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Question 1

You are going to investigate the effect of temperature on the activity of amylase.

Amylase is an enzyme that catalyses the breakdown of starch to form reducing sugars.

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 Use the pen to draw a line down the middle of the spotting tile. Write the numbers and the letters C and H on the spotting tile, as shown in Fig. 1.1.

Step 2 Place one drop of iodine solution into each of the labelled dimples on the spotting tile.
Step 3 Label one test-tube C and the other test-tube H. Place the test-tubes in the test-tube rack.
Step 4 Use the syringe to put 2 cm3 of starch suspension into test-tube C and into test-tube H.
Step 5 Place test-tube C and the test-tube labelled amylase C into the water in beaker C.
Start the stop-clock.
Step 6 After three minutes, stop the stop-clock and pour the contents of test-tube amylase C into test-tube C.
Restart the stop-clock, wait for 30 seconds and then continue to step 7.
Step 7 Use a pipette to remove a sample of liquid from test-tube C.
Place two drops of this sample into the dimple labelled C1 on your spotting tile. Return the rest of the sample in the pipette to test-tube C.
Step 8 Observe the colour of the liquid in dimple C1. Record this colour in your table in 1(a)(i).
Step 9 Repeat step 7 and step 8 at 30-second intervals, using the dimples labelled C2, C3, C4, C5 and C6.
Step 10 Raise your hand when you are ready for hot water to be added to beaker H.
Step 11 Place test-tube H and test-tube amylase H into the hot water in beaker H. Reset the stop-clock to zero. Start the stop-clock again.
Step 12 After three minutes, stop the stop-clock and pour the contents of test-tube amylase H into test-tube H. Restart the stop-clock, wait for 30 seconds and then continue to step 13.
Step 13 Use a pipette to remove a sample of the liquid from test-tube H.
Place two drops of this sample into the dimple labelled H1 on your spotting tile. Return the rest of the sample in the pipette to test-tube H.
Step 14 Observe the colour of the liquid in dimple H1. Record this colour in your table in 1(a)(i).
Step 15 Repeat step 13 and step 14 at 30-second intervals, using the dimples labelled H2, H3, H4, H5 and H6.

(a) (i) Prepare a table for your results.

(ii) State the colour of iodine solution when starch is present.

(iii) State a conclusion for your results.

(iv) State the independent variable in this investigation.

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

(b) (i) Explain why the method used in this investigation does not allow you to obtain an accurate time for the breakdown of starch.

(ii) The temperature of the water in the beakers during the investigation was a source of error. Describe how you could improve the method to reduce this error.

▶️ Answer/Explanation
Solution

(a) (i)

The table should have:

  • Three columns (Time, Colour at C, Colour at H)
  • Row headers for each 30-second interval (0s, 30s, 60s, 90s, 120s, 150s)
  • Space to record the colour change observations for both test tubes (C and H)

Explanation: A proper table is essential for recording experimental data systematically. The table should clearly show the time intervals and the corresponding colour changes in both the control (C) and heated (H) samples.

(a) (ii) Blue-black

Explanation: Iodine solution turns blue-black in the presence of starch. This is a classic chemical test for starch identification.

(a) (iii) The higher temperature increased the activity of amylase, causing faster breakdown of starch.

Explanation: Enzymes like amylase work faster at higher temperatures (up to their optimum) because the molecules have more kinetic energy, leading to more frequent successful collisions between enzyme and substrate.

(a) (iv) Temperature

Explanation: The independent variable is what you deliberately change in an experiment. Here, we’re comparing the enzyme activity at different temperatures (room temperature vs. heated).

(a) (v) Any two from:

  • Volume/concentration of amylase solution
  • Volume/concentration of starch solution
  • Number of iodine drops used
  • Time intervals between sampling

Explanation: Controlled variables are essential for a fair test. These factors must be kept constant to ensure any observed differences are solely due to the temperature change (independent variable).

(b) (i) The method doesn’t allow accurate timing because:

  • The colour change might occur between sampling times (30s intervals are too long)
  • The endpoint is subjective (judging colour changes visually)

Explanation: Enzyme reactions can be very rapid, and 30-second intervals might miss the exact moment of complete starch breakdown. Also, different people might judge the colour change differently.

(b) (ii) Improvements could include:

  • Using a thermostatically controlled water bath
  • Insulating the beakers to maintain constant temperature
  • Regularly monitoring temperature with a thermometer

Explanation: Temperature fluctuations can significantly affect enzyme activity. A water bath with thermostat maintains precise temperature control, while insulation minimizes heat loss to the surroundings.

Question 2

(a) Milk contains fats. The enzyme lipase catalyses the breakdown of fats to form fatty acids and glycerol. The fatty acids cause the pH of the milk to decrease.

Plan an investigation to determine the effect of lipase concentration on the breakdown of fats in milk.

(b) The emulsion test is used to test a sample of food for fat.

Describe the method you would use to do the emulsion test.

▶️ Answer/Explanation
Solution

(a)

Investigation Plan:

  1. Independent Variable: Use at least two different concentrations of lipase enzyme (e.g., 0.5%, 1%, 2% solutions).
  2. Dependent Variable: Measure either:
    • The pH after a set time period (e.g., 10 minutes) using a pH meter or pH paper
    • OR the time taken to reach a specific pH value (e.g., pH 4.5)
  3. Control Variables:
    • Keep the volume of milk constant for all tests
    • Use the same type/source of milk (same fat content)
    • Maintain constant temperature (e.g., using water bath at 37°C)
    • Use the same volume of lipase solution for each concentration
  4. Control Experiment: Include a negative control with boiled lipase (to show reaction requires active enzyme) or water instead of enzyme.
  5. Method Details:
    • Measure initial pH of milk before adding lipase
    • Add measured volume of lipase solution to milk and start timer
    • Record pH at regular intervals or note time when target pH is reached
  6. Repeats: Perform each concentration test at least twice (three trials total) for reliability
  7. Safety: Wear goggles and gloves when handling enzymes

(b)

Emulsion Test Method:

  1. Take a small sample of the food to be tested (solid foods should be crushed first).
  2. Add the food sample to a test tube containing about 2 cm³ of ethanol.
  3. Shake the test tube vigorously for about 1 minute to dissolve any fats present.
  4. Allow any solid particles to settle or filter the mixture.
  5. Pour the ethanol solution into another test tube containing an equal volume of distilled water.
  6. Observe the result:
    • Positive result: A cloudy white emulsion forms at the top of the liquid, indicating presence of fat
    • Negative result: The solution remains clear, indicating no fat present

Explanation: The emulsion test works because fats are soluble in ethanol but not in water. When the ethanol solution is added to water, the fats come out of solution, forming tiny droplets that scatter light, creating the characteristic cloudy emulsion. The cloudiness is directly proportional to the amount of fat present in the sample.

Question 3

Fig. 3.1 is a photograph of a type of seaweed called bladder wrack. The bladders help the seaweed float in water.

(a) (i) Draw a large diagram of the bladder wrack seaweed shown in Fig. 3.1.

(ii) Line PQ on Fig. 3.1 represents the length of one bladder on the bladder wrack seaweed. The actual length of the bladder is 19 mm.

Measure the length of line PQ on Fig. 3.1.

length of line PQ …… mm

Calculate the magnification of the photograph using the formula and your measurement.

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

Give your answer to two decimal places.

(iii) Seaweeds are species of algae that live in the sea. Fig. 3.2 shows photographs of bladder wrack seaweed and a different species of seaweed called egg wrack. The photographs are the same magnification.

State two ways, visible in Fig. 3.2, that bladder wrack is different from egg wrack.

(b) Bladder wrack is found on the seashore and is exposed to the air when it is not covered by water at certain times of day.

Students investigated how rapidly bladder wrack lost water. They used this method:

  • Three samples of bladder wrack were collected.
  • The samples were blotted with tissue to remove any water on the surface of the seaweed.
  • The initial mass of each sample was recorded.
  • The samples were hung from a piece of string stretched between two stands.
  • The mass of each sample was recorded every 30 minutes for the first two hours and then every hour for a further three hours.

(i) Suggest two variables that the students should keep constant during their investigation to ensure that the results are valid.

Table 3.1 shows the initial masses recorded by the students and the final masses recorded after five hours.

(ii) One of the final masses recorded is anomalous. State what is meant by an anomalous result.

(iii) Describe how the students calculated the mean value for the final mass of the bladder wrack.

(iv) Using the information in Table 3.1, calculate the mean percentage decrease in the mass of the bladder wrack samples after five hours. Give your answer to two significant figures.

(c) The students repeated their investigation using egg wrack seaweed. Table 3.2 shows the mean percentage decrease in the mass of the egg wrack samples throughout the investigation.

Using the data in Table 3.2, plot a line graph on the grid to show the effect of time on the mean percentage decrease in the mass of the egg wrack.

(d) Many people eat seaweed. State the names of the reagents that can be used to test seaweed for protein and vitamin C.

▶️ Answer/Explanation
Solution

(a) (i)

Answer: The diagram should have:

  • A single clear outline with no shading
  • Frond length at least 114 mm
  • 7 bladders drawn
  • Central midrib shown as a double line at least up to the point where it branches

(a) (ii)

Answer:
Measured length of line PQ: 12 mm (accept 11-13 mm)
Magnification calculation: \[ \frac{12}{19} = 0.63 \] (to two decimal places)

Explanation: First, measure line PQ carefully using a ruler. The actual length is given as 19 mm. Magnification is calculated by dividing the measured length by the actual length. Here, 12 mm ÷ 19 mm = 0.63 when rounded to two decimal places.

(a) (iii)

Answer: Two differences from:

  • Bladder wrack has more bladders
  • Bladders are closer together in bladder wrack
  • Bladder wrack has bladders in pairs on either side of midrib (egg wrack has single bladders)
  • Bladder wrack has a central midrib
  • Bladder wrack divides into two at the end (more branched)

Explanation: When comparing the two seaweeds, bladder wrack shows distinct features like paired bladders along a central midrib, while egg wrack has fewer, more spaced-out bladders without the prominent midrib structure.

(b) (i)

Answer: Two variables to keep constant:

  • Temperature
  • Humidity
  • Wind speed
  • Species/type of seaweed
  • Size/length/surface area of samples

Explanation: To ensure valid results, environmental conditions like temperature and humidity must be constant as they affect water loss. Also, using the same species and similar sized samples eliminates variability due to biological differences.

(b) (ii)

Answer: A result that doesn’t fit the pattern/trend.

Explanation: Sample 2’s final mass of 144g is anomalous because it’s much higher than the other two samples (82g and 70g), breaking the consistent pattern of water loss shown by the other samples.

(b) (iii)

Answer: They excluded the anomalous result (sample 2) and calculated the mean of samples 1 and 3 only.

Explanation: The mean was calculated as (82 + 70) ÷ 2 = 76g, intentionally leaving out the 144g value from sample 2 which was clearly inconsistent with the other results.

(b) (iv)

Answer: 57%

Explanation: Calculation steps:
1. Mean initial mass = 176g (given)
2. Mean final mass = 76g (excluding anomalous result)
3. Mass decrease = 176 – 76 = 100g
4. Percentage decrease = (100 ÷ 176) × 100 = 56.81%
Rounded to two significant figures: 57%

(c)

Answer: The graph should have:

  • X-axis labeled “time/minutes” with scale 0-300
  • Y-axis labeled “mean percentage decrease in mass” with scale 0-60%
  • All seven points accurately plotted (±½ small square)
  • A smooth curve connecting the points

Explanation: The graph shows a gradual increase in water loss over time, starting at 0% and reaching 51% after 300 minutes. The curve should show the rate slowing down as time progresses.

(d)

Answer:
Protein: Biuret reagent
Vitamin C: DCPIP

Explanation: Biuret reagent turns purple in the presence of proteins. DCPIP (dichlorophenolindophenol) changes from blue to colorless when reduced by vitamin C, making it a specific test for this vitamin.

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