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Question 1

You are going to investigate the effect of temperature on the rate of respiration in yeast cells.

When yeast cells respire they release carbon dioxide gas.

Read all the instructions but DO NOT DO THEM until you have drawn a table for your results in the space provided in 1(a)(i).

You should use the safety equipment provided while you are doing the practical work.

Step 1 Stir the yeast suspension with the glass rod and fill the syringe with 10 cm3 of yeast suspension. Ensure you place the syringe nozzle under any foam that is present on top of the yeast suspension so that the foam is not drawn up into the syringe.
Step 2 You have been provided with a small piece of modelling clay. Shape the modelling clay around the end of the syringe plunger as shown in Fig. 1.1. It is important that the modelling clay is firmly attached to the plunger so that it does not fall off.

Step 3 Gently lower the syringe into the measuring cylinder, as shown in Fig. 1.2. If necessary, reshape your modelling clay so that it fits inside the cylinder but do not remove any modelling clay.
Step 4 Fill the measuring cylinder with cold water. Do not let the water in the measuring cylinder overflow.
The nozzle of the syringe should be below the surface of the water in the measuring cylinder.

Step 5 Measure the temperature of the water in the measuring cylinder.
Record this measurement in your table in 1(a)(i).
Step 6 Start the stop-clock and wait for two minutes.
Step 7 Reset the stop-clock to zero.
Step 8 Start the stop-clock again and count the number of bubbles produced by the yeast in three minutes.
Record this number in your table in 1(a)(i).
Step 9 Carefully pour the cold water in the measuring cylinder into the waste container. The syringe containing the yeast suspension should remain in the measuring cylinder.
Step 10 Raise your hand when you are ready for hot water to be poured into your measuring cylinder. Ensure that the water level is above the nozzle of the syringe but not overflowing.
Step 11 Repeat steps 5 to 8.

(a) (i) Prepare a table to record your results in the space provided.

(ii) State a conclusion for your results.

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

(iv) State the independent variable in this investigation.

(v) State one variable that was kept constant in this investigation.

(vi) Suggest why you were instructed to wait for two minutes in step 6 before starting to count the number of bubbles.

(vii) Suggest why counting bubbles is not an 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 and measuring a volume of gas.

Fig. 1.3 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. 1.3 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. 1.4 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)

A suitable table would include:

  • Two columns with headers: “Temperature (°C)” and “Number of bubbles (in 3 minutes)”
  • Rows for recording data at both cold and hot water temperatures
  • Units clearly indicated
  • Space for recording the calculated rate (bubbles per minute)

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

Explanation: Yeast respiration is an enzyme-controlled process, and like most enzymes, their activity increases with temperature up to a certain point. The results should show more bubbles (indicating more CO₂ production) at higher temperatures.

(a) (iii) Rate = Number of bubbles ÷ 3 minutes

Explanation: For example, if you counted 15 bubbles in cold water, the rate would be 5 bubbles per minute. If you counted 30 bubbles in hot water, the rate would be 10 bubbles per minute.

(a) (iv) Temperature.

Explanation: The independent variable is what you deliberately change in the experiment – in this case, the temperature of the water surrounding the yeast suspension.

(a) (v) Any one of: volume of yeast suspension, concentration of yeast, time for counting bubbles, type of yeast used.

Explanation: Controlled variables are what you keep the same to ensure a fair test. The volume of yeast (10 cm³) was kept constant, as was the counting time (3 minutes).

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

Explanation: This equilibration time ensures the yeast cells have adjusted to the experimental temperature before measurements begin, giving more accurate results.

(a) (vii) Bubbles can vary in size, making counting an unreliable measure of gas volume.

Explanation: Some bubbles might be large while others are small, yet each would count as one bubble. Also, bubbles might merge or be missed during counting, introducing errors.

(b) The missing apparatus are:

  1. A delivery tube connecting the bung to the collecting apparatus
  2. An inverted measuring cylinder or burette filled with water to collect the gas

Explanation: The gas produced would travel through the delivery tube into the inverted measuring cylinder, displacing water. The volume of displaced water equals 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/red indicates reducing sugars)

Explanation: Benedict’s test is specific for reducing sugars. The color change occurs because reducing sugars can donate electrons to the copper ions in Benedict’s solution, causing them to reduce from Cu²⁺ (blue) to Cu⁺ (red precipitate).

(d) Investigation plan:

  1. Independent variable: Different masses of sodium chloride (e.g., 0g, 0.5g, 1g, 1.5g, 2g)
  2. Dependent variable: Volume of dough (measured by height increase or displacement method)
  3. Control variables:
    • Same mass/volume of flour, water, and yeast in each trial
    • Same type and brand of ingredients
    • Same kneading time and method
    • Same temperature conditions
    • Same time allowed for rising
  4. Method:
    • Prepare dough mixtures with different salt masses
    • Place in identical containers marked with volume measurements
    • Measure initial volume
    • Allow to rise for set time
    • Measure final volume
    • Calculate volume increase
  5. Repeat each salt mass at least 3 times for reliability

Explanation: This plan systematically tests how salt affects yeast respiration (indicated by dough rising). Controlling other variables ensures only salt mass affects the results. Repeats improve reliability.

 

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.

length of PQ …… mm

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.

Space for working.

(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.

Show on the graph how you obtained your estimate.

(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.

Space for working.

(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 clear outline with a single continuous line
  • Width (PQ) of at least 109 mm
  • Three or more distinct layers visible
  • A white line layer with a point at approximately the 9 o’clock position

Explanation: When drawing the cross-section, it’s important to maintain proportion and clearly show the different tissue layers. The outermost layer (epidermis) should be clearly visible, followed by the cortex, and then the vascular tissue in the center. The white line layer represents the cambium, which is responsible for secondary growth.

(a)(ii)

Answer: Measured length of PQ: 109 mm ± 1 mm
Actual diameter calculation: 18.2 mm (for 109 mm measurement)

Explanation: Using the formula provided, we calculate the actual diameter by dividing the measured length by the magnification. Assuming the magnification is 6× (as typical for such images), the calculation would be: 109 mm ÷ 6 = 18.166… mm, which rounds to 18.2 mm to one decimal place. This represents the real size of the carrot root’s diameter.

(b)(i)

Answer: Mass of carrot cubes

Explanation: The dependent variable is what is being measured in the experiment – in this case, the change in mass of the carrot cubes after being placed in different salt solutions. This is the variable that is expected to change in response to the different salt concentrations.

(b)(ii)

Answer: Any two from:
1. Initial size/surface area/volume of carrot cubes
2. Soaking time (1 hour)
3. Drying method
4. Type of plant tissue/carrot

Explanation: Controlled variables are essential for a fair test. The size of the cubes must be the same (1 cm sides) to ensure equal surface area for osmosis. The soaking time must be consistent (1 hour) to compare results accurately. The drying method must be uniform (paper towel) to remove excess water equally from all samples.

(b)(iii)

Answer: To remove any excess solution/water that would 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 experiment aims to measure only the water that entered or left the carrot cells through osmosis, not the water clinging to the surface. Consistent drying ensures only the actual mass change due to osmosis is recorded.

(b)(iv)

Answer: The graph should have:
– X-axis labeled “Concentration of salt solution (mol/dm³)” with appropriate scale
– Y-axis labeled “Change in mass (g)” with appropriate scale
– All seven points plotted accurately
– A suitable line drawn (likely a decreasing curve)

Explanation: When plotting the graph, the concentration should be on the x-axis (independent variable) and change in mass on the y-axis (dependent variable). The points should show a clear trend: as salt concentration increases, the change in mass decreases, becoming negative at higher concentrations. The line should show this inverse relationship clearly.

(b)(v)

Answer: Approximately 0.15 mol/dm³ (value depends on graph drawn)

Explanation: The point of no change is where the line crosses the x-axis (change in mass = 0). From the data, this occurs between 0.0 and 0.2 mol/dm³. By plotting the points and drawing the line of best fit, we can estimate the exact concentration where the mass neither increases nor decreases.

(b)(vi)

Answer: -6.25%

Calculation: \[ \text{Percentage change} = \left( \frac{\text{Final mass} – \text{Initial mass}}{\text{Initial mass}} \right) \times 100 \] \[ = \left( \frac{0.90 – 0.96}{0.96} \right) \times 100 = -6.25\% \]

Explanation: The negative percentage indicates a mass loss. The carrot cube lost 6.25% of its initial mass in the 0.4 mol/dm³ solution. This occurs because the salt solution is hypertonic compared to the carrot cells, causing water to leave the cells by osmosis.

(b)(vii)

Answer: To identify anomalous results and improve reliability

Explanation: Repeating experiments is crucial in science. Multiple trials help identify outliers or errors in individual measurements. They allow calculation of averages, making results more reliable. With only one set, we can’t tell if an unusual result is due to experimental error or represents the true relationship. Repeats increase confidence in the conclusions.

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