Edexcel iGCSE Biology 4BI1 - Paper 1B -Variety of living organisms- Exam Style Questions- New Syllabus

Question

Euglena is a microorganism that lives in water. The diagram shows Euglena with some structures labelled.

(a) (i) Which group contains Euglena?

A) bacteria
B) fungi
C) protocysts
D) viruses

(a) (ii) Which structure is the site of respiration?

A) P
B) R
C) T
D) U

(a) (iii) Which structure contains chromosomes?

A) P
B) Q
C) R
D) T

(a) (iv) Which structure is the cell membrane?

A) P
B) Q
C) R
D) U

(b) Euglena has some structures that are found in plant cells but not in animal cells. Which labelled structure is found in plant cells but not in animal cells?

A) P
B) Q
C) S
D) T

(c) Some of the labelled structures are known as organelles. State what is meant by the term organelle.

(d) Explain which two labelled structures allow Euglena to move through water. Refer to the diagram and your own knowledge in your answer.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(i)
• 2(b): Cell structure — parts (a)(ii), (a)(iii), (a)(iv), (b), (c)
• 2(f): Respiration — part (a)(ii)
• 2(b): Cell differentiation and specialized cells — part (d)

▶️ Answer/Explanation

Solution

(a)(i) C (protocysts)
A is not the answer as Euglena is not in bacteria
B is not the answer as Euglena is not in fungi
D is not the answer as Euglena is not in viruses

(a)(ii) D (U)
A is not the answer as P is the membrane
B is not the answer as R is the flagellum
D is not the answer as T is the nucleus

(a)(iii) D (T)
A is not the answer as P is the membrane
B is not the answer as Q is the cytoplasm
C is not the answer as R is the flagellum

(a)(iv) A (P)
B is not the answer as Q is the cytoplasm
C is not the answer as R is the flagellum
D is not the answer as U is the mitochondrion

(b) C (S)
A is not the answer as P is the cell membrane
B is not the answer as Q is the cytoplasm
D is not the answer as T is the nucleus

(c) • structure / part of a cell with particular function / purpose / role (s) /eq

(d) An explanation that makes reference to three of the following:
• mitochondria / U (1)
• (mitochondrion) respiration / releases energy / provides ATP / eq (1)
• flagellum / R / tail / eq (1)

Question

The photograph shows a fungus called Mucor growing on an agar plate. Cells within Mucor contain nuclei and mitochondria.

(a) (i) Which of these is a correct statement about the fungus, Mucor?

A) Mucor is a eukaryotic organism with cell walls made of chitin
B) Mucor is a eukaryotic organism with cell walls made of cellulose
C) Mucor is a prokaryotic organism with cell walls made of chitin
D) Mucor is a prokaryotic organism with cell walls made of cellulose

(ii) Plant cells contain chloroplasts, cytoplasm and starch. How many of these are also present in a fungus such as Mucor?

A) 0
B) 1
C) 2
D) 3

(iii) The fungus in the diagram is circular with a radius of 45 mm. Calculate the area, in mm², of this circle.
Give your answer to 2 significant figures. [area = πr² π = 3.14]

(b) Mucor can reproduce using asexual or sexual reproduction.

(i) State two differences between asexual and sexual reproduction.
(ii) Explain why it is an advantage for Mucor to be able to reproduce using sexual reproduction.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — parts (a)(i), (a)(ii)
• 2(b): Cell structure — part (a)(ii)
• Appendix 3: Mathematical skills — part (a)(iii)
• 3(a): Reproduction — parts (b)(i), (b)(ii)

▶️ Answer/Explanation

Solution

(a)(i) A (Mucor is a eukaryotic organism with cell walls made of chitin)
B is incorrect as Mucor does not have cellulose
C is incorrect as Mucor is not a prokaryote
D is incorrect as Mucor is not a prokaryote

(a)(ii) B (1)
A is incorrect as Mucor has cytoplasm
C is incorrect as Mucor does not have chloroplasts or starch
D is incorrect as Mucor does not have chloroplasts or starch

(a)(iii)
• \(3.14 \times 45^2 = 6358.5\)
• to two sig figs = 6400

(b)(i) An answer that makes reference to two of the following:
• asexual produces genetically identical offspring / clones (1)
• asexual has no fertilisation / no gametes (1)
• asexual only has one parent cell (1)
• asexual uses (only) mitosis / asexual does not use meiosis (1)

(b)(ii) An explanation that makes reference to the following:
• sexual reproduction produces (genetic) variation (1)
• some may survive environmental changes / not all killed by disease / are more adaptable / natural selection can occur (1)

Question

The diagram shows part of a food web for the Arctic Ocean.

(a)(i) Phytoplankton are protocysts that photosynthesise.
Which of the following features do phytoplankton share with plants?

chitin cell wall
chloroplast
nucleus

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

(ii) Name two organisms in this food web that can feed as tertiary consumers.
(iii) Draw the food chain with the most trophic levels in this food web.

(b) Scientists measure the changes in the biomasses of phytoplankton and zooplankton in one year.

(i) During a month in spring, the scientists found that the biomass of phytoplankton in the water increased from \(1.2\ \text{mg per dm}^3\) to \(12.6\ \text{mg per dm}^3\).
Calculate the percentage increase of these phytoplankton in this month.

(ii) During the year, the scientists also measured the change in the number of hours of light each day and the change in nitrate concentration in the water near the surface of the ocean.
The graph shows their results.

Discuss the reasons for the changes in the biomasses of phytoplankton and zooplankton during the year.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(i)
• 5(b): Feeding relationships — parts (a)(ii), (a)(iii)
• 5(a): The organism in the environment — part (b)(ii)
• 2(e): Nutrition (Photosynthesis) — part (b)(ii)
• Appendix 3: Mathematical skills — part (b)(i)

▶️ Answer/Explanation

Solution

(a)(i) C (2 and 3 only)
A is incorrect because chitin is not present in either.
B is incorrect because chitin is not present in either.
D is incorrect because both have chloroplasts.

(a)(ii) Any two of: polar bear, seal, grey whale, predatory fish.

(a)(iii) phytoplankton → zooplankton → plankton-eating fish → predatory fish → seal → polar bear
One mark for correct organisms in order. One mark for correct arrow direction.

(b)(i) 950%
Working:
Increase = \(12.6 – 1.2 = 11.4\ \text{mg per dm}^3\)
Percentage Increase = \(\frac{11.4}{1.2} \times 100 = 950\%\)

(b)(ii) An answer that makes reference to points such as:
• Phytoplankton biomass is low in winter due to low light and temperature, reducing photosynthesis.
• Biomass increases in spring as light intensity and duration increase, enhancing photosynthesis.
• Increased photosynthesis produces more glucose, allowing phytoplankton growth.
• Zooplankton biomass increases after phytoplankton increase, as they feed on phytoplankton.
• Phytoplankton decrease in late summer/autumn as zooplankton population peaks and consume them, and light/nitrate levels fall.
• Nitrate concentration decreases in spring/summer as phytoplankton absorb nitrates to make amino acids and proteins.
• The biomass of both is limited by abiotic factors (light, nitrates) and biotic factors (predation).

Question

The diagram shows part of a food web from an oak woodland.

(a) (i) Which is the producer in this food web?

A) beetle
B) deer
C) oak tree
D) tick

(ii) Draw a food chain, from this web, that includes the mouse and contains four trophic levels.

(iii) Which one of these organisms is in two different trophic levels in this food web?

A) ant
B) blue jay
C) caterpillar
D) mouse

(b) A tick is a small spider-like organism that bites and then takes in blood from the mammals as it feeds.

This is a magnified image of a tick.

(i) The actual length of the tick, as shown by line A-B, is 3.5 mm. Calculate the magnification of the image of the tick.

(ii) The tick absorbs substances from the mammal’s blood it has taken in. Give the function of two named substances absorbed by the tick.

(iii) Ticks can pass diseases between organisms. Suggest how ticks can pass diseases from one organism to another.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 5(b): Feeding relationships — parts (a)(i), (a)(ii), (a)(iii)
• 5(a): The organism in the environment — part (a)(i)
• Mathematical Skills (Appendix 3): Arithmetic and numerical computation — part (b)(i)
• 2(e): Nutrition (Humans) — part (b)(ii)
• 1(b): Variety of living organisms / Pathogens — part (b)(iii)

▶️ Answer/Explanation

Solution

(a)(i) C (oak tree)
A is not the answer as beetle is not the producer
B is not the answer as deer is not the producer
D is not the answer as tick is not the producer

(a)(ii)
oak tree → caterpillar → mouse → tick
Allow 1 mark for correct order. No credit for pyramids.

(a)(iii) D (mouse)
A is not correct as the ant is not at two levels
B is not correct as the blue jay is not at two levels
C is not correct as the caterpillar is not at two levels

(b)(i)
Measurement of line = \(10.4 \text{ cm} = 104 \text{ mm}\)
Magnification = \(\frac{\text{image size}}{\text{actual size}} = \frac{104}{3.5}\)
Magnification = \(29.7\) (range \(29.0 – 30.0\) accepted)
Allow 1 mark for correct measurement of line (10.3-10.5 cm or 103-105 mm) with units.

(b)(ii)
An explanation makes reference to four of the following points (substance + matching function):
• Glucose (1) — for energy / respiration (1)
• Iron (1) — for haemoglobin / red blood cells (1)
• Amino acids (1) — for protein (synthesis) / growth (1)
• Fatty acids / lipids (1) — for energy / insulation (1)
• Water (1) — for keeping body hydrated / transport / solvent (1)
• Vitamin C (1) — for prevents scurvy (1)
Allow named vitamin or mineral for one mark each. Function must match substance.

(b)(iii)
An answer that makes reference to two of the following:
• Tick picks up / bites / sucks up / absorbs blood from infected animal / animal with disease (1)
• Bacteria / virus / pathogen is present in the blood (1)
• Tick then feeds on / bites a new / uninfected animal (1)
• Pathogen is transferred to the new host (1)
Allow ‘transfer’ if reference to biting is included.

Question

The passage describes the process used to produce yoghurt. Complete the passage by writing a suitable word or words in each blank space.

Yoghurt is made by heating to a high temperature.

This heating process is known as .

This ensures that bacteria present in the liquid are .

The liquid is then cooled to between \(40^\circ C\) and \(46^\circ C\).

A type of bacteria called is then added.

These bacteria use a sugar called for respiration.

The pH of the yoghurt decreases because this respiration produces a substance called .

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 6(a): Use of biological resources – Food production (Micro-organisms) — specifically 6.7, 6.8
• 2(f): Respiration — specifically 2.36, 2.38 (anaerobic respiration)
• 1(b): Variety of living organisms — specifically 1.3 (Bacteria)

▶️ Answer/Explanation

Solution

Completed Passage:

Yoghurt is made by heating milk to a high temperature.

This heating process is known as pasteurisation or sterilisation.

This ensures that bacteria present in the liquid are killed.

The liquid is then cooled to between \(40^\circ C\) and \(46^\circ C\).

A type of bacteria called Lactobacillus (or Streptococcus) is then added.

These bacteria use a sugar called lactose for

anaerobic respiration.

The pH of the yoghurt decreases because this respiration produces a substance called

lactic acid (or lactate).

Topic Mapping:

• 6(a) Food production (Micro-organisms) – 6.7, 6.8: The question directly assesses knowledge of yoghurt production using bacteria (Lactobacillus) and the conditions in a fermenter (heating, cooling).

• 2(f) Respiration – 2.36, 2.38: The question involves the type of respiration (anaerobic) and its product (lactic acid).

• 1(b) Variety of living organisms (Bacteria) – 1.3: Requires naming a specific bacterium used in food production.

Question

Biologists classify organisms into different groups. Two of these groups are bacteria and fungi.

(a) Complete the passage about bacteria by writing a suitable word or words in each blank space.

Bacteria are small …… celled organisms. They have a cell membrane and a cell ……, made of peptidoglycan. Some bacteria are able to use the energy from the Sun to carry out …… but other bacteria are …… and feed on dead and decaying organisms. Bacterial cells do not have a nucleus but most of their genetic material is contained in a circular …… of DNA. They also have small circles of DNA called …… in their cytoplasm. Some bacteria are used by humans in food production. An example is using …… to make yoghurt. Other bacteria, such as Pneumococcus, cause disease and so are called ……

(b) A microscopic single-celled fungus, yeast, is used in the production of bread. Describe how yeast is used to produce bread.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)
• 2(f): Respiration — part (b)

▶️ Answer/Explanation

Solution

(a) The completed passage:

Bacteria are small single / one / unicellular celled organisms. They have a cell membrane and a cell wall, made of peptidoglycan. Some bacteria are able to use the energy from the Sun to carry out photosynthesis but other bacteria are decomposers / saprophytic / saprophytes / saprotrophic and feed on dead and decaying organisms. Bacterial cells do not have a nucleus but most of their genetic material is contained in a circular chromosome / nucleoid of DNA. They also have small circles of DNA called plasmids in their cytoplasm. Some bacteria are used by humans in food production. An example is using Lactobacillus / Streptococcus to make yoghurt. Other bacteria, such as Pneumococcus, cause disease and so are called pathogens / pathogenic.

Explanation: This question tests knowledge of bacterial structure, nutrition, genetics, and their roles (both beneficial and harmful). Bacteria are prokaryotes with a cell wall, some can photosynthesize, and many are decomposers. Their genetic material is in a circular chromosome and plasmids. Lactobacillus is used in yoghurt production, while Pneumococcus is a pathogen.

(b) A description that makes reference to four of the following points:

Yeast respires (aerobically) initially / eq (1)
Yeast (then) respires anaerobically / ferments / eq (1)
Converts starch to maltose / maltose to glucose / starch to glucose / eq (1)
Using amylase / maltase / eq (1) (enzymes present in flour or added)
Releases carbon dioxide / eq (1)
Causes (dough / bread) to rise / bubbles trapped / volume increases / eq (1)

Explanation: Yeast is a single-celled fungus used in baking. It carries out anaerobic respiration (fermentation) in the dough, using sugars (from starch breakdown by enzymes) as a substrate. The process produces carbon dioxide gas, which gets trapped in the dough, causing it to rise and giving bread its spongy texture.

Question

Lactobacillus is a type of bacterium used to make yoghurt.
The diagram shows a single Lactobacillus bacterium.

(a) (i) What is the function of ribosomes?

A. photosynthesis
B. protein synthesis
C. respiration
D. storage of cell sap

(a) (ii) Which row of the table gives the components that are found in all bacteria and in all eukaryotic cells?

(a) (iii) Lactobacillus cells do not burst when placed in distilled water. Red blood cells burst when placed in distilled water. Explain why the Lactobacillus cells do not burst but the red blood cells do burst.

(b) Describe how Lactobacillus bacteria produce yoghurt from milk.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — parts (a)(i), (a)(ii)
• 2(b): Cell structure — parts (a)(i), (a)(ii)
• 2(d): Movement of substances into and out of cells — part (a)(iii)
• 5(a): Use of biological resources — part (b)

Question

Yeast is a fungus that can respire anaerobically.

(a) (i) What substance is the cell wall of a yeast cell made of?

A. cellulose
B. chitin
C. glycogen
D. starch

(a) (ii) Which row of the table shows the substances produced by yeast during anaerobic respiration?

(b) A student uses this apparatus to investigate the effect of temperature on anaerobic respiration by yeast.

This is the student’s method.

place yeast cells and glucose solution into a test tube
keep the test tube at a temperature of 25°C
cover the yeast and glucose solution with paraffin oil
count the number of bubbles produced each minute for 20 minutes
repeat at a temperature of 37°C

The student draws a graph to show the total number of bubbles that have been produced by the end of each minute for the 20-minute period.

The graph shows the student’s results.

(i) State the function of the paraffin oil.

(ii) Describe a method the student could use to keep the temperature of the yeast and glucose mixture constant.

(iii) Calculate the rate of bubble production from 0 minutes to 8 minutes when the yeast is at 37°C.

(iv) Explain the change in the total number of bubbles produced from 0 minutes to 20 minutes at 25°C.

(v) Explain the difference in the rate of bubble production between 0 and 10 minutes at 37°C compared with 25°C.

(vi) Describe how the student could modify the experiment to give more accurate results.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(i)
• 2(b): Cell structure — part (a)(i)
• 2(f): Respiration — parts (a)(ii), (b)(i)–(b)(vi)
• 5(a): Food production (Micro-organisms) — part (b) experimental context
• Experimental skills: Planning, data analysis, evaluation — parts (b)(ii), (b)(iii), (b)(iv), (b)(v), (b)(vi)

▶️ Answer/Explanation

Solution

(a)(i) B (chitin)

Explanation: The cell walls of fungal cells, including yeast, are primarily composed of chitin, a strong and flexible polysaccharide. Cellulose is found in plant cell walls, while glycogen and starch are storage polysaccharides and are not structural components of cell walls.

(a)(ii) A

Explanation: Yeast undergoes alcoholic fermentation during anaerobic respiration. The products of this process are ethanol and carbon dioxide. Lactic acid is not produced; it is a product of anaerobic respiration (lactic acid fermentation) in certain bacteria and animal muscle cells.

(b)(i) To stop oxygen getting in / to prevent aerobic respiration.

Explanation: The layer of paraffin oil acts as a barrier, sealing the yeast and glucose mixture from the air. This prevents oxygen from dissolving into the solution, ensuring that the yeast is forced to respire anaerobically (ferment) rather than aerobically, which would produce different products and potentially a different rate of gas production.

(b)(ii) Use a water bath. Monitor the temperature with a thermometer.

Explanation: To maintain a constant temperature, the test tube containing the yeast mixture should be placed in a water bath set to the desired temperature (e.g., 25°C or 37°C). The large volume of water helps to buffer against rapid temperature changes. The student should then use a thermometer to regularly check the temperature of the water bath (or the mixture itself) and make minor adjustments if necessary to keep it constant throughout the experiment.

(b)(iii) 4 bubbles per minute

Explanation: The calculation for rate is: Rate = Total number of bubbles / Time. According to the graph, at 37°C, the total number of bubbles produced by 8 minutes is 32. Therefore, the rate is 32 bubbles / 8 minutes = 4 bubbles per minute.

(b)(iv) The total number increases up to around 16–18 minutes and then levels off. This is because initially glucose is not limiting, but later it runs out or the yeast is poisoned by ethanol.

Explanation: The graph line for 25°C shows a steady increase in the cumulative number of bubbles up to a certain point (around 16–18 minutes), after which the line becomes horizontal. The initial increase indicates that anaerobic respiration is occurring at a constant rate, as there is plenty of glucose substrate available. The leveling off occurs because the glucose supply eventually becomes depleted. Additionally, the ethanol produced as a waste product can reach a concentration that becomes toxic to the yeast cells, inhibiting further respiration and stopping bubble production.

(b)(v) The rate is faster at 37°C. This is because 37°C is closer to the optimum temperature for enzymes involved in respiration.

Explanation: The graph shows a steeper gradient (more bubbles produced in the same time) for the 37°C line compared to the 25°C line between 0 and 10 minutes. Enzyme‑controlled reactions, like those in respiration, have an optimum temperature. 37°C is closer to this optimum than 25°C. The increased thermal energy at 37°C causes enzymes and substrate molecules to move faster and collide more frequently and with more energy. This increases the rate of successful collisions and the formation of enzyme‑substrate complexes, thus speeding up the reaction. The faster rate also means the glucose substrate is used up more quickly at 37°C, which is why the curve begins to level off earlier.

(b)(vi) Collect and measure the volume of gas produced instead of counting bubbles. Use a gas syringe or a measuring cylinder in a water trough.

Explanation: Counting bubbles is an inaccurate method because bubbles can vary significantly in size. A small bubble and a large bubble would each count as one, giving a misleading measure of the actual quantity of gas produced. A more accurate method would be to replace the delivery tube in the diagram with one that leads into an inverted measuring cylinder (or burette) filled with water, or to use a gas syringe. This apparatus would collect the gas and allow the student to measure its volume directly, which is a more reliable and quantitative measure of the rate of respiration.

Question

(a) All living organisms share characteristics.

(i) State two characteristics that all living organisms share.

(ii) Some organisms are pathogens. Which of these organisms can cause a bacterial disease in humans?

A. Chlorella
B. Lactobacillus bulgaricus
C. Mucor
D. Pneumococcus

(b) Viruses are pathogens but not living organisms.

(i) Describe the effect of a named virus that infects plants.

(ii) Give three differences between the structure of viruses and bacteria.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(a): Characteristics of living organisms — part (a)(i)
• 1(b): Variety of living organisms — part (a)(ii)
• 1(b): Pathogens (viruses, bacteria) — parts (b)(i), (b)(ii)

▶️ Answer/Explanation

Solution

(a)(i) Answer: Any two of: Nutrition, Respiration, Excretion, Sensitivity, Movement, Homeostasis, Reproduction, Growth.

Explanation: All living organisms share these eight characteristics, as outlined in Topic 1(a) of the specification.

(a)(ii) Answer: D (Pneumococcus)

Explanation: Pneumococcus is a bacterium that causes pneumonia. Chlorella is a protocyst, Lactobacillus is a beneficial bacterium, and Mucor is a fungus (Topic 1(b)).

(b)(i) Answer: Named Virus: Tobacco Mosaic Virus (TMV). Effect: Causes a mosaic pattern of discolouration on the leaves by damaging chloroplasts.

Explanation: TMV is a plant virus specified in Topic 1(b) that prevents chloroplast formation, leading to leaf discolouration.

(b)(ii) Answer: Any three of: 1. Viruses are smaller. 2. Viruses have a protein coat; bacteria have a cell wall. 3. Viruses lack cytoplasm and organelles; bacteria have them. 4. Viruses have either DNA or RNA; bacteria have both. 5. Viruses cannot reproduce independently; bacteria can.

Explanation: These structural and functional differences are covered in Topic 1(b), distinguishing viruses (non-living pathogens) from bacteria (living prokaryotes).

Question

Tobacco mosaic virus infects plant cells.

The photograph shows some of the virus particles.

(a) (i) Tobacco mosaic virus particles consist of a molecule of RNA surrounded by a coat. Which substance is the coat made from?

A. cellulose
B. chitin
C. protein
D. starch

(a) (ii) The virus particle labelled P has an actual length of 0.3 μm. Calculate the magnification of this virus particle. [1 mm = 1000 μm]

(b) The photograph shows the leaves of a plant infected by tobacco mosaic virus.

Plants cells infected with the virus stop making chloroplasts. Explain why plants that are infected with the virus grow more slowly than uninfected plants.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(i)
• 2(b): Cell structure — part (b)
• 2(e): Nutrition — part (b)
• Appendix 4: Mathematical skills — part (a)(ii)

▶️ Answer/Explanation

Solution

(a)(i) C (protein)

Explanation: The outer coat or capsid of a virus is primarily made up of protein subunits called capsomeres. This protein coat protects the viral genetic material inside (which, in this case, is RNA). Cellulose (A) is a polysaccharide that makes up plant cell walls. Chitin (B) is a polysaccharide found in the exoskeletons of insects and fungal cell walls. Starch (D) is a carbohydrate storage molecule in plants. Viruses do not contain these substances in their structure.

(a)(ii) Magnification = × 76,700 (or approximately × 77,000)

Explanation: To calculate magnification, we use the formula:

Magnification = Size of Image ÷ Size of Real Object

First, measure the length of the virus particle in the photograph (the image size). Assuming the measured length is 23 mm (this value can vary slightly between 22–24 mm).

Convert to micrometers (μm): 23 mm = 23,000 μm.

Actual size = 0.3 μm.

Magnification = 23,000 μm ÷ 0.3 μm ≈ 76,667.

Therefore, magnification ≈ × 76,700 (or × 77,000 rounded).

(b) Infected plants grow more slowly due to a cascade of effects caused by the virus halting chloroplast production. Chloroplasts are the organelles where photosynthesis occurs. Photosynthesis is the process that uses light energy to convert carbon dioxide and water into glucose, providing the plant with chemical energy and the building blocks for growth.

With fewer or no chloroplasts, the plant cannot perform photosynthesis effectively. This leads to a severe reduction in glucose production. Glucose is crucial for respiration (releasing energy for cellular processes) and for synthesizing starch and cellulose. A deficiency in energy and structural materials directly impedes growth.

Question

Yeast can be used in food production.

(a) (i) Which group of organisms does yeast belong to?

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

(ii) Which substance is the cell wall of a yeast cell made of?

A. cellulose
B. chitin
C. sucrose
D. starch

(b) A teacher sets up an experiment to investigate the effect of temperature on the rate of respiration in yeast. The diagram shows part of the apparatus the teacher uses.

(i) Explain what additional apparatus the teacher will need to investigate the effect of temperature on the rate of respiration in this experiment.

(ii) State the purpose of the liquid paraffin on the surface of the glucose solution.

(iii) Give the name of a suitable chemical that could be used as indicator X to show that the yeast is respiring.

(iv) Diazine green changes colour from blue to pink as the oxygen levels in the yeast and glucose solution reduce. Explain how this colour change gives information about the respiration in the yeast.

(c) Explain why the rate of respiration in the yeast will change as the temperature is increased.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(i)
• 2(b): Cell structure — part (a)(ii)
• 5(a): Food production (Micro-organisms) — part (a)(i), (a)(ii), (b) context
• 2(f): Respiration — parts (b)(ii), (b)(iii), (b)(iv), (c)
• 2(j): Co-ordination and response (Homeostasis) — part (c) linked to enzyme temperature effects
• 5.6 practical: Investigate the role of anaerobic respiration by yeast — part (b) experimental context

▶️ Answer/Explanation

Solution

(a)(i) B (fungi)
Explanation: Yeast is a single-celled organism that belongs to the kingdom Fungi. Unlike bacteria (prokaryotes), yeast are eukaryotic cells. They are not plants as they do not perform photosynthesis and lack chloroplasts, and they are not protoctists, which is a kingdom typically containing diverse, often unicellular, eukaryotes that don’t fit into the other kingdoms.

(a)(ii) B (chitin)
Explanation: The cell wall of a yeast cell is primarily made of a complex carbohydrate called chitin, which is also found in the exoskeletons of insects and the cell walls of other fungi. This is different from plant cell walls, which are made of cellulose (A). Sucrose (C) is a sugar and starch (D) is a storage polysaccharide; neither are structural components of cell walls.

(b)(i) The teacher would need additional apparatus to control and measure temperature. This includes a water bath (or a beaker of water heated by a Bunsen burner) to contain the test tube and allow for precise temperature variation. A thermometer is essential to accurately measure the temperature of the yeast and glucose solution. A stopwatch or timer is also needed to measure the rate of the reaction, for example, by timing how long it takes for a colour change to occur or measuring the rate of gas production if that was being monitored.

(b)(ii) The layer of liquid paraffin acts as a seal on the surface of the glucose solution. Its purpose is to prevent oxygen from the air dissolving into the solution. This creates anaerobic conditions, ensuring that any respiration detected is anaerobic respiration (fermentation) rather than aerobic respiration. This is crucial for investigating this specific type of respiratory pathway.

(b)(iii) A suitable chemical indicator would be limewater (calcium hydroxide solution). Alternatively, hydrogencarbonate indicator could be used.
Explanation: Limewater is used to test for the product carbon dioxide (CO₂) of respiration. CO₂ turns limewater cloudy. Hydrogencarbonate indicator changes colour (from red to yellow) in the presence of increased carbon dioxide levels.

(b)(iv) Diazine green is an indicator that is blue when oxygen is present and pink when it is absent (reduced). The initial blue colour indicates there is oxygen dissolved in the solution, and the yeast is likely respiring aerobically. As the yeast cells respire, they use up the oxygen in the solution for aerobic respiration. The change in colour from blue to pink indicates that the oxygen concentration is decreasing. Once the oxygen is depleted and the solution turns pink, it shows that the yeast has switched to anaerobic respiration (fermentation) to continue producing energy.

(c) The rate of respiration in yeast changes with temperature due to the effect on enzymes, which control the metabolic reactions of respiration.
As temperature increases from a low value, the kinetic energy of the enzyme and substrate molecules increases. This causes them to move faster and collide more frequently. More collisions lead to the formation of more enzyme-substrate complexes, which increases the rate of respiration.
This increase continues up to an optimum temperature, where the respiration rate is at its maximum because enzymes are working most efficiently.
Beyond this optimum temperature, the high energy causes the hydrogen and ionic bonds that hold the enzyme’s shape to break. This alters the specific 3D shape of the enzyme’s active site. The substrate can no longer fit into the denatured active site, meaning fewer enzyme-substrate complexes can form. This causes the rate of respiration to decrease rapidly and eventually stop.

Question

The diagram shows a yeast cell.

(a) (i) In which of the labelled structures in the yeast cell does aerobic respiration occur?

A. mitochondrion
B. nucleus
C. ribosome
D. vacuole

(ii) What substance is the yeast cell wall made of?

A. cellulose
B. chitin
C. glycogen
D. starch

(iii) Which of these structures found in the yeast cell would also be present in a prokaryotic cell?

A. cell membrane
B. mitochondrion
C. nucleus
D. vacuole

(b) Yeast cells are often used when making bread.
A student uses this method to investigate the effect of temperature on the height that bread dough rises.
• place dough containing flour, sugar, water and yeast into a measuring cylinder
• measure the height of the dough
• place the measuring cylinder in a 25°C water bath
• measure the height of the dough after two hours
• repeat the experiment at temperatures of 35°C and 65°C
The diagram shows the student’s apparatus.

The table shows the student’s results.

(b) (i) Calculate the percentage increase in the height of the dough after two hours at a temperature of 35°C.

(ii) Explain why yeast causes the bread dough to increase in height.

(iii) Explain why the dough rises to a different height at 25 °C compared with the height at 35 °C.

(iv) Explain why the dough rises to a different height at 35 °C compared with the height at 65 °C.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(ii)
• 2(b): Cell structure — parts (a)(i), (a)(iii)
• 2(f): Respiration — part (a)(i), (b)(ii)
• 2.10–2.14B: Enzymes and metabolism — parts (b)(iii), (b)(iv)
• 5(a): Food production (Micro-organisms) — part (b)(ii)
• Appendix 4: Mathematical skills — part (b)(i)

▶️ Answer/Explanation

Solution

(a)(i) A mitochondrion

Explanation: Aerobic respiration is the process that releases energy from glucose in the presence of oxygen. In eukaryotic cells, like yeast cells, this process occurs specifically in the mitochondria. The mitochondria are often called the “powerhouses” of the cell because they generate most of the cell’s supply of ATP, the energy currency. The nucleus (B) contains genetic material, the ribosome (C) is the site of protein synthesis, and the vacuole (D) is used for storage; none of these are the primary site for aerobic respiration.

(a)(ii) B chitin

Explanation: The cell wall of fungi, which includes yeast, is primarily made of a complex carbohydrate called chitin. Chitin is a strong and flexible polysaccharide that also forms the exoskeletons of insects. Cellulose (A) is the material that makes up plant cell walls. Glycogen (C) is a polysaccharide used for energy storage in animals and fungi, and starch (D) is used for energy storage in plants.

(a)(iii) A cell membrane

Explanation: Prokaryotic cells, such as bacteria, lack membrane-bound organelles. Therefore, they do not have a true nucleus (C), mitochondria (B), or vacuoles (D) like eukaryotic cells do. However, all cells, both prokaryotic and eukaryotic, are surrounded by a cell membrane (also called the plasma membrane). This phospholipid bilayer controls the movement of substances in and out of the cell and is a fundamental structure for life.

(b)(i) 80%

Explanation: The percentage increase is calculated using the formula:
\[ \text{Percentage Increase} = \left( \frac{\text{Final Value} – \text{Initial Value}}{\text{Initial Value}} \right) \times 100\]
Plugging in the values from the experiment at 35°C:
\[ \left( \frac{45 – 25}{25} \right) \times 100 = \left( \frac{20}{25} \right) \times 100 = 0.8 \times 100 = 80\%\]
So, the dough increased in height by 80% after two hours at 35°C.

(b)(ii) Yeast respires anaerobically (a process called fermentation) using the sugar in the dough as a substrate. This respiration produces carbon dioxide (\(CO_2\)) gas. The gas bubbles become trapped within the stretchy, elastic network of gluten proteins in the dough. As more and more gas is produced, these bubbles expand, causing the entire dough mixture to inflate and rise in height.

(b)(iii) Temperature affects the rate of enzyme-controlled reactions, such as respiration in yeast. At 25°C, the temperature is further from the optimum temperature for the yeast’s enzymes. The yeast cells and their enzymes have less kinetic energy, leading to fewer successful collisions between enzymes and substrates. This results in a slower rate of respiration, less carbon dioxide gas being produced, and consequently, a smaller rise in the dough (40% increase) compared to the rise at 35°C (80% increase).

(b)(iv) At 65°C, the temperature is likely too high and has denatured the enzymes in the yeast. Denaturation is a process where the enzyme’s active site changes shape permanently, and it can no longer bind to its substrate. With the enzymes denatured, respiration stops entirely or occurs at a very minimal rate. Very little carbon dioxide is produced, so the dough rises very little (only 8%). In contrast, at 35°C, the temperature is likely close to the optimum for the yeast’s enzymes, allowing for a high rate of respiration and gas production, leading to the greatest rise.

Question

Yeast cells can be genetically modified to produce the proteins found on the outside of viruses.

(a) (i) Yeast is a single-celled organism. A yeast cell has a nucleus, and the cell wall is made of chitin. Which group of organisms does yeast belong to?

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

(ii) Give a reason why viruses are not considered to be living organisms.

(b) A species of yeast is genetically modified to produce a protein found on the outside of a hepatitis B virus. This protein is used to make vaccines to prevent people being infected with hepatitis B. The gene for this protein is inserted into a plasmid. This plasmid is then used to modify the yeast cells. Give the roles of two named enzymes used to produce plasmids containing the gene for the hepatitis B protein.

(c) The diagram shows an industrial fermenter that can be used to grow large quantities of genetically modified yeast.

(i) Explain the function of the temperature monitor and cooling jacket.

(ii) Explain why air is needed in the fermenter.

(iii) Explain why the air is filtered before going into the fermenter.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(i)
• 1(b): Variety of living organisms — part (a)(ii)
• 5(c): Genetic modification (genetic engineering) — part (b)
• 5(a): Food production — parts (c)(i), (c)(ii), (c)(iii)

▶️ Answer/Explanation

Solution

(a)(i) B (fungi)

Explanation: Yeast is classified as a fungus because it is a single-celled organism with a nucleus (making it eukaryotic) and a cell wall made of chitin. Animals do not have cell walls, plants have cell walls made of cellulose, and protoctists do not typically have chitin in their cell walls.

(a)(ii) Viruses do not carry out all seven life processes independently / They cannot reproduce without a host cell.

Explanation: Living organisms are typically defined by characteristics such as growth, respiration, sensitivity, movement, excretion, nutrition, and reproduction. Viruses lack many of these; most notably, they cannot reproduce on their own and require a host cell to replicate. They also do not carry out metabolic processes like respiration.

(b) Restriction enzymes and Ligase.

Explanation: In genetic engineering, specific enzymes are used to insert a gene into a plasmid. Restriction enzymes are used to cut the DNA at specific sequences, both to remove the desired gene (e.g., for the hepatitis B protein) and to open up the plasmid. Ligase is then used to join the pieces of DNA together, effectively inserting the new gene into the plasmid by forming bonds between the sugar-phosphate backbones of the DNA strands.

(c)(i) The temperature monitor and cooling jacket work together to maintain an optimal temperature.

Explanation: The fermentation process, which involves yeast respiration and other chemical reactions, releases heat. If the temperature rises too high, it can denature the enzymes inside the yeast cells, slowing down or stopping the production of the desired protein. The temperature monitor detects the temperature inside the fermenter. If it gets too high, the cooling system is activated, and cool water is circulated through the cooling jacket, which absorbs the excess heat and carries it away, thus maintaining a constant, ideal temperature for the yeast to thrive.

(c)(ii) Air is needed to provide oxygen for aerobic respiration.

Explanation: Yeast, like many organisms, can respire aerobically (with oxygen) to produce energy. Aerobic respiration is much more efficient than anaerobic respiration, yielding more energy (ATP) per glucose molecule. This abundant energy allows the genetically modified yeast cells to grow rapidly and produce large quantities of the viral protein. Without oxygen, the yeast would switch to less efficient anaerobic respiration, resulting in slower growth and lower product yield.

(c)(iii) The air is filtered to prevent contamination.

Explanation: Unfiltered air contains bacteria, fungi (other than the desired yeast), and other microorganisms. If these contaminants entered the fermenter, they would compete with the genetically modified yeast for nutrients and space. Furthermore, they could potentially produce unwanted waste products or even consume the desired protein, drastically reducing the yield and purity of the vaccine product. The filter acts as a physical barrier, ensuring that only sterile air enters the fermenter, which is crucial for maintaining a pure and efficient culture.

Question

Biologists classify organisms into different groups. One group of organisms is fungi.

(a) Complete the passage about fungi by writing a suitable word or words in each blank space.

Fungi do not carry out photosynthesis. Their body is usually organised into a mycelium made from thread-like structures called ______.

Fungal cell walls are made of ______.

Fungi feed by extracellular secretion of ______ onto food material and absorption of the organic products. This is known as ______ nutrition.

(b) A student investigates the effect of temperature on the rate of anaerobic respiration in yeast.

The student measures the rate of gas produced in cm³ per minute.

The graph shows their results.

Graph of gas production rate vs temperature for yeast

(b)(i) Name the gas produced by yeast during anaerobic respiration.

(b)(ii) Explain the effect that increasing temperature has on the rate of gas production by the yeast.

(b)(iii) Describe how the student could measure the rate of gas production in this experiment.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)
• 2(f): Respiration — part (b)(i)
• 2(d): Movement of substances into and out of cells — part (b)(ii)
• 5(a): Use of biological resources — part (b)(iii)
• Appendix 6: Suggested practical investigations — part (b)(iii)

▶️ Answer/Explanation

Solution

(a) Fungi do not carry out photosynthesis. Their body is usually organised into a mycelium made from thread-like structures called hyphae.

Fungal cell walls are made of chitin.

Fungi feed by extracellular secretion of enzymes onto food material and absorption of the organic products. This is known as saprotrophic nutrition.

Explanation: Fungi are heterotrophic organisms that decompose organic matter. Their body structure consists of a network of hyphae that form a mycelium. The cell walls contain chitin, a strong polysaccharide also found in insect exoskeletons. Fungi digest food externally by secreting enzymes that break down complex molecules into simpler ones, which are then absorbed. This feeding method is called saprotrophic nutrition.

(b)(i) Carbon dioxide / CO₂

Explanation: During anaerobic respiration (fermentation), yeast converts glucose into ethanol and carbon dioxide. The gas bubbles produced are CO₂, which is what the student measures in this experiment.

(b)(ii) An explanation that makes reference to the following:

Increasing (kinetic) energy / molecules move more
More collisions / more enzyme-substrate complexes formed
Above 37/40°C / optimum / eventually / at higher temperature enzyme denatures / change in active site / substrate and enzyme can no longer bind

Explanation: As temperature increases from low to moderate levels, the rate of gas production increases because the reactant molecules (enzymes and substrates) have more kinetic energy. This causes them to move faster and collide more frequently, leading to more enzyme-substrate complexes and a higher reaction rate. However, beyond the optimum temperature (around 37-40°C for many enzymes), the enzyme structure begins to denature. The hydrogen bonds holding the enzyme’s shape break, causing the active site to change shape. When this happens, substrates can no longer fit into the active site, and the reaction rate decreases dramatically.

(b)(iii) A description that makes reference to two of the following:

Use (collect gas in) measuring cylinder / (gas) syringe
Measure volume / cm³ in time / or time to produce volume

Explanation: To measure the rate of gas production, the student should collect the gas produced in a gas syringe or by displacing water in an inverted measuring cylinder. The volume of gas collected should be measured at regular time intervals (e.g., every minute). The rate can then be calculated as volume of gas produced per unit time (e.g., cm³/min). This method allows for quantitative measurement of the respiration rate at different temperatures.

Question

Sickle cell anaemia is a condition in which some of the person’s red blood cells develop abnormally.

The diagram shows red blood cells from a healthy person and red blood cells from a person with sickle cell anaemia.

Red blood cells comparison

(a) A person with sickle cell anaemia often suffers pain as some of their blood vessels become blocked by the sickle cells.

(i) Suggest why the person’s blood vessels may become blocked.

(ii) People with sickle cell anaemia have symptoms of tiredness and joint pain that get worse if they are exposed to cold temperatures and high altitudes.
Suggest why these symptoms get worse.

(b) Sickle cell anaemia is caused by a single recessive allele.

(i) State what is meant by the term recessive.

(ii) A man and a woman who are both heterozygous for the sickle cell allele have a child.
Calculate the probability that the child will be female and not have sickle cell anaemia.

(c) Sickle cell anaemia is more common in countries where malaria is found. This is because having an allele for sickle cell anaemia can reduce the likelihood of developing malaria.

Which type of organism causes malaria?

A. bacterium
B. fungus
C. plant
D. protoctist

(d) What is the name of the pigment found in red blood cells?

A. chlorophyll
B. haemoglobin
C. iron
D. magnesium

(e) Give two differences in structure between red blood cells and white blood cells.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 2(h): Transport — parts (a)(i), (a)(ii), (d), (e)
• 3(b): Inheritance — parts (b)(i), (b)(ii)
• 1(b): Variety of living organisms — part (c)
• 2(b): Cell structure — part (e)

▶️ Answer/Explanation

Solution

(a)(i) Sickle shaped red blood cells stick to each other / caught / trapped in walls of blood vessels.

Explanation: In sickle cell anaemia, the red blood cells become rigid and crescent-shaped instead of being flexible and biconcave. These abnormally shaped cells are less flexible and can get stuck to each other or to the walls of small blood vessels. This creates blockages that restrict blood flow to various parts of the body, leading to pain and potential tissue damage.

(a)(ii) An explanation that includes:

Cold temperatures reduce blood flow / cause more sickling
Less oxygen at high altitude
Less respiration / more anaerobic respiration
More lactic acid
Less energy / ATP

Explanation: Cold temperatures cause blood vessels to constrict, reducing blood flow and potentially triggering more red blood cells to sickle. At high altitudes, the lower oxygen concentration means less oxygen is available to tissues. This reduced oxygen supply leads to less efficient aerobic respiration in cells, forcing them to rely more on anaerobic respiration which produces lactic acid. The combination of reduced energy production (ATP) from less efficient respiration and the buildup of lactic acid exacerbates symptoms like tiredness and joint pain.

(b)(i) Only expressed when homozygous / two copies / no dominant allele present / not expressed in heterozygote.

Explanation: A recessive allele is one whose effect is masked or “hidden” when a dominant allele is present. For a recessive trait to be physically expressed in an individual, that person must inherit two copies of the recessive allele (one from each parent), making them homozygous recessive. If they have one dominant and one recessive allele (heterozygous), the dominant allele will determine the trait, and the recessive allele will not be visible but can be passed to offspring.

(b)(ii) 0.375 or 3/8 or 37.5%

Explanation: First, we need to find the probability of the child not having sickle cell anaemia. Since both parents are heterozygous (HbA Hbs), we can use a Punnett square:

Parental genotypes: HbA Hbs × HbA Hbs

Possible offspring genotypes:

HbA HbA (normal) – 1/4 probability
HbA Hbs (carrier, no disease) – 2/4 probability
Hbs Hbs (sickle cell anaemia) – 1/4 probability

Probability of not having sickle cell anaemia = 1/4 + 2/4 = 3/4

Probability of being female = 1/2

Since these are independent events, we multiply the probabilities:

3/4 × 1/2 = 3/8 = 0.375 = 37.5%

(c) D protoctist

Explanation: Malaria is caused by parasites belonging to the genus Plasmodium, which are single-celled eukaryotic organisms classified as protoctists (or protists). These are not bacteria (which are prokaryotic), fungi, or plants. The Plasmodium parasite is transmitted to humans through the bite of infected Anopheles mosquitoes.

(d) B haemoglobin

Explanation: Haemoglobin is the iron-containing protein pigment found in red blood cells that gives them their characteristic red color. Its crucial function is to bind with oxygen in the lungs and transport it to tissues throughout the body, while also helping to carry some carbon dioxide back to the lungs. Chlorophyll is found in plants, iron is a mineral component of haemoglobin but not the pigment itself, and magnesium is not the primary pigment in red blood cells.

(e) Two differences:

Red blood cells are smaller than white blood cells
Red blood cells have no nucleus while white blood cells do
Red blood cells are biconcave in shape while white blood cells are spherical or irregular

Explanation: Red blood cells (erythrocytes) and white blood cells (leukocytes) have distinct structural differences that relate to their different functions. Red blood cells are smaller and lack a nucleus, which allows more space for haemoglobin and makes them more flexible for moving through narrow capillaries. Their biconcave disc shape provides a large surface area for efficient gas exchange. In contrast, white blood cells are generally larger, have a nucleus (essential for their immune functions), and can change shape to engulf pathogens or move through tissues.

Question

Plant and animal cells have some features in common and some differences.

(a) (i) Which of these structures is not found in animal cells?

A) cell membrane
B) cell wall
C) mitochondrion
D) nucleus

(a) (ii) Which of these substances is a carbohydrate stored in plant cells?

A) chlorophyll
B) glucose
C) glycogen
D) starch

(b) The diagram shows a leaf palisade mesophyll cell.

(i) Describe the function of the parts labelled A, B, C and D.

(ii) Explain two ways that the structure of this palisade mesophyll cell is adapted for its function.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(ii)
• 2(b): Cell structure — parts (a)(i), (b)(i), (b)(ii)
• 2(c): Biological molecules — part (a)(ii)
• 2(e): Nutrition (in plants) — part (b)(ii)

▶️ Answer/Explanation

Solution

(a)(i) B (cell wall)
A is incorrect because it is not the cell membrane.
C is incorrect because it is not mitochondria.
D is incorrect because it is not the nucleus.

(a)(ii) D (starch)
A is incorrect because it is not chlorophyll.
B is incorrect because it is not glucose.
C is incorrect because it is not glycogen.

(b)(i) An answer that makes reference to the following points:

A: (chloroplasts absorb light) for photosynthesis / absorb light energy to make carbohydrate / eq
B: (nucleus) controls protein synthesis / contains DNA / contains genes / controls cell / eq
C: (vacuole) contains cell sap eq
D: (cytoplasm) where chemical reactions occur

Additional guidance: Allow starch / glucose / sugar for A; allow maintains turgor / stores water / salts / pigments / toxins for C; allow where protein synthesis occurs / respiration occurs / medium for reactions for D.

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

contains chloroplasts to absorb light / for photosynthesis eq
long / arranged in a vertical plane / large surface area / rectangular shape, to absorb most light / eq
large vacuole to store water

Question

The bacterium H. pylori causes stomach ulcers.

(a) The diagram shows this bacterium.

(i) Which of these is found in this bacterium?

A. cellulose
B. chitin
C. cytoplasm
D. nucleus

(ii) The bacterium has evolved to release an enzyme called urease.
The action of the bacterium neutralises the acid in the stomach.
What is the pH changed to?

A. 1
B. 2
C. 7
D. 12

(iii) Use the theory of evolution by natural selection to explain how H. pylori bacteria could have evolved to produce urease.

(b) Probiotics are live microorganisms that can have health benefits when consumed.

Scientists investigate the ability of probiotics and cranberry juice to reduce the growth of H. pylori.

The scientists give various treatments to a group of people who have H. pylori.

The treatments are given daily for three weeks.

The scientists measure the mean percentage reduction of H. pylori for each treatment.

The table shows the scientists’ results.

Give two conclusions from these results.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(i)
• 2(c): Biological molecules (Enzymes) — part (a)(ii)
• 4(b): Inheritance (Theory of evolution by natural selection) — part (a)(iii)
• 6(a): Use of biological resources (Food production – Micro-organisms) — part (b)
• Practical Investigation: Data analysis and evaluation — part (b)

▶️ Answer/Explanation

Solution

(a)(i) C cytoplasm

Explanation: Bacteria are prokaryotic cells, which means they lack a true nucleus (option D) and membrane-bound organelles. Their genetic material is found in the cytoplasm. Cellulose (option A) is a structural component of plant cell walls, and chitin (option B) is found in the cell walls of fungi and the exoskeletons of arthropods. Bacterial cell walls are typically made of peptidoglycan, not cellulose or chitin. Therefore, the only component from the list that is definitively found in a bacterial cell is the cytoplasm.

(a)(ii) C 7

Explanation: The stomach is highly acidic, with a pH typically around 1.5 to 3.5 due to hydrochloric acid. Neutralization means bringing the pH to a neutral level. On the pH scale, 7 is neutral. Urease produced by H. pylori breaks down urea to produce ammonia, which is alkaline, thereby neutralizing the stomach acid in its immediate vicinity and raising the pH towards 7. Options A (1) and B (2) are acidic, and option D (12) is strongly alkaline, none of which represent a neutral pH.

(a)(iii)

Explanation: According to the theory of evolution by natural selection, the evolution of urease production in H. pylori can be explained step-by-step. Initially, within a population of ancestral H. pylori bacteria, there would have been genetic variation. Some of this variation arose from random mutations in the bacterial DNA. One such mutation might have resulted in a bacterium that could produce the enzyme urease. This urease enzyme conferred a significant advantage in the harsh, acidic environment of the stomach. By neutralizing the acid around it, this bacterium was more likely to survive and live longer than its peers that lacked the mutation. With increased survival, this bacterium had a greater chance to reproduce asexually (through binary fission) and pass on the gene for urease production to its offspring. Over many generations, the allele for urease production became more and more common in the population because individuals possessing it were consistently selected for by the environmental pressure of stomach acidity. Eventually, the entire population of H. pylori evolved to produce urease.

(b)

Two conclusions:

All three treatments (probiotics, cranberry juice, and their combination) cause a greater reduction in H. pylori compared to the control, meaning they are all more effective than no treatment.
The combination of probiotics and cranberry juice results in a greater reduction of H. pylori than either treatment used alone, suggesting a synergistic effect where the two work better together.

Explanation: By analyzing the data in the table, we can draw clear comparisons. The control group, which likely received a placebo or no treatment, showed only a 1.5% reduction, establishing a baseline. The probiotics alone led to a 14.9% reduction, and cranberry juice alone led to a 16.9% reduction. Both are substantially higher than the control, indicating they are effective treatments. However, when used together, the reduction jumps to 22.9%, which is higher than the sum of their individual effects if they were simply additive. This indicates that the two treatments may work well in combination, potentially enhancing each other’s effectiveness against H. pylori.

Question

Bread contains starch.

(a) Describe how you would test a piece of bread to show it contains starch.

(b) The diagram shows the mycelium of an organism growing on a piece of bread.

(i) Which type of organism is shown growing on the bread?

A. bacterium
B. fungus
C. protoctist
D. virus

(ii) Which enzyme is released by the organism to digest starch?

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

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (b)(i)
• 2(c): Biological molecules — parts (a), (b)(ii)
• 2(e): Nutrition (in humans and flowering plants) — part (b)(ii)
• 2.9: Practical investigation of food samples — part (a)

▶️ Answer/Explanation

Solution

(a) Add iodine solution to the bread sample. If starch is present, the color will change to blue-black.

Explanation: To test for starch, you would first obtain a small sample of the bread. Then, you would add a few drops of iodine solution directly onto the bread sample. Iodine solution is typically a brownish-yellow color. If starch is present in the bread, a chemical reaction occurs between the iodine and the starch molecules, resulting in a distinct color change to blue-black or sometimes a very dark blue-purple. This is a standard biochemical test for starch.

(b)(i) B fungus

Explanation: The diagram shows a mycelium, which is a characteristic feature of fungi. A mycelium is a network of fine, thread-like structures called hyphae that fungi use to absorb nutrients from their food source, in this case, the bread. Bacteria are single-celled and do not form mycelia. Protoctists are a diverse group, but none form a mycelium like this. Viruses are not considered living organisms and cannot grow on bread in this way.

(b)(ii) A amylase

Explanation: Starch is a large, complex carbohydrate polymer. To digest it, the fungus secretes the enzyme amylase. Amylase works by breaking down the chemical bonds in starch, converting it into smaller sugar molecules like maltose, which the fungus can then absorb and use for energy. Ligase is an enzyme involved in joining DNA fragments, not digesting food. Lipase breaks down lipids (fats), and protease breaks down proteins, neither of which is the correct enzyme for starch digestion.

Question

The diagram shows a yeast cell.

(a) (i) Which row of the table is correct for this yeast cell?

(ii) Which type of organism is a yeast cell?

A. a bacterium
B. a fungus
C. a plant
D. a protoctist

(b) Biofuel is made from ethanol.

Scientists use genetically modified (GM) yeast to produce biofuel.

The GM yeast contains an enzyme that digests plant cell walls to produce glucose.

The yeast uses the glucose in respiration to produce ethanol.

(i) Which of these equations shows the respiration in the yeast?

A. glucose → ethanol
B. glucose → ethanol + carbon dioxide
C. glucose + oxygen → ethanol
D. glucose + oxygen → ethanol + carbon dioxide

(ii) Name an enzyme used by scientists to genetically modify the yeast.

(iii) The GM yeast is a recombinant strain.

State what is meant by the term recombinant.

(iv) Suggest why biofuel produced using glucose from plants could reduce global warming.

(c) The graph shows the mass of ethanol produced by GM yeast and by normal yeast over a period of 3 days.

(i) Calculate the percentage increase in the mass of ethanol produced by GM yeast compared to normal yeast after 1 day.

(ii) Give two reasons why the rate of ethanol production decreases after 1 day.

Most-appropriate topic codes (Edexcel IGCSE Biology):

• 1(b): Variety of living organisms — part (a)(i), (a)(ii)
• 5(a): Food production — part (b)
• 2(f): Respiration — part (b)(i)
• 5(c): Genetic modification (genetic engineering) — parts (b)(ii), (b)(iii)
• 4(d): Human influences on the environment — part (b)(iv)
• 4: Mathematical skills (Arithmetic and numerical computation): — part (c)(i)
• 2(f): Respiration — part (c)(ii)

▶️ Answer/Explanation

Solution

(a)(i) C (chitin and glycogen)

Explanation: Yeast is a fungus, and fungal cell walls are primarily composed of chitin, not cellulose which is found in plants. For energy storage, fungi like yeast store glycogen in their cytoplasm, similar to animals, rather than starch which is typical of plants.

(a)(ii) B (a fungus)

Explanation: Yeast is classified as a unicellular fungus. It is not a bacterium (which are prokaryotic), a plant (which are multicellular and photosynthetic), or a protoctist (which is a diverse group including algae and protozoa).

(b)(i) B (glucose → ethanol + carbon dioxide)

Explanation: The respiration described is anaerobic respiration (fermentation) in yeast. The correct chemical equation for this process is glucose being broken down into ethanol and carbon dioxide, without the use of oxygen. Option A is incomplete, and options C and D involve oxygen, which characterizes aerobic respiration.

(b)(ii) Restriction enzyme / endonuclease / ligase

Explanation: To genetically modify the yeast, scientists use enzymes like restriction endonucleases (which cut DNA at specific sequences) and DNA ligase (which joins DNA fragments together). These enzymes are essential tools in genetic engineering for inserting new genes into an organism’s genome.

(b)(iii) Contains new / foreign DNA / gene from another organism.

Explanation: A recombinant organism is one that has had its genetic material altered by the insertion of DNA from a different source. In this case, the GM yeast has been modified to contain a gene from another organism that codes for an enzyme capable of digesting plant cell walls.

(b)(iv)

Explanation: Biofuel from plants can help reduce global warming primarily because it is part of a carbon-neutral cycle. The carbon dioxide released when the biofuel is burned is approximately equal to the carbon dioxide that the plants absorbed from the atmosphere during photosynthesis. This contrasts with burning fossil fuels, which releases carbon that has been locked away for millions of years, thereby increasing the net concentration of CO₂ in the atmosphere. Additionally, using biofuels can reduce our dependence on fossil fuels, leading to lower overall emissions of greenhouse gases.

(c)(i) 76% (accept range 69-80%)

Explanation: To calculate the percentage increase, we use the formula: \[ \text{Percentage Increase} = \frac{\text{Mass from GM yeast} – \text{Mass from normal yeast}}{\text{Mass from normal yeast}} \times 100\% \] From the graph, after 1 day, the mass from GM yeast is approximately 2.2 g and from normal yeast is approximately 1.25 g. \[ \text{Increase} = 2.2 – 1.25 = 0.95 \text{ g} \] \[ \text{Percentage Increase} = \frac{0.95}{1.25} \times 100\% = 76\% \] Slight variations in reading the graph values can lead to answers between 69% and 80% being accepted.

(c)(ii)

Explanation: The rate of ethanol production decreases after 1 day likely due to two main factors. First, the glucose (substrate) that the yeast uses for respiration begins to run out or become depleted, slowing down the metabolic process. Second, the ethanol produced is actually toxic to the yeast in high concentrations. As ethanol builds up in the surrounding environment, it can inhibit the yeast’s enzymes and eventually kill the cells, leading to a decline in the production rate.

Edexcel iGCSE Biology 4BI1 - Paper 1B -Variety of living organisms- Exam Style Questions- New Syllabus

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