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Question 1

Flour and yeast are used to make bread dough, as shown in Fig. 1.1. Respiration in yeast produces carbon dioxide gas bubbles that make the bread dough increase in volume.

A student used bread dough to investigate the effect of temperature on the rate of respiration in yeast. They used this method:

Place three test-tubes labelled C, R and H into a test-tube rack.
Add 10 cm³ of water to each of the test-tubes.
Place test-tube C into a cold water-bath at 5°C.
Place test-tube R into a water-bath at 20°C.
Place test-tube H into a hot water-bath at 40°C.
Leave the test-tubes in the water-baths for five minutes.
Label three transparent cups C, R and H.
Use a spoon to put some of the flour and yeast mixture into each cup to a depth of approximately 1 cm.
Pour the water from test-tube C into cup C. Use the glass rod to stir the mixture to form a dough.
Pour the water from test-tube R into cup R. Use the glass rod to stir the mixture to form a dough.
Pour the water from test-tube H into cup H. Use the glass rod to stir the mixture to form a dough.
Draw a line on each cup to mark the position of the top of the dough.
Leave the three cups of dough for 20 minutes.
After 20 minutes, draw a second line on each cup to mark the new position of the top of the dough.

Fig. 1.3 shows the cups after 20 minutes.

(a) (i) Draw a line on each cup in Fig. 1.3 to show the new position of the top of the dough.

(ii) Measure the change in the height of the dough in each cup in Fig. 1.3. Prepare a table and record these measurements in your table.

(iii) State a conclusion for these results.

(iv) State the independent variable in the investigation.

(v) Identify one possible source of error with the method used to measure the dependent variable.

(vi) State a piece of laboratory equipment that could be used to improve the method in step 8.

(vii) Explain why test-tubes C, R and H were kept in the water-baths for five minutes in step 6.

(viii) This investigation was done by another student using three cups which had different diameters, as shown in Fig. 1.4. Explain why using different-sized cups caused an error in the results.

(b) Yeast uses the reducing sugar glucose for respiration. This reaction produces carbon dioxide.

(i) A sample of flour was tested for reducing sugars using the Benedict’s test. State the result of a positive test.

(ii) Identify a hazard when doing the Benedict’s test.

(iii) State the name of a reagent that can be used to test for carbon dioxide.

(iv) Salt (sodium chloride) is usually added to the flour and yeast mixture when making bread but it can reduce the rate of respiration in yeast. The effect of salt concentration on the volume of carbon dioxide gas produced by yeast is shown in Table 1.1.

Calculate the percentage change in the volume of carbon dioxide gas produced from a salt concentration of 0g per dm³ to 10g per dm³. Give your answer to one decimal place.

(c) Humans can use the energy released during respiration to exercise. Plan an investigation to determine the effect of exercising at different intensities on breathing rate.

▶️ Answer/Explanation

Solution

(a)(i) Lines should be drawn on each cup that would intersect the dough line if drawn all the way across.

Explanation: The lines should be drawn at the new height of the dough after 20 minutes, showing how much it has risen due to CO₂ production from yeast respiration.

(a)(ii) Example table:

Cup	Change in dough height (mm)
C (5°C)	[measured value]
R (20°C)	[measured value]
H (40°C)	[measured value]

Explanation: The table should clearly show the change in dough height for each temperature condition, with appropriate units (mm). The actual values would depend on measurements from Fig. 1.3.

(a)(iii) As temperature increases (the rate of) respiration increases.

Explanation: The conclusion should relate the temperature (independent variable) to the rate of respiration (dependent variable), which is indicated by the dough rising. Higher temperatures generally increase enzyme activity in yeast up to a point, leading to more CO₂ production.

(a)(iv) Temperature.

Explanation: The independent variable is the factor deliberately changed in the experiment, which in this case is the temperature of the water baths (5°C, 20°C, 40°C).

(a)(v) The dough was rounded/not flat/not smooth.

Explanation: An uneven dough surface would make it difficult to accurately measure the height change, as the measurement might vary depending on where you measure from. This introduces variability in the results.

(a)(vi) (Electronic) balance / measuring cylinder.

Explanation: Using a balance would allow precise measurement of the flour and yeast mixture by mass rather than volume, making the amounts more consistent. A measuring cylinder could be used to measure the water more accurately than just pouring 10 cm³.

(a)(vii) To equilibrate the water / so that the water is the same temperature as the water-baths.

Explanation: The five-minute wait ensures the water reaches the exact temperature of each water bath before being added to the dough. Without this step, the water might not be at the intended temperature, affecting the yeast activity.

(a)(viii) The initial amount/mass/volume of dough is different in each cup.

Explanation: Different cup diameters mean that the same 1 cm depth represents different volumes of dough mixture in each cup. This introduces another variable (amount of dough) besides temperature, making it impossible to know if results are due to temperature differences or dough quantity differences.

(b)(i) (Brick-)red precipitate.

Explanation: Benedict’s test turns from blue to green/yellow/orange/red (with red being the strongest positive) in the presence of reducing sugars like glucose. A brick-red color indicates a high concentration of reducing sugars.

(b)(ii) Heating (the reagent, risk described) / Benedict’s reagent is toxic/harmful/irritant/corrosive.

Explanation: The Benedict’s test requires heating the mixture in a water bath, which poses burn risks. Also, Benedict’s solution contains copper sulfate which is toxic if ingested and can irritate skin and eyes.

(b)(iii) Limewater.

Explanation: Limewater (calcium hydroxide solution) turns cloudy/milky in the presence of carbon dioxide due to formation of calcium carbonate precipitate.

(b)(iv) -94.3%

Calculation:
Initial volume at 0g/dm³ = 5.3 cm³/min
Final volume at 10g/dm³ = 0.3 cm³/min
Change = 0.3 – 5.3 = -5.0 cm³/min
Percentage change = (Change/Initial) × 100 = (-5.0/5.3) × 100 = -94.3396…%
Rounded to one decimal place = -94.3%

Explanation: The negative sign indicates a decrease. There’s a 94.3% reduction in CO₂ production when salt concentration increases from 0 to 10g/dm³, showing salt strongly inhibits yeast respiration.

(c) Investigation plan should include:

Independent variable: At least two different exercise intensities (e.g., walking vs. running)
Dependent variable: Breathing rate (count breaths per minute)
Method of changing intensity: Same exercise type at different speeds or using different equipment (treadmill settings, etc.)
Measure initial/resting breathing rate before exercise
Control variables: Same participant, same environmental conditions, same duration of exercise
Repeats: Multiple trials at each intensity
Safety: Ensure participant is healthy, proper warm-up, etc.

Explanation: A good plan would specify how to measure breathing rate (e.g., counting chest rises for 30 seconds and doubling), how to standardize exercise intensity (e.g., specific treadmill speeds), and how to ensure reliability (multiple trials). It should control for other factors that affect breathing rate like temperature, time of day, and participant fitness level.

Question 2

(a) Fig. 2.1 is a photomicrograph of two guard cells in the lower epidermis of a leaf.

(i) Make a large drawing of the two guard cells and the two complete epidermal cells shown in Fig. 2.1.

(ii) Line PQ represents the length of the guard cells in Fig. 2.1.

Measure the length of line PQ in Fig. 2.1.

Calculate the actual length of the guard cells using the formula and your measurement.

\[ \text{magnification} = \frac{\text{length of line PQ}}{\text{actual length of the guard cells}} \]

Give your answer to two significant figures.

(b) Transpiration is the loss of water vapour from the leaves of a plant. A student used a potometer to investigate the effect of wind speed on transpiration.

The student used this method:

Cut a leafy shoot from a plant.
Place the leafy shoot into the potometer.
Measure the distance the air bubble moves in five minutes at different wind speeds.
Maintain the temperature in the room at 25°C and the relative humidity at 60%.
Repeat the investigation using five different leafy shoots. Each shoot must have same number of leaves.

(i) State the dependent variable in the investigation.

(ii) State two factors that were kept constant in the investigation described in 2(b).

(iii) Suggest the purpose of the petroleum jelly shown in Fig. 2.2.

(iv) Explain why the student repeated the investigation.

(v) Plot a line graph on the grid of the data in Table 2.1.

(vi) Using your graph, estimate the distance the air bubble moved in five minutes when the wind speed was 15 km per hour.

▶️ Answer/Explanation

Solution

(a)(i)

Answer: Drawing should show:

Outline as a single clear line with no shading
Size of the guard cell width at least 31 mm
At least four nuclei (2 roughly circular for epidermal cells and 2 longer than wide for guard cells)
Only two epidermal cells touching each other and touching two guard cells

(a)(ii)

Answer:

Length of line PQ: 31 ± 1 mm

Actual length calculation: 0.019 mm

Explanation: Using the magnification formula, if line PQ measures 31 mm at ×1650 magnification, the actual length would be 31 ÷ 1650 = 0.0187878 mm, which rounds to 0.019 mm to two significant figures.

(b)(i)

Answer: Distance (air bubble) moved in five minutes / in a set time

Explanation: The dependent variable is what’s being measured in response to changes in wind speed, which is how far the air bubble moves in the potometer tubing over a fixed time period.

(b)(ii)

Answer: Any two from:

Temperature
Same potometer
Number of leaves
Humidity
Time measured

Explanation: These factors were kept constant to ensure that only wind speed affects the results, making it a fair test. If these varied, they could influence transpiration rate independently of wind speed.

(b)(iii)

Answer: Airtight seal / prevent water loss

Explanation: The petroleum jelly creates a watertight seal around the bung where the shoot enters the potometer, preventing water from escaping at this point which would give false readings.

(b)(iv)

Answer: To identify anomalous results

Explanation: Repeating the investigation with multiple shoots helps identify any unusual results that might occur due to variations between individual plants or experimental errors. This increases the reliability of the conclusions.

(b)(v)

Answer: Graph should show:

Axes labelled with units (wind speed in km/hour and distance moved in mm)
Suitable scale occupying at least half the grid
All five points plotted accurately
Smooth curve or line of best fit drawn

(b)(vi)

Answer: Value from graph (approximately 18 mm)

Explanation: From the plotted graph, you would find the point where 15 km/hour intersects the curve and read across to the y-axis to estimate the distance moved. The exact value would depend on how the graph was drawn, but should be between the 10 km/h (13.0 mm) and 20 km/h (23.0 mm) points.

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