(b)(ii) The peas in water at 4°C showed only slight cloudiness because the low temperature significantly slowed down fungal growth. At 4°C, the enzymes in the saprotrophic fungi work much more slowly due to reduced kinetic energy, leading to fewer enzyme-substrate collisions. This results in less digestion of the peas, less fungal respiration, and therefore less spoilage and cloudiness.

(b)(iii) The peas in vinegar at 37°C showed no cloudiness because the acidic conditions denatured the enzymes of the saprotrophic fungi. Vinegar, being a weak acid, lowers the pH. This change in pH alters the shape of the enzymes’ active sites, preventing them from binding to their substrates and catalyzing the digestive reactions. Consequently, fungal growth is inhibited or the fungi are killed, preventing spoilage and resulting in clear solution.

Question

The diagram shows a single-celled organism called Chlorella that lives in fresh water.

Chlorella has a chloroplast and can photosynthesise.

(a) (i) Which of these groups of organisms contains Chlorella?

A. animals
B. bacteria
C. plants
D. protoctists

(ii) Which of these labelled structures would also be present in an animal cell?

A. cell membrane and chloroplast
B. cell membrane and mitochondrion
C. cell wall and chloroplast
D. cell wall and mitochondrion

(b) Complete the balanced chemical symbol equation for photosynthesis.

__________ + __________ → C₆H₁₂O₆ + 6O₂

(c) The graph shows the effect of light intensity on gas exchange by Chlorella.

(i) Explain why Chlorella takes in oxygen at light intensities below 10 arbitrary units.

(ii) Explain the changes in the volume of oxygen released as the light intensity increases from 10 arbitrary units.

(iii) The volume of oxygen released by Chlorella is the difference between the oxygen produced by photosynthesis and the oxygen taken in.

Use the graph to calculate the volume of oxygen produced in five minutes by photosynthesis at a light intensity of 50 arbitrary units.

(d) Describe how hydrogen-carbonate indicator could be used to investigate the effect of light intensity on carbon dioxide exchange by Chlorella.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(i)
• 2(b): Cell structure — part (a)(ii)
• 2(e): Nutrition (photosynthesis) — parts (b), (c), (d)
• 2(f): Respiration — parts (c)(i), (c)(ii), (c)(iii)
• 2(g): Gas exchange — part (d)

▶️ Answer/Explanation

Solution

(a) (i) D (protoctists)

Explanation: Chlorella is a single-celled, photosynthetic organism with a nucleus and chloroplasts. It is not an animal (A) because it has chloroplasts and a cell wall. It is not a bacterium (B) because it has a true nucleus and membrane-bound organelles like chloroplasts and mitochondria. It is not a plant (C) because it is unicellular, whereas plants are multicellular. Therefore, it belongs to the kingdom Protoctista (D), which contains various unicellular and simple multicellular eukaryotes, including algae.

(a) (ii) B (cell membrane and mitochondrion)

Explanation: Animal cells have a cell membrane and mitochondria. They do not have chloroplasts (so A and C are incorrect) and they do not have a cell wall (so D is incorrect). Both animal cells and Chlorella require mitochondria for respiration to release energy.

(b) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Explanation: The balanced equation for photosynthesis shows that six molecules of carbon dioxide (6CO₂) and six molecules of water (6H₂O), in the presence of light energy and chlorophyll, react to produce one molecule of glucose (C₆H₁₂O₆) and six molecules of oxygen (6O₂). The reactants must be placed on the left-hand side of the arrow.

(c) (i)

Explanation: At low light intensities (below 10 arbitrary units), the rate of photosynthesis is very low because there is insufficient light energy. However, respiration continues at all times to release energy for cell processes. Therefore, the oxygen produced by photosynthesis is less than the oxygen consumed by respiration. This results in a net uptake of oxygen from the surroundings, which is why the graph shows a negative value for oxygen exchange (indicating net intake).

(c) (ii)

Explanation: As light intensity increases from 10 arbitrary units, the rate of photosynthesis also increases because light is a key factor for the light-dependent reactions. At 10 arbitrary units, the compensation point is reached where the rate of photosynthesis equals the rate of respiration, so there is no net gas exchange. Above this point, the rate of photosynthesis becomes greater than the rate of respiration. This means more oxygen is produced by photosynthesis than is consumed by respiration, leading to a net release of oxygen, which is shown by the positive values on the graph. The curve eventually levels off because another factor, such as carbon dioxide concentration or temperature, becomes limiting and prevents the rate of photosynthesis from increasing further, even with more light.

(c) (iii) 48 mm³

Explanation: The graph shows the net oxygen released, which is the oxygen from photosynthesis minus the oxygen used in respiration. At 50 arbitrary units, the net oxygen released is approximately 38 mm³. We are told that the oxygen taken in (used in respiration) is 10 mm³ (this value is consistent across light intensities as respiration rate is relatively constant). To find the gross oxygen produced by photosynthesis, we add the oxygen used in respiration to the net oxygen released: 38 mm³ + 10 mm³ = 48 mm³.

(d)

Explanation: To investigate the effect of light intensity on carbon dioxide exchange, you could set up the following experiment. Place equal volumes or masses of Chlorella in several test tubes containing the same volume of hydrogen-carbonate indicator solution. Seal the tubes. Hydrogen-carbonate indicator changes color with carbon dioxide concentration: it turns yellow when carbon dioxide levels are high, red at atmospheric levels, and purple when carbon dioxide levels are low. You would then place the tubes at different distances from a light source to create different light intensities (e.g., 10 cm, 20 cm, 30 cm away). A control tube with no Chlorella should be set up to show that any color change is due to the organism. You would also need to control other variables, such as temperature and the initial concentration of the algae and indicator. After leaving the tubes for a set period, you would observe and record the final color of the indicator in each tube. In high light, photosynthesis would be high, so carbon dioxide would be absorbed, and the indicator would turn purple. In low light or darkness, respiration would dominate, releasing carbon dioxide, and the indicator would turn yellow.

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

P. multocida is a bacterium that causes cholera in chickens.

The diagram shows the bacterium.

(a) Give two structures in this bacterium that are also found in all eukaryotic cells.

(b) Scientists investigated the survival of chickens injected with normal P. multocida or with weakened P. multocida.

The table shows the scientists’ results.

(i) What is a correct conclusion about P. multocida from these results?

A. they are decomposers
B. they are pathogens
C. they are microscopic
D. they are non-living

(ii) The scientists took the living chickens that had been injected with weakened P. multocida and then injected them with normal P. multocida.

The chickens did not die, as they were now immune.

Explain why these chickens did not die.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)
• 1.3: Prokaryotic organisms (bacteria) — part (a)
• 1.4: Pathogens — part (b)(i)
• 2(h): Transport — part (b)(ii) – Immune response
• 2.62/2.63B: Immune response and vaccination — part (b)(ii)

▶️ Answer/Explanation

Solution

(a) 1. Cell membrane
2. Ribosomes
(Also accept: Cytoplasm)

Explanation: Prokaryotic cells, like this bacterium, and eukaryotic cells share some fundamental structures necessary for basic life functions. The cell membrane is a universal feature, acting as a semi-permeable barrier that controls the movement of substances in and out of the cell. Ribosomes are also found in both, as they are essential for protein synthesis, although bacterial ribosomes are slightly smaller. Cytoplasm is another correct answer, as it is the gel-like substance that fills the cell and houses the organelles and other cellular components.

(b)(i) B they are pathogens

Explanation: The results show that the normal form of the bacterium causes disease (chickens die), while the weakened form does not. An organism that causes disease is defined as a pathogen. While the bacteria are likely decomposers and microscopic, these specific results do not provide direct evidence for those conclusions. The results definitively show they are living organisms capable of causing disease.

(b)(ii)

Explanation: The initial injection with weakened P. multocida acted as a vaccination. These weakened bacteria contained the same antigens (specific proteins on their surface) as the normal, harmful bacteria. When injected, the chicken’s immune system recognized these antigens as foreign and mounted a primary immune response. This involved the production of specific antibodies to fight the infection and, crucially, the creation of memory cells. When the chickens were later injected with the normal, pathogenic bacteria, these memory cells recognized the antigens immediately. This triggered a much faster and stronger secondary immune response, producing a large number of the correct antibodies very quickly to destroy the bacteria before they could cause disease and kill the chicken.

Question

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

Schistosomiasis

Schistosomiasis is an infection caused by a parasitic worm called a schistosome. The immature or larvae of the worm live in rivers and lakes in tropical parts of the world. The diagram shows the life cycle of the schistosome worm.

Some people in tropical parts of the world use water from these rivers and lakes for their daily lives. These people risk infection because the larvae burrow into their skin. The larvae are then transported to other parts of the body where they damage organs such as the kidneys, intestines, lungs and brain. The larvae develop into adult worms.

The adult worms lay eggs in the human body. Some of these eggs are destroyed by the immune system, but most eggs survive. These eggs can get into water if faeces or urine from infected people pass into rivers or lakes. In the water, the eggs develop into small larvae which grow inside freshwater snails. Larvae are released from the snails and infect any person they contact. These larvae develop into adult worms inside the human body.

Doctors diagnose schistosomiasis when they find eggs in the faeces or urine of infected people. Infected people also have blood cells in their urine and antibodies for the pathogen in their blood.

At present, the drug praziquantel is used to kill the worms. A dose of \(0.040\ \text{g}\) per kg of body mass is usually effective. With no treatment, affected organs can be permanently damaged, leading to death. It is estimated that 240 million people (in the world) have schistosomiasis. Every year \(8 \times 10^{-4}\) per cent of infected people die from the disease.

A vaccine is being developed using a plasmid. The plasmid has DNA inserted that makes a protein found on the body surface of the adult schistosome worm.

In one investigation, a vaccine made using the DNA plasmid was given to a group of infected people. The results showed a mean number of 21.53 worms per person in this group. In the control group, a mean number of 40.53 worms per person was found.

(a) Which process is affected if kidneys are damaged (line 8)?

A) digestion
B) mutation
C) ultrafiltration
D) vaccination

(b) Suggest three ways to reduce the risk of being infected by schistosomes.

(c) Name two different blood cells that would be found in the urine of infected people (line 17).

(d) An infected person has a body mass of \(120\ \text{kg}\). What dose of drugs would be effective for this person (lines 19 to 20)?

A) \(0.04\ \text{mg}\)
B) \(4.8\ \text{mg}\)
C) \(40\ \text{mg}\)
D) \(4800\ \text{mg}\)

(e) Using the estimated number of people in the world who have schistosomiasis (lines 21 to 22), calculate the number of people who die each year from schistosomiasis.

(f) Which of these is the correct description of a plasmid?

A) a circle of DNA
B) a circle of mRNA
C) a circle of protein
D) a circle of tRNA

(g) Explain how a vaccine could protect people from schistosomiasis (lines 24 to 25).

(h)(i) Suggest what is given to the control group (lines 27 to 29).

(ii) A scientist claims that the investigation proves the vaccine is effective against schistosomiasis (lines 27 to 29). Comment on this claim.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 2(h): Transport — parts (c) [Blood composition], (g), (h)(i) [Vaccination and immune response]
• 2(i): Excretion — part (a) [Kidney function]
• 1(b): Variety of living organisms / Pathogens — context of parasitic infection
• 5(c): Genetic modification — part (f) [Plasmids as vectors]
• Mathematical Skills (Appendix 3): — parts (d), (e) [Calculations]
• Experimental Skills & Evaluation (Section 4): — part (h)(ii) [Evaluating an investigation]

▶️ Answer/Explanation

Solution

(a) C (ultrafiltration)
A is incorrect because digestion is not a process in the kidneys.
B is incorrect because mutation is not a process in the kidneys.
D is incorrect because vaccination is not a process in the kidneys.

(b) An answer that makes reference to three of the following points:
• treat drinking water / boil water (before drinking) / do not drink water / drink bottled water / eq (1)
• sanitation / no faeces in water / no urine in water / eq (1)
• remove snails / eq (1)
• vaccination (1)
Additional guidance: Allow “do not go in infected rivers or lakes / cover skin when in water / avoid contact with affected water / only wash in clean water”.

(c) An answer that makes reference to two of the following points:
• red blood cells / rbc (1)
• white blood cells / wbc (1)
• lymphocytes (1)
• phagocytes / macrophages (1)

(d) D (4800 mg)
Calculation: \(120\ \text{kg} \times 0.040\ \text{g/kg} = 4.8\ \text{g} = 4800\ \text{mg}\)
A is incorrect because it is the wrong value.
B is incorrect because it is the wrong value.
C is incorrect because it is the wrong value.

(e) \(1920\) people
Working:
• \(8 \times 10^{-4}\% = 0.0008\%\)
• \(0.0008\% \text{ of } 240,000,000 = 0.000008 \times 240,000,000 = 1920\)
Allow 1 mark for: \(19200000 / 1920000 / 192000 / 192200 / 192 / 19.2 / 1.92 / 0.192 / 0.0192\)
Award full marks for correct numerical answer without working.

(f) A (a circle of DNA)
B is incorrect because it is not RNA.
C is incorrect because it is not a protein.
D is incorrect because it is not RNA.

(g) An explanation that makes reference to three of the following points:
• antigen (1)
• memory cells / lymphocytes (1)
• (secondary) immune response (1)
• more antibodies / antibodies made sooner / faster / faster immune response / eq (1)

(h)(i) • (a treatment with) no plasmid / no protein / only water / saline / eq (1)
Allow placebo vaccine / a placebo / plasmid with no gene / plasmid with no DNA / different DNA.

(h)(ii) An answer that makes reference to three of the following points:
• reduced numbers / eq (1)
• by 19 or by 47% / about 50% (1)
• schistosomes / worms, still present in body (1)
• no idea of group size / needs to be repeated (1)
• no idea of age / sex / health (1)
Additional Guidance:
Allow “reduces numbers of worms / worms decrease / lower number of worms after vaccine”.
Allow “more worms in control group”.
Allow “does not completely get rid of them”.
Allow “more testing / more people tested”.

Question

Human populations are at risk from infectious disease.

(a) Describe the different types of pathogen. Refer to a disease that each type of pathogen causes in your answer.

(b) Explain how vaccination protects humans from pathogens.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms / Pathogens — part (a) [Describing viruses, bacteria, fungi, protocysts and related diseases]
• 1.4: Understand the term pathogen — part (a) [Context]
• 2(h): Transport — part (b) [Vaccination and immune response, memory cells]
• 3.13B: Understand how vaccination results in the manufacture of memory cells — part (b) [Core explanation]

▶️ Answer/Explanation

Solution

(a) A description that makes reference to six of the following points (typically three pathogen types with their descriptions and matching diseases):

Virus: non-living organisms / small particles / protein coat (capsid) / relies on other organisms for reproduction / eq (1)
Disease: AIDS / Influenza / HIV (1) [Disease mark must match pathogen type. Allow plant disease e.g., Tobacco Mosaic Virus (TMV).]
Bacteria: microscopic single-celled / prokaryotic / no nucleus / have nucleoid / plasmids (1)
Disease: Pneumonia / Cholera / Tuberculosis (1)
Fungus: not able to carry out photosynthesis / saprotrophic nutrition / single-celled or hyphal / cell wall made of chitin / eq (1)
Disease: Athlete’s foot / Ringworm (1)
Protocysts (Protozoa): microscopic single-celled / eukaryotic (1)
Disease: Malaria (caused by Plasmodium) (1)

Scoring Note: The first mark for each pathogen is for its description, the second is for naming a correct associated disease. A pair (description + disease) for one pathogen type scores 2 marks. “HIV / AIDS” scores the disease mark for a virus but not the pathogen description mark if the description is incorrect. “Virus non-living causing cholera” scores the pathogen description mark but not the disease mark (cholera is bacterial).

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

dead / weakened / harmless / attenuated pathogen / or its antigens are introduced (into the body) / eq (1) [Allow ‘weakened strain’.]
this stimulates the production of memory cells / lymphocytes (1)
this leads to a (secondary) immune response upon future infection with the actual pathogen (1)
antibodies are produced faster / in greater quantity / sooner (1)

Edexcel iGCSE Biology 4BI1 - Paper 2B -Variety of living organisms- Exam Style Questions- New Syllabus

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