Biology (9-1) 0970 | Paper 4 Mock 01 | Theory (Extended)
Question 1 Report
| Day | Number of bacteria killed by the standard dose |
|---|---|
| 1 | 980 |
| 2 | 910 |
| 3 | 640 |
| 4 | 320 |
| 5 | 90 |
Bacteria can reproduce very rapidly, and a large population may contain a few individuals that differ from the rest because of random changes in their genes. When an antibiotic is used to treat an infection, these differences can affect whether a bacterium survives. A scientist grew a population of bacteria and treated it with the same dose of an antibiotic every day for five days, counting the number of bacteria killed by the standard dose each day. The results of this investigation are shown in Table 10.1.
(a) Using the data in Table 10.1, describe how the number of bacteria killed by the antibiotic changed over the five days. [2]
(b) State the name of the process, first described by Darwin, by which the resistant bacteria became more common in the population. [1]
(c) Explain, in terms of variation and survival, why the population of bacteria became resistant to the antibiotic. [4]
(d) Suggest one way in which the spread of antibiotic-resistant bacteria between human patients could be reduced. [2]
(e) State the source of the original variation that allowed a few bacteria to survive the antibiotic. [2]
This question tests natural selection using antibiotic-resistance data: variation exists, the antibiotic selects the resistant survivors, and they pass on the resistance.
(a) Using the data, the number of bacteria killed fell each day [1], from 980 on day 1 to 90 on day 5 (a large decrease) [1]. Fewer are killed because more of the survivors are resistant.
(b) The process, first described by Darwin, is natural selection [1].
(c) Why the population became resistant, in terms of variation and survival: there is variation among the bacteria [1]; a few have an allele that gives resistance to the antibiotic [1]; these resistant bacteria survive and reproduce while the non-resistant ones are killed [1]; and the resistance allele is passed on to the offspring, so resistant bacteria become more common in the population [1].
(d) One way to reduce the spread of resistant bacteria between patients (clear points, max 2): doctors prescribe antibiotics only when they are really needed; patients always finish the full course; or good hygiene and isolation of infected patients [2].
(e) The source of the original variation was mutation [1]: a random change in the genes / DNA of a bacterium [1].
Answer Details
This question tests natural selection using antibiotic-resistance data: variation exists, the antibiotic selects the resistant survivors, and they pass on the resistance.
(a) Using the data, the number of bacteria killed fell each day [1], from 980 on day 1 to 90 on day 5 (a large decrease) [1]. Fewer are killed because more of the survivors are resistant.
(b) The process, first described by Darwin, is natural selection [1].
(c) Why the population became resistant, in terms of variation and survival: there is variation among the bacteria [1]; a few have an allele that gives resistance to the antibiotic [1]; these resistant bacteria survive and reproduce while the non-resistant ones are killed [1]; and the resistance allele is passed on to the offspring, so resistant bacteria become more common in the population [1].
(d) One way to reduce the spread of resistant bacteria between patients (clear points, max 2): doctors prescribe antibiotics only when they are really needed; patients always finish the full course; or good hygiene and isolation of infected patients [2].
(e) The source of the original variation was mutation [1]: a random change in the genes / DNA of a bacterium [1].
Question 2 Report
In one kind of cattle, coat colour is set by a single gene. The gene has two alleles. The allele for a black coat is dominant. We show it as B. The allele for a red coat is recessive. We show it as b. A farmer kept a herd of cattle. The farmer wanted many black calves. Black cattle sold for more money. The farmer crossed a black bull with red cows. Some calves were black. Some calves were red. This surprised the farmer. The farmer thought a black bull would give only black calves. A student used a genetic diagram to explain the result.
(a) State what is meant by the term recessive allele. [1]
(b) State the genotype of a red cow. [1]
(c) The black bull produced both black and red calves when crossed with red cows. State the genotype of the black bull and explain your answer. [2]
(d) Draw a genetic diagram to show this cross and the expected ratio of black to red calves. [4]
(e) Explain why the actual numbers of black and red calves may not be exactly in the expected ratio. [2]
(a) A recessive allele is one whose characteristic is only shown in the phenotype when two copies are present (i.e. when no dominant allele is present) [1]. Here a red coat only appears in a bb animal.
(b) Red is recessive, so a red cow must carry two red alleles: its genotype is bb [1].
(c) The black bull must be Bb (heterozygous) [1]. This is because when he was crossed with red (bb) cows some calves were red (bb); a red calf can only inherit a b allele from each parent, so the bull must carry a b allele as well as the B allele that makes him black [1]. A pure-breeding BB bull crossed with bb cows would give only black (Bb) calves.
(d) The genetic diagram for the cross Bb (black bull) x bb (red cow) is set out below [parents 1; gametes 1; offspring genotypes 1; ratio 1]:
| Black bull | Red cow | |
|---|---|---|
| Parent phenotypes | black | red |
| Parent genotypes | Bb | bb |
| Gametes | B or b | b or b |
Punnett square (bull's gametes across the top, cow's gametes down the side):
| B | b | |
|---|---|---|
| b | Bb (black) | bb (red) |
| b | Bb (black) | bb (red) |
Offspring genotypes are Bb, Bb, bb, bb, giving an expected ratio of 1 black : 1 red \(( = 2\ \text{Bb} : 2\ \text{bb})\) [ratio 1].
(e) The 1:1 ratio is only a prediction of probabilities. Fertilisation is random: which sperm (carrying B or b) fuses with each egg is a matter of chance [1]. So with a real, especially small, number of calves the actual numbers vary from the predicted ratio; the ratio only approaches 1:1 when very many offspring are produced [1].
Answer Details
(a) A recessive allele is one whose characteristic is only shown in the phenotype when two copies are present (i.e. when no dominant allele is present) [1]. Here a red coat only appears in a bb animal.
(b) Red is recessive, so a red cow must carry two red alleles: its genotype is bb [1].
(c) The black bull must be Bb (heterozygous) [1]. This is because when he was crossed with red (bb) cows some calves were red (bb); a red calf can only inherit a b allele from each parent, so the bull must carry a b allele as well as the B allele that makes him black [1]. A pure-breeding BB bull crossed with bb cows would give only black (Bb) calves.
(d) The genetic diagram for the cross Bb (black bull) x bb (red cow) is set out below [parents 1; gametes 1; offspring genotypes 1; ratio 1]:
| Black bull | Red cow | |
|---|---|---|
| Parent phenotypes | black | red |
| Parent genotypes | Bb | bb |
| Gametes | B or b | b or b |
Punnett square (bull's gametes across the top, cow's gametes down the side):
| B | b | |
|---|---|---|
| b | Bb (black) | bb (red) |
| b | Bb (black) | bb (red) |
Offspring genotypes are Bb, Bb, bb, bb, giving an expected ratio of 1 black : 1 red \(( = 2\ \text{Bb} : 2\ \text{bb})\) [ratio 1].
(e) The 1:1 ratio is only a prediction of probabilities. Fertilisation is random: which sperm (carrying B or b) fuses with each egg is a matter of chance [1]. So with a real, especially small, number of calves the actual numbers vary from the predicted ratio; the ratio only approaches 1:1 when very many offspring are produced [1].
Question 3 Report
Human reproduction is sexual. A new life starts when two gametes join. The male gametes are the sperm. They are made in the testes. The female gametes are the egg cells. They are made in the ovaries. About once a month an egg is released. It leaves one of the ovaries. It travels along the oviduct. It moves towards the uterus. Here the egg may meet sperm. If a sperm fertilises the egg, it travels on. It reaches the uterus. It sinks into the thick lining. There it can grow into a baby. If it is not fertilised, the lining breaks down. A student was asked about the egg's journey.
(a) Name the male gamete and the organ in which it is made. [2]
(b) Name the part of the female reproductive system in which fertilisation usually takes place. [1]
(c) State what is meant by the term fertilisation. [2]
(d) Describe what happens to the fertilised egg from the moment of fertilisation until it becomes attached to the uterus. [3]
(e) Explain why the lining of the uterus becomes thick each month. [2]
This question is about human gametes, fertilisation and early development.
(a) The male gamete is the sperm [1]; it is made in the testis / testes [1].
(b) Fertilisation usually takes place in the oviduct (fallopian tube) [1].
(c) Fertilisation is the joining / fusion of a male gamete (sperm) nucleus [1] with a female gamete (egg) nucleus [1] to form a zygote.
(d) From fertilisation until attachment to the uterus: the fertilised egg (zygote) divides repeatedly by mitosis to form a ball of cells [1]; it travels down the oviduct towards the uterus [1]; and then becomes attached to (implants in) the thick lining of the uterus [1].
(e) The lining of the uterus becomes thick each month because the thickened lining develops a good blood supply [1], so that it is ready to receive and nourish a fertilised egg / developing embryo if one implants [1].
Exam tip: in part (c) the marks are specifically for the fusion of the two nuclei; writing only "sperm meets egg" without the idea of nuclei fusing risks losing a mark.
Answer Details
This question is about human gametes, fertilisation and early development.
(a) The male gamete is the sperm [1]; it is made in the testis / testes [1].
(b) Fertilisation usually takes place in the oviduct (fallopian tube) [1].
(c) Fertilisation is the joining / fusion of a male gamete (sperm) nucleus [1] with a female gamete (egg) nucleus [1] to form a zygote.
(d) From fertilisation until attachment to the uterus: the fertilised egg (zygote) divides repeatedly by mitosis to form a ball of cells [1]; it travels down the oviduct towards the uterus [1]; and then becomes attached to (implants in) the thick lining of the uterus [1].
(e) The lining of the uterus becomes thick each month because the thickened lining develops a good blood supply [1], so that it is ready to receive and nourish a fertilised egg / developing embryo if one implants [1].
Exam tip: in part (c) the marks are specifically for the fusion of the two nuclei; writing only "sperm meets egg" without the idea of nuclei fusing risks losing a mark.
Question 4 Report
Fig. 3.1 shows a transverse section through the stem of a young dicotyledonous plant. The vascular bundles contain both xylem and phloem tissue. The phloem transports dissolved food substances made in the leaves to other parts of the plant in a process called translocation.
(a) Define the term translocation. [2]
(b) State the plant tissue that carries out translocation. [1]
(c) Name two substances that are transported in the phloem. [2]
(d) State what is meant by the terms source and sink. [2]
(e) Explain why a growing root tip is described as a sink. [3]
(f) Describe two ways in which the structure of phloem differs from that of xylem. [2]
(g) Explain why translocation requires energy from respiration. [3]
This question is about translocation, the transport of food in the phloem, and how phloem differs from xylem. Because the figure only shows the arrangement of xylem and phloem in the vascular bundles, base your answers on named tissues and functions rather than on any measurement.
(a) Translocation is the movement of sucrose and amino acids [1] through the phloem, from sources to sinks [1]. A full definition names both the substances and the direction.
(b) The tissue that carries out translocation is the phloem [1].
(c) Two substances transported in the phloem are any two of sucrose, amino acids or other soluble sugars [2]. Note that sugar travels as sucrose, not as glucose.
(d) A source is a part that makes or releases food/sucrose, such as a leaf [1], while a sink is a part that uses or stores food, such as a root or fruit [1].
(e) A growing root tip is a sink because it is growing and dividing rapidly [1], so it uses sugars for respiration and to build new cells [1]. Since it cannot photosynthesise underground, food must be delivered to it rather than made there [1].
(f) Two structural differences are any two of: phloem cells are living whereas xylem vessels are dead [1]; phloem has sieve plates and companion cells, whereas xylem has lignified walls and no cross walls [1].
(g) Translocation needs energy because it is an active process [1]: energy is required to load sucrose into the phloem [1], and this energy comes from respiration in the companion cells [1]. This is why phloem, unlike xylem, must be made of living cells.
Exam tip: "source to sink" is the phrase examiners look for; always name the transported substances as sucrose and amino acids, not simply "food".
Answer Details
This question is about translocation, the transport of food in the phloem, and how phloem differs from xylem. Because the figure only shows the arrangement of xylem and phloem in the vascular bundles, base your answers on named tissues and functions rather than on any measurement.
(a) Translocation is the movement of sucrose and amino acids [1] through the phloem, from sources to sinks [1]. A full definition names both the substances and the direction.
(b) The tissue that carries out translocation is the phloem [1].
(c) Two substances transported in the phloem are any two of sucrose, amino acids or other soluble sugars [2]. Note that sugar travels as sucrose, not as glucose.
(d) A source is a part that makes or releases food/sucrose, such as a leaf [1], while a sink is a part that uses or stores food, such as a root or fruit [1].
(e) A growing root tip is a sink because it is growing and dividing rapidly [1], so it uses sugars for respiration and to build new cells [1]. Since it cannot photosynthesise underground, food must be delivered to it rather than made there [1].
(f) Two structural differences are any two of: phloem cells are living whereas xylem vessels are dead [1]; phloem has sieve plates and companion cells, whereas xylem has lignified walls and no cross walls [1].
(g) Translocation needs energy because it is an active process [1]: energy is required to load sucrose into the phloem [1], and this energy comes from respiration in the companion cells [1]. This is why phloem, unlike xylem, must be made of living cells.
Exam tip: "source to sink" is the phrase examiners look for; always name the transported substances as sucrose and amino acids, not simply "food".
Question 5 Report
Green plants make their own food. They use light energy from the Sun. This happens mostly in the leaves. Leaf cells hold a green pigment. It is called chlorophyll. In the leaf the plant takes in carbon dioxide. It takes up water from the soil. It uses light to change these into glucose and oxygen. The oxygen goes into the air. The glucose is used or stored as starch. A gardener grew tomatoes in a greenhouse. They grew a bigger crop than plants outside. The gardener thought the warmth and carbon dioxide helped. A student used limiting factors to explain this.
(a) Write the word equation for photosynthesis. [2]
(b) State the part of the plant cell in which photosynthesis takes place. [1]
(c) State two uses the plant makes of the glucose produced in photosynthesis. [2]
(d) Explain, using the idea of limiting factors, why the tomato plants in the warm, carbon-dioxide-rich greenhouse grew better. [4]
(e) Suggest one other factor the gardener could increase to raise the rate of photosynthesis further. [1]
This question tests the photosynthesis equation, where it happens, the fate of glucose, and how limiting factors control the rate.
(a) The word equation for photosynthesis [2] (1 mark for the correct reactants and glucose, 1 mark for oxygen):
carbon dioxide + water \(\xrightarrow{\text{light, chlorophyll}}\) glucose + oxygen
Light energy and chlorophyll are the conditions that drive the reaction and are shown above the arrow, not as raw materials.
(b) Photosynthesis takes place in the chloroplast, which contains the chlorophyll that absorbs light. [1]
(c) Any two uses of the glucose (1 mark each) [2]: used in respiration to release energy; converted to starch for storage; or used to make cellulose, sucrose, or (with nitrogen) amino acids and proteins.
(d) Using limiting factors [4]: the rate of photosynthesis is limited by whichever factor is in shortest supply [1]. Adding more carbon dioxide supplies more raw material for the reaction [1], and the extra warmth increases the rate of the enzyme-controlled reactions [1], so photosynthesis proceeds faster and more glucose is made for growth, giving a bigger crop. [1]
(e) The gardener could increase the light intensity to raise the rate of photosynthesis further. [1]
Answer Details
This question tests the photosynthesis equation, where it happens, the fate of glucose, and how limiting factors control the rate.
(a) The word equation for photosynthesis [2] (1 mark for the correct reactants and glucose, 1 mark for oxygen):
carbon dioxide + water \(\xrightarrow{\text{light, chlorophyll}}\) glucose + oxygen
Light energy and chlorophyll are the conditions that drive the reaction and are shown above the arrow, not as raw materials.
(b) Photosynthesis takes place in the chloroplast, which contains the chlorophyll that absorbs light. [1]
(c) Any two uses of the glucose (1 mark each) [2]: used in respiration to release energy; converted to starch for storage; or used to make cellulose, sucrose, or (with nitrogen) amino acids and proteins.
(d) Using limiting factors [4]: the rate of photosynthesis is limited by whichever factor is in shortest supply [1]. Adding more carbon dioxide supplies more raw material for the reaction [1], and the extra warmth increases the rate of the enzyme-controlled reactions [1], so photosynthesis proceeds faster and more glucose is made for growth, giving a bigger crop. [1]
(e) The gardener could increase the light intensity to raise the rate of photosynthesis further. [1]
Question 6 Report
| Feature | Plant shoot and light | Human and hot object |
|---|---|---|
| Speed of response | slow | fast |
| Response coordinated by | ................... | nervous system |
| Type of movement | growth (tropism) | ................... |
Both plants and animals are able to respond to changes in their environment, known as stimuli. Plants usually respond by slow growth movements called tropisms, whereas animals often respond rapidly using nerves and muscles. Table 5.1 compares the response of a plant shoot to light with the response of a human who touches a hot object.
| Feature | Plant shoot and light | Human and hot object |
|---|---|---|
| Speed of response | slow | fast |
| Response coordinated by | ................... | nervous system |
| Type of movement | growth (tropism) | ................... |
(a) Complete Table 5.1 by filling in the two empty boxes. [2]
(b) State what is meant by the terms stimulus and response. [2]
(c) State the name of the response of a shoot growing towards light. [1]
(d) Explain why a growth response in a plant is usually slower than a response in an animal. [2]
(e) Suggest one advantage to a plant of being able to respond to the direction of light. [2]
(f) Describe how auxin brings about the bending of a shoot towards light. [3]
(g) State one stimulus, other than light, to which plant shoots or roots respond. [1]
This question compares a slow plant growth response (a tropism) with a fast animal nervous response. The completed Table 5.1 is:
| Feature | Plant shoot and light | Human and hot object |
|---|---|---|
| Speed of response | slow | fast |
| Response coordinated by | auxin (a plant hormone) | nervous system |
| Type of movement | growth (tropism) | muscle contraction |
(a) Plant response coordinated by: auxin / a plant hormone [1]; human type of movement: muscle contraction (muscles move a limb) [1]. [2]
(b) A stimulus is a change in the environment that is detected [1]; a response is a change in an organism produced as a result of a stimulus [1]. [2]
(c) A shoot growing towards light shows (positive) phototropism. [1]
(d) A plant response depends on cells growing and dividing, which is a slow process [1], whereas an animal response is carried by fast electrical nerve impulses [1]. [2]
(e) One advantage: the shoot grows towards the light so the leaves face the light [1], allowing it to absorb more light for photosynthesis and making food [1]. [2]
(f) Auxin is made at the shoot tip and collects on the shaded side of the shoot [1]; auxin makes the cells on the shaded side elongate more [1], so the shaded side grows faster than the lit side and the shoot bends towards the light [1]. [3] (max 3)
(g) One other stimulus: gravity (also accept water or touch). [1]
Answer Details
This question compares a slow plant growth response (a tropism) with a fast animal nervous response. The completed Table 5.1 is:
| Feature | Plant shoot and light | Human and hot object |
|---|---|---|
| Speed of response | slow | fast |
| Response coordinated by | auxin (a plant hormone) | nervous system |
| Type of movement | growth (tropism) | muscle contraction |
(a) Plant response coordinated by: auxin / a plant hormone [1]; human type of movement: muscle contraction (muscles move a limb) [1]. [2]
(b) A stimulus is a change in the environment that is detected [1]; a response is a change in an organism produced as a result of a stimulus [1]. [2]
(c) A shoot growing towards light shows (positive) phototropism. [1]
(d) A plant response depends on cells growing and dividing, which is a slow process [1], whereas an animal response is carried by fast electrical nerve impulses [1]. [2]
(e) One advantage: the shoot grows towards the light so the leaves face the light [1], allowing it to absorb more light for photosynthesis and making food [1]. [2]
(f) Auxin is made at the shoot tip and collects on the shaded side of the shoot [1]; auxin makes the cells on the shaded side elongate more [1], so the shaded side grows faster than the lit side and the shoot bends towards the light [1]. [3] (max 3)
(g) One other stimulus: gravity (also accept water or touch). [1]
