2025 Cambridge IAS-Level Biology (9700) 模擬試題連答案詳解

An experiment was carried out to investigate the effect of a reversible inhibitor on the rate of reaction of an enzyme. The table shows the maximum rate of reaction (\(V_{max}\)) and the Michaelis-Menten constant (\(K_m\)) of the enzyme in the presence and absence of the inhibitor.

| Condition | \(V_{max}\) / arbitrary units | \(K_m\) / \(\text{mmol dm}^{-3}\) |
| :--- | :---: | :---: |
| No inhibitor | 100 | 2.0 |
| With inhibitor | 50 | 2.0 |

Which statement about this inhibitor is correct?

An enzyme with an optimum temperature of \(37^\circ\text{C}\) was used to catalyze a reaction at \(37^\circ\text{C}\) and at \(50^\circ\text{C}\). All other conditions, including substrate concentration (which was in excess), were kept constant.

Which row correctly describes the initial rate of reaction and the total yield of product after 30 minutes at \(50^\circ\text{C}\) compared to \(37^\circ\text{C}\)?

| Row | Initial rate of reaction at \(50^\circ\text{C}\) | Total yield of product after 30 minutes at \(50^\circ\text{C}\) |
| :--- | :--- | :--- |
| **A** | higher | higher |
| **B** | higher | lower |
| **C** | lower | higher |
| **D** | lower | lower |

Which row correctly describes the mechanisms involved in the active loading of sucrose into companion cells and its subsequent movement into sieve tube elements at a source?

| Row | Movement of \(\text{H}^+\) out of companion cell | Movement of sucrose into companion cell | Movement of sucrose from companion cell to sieve tube element |
| :--- | :--- | :--- | :--- |
| **A** | active transport | co-transport against its concentration gradient | diffusion through plasmodesmata |
| **B** | active transport | co-transport down its concentration gradient | active transport through plasmodesmata |
| **C** | facilitated diffusion | co-transport against its concentration gradient | facilitated diffusion through carrier proteins |
| **D** | facilitated diffusion | co-transport down its concentration gradient | active transport through carrier proteins |

A student used a potometer to measure the rate of transpiration of a leafy shoot. Under initial laboratory conditions, the air bubble in the capillary tube moved a distance of \(60\text{ mm}\) in \(15\text{ minutes}\). The student then changed one environmental factor, keeping all other factors constant, and measured the movement of the bubble again. Under these new conditions, the bubble moved a distance of \(40\text{ mm}\) in \(5\text{ minutes}\).

Which change in environmental factor could explain these results?

Which row correctly identifies the type of pathogen, the primary method of transmission, and a key prevention or control method for tuberculosis (TB)?

| Row | Pathogen type | Primary transmission method | Key prevention or control method |
| :--- | :--- | :--- | :--- |
| **A** | bacterium | vector-borne | vaccination with BCG |
| **B** | bacterium | airborne droplets | contact tracing and antibiotic treatment |
| **C** | virus | airborne droplets | vaccination with BCG |
| **D** | protoctist | water-borne | water treatment and improved sanitation |

The tertiary structure of a protein is maintained by several types of chemical bonds.

Which of these bonds are disrupted when a protein is denatured by a decrease in pH to 2.0?

1. Disulfide bonds
2. Hydrogen bonds
3. Ionic bonds
4. Peptide bonds

Carbon dioxide produced by actively respiring tissues diffuses into red blood cells, where it undergoes a series of reactions.

Which statement correctly describes a reaction or event occurring in the red blood cells in respiring tissues?

A student calibrated an eyepiece graticule using a stage micrometer. At a magnification of \(\times100\), 100 eyepiece graticule units (epu) aligned precisely with a length of \(1.0\text{ mm}\) on the stage micrometer.

The student then changed the objective lens so that the total magnification was increased to \(\times400\).

What is the actual length represented by 1 eyepiece graticule unit (epu) at this new magnification?

An enzyme-catalysed reaction is investigated. The rate of reaction is measured at different substrate concentrations in the presence and absence of a chemical substance, \(X\).

At high substrate concentrations, the rate of reaction in the presence of \(X\) reaches the same maximum velocity (\(V_{\max}\)) as without \(X\), but the Michaelis-Menten constant (\(K_m\)) is increased.

What can be concluded about substance \(X\) and the enzyme?

What is the correct sequence of events that occurs during the active loading of sucrose into phloem companion cells and its subsequent movement into sieve tube elements?

1. Sucrose is co-transported with hydrogen ions into the companion cell.
2. Hydrogen ions are actively pumped out of the companion cell into the cell wall using ATP.
3. Sucrose diffuses through plasmodesmata from the companion cell into the sieve tube element.
4. A high concentration of hydrogen ions is established in the cell wall of the companion cell.

Which row correctly describes the pathogen type, method of transmission, and a major control method for cholera?

Haemoglobin is a globular protein with a quaternary structure. Which statement correctly describes the bonds or interactions stabilizing the structure of a haemoglobin molecule?

Which row correctly describes the net movement of fluid and the main reason for this movement at the arteriole end of a capillary bed?

A student uses a light microscope with an eyepiece graticule and a stage micrometer to measure the diameter of a plant cell.

- The stage micrometer has divisions spaced \(0.01\text{ mm}\) apart.
- When the stage micrometer is focused, 100 divisions of the eyepiece graticule align with 25 divisions of the stage micrometer.
- The stage micrometer is replaced with a slide of plant cells, and a cell is measured to be 24 eyepiece graticule units in diameter.

What is the actual diameter of the plant cell in micrometres (\(\mu\text{m}\))?

An experiment was carried out to investigate the effect of pH on the activity of salivary amylase, which has an optimum pH of 7.0. Two tubes were prepared:

- Tube P contains salivary amylase and starch buffered at pH 7.0.
- Tube Q contains salivary amylase and starch buffered at pH 2.0.

Both tubes are incubated at \(37^\circ\text{C}\) for 10 minutes.

Which statement correctly explains why the rate of reaction in Tube Q is much lower than in Tube P?

An enzyme-catalysed reaction is carried out at \(25^\circ\text{C}\) and at \(37^\circ\text{C}\) (the optimum temperature for this enzyme), with all other conditions kept constant. Both reactions start with the same concentration of substrate.

Which statement correctly describes the curves for the concentration of product against time for these two reactions?

An investigation was carried out into the effect of two different inhibitors, X and Y, on an enzyme-catalyzed reaction. The rate of reaction was measured at different substrate concentrations.

The results obtained showed:
- Substrate concentration at which half-maximum velocity (\(K_m\)) is achieved:
- Control (no inhibitor): \(2.0 \text{ mmol dm}^{-3}\)
- With inhibitor X: \(2.0 \text{ mmol dm}^{-3}\)
- With inhibitor Y: \(5.5 \text{ mmol dm}^{-3}\)
- The maximum rate of reaction (\(V_{max}\)) achieved:
- Control (no inhibitor): \(100 \text{ arbitrary units (a.u.)}\)
- With inhibitor X: \(60 \text{ a.u.}\)
- With inhibitor Y: \(100 \text{ a.u.}\)

Which statement correctly identifies the types of inhibition shown by X and Y?

Which process occurs during the active loading of sucrose into companion cells in the phloem of a photosynthesizing leaf?

Which row correctly matches the pathogen, the transmission method, and the primary site of action of the pathogen for cholera?

Which of the following features describe the quaternary structure of a molecule of haemoglobin?

1. The presence of four polypeptide chains.
2. The presence of ionic bonds, hydrogen bonds, and hydrophobic interactions holding the subunits together.
3. The presence of four haem groups, each containing an iron ion (\(\text{Fe}^{2+}\)).
4. The alpha-helical regions within each individual polypeptide chain.

The table shows the hydrostatic and oncotic pressures of the blood inside a capillary and the tissue fluid outside the capillary at both the arteriole and venule ends of a capillary bed.

| Pressure type | At the arteriole end / kPa | At the venule end / kPa |
| :--- | :--- | :--- |
| Hydrostatic pressure of blood | +4.6 | +2.3 |
| Hydrostatic pressure of tissue fluid | +1.2 | +1.2 |
| Oncotic pressure of blood | -3.3 | -3.3 |
| Oncotic pressure of tissue fluid | -0.2 | -0.2 |

*Note: Positive (+)* values represent pressures forcing fluid out of the capillary; *negative (-)* values represent pressures drawing fluid into the capillary.

What is the net filtration pressure at the arteriole end and the net reabsorption pressure at the venule end?

A student calibrated an eyepiece graticule using a stage micrometer.

The stage micrometer had scale divisions of \(0.1\text{ mm}\).

At a magnification of \(\times 100\), 40 divisions of the eyepiece graticule aligned perfectly with 8 divisions of the stage micrometer.

The student then replaced the stage micrometer with a slide of plant tissue and observed a cell using a magnification of \(\times 400\).

The cell was measured to be 15 eyepiece graticule divisions in width.

What is the actual width of the plant cell in micrometres (\(\mu\text{m}\))?

A sample of double-stranded DNA was analyzed and found to contain 34% cytosine.

During transcription of a section of this DNA, an mRNA molecule was synthesized. The template strand of the DNA used for transcription contained 18% adenine.

What is the percentage of uracil in the resulting mRNA molecule?

Which row correctly identifies an example of passive artificial immunity and the reason for its duration of protection?

An investigation was carried out to determine the effect of two different inhibitors, X and Y, on the activity of an enzyme. The table shows the maximum rate of reaction (\(V_{max}\)) and the Michaelis-Menten constant (\(K_m\)) values obtained.

| Condition | \(V_{max}\) / arbitrary units | \(K_m\) / \(\text{mmol dm}^{-3}\) |
|---|---|---|
| Enzyme alone | 100 | 2.5 |
| Enzyme + Inhibitor X | 100 | 5.0 |
| Enzyme + Inhibitor Y | 50 | 2.5 |

Which statement correctly identifies the types of inhibition shown by X and Y?

Which row correctly describes the changes in the companion cell and the sieve tube element shortly after the application of a metabolic poison that inhibits cellular respiration?

Which of the following options correctly pairs an infectious disease with its causative pathogen, the cellular nature of that pathogen, and its principal method of transmission?

Which of the statements about the structural properties of haemoglobin and collagen are correct?

1. Haemoglobin is a globular protein whereas collagen is a fibrous protein.
2. Both proteins contain a non-protein prosthetic group essential for their function.
3. Every third amino acid in the primary structure of a collagen polypeptide is glycine, whereas haemoglobin does not have this repeating pattern.
4. High temperatures break the covalent peptide bonds in haemoglobin but not in collagen.

Which of the following describes the correct sequence of events occurring in red blood cells as blood flows through actively respiring muscle tissue?

A photomicrograph of a plant cell nucleus shows a nucleolus with a measured diameter of \(6\text{ mm}\). If the actual diameter of the nucleolus is \(1.5\ \mu\text{m}\), what is the magnification of this image, and which type of microscope would be required to resolve two distinct structures inside this nucleolus that are only \(50\text{ nm}\) apart?

| | Magnification | Type of microscope |
|---|---|---|
| **A** | \(\times 4000\) | Electron microscope |
| **B** | \(\times 4000\) | Light microscope |
| **C** | \(\times 400\) | Electron microscope |
| **D** | \(\times 400\) | Light microscope |

A double-stranded DNA molecule is found to contain 28% thymine bases. This DNA is allowed to replicate semi-conservatively. What will be the percentage of guanine bases in the newly synthesized complementary strands?

A patient is bitten by a venomous snake and is immediately injected with an antivenom containing specific antibodies. Two months later, the patient is bitten by the same species of snake but is not immune and requires another injection. What type of immunity is provided by the antivenom, and why is long-term immunity not achieved?

An investigation was carried out to determine the effect of two different inhibitors, X and Y, on the activity of an enzyme. The rate of reaction was measured at different substrate concentrations.
The table shows the values of \(V_{\text{max}}\) and \(K_{\text{m}}\) obtained:

| Condition | \(V_{\text{max}}\) / arbitrary units | \(K_{\text{m}}\) / mmol dm\({}^{-3}\) |
|---|---|---|
| Enzyme alone | 100 | 2.5 |
| Enzyme + Inhibitor X | 100 | 5.0 |
| Enzyme + Inhibitor Y | 50 | 2.5 |

Which statement correctly identifies the type of inhibition and explains its effect?

During the active loading of sucrose into a phloem companion cell, several transport steps are involved. Which row correctly describes these processes?

Which row correctly matches the infectious disease with its pathogen type, method of transmission, and primary site of infection in the human body?

Which of the following describes a structural feature of collagen that distinguishes it from haemoglobin?

At which point in the mammalian cardiac cycle does the left atrioventricular (bicuspid) valve close, and at which point does the left semilunar (aortic) valve open?

An electron micrograph shows a mitochondrion that has an image length of 60 mm.
If the actual length of the mitochondrion is 1.5 \(\mu\text{m}\), what is the magnification of the micrograph?

Bacteria were grown in a medium containing heavy nitrogen (\({}^{15}\text{N}\)) until all of their DNA was labeled with \({}^{15}\text{N}\).
They were then transferred to a medium containing light nitrogen (\({}^{14}\text{N}\)) and allowed to replicate their DNA.
What would be the composition of the DNA molecules after two rounds of semi-conservative replication in the \({}^{14}\text{N}\) medium?

A child is bitten by a venomous snake. At the hospital, they are immediately injected with antivenom containing pre-formed antibodies against the venom.
Several years later, the child is bitten by the same species of snake but is not immune and requires another injection of antivenom.
Which type of immunity is provided by the antivenom, and why does it not provide long-term protection?

(a) Describe how competitive and non-competitive inhibitors affect the Michaelis-Menten constant (\(K_m\)) and the maximum rate of reaction (\(V_{max}\)). [4]

(b) A student investigated the rate of reaction of amylase with different concentrations of starch, with and without the addition of a chemical called phaseolamin. The results are shown in Table 1.1.

Table 1.1:
- Starch concentration / %: 0.1, 0.2, 0.4, 0.6, 0.8, 1.0
- Rate of reaction without phaseolamin / arbitrary units (a.u.): 12, 22, 38, 45, 48, 48
- Rate of reaction with phaseolamin / arbitrary units (a.u.): 4, 8, 15, 22, 28, 32

(i) Explain whether phaseolamin acts as a competitive or a non-competitive inhibitor, using the data in Table 1.1 to support your answer. [3]

(ii) Outline how the student could determine the \(K_m\) value for amylase in the absence of the inhibitor. [3]

(a) Describe three structural features of a sieve tube element that adapt it for the mass flow of organic solutes. [3]

(b) Explain how sucrose is loaded into the companion cells and sieve tube elements at a source. [5]

(c) State two differences between the transport of water in xylem vessels and the transport of organic solutes in phloem sieve tubes. [2]

(a) Describe how the bacterium Vibrio cholerae is transmitted from an infected person to an uninfected person. [2]

(b) Explain the sequence of events that occurs once V. cholerae colonises the small intestine, leading to severe watery diarrhoea. [5]

(c) Oral rehydration therapy (ORT) is used to treat cholera. Explain how ORT rehydrates the body, referencing the components of the rehydration solution. [3]

(a) State three differences between the quaternary structure of collagen and the quaternary structure of haemoglobin. [3]

(b) Explain how the primary structure of collagen determines its secondary and tertiary structures, and how this relates to its function. [5]

(c) Suggest why the replacement of a hydrophilic amino acid (such as glutamic acid) with a hydrophobic amino acid (such as valine) on the surface of a haemoglobin molecule (as in sickle cell anaemia) affects its solubility. [2]

(a) Define the terms systole and diastole. [2]

(b) In an investigation, the pressure changes in the left side of the heart of a mammal were monitored over a single cardiac cycle lasting 0.8 seconds. The following events were observed:
- At 0.15 seconds, the pressure in the left ventricle rises above the pressure in the left atrium.
- At 0.35 seconds, the pressure in the left ventricle rises above the pressure in the aorta.
- At 0.55 seconds, the pressure in the left ventricle falls below the pressure in the aorta.
- At 0.70 seconds, the pressure in the left ventricle falls below the pressure in the left atrium.

(i) State which valve opens or closes at:
- 0.15 seconds: [1]
- 0.35 seconds: [1]
- 0.55 seconds: [1]

(ii) Calculate the heart rate of this mammal in beats per minute (bpm). Show your working. [2]

(c) Explain why the maximum pressure reached in the left ventricle is much higher than the maximum pressure reached in the right ventricle. [3]

(a) An electron micrograph of a liver cell shows a mitochondrion. The actual length of this mitochondrion is 2.5 \(\mu\text{m}\). In the micrograph, the mitochondrion measures 45 mm in length.

Calculate the magnification of this micrograph. Show your working and give your answer to the nearest whole number. [2]

(b) Explain why an electron microscope can resolve details of cell structure that are not visible using a light microscope. [3]

(c) Describe the roles of the rough endoplasmic reticulum (RER) and the Golgi apparatus in the production and secretion of extracellular enzymes. [5]

### Question 1: Investigation of Enzyme Inhibition

Catalase is an enzyme found in many living organisms, including yeast. It catalyzes the breakdown of hydrogen peroxide (\(\text{H}_2\text{O}_2\)) into oxygen and water:

\[2\text{H}_2\text{O}_2 \xrightarrow{\text{Catalase}} 2\text{H}_2\text{O} + \text{O}_2\text{ (g)}\]

Copper sulfate (\(\text{CuSO}_4\)) can act as an inhibitor of catalase. In this investigation, you will investigate the effect of different concentrations of copper sulfate solution on the rate of catalase activity.

You are provided with:
- **Y**: A 2.0% suspension of active yeast cells (source of catalase)
- **H**: A 3.0% hydrogen peroxide solution
- **I**: A 1.0% copper sulfate solution (the inhibitor)
- **W**: Distilled water

---

### Part (a) Preparing dilutions

(i) You need to prepare 10.0 cm³ of each of the following concentrations of copper sulfate solution: 0.8%, 0.6%, 0.4%, and 0.2%, using the 1.0% inhibitor solution **I** and distilled water **W**.

Complete **Table 1.1** below to show the volumes of **I** and **W** you will use to prepare each concentration. Ensure all volumes are recorded to one decimal place. [3 marks]

**Table 1.1**
| Final concentration of inhibitor / % | Volume of 1.0% inhibitor solution **I** / cm³ | Volume of distilled water **W** / cm³ | Total volume / cm³ |
| :---: | :---: | :---: | :---: |
| 1.0 | 10.0 | 0.0 | 10.0 |
| 0.8 | | | 10.0 |
| 0.6 | | | 10.0 |
| 0.4 | | | 10.0 |
| 0.2 | | | 10.0 |

(ii) State the volume of distilled water **W** and 1.0% inhibitor solution **I** needed to prepare 10.0 cm³ of the 0.0% control solution. [1 mark]

---

### Part (b) Investigation

The rate of catalase activity can be measured using a paper disc flotation method. Paper discs are soaked in the yeast suspension containing the inhibitor, then dropped into hydrogen peroxide. As oxygen gas is produced, bubbles become trapped in the fibers of the paper disc, causing it to float to the surface. The time taken (\(t\)) for the disc to rise is inversely proportional to the rate of reaction.

**Procedure:**
1. Label six test-tubes with the concentrations of inhibitor: 1.0%, 0.8%, 0.6%, 0.4%, 0.2%, and 0.0%.
2. Add 2.0 cm³ of yeast suspension **Y** and 2.0 cm³ of the corresponding inhibitor solution to each test-tube. Swirl gently and leave to incubate for 3 minutes.
3. Label another six test-tubes with the same concentrations and add 10.0 cm³ of hydrogen peroxide solution **H** to each.
4. Using forceps, submerge a clean filter paper disc in the 1.0% yeast-inhibitor mixture for exactly 10 seconds.
5. Transfer the disc to the bottom of the corresponding tube of hydrogen peroxide **H**. Start a stopwatch immediately.
6. Measure the time taken (\(t\)), in seconds, for the disc to rise to the surface of the liquid. If the disc does not rise within 180 seconds, stop and record '>180'.
7. Repeat steps 4–6 for each concentration.

(i) State one control variable (other than temperature, concentration/volume of yeast, or concentration/volume of hydrogen peroxide) that must be kept constant to ensure valid results, and describe how you would control it. [2 marks]

(ii) Prepare a table in the space below and record all your results from your practical work. [5 marks]

---

### Part (c) Analysis and explanation

(i) Explain the biological effect of increasing copper sulfate concentration on the time taken for the filter paper disc to rise. [3 marks]

---

### Part (d) Graph plotting and interpretation

In a separate investigation, the effect of pH on the rate of reaction of yeast catalase was studied. The results are shown in **Table 1.2**.

**Table 1.2**
| pH of buffer solution | Rate of reaction / arbitrary units (a.u.) |
| :---: | :---: |
| 4.0 | 6.5 |
| 5.0 | 15.0 |
| 6.0 | 32.5 |
| 7.0 | 48.0 |
| 8.0 | 22.0 |
| 9.0 | 4.5 |

(i) Plot a graph of the data in Table 1.2 on a grid, using suitable scales. [4 marks]

(ii) Based on your graph, estimate the optimum pH for this catalase and describe how the investigator could modify the experimental method to determine the optimum pH more precisely. [2 marks]

---

### Part (e) Evaluation

Identify two potential sources of error in the method used in Part (b) that could affect the accuracy or reliability of your results, and suggest an improvement for each. [2 marks]

Section 2: Microscopy and drawings. M1 is a slide of a transverse section through a herbaceous dicotyledonous stem. (a) Draw a large plan diagram of a sector of the stem on M1 to show the distribution of tissues. Your drawing should represent a wedge-shaped sector containing at least two entire vascular bundles. Do not draw any individual cells. [5] (b) Using the high-power lens of your microscope, select a group of three adjacent xylem vessel elements from one of the vascular bundles in M1. Make a large drawing of this group of three cells. Label one xylem vessel element on your drawing. [4] (c) An eyepiece graticule can be calibrated using a stage micrometer. At high-power magnification, a student determined that 1 eyepiece graticule unit (epu) is equal to 2.4 micrometers. (i) The student used the same high-power magnification to measure the thickness of the sclerenchyma cap (supporting tissue) of a vascular bundle on M1. The thickness was found to be 32 epu. Calculate the actual thickness of the sclerenchyma cap in micrometers. Show your working. [2] (ii) The student made a high-power drawing of the vascular bundle. In this drawing, the sclerenchyma cap was measured to be 38 mm thick. Calculate the magnification of the student's drawing. Show your working and present your final answer to the nearest whole number. [3] (d) Figure 1.1 is a photomicrograph of a transverse section through the stem of a different plant species, Zea mays (a monocotyledon). In Zea mays, the vascular bundles are scattered throughout the ground tissue, there is no cambium, and there is no distinct division into cortex and pith. In contrast, the stem on M1 has vascular bundles arranged in a ring, contains cambium, and has a distinct cortex and central pith. Prepare a table to compare the observable structures of the stem in M1 with those in Figure 1.1. [4]