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Question 1

A student investigated the effect of temperature on the rate of respiration in yeast cells. When yeast cells respire they release carbon dioxide gas.

The student used this method:

  1. Fill a syringe with 15 cm³ of yeast suspension.
  2. Gently lower the syringe into the measuring cylinder, as shown in Fig. 1.1.
  3. Fill the measuring cylinder with hot water. Ensure that the water level in the measuring cylinder is above the syringe nozzle.
  4. Measure the temperature of the water in the measuring cylinder.
  5. Start the stop-clock and wait for two minutes.
  6. After two minutes, reset the stop-clock to zero.
  7. Start the stop-clock again and count the number of bubbles produced by the yeast suspension in three minutes.
  8. Pour the hot water in the measuring cylinder into the waste container. The syringe containing the yeast suspension should remain in the measuring cylinder.
  9. Fill the measuring cylinder with cold water. Ensure that the water level in the measuring cylinder is above the syringe nozzle.
  10. Repeat steps 4 to 7.

Fig. 1.2 shows the thermometer during step 4 and step 10.

Fig. 1.3 shows the number of bubbles the student counted in three minutes in step 7 and step 10.

(a) (i) Prepare a table for the results and record the temperatures shown in Fig. 1.2 and the results shown in Fig. 1.3.

 

(ii) State a conclusion for the results.

(iii) Using the results, calculate the rate of bubble production in bubbles per minute for the yeast suspension in hot water and in cold water.

(iv) State the independent variable in this investigation.

(v) State two variables that were kept constant in this investigation.

(vi) Suggest why the yeast suspension was left for two minutes in step 5 before starting to count the number of bubbles.

(vii) Suggest why counting bubbles is not the most accurate method of determining the rate of respiration in yeast.

(b) Measuring the volume of a gas is more accurate than counting bubbles. Using a gas syringe is one method of collecting a volume of gas.

Fig. 1.4 shows part of the apparatus that can be used to measure the volume of a gas by a different method.

Complete the diagram in Fig. 1.4 by drawing and labelling the two pieces of apparatus that are missing.

 

(c) Yeast can respire reducing sugars. Describe the method you would use to test a substance for the presence of reducing sugars.

(d) Bread is made from flour, water and yeast which are mixed to form a dough.
Fig. 1.5 shows a person making bread.

The carbon dioxide gas produced by yeast causes the volume of the dough to increase.
Sodium chloride (salt) is often added to dough when making bread. The sodium chloride affects the rate at which the yeast respire.
Plan an investigation to determine the effect of the mass of sodium chloride on the volume of dough.

▶️ Answer/Explanation
Solution

(a)(i)

Temperature (°C)Number of bubbles in 3 minutes
4254
1812

Explanation: The table should have clear column headings with units. The temperatures are read from the thermometers (42°C for hot water and 18°C for cold water), and the bubble counts are taken directly from Fig. 1.3.

(a)(ii) The higher the temperature, the greater the rate of respiration in yeast (as shown by more bubbles of CO₂ being produced).

Explanation: The results clearly show that at 42°C, more bubbles were produced (54) compared to at 18°C (12 bubbles), indicating that respiration rate increases with temperature within this range.

(a)(iii)
Rate in hot water: 54 bubbles ÷ 3 minutes = 18 bubbles per minute
Rate in cold water: 12 bubbles ÷ 3 minutes = 4 bubbles per minute

Explanation: To calculate the rate, divide the total number of bubbles by the time period (3 minutes) for each temperature condition.

(a)(iv) Temperature (of the water).

Explanation: The independent variable is what the experimenter deliberately changes – in this case, the temperature of the water bath (hot vs cold).

(a)(v) Any two from:
– Volume of yeast suspension (15 cm³)
– Concentration of yeast suspension
– Time for counting bubbles (3 minutes)
– Waiting time before counting (2 minutes)

Explanation: These are variables that were kept the same between the hot and cold water tests to ensure a fair comparison.

(a)(vi) To allow the yeast suspension to reach the same temperature as the water bath.

Explanation: This equilibration time ensures that the yeast cells are actually at the test temperature when measurements begin, rather than still adjusting to the new temperature.

(a)(vii) Because bubbles can vary in size/volume, and it’s easy to miscount bubbles when they are being produced rapidly.

Explanation: Counting bubbles doesn’t account for differences in gas volume per bubble. Measuring the actual volume of gas produced would be more accurate.

(b) The missing apparatus are:
1. A delivery tube connecting the yeast container to the measuring cylinder
2. An inverted measuring cylinder filled with water to collect the gas

 

Explanation: The delivery tube carries the gas from the yeast suspension to the measuring cylinder, which is inverted in water to collect and measure the volume of gas produced.

(c) Method:
1. Add Benedict’s solution to the test substance
2. Heat the mixture in a water bath
3. Observe color change (blue → green/yellow/orange/red indicates reducing sugars)

Explanation: Benedict’s test is the standard method for detecting reducing sugars. The color change occurs due to the reduction of copper(II) ions to copper(I) oxide by the reducing sugars.

(d) Investigation plan:
1. Independent variable: Different masses of sodium chloride (e.g., 0g, 0.5g, 1g, 1.5g, 2g)
2. Dependent variable: Volume/height increase of dough
3. Control variables:
– Same initial volume/mass of dough
– Same type and amount of flour, yeast, and water
– Same temperature and time period
4. Method:
– Prepare dough mixtures with different salt masses
– Measure initial height/volume
– Leave to rise for set time
– Measure final height/volume
– Calculate volume increase
5. Repeat each salt mass multiple times for reliability

Explanation: A good investigation would systematically vary the salt mass while keeping all other factors constant, with multiple trials to ensure reliable results. The volume increase indicates respiration rate as it’s caused by CO₂ production.

Question 2

Fig. 2.1 is a photograph of a cross-section of a root from a carrot plant, Daucus carota.

(a) (i) Draw a large diagram of the carrot root cross-section that shows the layers visible in Fig. 2.1.

(ii) Line PQ on Fig. 2.1 represents the diameter of the carrot root cross-section.

Measure the length of line PQ on Fig. 2.1.

Calculate the actual diameter of the carrot root cross-section using the formula and your measurement.

\[ \text{magnification} = \frac{\text{length of line PQ in Fig. 2.1}}{\text{actual diameter of the carrot root cross-section}} \]

Give your answer to one decimal place.

(b) A student investigated the effect of the concentration of a salt solution on the mass of carrot cubes. The student used this method:

  • Carrots were cut into cubes. Each side of the cube was 1 cm in length.
  • The initial mass of each carrot cube was measured and recorded.
  • Each carrot cube was put into a different concentration of salt solution.
  • The carrot cubes were left in the salt solutions for one hour.
  • After one hour, the carrot cubes were removed from the salt solution and dried with a paper towel.
  • The final mass of each carrot cube was measured and recorded.

(i) State the dependent variable in the investigation described in 2(b).

(ii) State two variables that were kept constant in this investigation.

(iii) Explain why it was important to dry the carrot cubes before measuring the final mass.

The results of the investigation are shown in Table 2.1.

(iv) Using the data in Table 2.1, plot a line graph on the grid to show the effect of concentration of salt solution on the change in mass of the carrot cubes.
One axis has been started for you.

(v) Using your graph, estimate the concentration of salt solution at which there is no change in the mass of the carrot cube.

(vi) Using the information in Table 2.1, calculate the percentage change in mass of the carrot cube that was placed in the 0.4 mol per dm3 salt solution.

(vii) The student did not repeat the investigation and only collected one set of results. Explain why it is better to collect several sets of results.

▶️ Answer/Explanation
Solution

(a)(i)

Answer: The diagram should show:

  • A single clear outline of the carrot root cross-section
  • Width (PQ) at least 109 mm
  • At least 3 distinct layers visible
  • A white line layer with a point at approximately the 9 o’clock position

(a)(ii)

Answer:

Length of PQ = 109 ± 1 mm

Actual diameter calculation:

Using the formula: magnification = image size / actual size

Assuming standard magnification (6×):

Actual diameter = Image size / Magnification = 109 mm / 6 = 18.2 mm (rounded to one decimal place)

(b)(i)

Answer: The dependent variable is the mass of the carrot cubes.

Explanation: This is what is being measured and is expected to change in response to the different salt concentrations.

(b)(ii)

Answer: Any two from:

  • Initial size/surface area/volume of carrot cubes
  • Soaking time (1 hour)
  • Drying method (paper towel)
  • Type of plant tissue/carrot

Explanation: These variables need to be kept constant to ensure that any changes in mass are solely due to the different salt concentrations and not other factors.

(b)(iii)

Answer: To remove any excess solution/water that might affect the mass measurement.

Explanation: If the cubes weren’t dried, the surface water would add to the measured mass, making the results inaccurate. The drying ensures only the mass of the carrot tissue itself is measured.

(b)(iv)

Answer: The graph should have:

  • Axes correctly labeled with units (concentration in mol/dm³ and change in mass in g)
  • Appropriate linear scale using at least half the grid in both directions
  • All six points accurately plotted (± half a small square)
  • A suitable line drawn (likely a decreasing trend)

(b)(v)

Answer: Approximately 0.15 mol/dm³ (value may vary slightly based on graph)

Explanation: This is the concentration where the change in mass is zero (where the line crosses the x-axis). The exact value should be determined from the candidate’s graph.

(b)(vi)

Answer: -6.25%

Calculation:

Percentage change = [(Final mass – Initial mass) / Initial mass] × 100

= [(0.90 – 0.96) / 0.96] × 100

= (-0.06 / 0.96) × 100

= -6.25%

Explanation: The negative value indicates a decrease in mass, showing that the carrot cube lost water in this salt concentration.

(b)(vii)

Answer: To identify anomalous results and improve reliability.

Explanation: Repeating the investigation helps to:

  • Identify and eliminate anomalies caused by experimental errors
  • Calculate more reliable averages
  • Increase confidence in the results
  • Account for natural variations in biological material
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