Year 11 Paper 4: Defence, Genetics and Ecology
Covers cell structure and transport, digestion and the circulatory system, the body's defence systems and how disease is treated, genetics and inheritance, evolution and natural selection, and ecosystems and material cycles.
Year 11 here means a typical teaching order, not a syllabus rule. No exam board defines what belongs to Year 11, and schools sequence the course differently. Check it against your own scheme of work before using it to decide what a class has covered.
Questions
Question 1 [4 marks]
Cell Structure and Transport
A student views a sample of onion cells under a microscope.
The diameter of the field of view is 0.6 mm, and exactly 15 onion cells fit side by side across this diameter.
Calculate the mean width of one onion cell, in micrometres.
Question 2 [5 marks]
Evolution and Natural Selection
A population of cheetahs went through a period, thousands of years ago, when only a very small number of individuals survived (a genetic bottleneck), before the population later grew large again.
Explain why modern cheetah populations are thought to have unusually low genetic variation, and explain why this low genetic variation puts the species at greater risk today.
Question 3 [5 marks]
Ecosystems and Material Cycles
A student used a transect line across a rocky shore and placed a 0.5 m^2 quadrat every 2 metres along the line. In one quadrat, seaweed covered an estimated 60% of the quadrat's area.
Calculate the area, in m^2, of the quadrat that was covered by seaweed. If the whole study area along the shore covers 400 m^2 and this percentage cover is representative of the whole area, estimate the total area of the shore covered by seaweed.
Question 4 [5 marks]
Digestion and the Circulatory System
In the investigation above, the student then tested amylase at 60 deg C and found the reaction did not occur.
Explain, in terms of enzyme structure, why amylase does not break down starch at 60 deg C.
Question 5 [5 marks]
Human Defence Systems and Treating Disease
Herd immunity against a disease is achieved once 80% of a population is immune, making it very unlikely an unvaccinated person will come into contact with the pathogen.
In a town of 50,000 people, 32,000 are currently immune to the disease (through vaccination or previous infection). Calculate the percentage of the town's population that is currently immune, and calculate how many more people would need to become immune to reach the 80% herd immunity threshold.
Question 6 [6 marks]
Genetics and Inheritance
Human height is an example of continuous variation, showing a wide range of values influenced by both genes and environment. Human blood group (A, B, AB or O) is an example of discontinuous variation, controlled entirely by genes, with a small number of distinct categories.
Explain the difference between continuous and discontinuous variation, and explain why environmental factors can affect a characteristic controlled by continuous variation (such as height) but generally do not affect a characteristic controlled by discontinuous variation (such as blood group).
Question 7 [6 marks]
Ecosystems and Material Cycles
In a food chain, grass contains 50000 kJ of energy stored as biomass. Only 10% of this energy is transferred to the rabbits that eat the grass, and only 10% of the energy in the rabbits is transferred to the foxes that eat the rabbits.
Calculate the energy stored as biomass in the rabbits, and the energy stored as biomass in the foxes. Then explain why food chains rarely have more than four or five trophic levels.
Question 8 [6 marks]
Cell Structure and Transport
Exchange surfaces in living organisms, such as the alveoli in the lungs or gill filaments in fish, are adapted to maximise the rate of diffusion of substances across them.
Describe five features that a good exchange surface needs to maximise the rate of diffusion, explaining why each feature increases the rate.
Question 9 [6 marks]
Human Defence Systems and Treating Disease
Before the results of a new drug trial are accepted by the scientific and medical community, they are usually published in a peer-reviewed journal.
Explain what is meant by peer review, and explain why peer review is important before a new medical treatment is more widely accepted or used.
Question 10 [6 marks]
Ecosystems and Material Cycles
In a food chain of producer -> primary consumer -> secondary consumer -> tertiary consumer, only 10% of energy is transferred between each trophic level. A population of tertiary consumers (eagles) contains 80 kJ of energy stored as biomass.
Calculate the energy that must have been available at the secondary consumer, primary consumer and producer levels to support this, and explain why an ecosystem can typically support far fewer tertiary consumers than producers.
Question 11 [6 marks]
Genetics and Inheritance
A family tree shows a genetic disorder caused by a recessive allele. Two unaffected parents have one child with the disorder and one child without it.
A geneticist wants to confirm the disorder is recessive rather than dominant, and wants to determine both parents' genotypes.
Explain how the family tree evidence shows the allele must be recessive, and state the parents' genotypes using F for the dominant allele and f for the recessive allele.
Model solutions
| Question 1[4 marks] | |
|---|---|
| Answer or working | Marks |
| converting the field of view diameter to micrometres: 0.6 mm = 600 micrometres | M1 |
| using mean cell width = field of view diameter / number of cells | M1 |
| substituting 600 / 15 | M1 |
| 40 micrometres | A1 |
| Question 2[5 marks] | |
|---|---|
| Answer or working | Marks |
| when the population was reduced to a very small number of surviving individuals, only the alleles carried by those few individuals remained in the population | B1 |
| many alleles that had existed in the larger, earlier population were lost, because the individuals carrying them did not survive | B1 |
| as the population grew again from this small number of survivors, all of the new individuals shared alleles with this small starting group, so overall genetic variation remained low | B1 |
| low genetic variation means the population is less likely to already contain an allele giving resistance or an advantage against a new threat, such as a disease | B1 |
| this makes it more likely that a new disease or environmental change could affect a very large proportion of the population at once, increasing the risk of extinction | B1 |
| Final answer: A past bottleneck left only a few surviving individuals, losing most of the population's alleles; because today's cheetahs descend from this small group, genetic variation remains low, making it less likely the population already has an allele that could help it survive a new disease or environmental change, increasing extinction risk | |
| Question 3[5 marks] | |
|---|---|
| Answer or working | Marks |
| area covered in the quadrat = 60% of 0.5 m^2 | M1 |
| 0.3 m^2 | A1 |
| using the same percentage (60%) applied to the total area | M1 |
| substituting 60% of 400 m^2 | M1 |
| 240 m^2 | A1 |
| Final answer: 0.3 m^2 of the quadrat was covered by seaweed; an estimated 240 m^2 of the total 400 m^2 shore is covered by seaweed | |
| Question 4[5 marks] | |
|---|---|
| Answer or working | Marks |
| enzymes are proteins with a specific 3-D shape | B1 |
| the active site has a shape complementary to the substrate (starch) | B1 |
| at high temperatures the enzyme gains kinetic energy and vibrates more | B1 |
| this breaks the bonds holding the enzyme's shape, changing the shape of the active site | B1 |
| the substrate can no longer bind (fit) the active site, so the enzyme is denatured and cannot catalyse the reaction | B1 |
| Final answer: At 60 deg C the enzyme's active site changes shape (denatures) because heat breaks the bonds maintaining its structure, so the substrate no longer fits and the reaction cannot be catalysed | |
| Question 5[5 marks] | |
|---|---|
| Answer or working | Marks |
| using percentage immune = (32000/50000) x 100 | M1 |
| 64% | A1 |
| finding the number needed for herd immunity = 80% of 50000 | M1 |
| finding the further number needed = 40000 - 32000 | M1 |
| 8000 more people | A1 |
| Final answer: 64% of the town's population is currently immune; a further 8000 people would need to become immune to reach the 80% herd immunity threshold | |
| Question 6[6 marks] | |
|---|---|
| Answer or working | Marks |
| continuous variation showing a wide range of values between two extremes, with no distinct categories | B1 |
| discontinuous variation falling into a small number of distinct categories, with no intermediate values | B1 |
| continuous variation usually being controlled by a combination of many genes (polygenic) as well as the environment | B1 |
| discontinuous variation usually being controlled by a single gene, with limited or no environmental influence | B1 |
| height being influenced by many genes plus factors such as diet and exercise, so environmental factors can shift a person's height within the range set by their genes | B1 |
| blood group being determined entirely by which alleles a person inherits, so no environmental factor can change which blood group alleles are present | B1 |
| Final answer: Continuous variation (e.g. height) shows a range of values controlled by many genes plus environment, so environmental factors can affect it; discontinuous variation (e.g. blood group) falls into distinct categories controlled by a single gene, so it is unaffected by the environment | |
| Question 7[6 marks] | |
|---|---|
| Answer or working | Marks |
| rabbits' energy = 10% of 50000 | M1 |
| 5000 kJ | A1 |
| foxes' energy = 10% of 5000 | M1 |
| 500 kJ | A1 |
| so much energy being lost at each trophic level, as heat, in waste/egested material, and in movement and other life processes, that only a small percentage is passed on | B1 |
| after several trophic levels, so little energy remaining that there is not enough biomass/energy available to support a further trophic level | B1 |
| Final answer: Rabbits store 5000 kJ and foxes store 500 kJ; food chains rarely exceed four or five trophic levels because so much energy is lost at each level that too little remains to support further levels | |
| Question 8[6 marks] | |
|---|---|
| Answer or working | Marks |
| a large surface area, which provides more space for molecules to diffuse across at any one time | B1 |
| a thin membrane (short diffusion distance), which reduces the distance molecules must travel, increasing the rate | B1 |
| a good blood supply or ventilation, which maintains a steep concentration gradient by removing or supplying substances | B1 |
| maintaining a steep concentration gradient increases the rate of diffusion | B1 |
| a moist surface (in gas exchange organs), which allows gases to dissolve so they can diffuse across the membrane | B1 |
| a permeable surface, allowing the specific molecules being exchanged to pass through | B1 |
| Final answer: Good exchange surfaces have a large surface area, a short diffusion distance (thin walls), a good blood supply or ventilation to maintain a steep concentration gradient, and (in gas exchange) a moist, permeable surface | |
| Question 9[6 marks] | |
|---|---|
| Answer or working | Marks |
| peer review is the process by which other scientists who are independent of the original research, and who are experts in the same field, check a scientific report before it is published | B1 |
| the reviewers check the methods used are valid/appropriate and check the conclusions are supported by the data/results presented | B1 |
| without peer review, flawed, biased or fraudulent research could be published and accepted as reliable | B1 |
| this could lead to a new treatment being used that is actually unsafe or ineffective | B1 |
| peer review helps identify errors, or gaps that need further investigation, before other scientists, doctors or the public rely on the findings | B1 |
| it therefore increases confidence that published, accepted research (and its conclusions about a treatment) can be trusted | B1 |
| Final answer: Peer review is independent experts checking a study's methods and conclusions before publication; it helps catch errors, bias or unsupported claims, so the medical community and public can trust that an accepted new treatment has been properly scrutinised | |
| Question 10[6 marks] | |
|---|---|
| Answer or working | Marks |
| using previous level's energy = current level's energy / 10% | M1 |
| secondary consumer = 80 / 0.1 | M1 |
| 800 kJ | A1 |
| primary consumer = 800 / 0.1 = 8000 kJ | M1 |
| producer = 8000 / 0.1 = 80,000 kJ | A1 |
| because only about 10% of energy is transferred at each level, an enormous amount of energy is needed at the producer level to support even a small amount of biomass at the top of the food chain, so an ecosystem can support far more producers than tertiary consumers | B1 |
| Final answer: 800 kJ would be needed at the secondary consumer level, 8000 kJ at the primary consumer level, and 80,000 kJ at the producer level; because only about 10% of energy passes on at each level, a huge amount of producer biomass is needed to support even a little biomass of top consumers | |
| Question 11[6 marks] | |
|---|---|
| Answer or working | Marks |
| both parents are unaffected but have a child with the disorder | B1 |
| this means the allele causing the disorder cannot be dominant, because a dominant allele would be expressed and at least one parent would show the disorder | B1 |
| the allele must therefore be recessive, only causing the disorder when two copies are present | B1 |
| since neither parent shows the disorder but one child does, both parents must carry one copy of the recessive allele | B1 |
| both parents have the genotype Ff | A1 |
| the affected child has the genotype ff, and the unaffected child has genotype FF or Ff | B1 |
| Final answer: The allele is recessive because two unaffected parents produced an affected child; both parents must be carriers with genotype Ff | |