Biology 5090/62 — October/November 2011

Walkthrough

To construct the graph, the candidate places temperature on the horizontal x-axis and depth of foam on the vertical y-axis. Both axes must use linear scales that cover the range of the data and utilise at least half the grid in each direction. The six data points from Table 1.1 are plotted accurately. Finally, a smooth curve of best fit is drawn through the points. The curve should show an initial rise in foam depth as temperature increases, peak between 35 and 45 $^\circ\text{C}$, and then fall as the temperature continues to rise.

Key Takeaways

Graphs must have correctly labelled axes with units, linear scales, and accurate plots. A line of best fit for non-linear data (like enzyme activity against temperature) must be a smooth curve, not straight line segments connecting the points.

Common Mistakes

• Connecting the points with straight line segments instead of drawing a smooth curve.
• Forgetting to include units on the axis labels (e.g., writing "temperature" instead of "temperature / $^\circ\text{C}$").
• Using non-linear scales (e.g., skipping numbers) or scales that do not start at zero when appropriate.
• Plotting the axes the wrong way round (depth on x, temperature on y).

Things to Be Careful About

Ensure the scales on both axes are linear and cover the full range of data. The mark scheme awards marks for the labels including units (temperature / $^\circ\text{C}$ and depth / mm). Plots must be clear and accurate, ideally plotted to within half a small square. The curve of best fit should not be forced through every single point if there is natural variation, but should follow the overall trend.

Walkthrough

The optimum temperature is the temperature at which the enzyme catalase is most active, producing the greatest depth of foam. On the graph, this is the peak of the curve. By reading the value at the highest point of the curve, the optimum temperature is found to be between 36 $^\circ\text{C}$ and 40 $^\circ\text{C}$ (e.g., 38 $^\circ\text{C}$). The corresponding depth of foam on the y-axis at this peak is between 43 mm and 45 mm (e.g., 44 mm).

Key Takeaways

The optimum temperature for an enzyme is the point on the activity curve where the reaction rate is highest. This is read directly from the peak of the graph.

Common Mistakes

• Reading the temperature of the highest plotted point (35 $^\circ\text{C}$) instead of the peak of the curve of best fit.
• Failing to read the corresponding y-value accurately from the peak.

Things to Be Careful About

The answers must be consistent with the graph drawn in part (a)(i). If a candidate draws a peak at 37 $^\circ\text{C}$ and 43 mm, those values are accepted. The mark scheme allows a range (36–40 $^\circ\text{C}$ and 43–45 mm) to account for slight variations in how candidates draw the curve.

Walkthrough

At 90 $^\circ\text{C}$, the temperature is far above the optimum and even above the temperatures where activity is already dropping (55 $^\circ\text{C}$ and 60 $^\circ\text{C}$). Enzymes are proteins, and at very high temperatures, the hydrogen and disulfide bonds that maintain their three-dimensional shape are broken. This process is called denaturation. When the enzyme denatures, the shape of the active site changes (deforms), meaning the substrate can no longer fit into it (lock-and-key model). Therefore, no catalase reaction occurs, and no oxygen gas is produced to form foam.

Key Takeaways

Enzymes have an optimum temperature. Above this, activity decreases rapidly, and at sufficiently high temperatures (often around 70–90 $^\circ\text{C}$ depending on the enzyme), the enzyme denatures and the reaction stops completely.

Common Mistakes

• Saying the enzyme is "killed" (enzymes are not living, so they cannot be killed; they are denatured or deactivated).
• Saying the substrate is destroyed.
• Giving the observation (no foam) but failing to explain it by mentioning denaturation or the change in the active site shape.

Things to Be Careful About

The explanation must link the high temperature to the structural change in the enzyme (denaturation / active site deformed) and the resulting loss of function. Vague answers like "the enzyme stops working" without the biological reason will not score.

Walkthrough

Catalase is an intracellular enzyme, meaning it is contained inside the liver cells. To access it, the cells must be broken open. Grinding the liver with sand provides an abrasive action. The hard particles of sand increase friction between the cells, physically rupturing the cell membranes and cell walls, thereby releasing the enzyme into the surrounding water.

Key Takeaways

In enzyme extraction experiments, grinding tissue with sand (or silica) is a standard technique to mechanically break cells and release their contents.

Common Mistakes

• Saying sand is to "mix" the liver (too vague).
• Saying sand is to "absorb" water or "dry" the liver.
• Not mentioning the breaking of cells or release of the enzyme.

Things to Be Careful About

The mark scheme specifically looks for "increase friction / abrasion" and "to break cells open / release enzyme". Both concepts are needed for the full 2 marks.

Walkthrough

The decomposition of hydrogen peroxide by catalase produces water and oxygen gas. The standard biological test for oxygen is the glowing splint test. A wooden splint is lit and then blown out so that it is glowing red hot (smouldering). When this splint is introduced to an environment rich in oxygen, the oxygen supports combustion, causing the splint to burst back into flame (relight).

Key Takeaways

Oxygen supports combustion. The glowing splint test is a definitive test for oxygen gas in biological experiments.

Common Mistakes

• Using a "lit splint" (this tests for hydrogen, which produces a 'pop').
• Saying the splint "goes out" (this tests for carbon dioxide or oxygen lack).
• Not specifying that the splint must be "glowing" or "smouldering", not fully lit.

Things to Be Careful About

Be precise with the terminology: "glowing splint" and "relights". Do not say "lit splint relights".

Walkthrough

The initial investigation used wide temperature intervals (10 $^\circ\text{C}$ steps). The peak activity is known to be between 35 $^\circ\text{C}$ and 45 $^\circ\text{C}$, but the exact optimum could be 36, 38, or 40 $^\circ\text{C}$. To find it more accurately, the candidate should narrow the range to 35–45 $^\circ\text{C}$ and test at smaller intervals (e.g., 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45 $^\circ\text{C}$). Repeating and taking a mean also improves accuracy by reducing the effect of random errors.

Key Takeaways

When an approximate optimum is known, accuracy is improved by narrowing the range and using smaller intervals for the independent variable.

Common Mistakes

• Suggesting to test a wider range (e.g., 0 to 100 $^\circ\text{C}$) which does not improve accuracy around the known optimum.
• Not mentioning smaller intervals or a narrower range.

Things to Be Careful About

Part (c)(i) asks for accuracy of the optimum temperature figure. Part (c)(ii) asks for reliability of the results. Do not repeat the answer from (c)(ii) (like repeating to find a mean) as the primary answer here, although repeating is good practice. The key improvement for accuracy of the optimum is smaller temperature intervals.

Walkthrough

Reliability refers to the consistency of results. To improve reliability, the candidate must suggest ways to reduce random errors and control variables so that only temperature affects the outcome.

1. Repeats and mean: Taking multiple readings at each temperature and calculating a mean makes the data more reliable by smoothing out anomalies.
2. Better measurement: Measuring foam depth is subjective and depends on the density of the foam. Measuring the actual volume of gas evolved using a gas syringe is more objective and reliable.
3. Controlled variables: The rate of reaction depends on substrate concentration, enzyme concentration, and pH. These must be kept constant (controlled) so that any change in rate is solely due to temperature. This means using the same volume and concentration of hydrogen peroxide and the same volume and concentration of liver extract for every trial.
4. Temperature control: The mixture must be kept at the set temperature during the reaction, not just before mixing. Using water baths ensures this.
5. Time control: The depth of foam is measured after a specific time. If the timing is inaccurate or varies between trials, the results will not be comparable. Use a stopwatch and ensure the same reaction time is used.

Key Takeaways

Reliability is improved by repeating experiments, calculating means, controlling all variables except the independent variable, and using more objective measurement techniques.

Common Mistakes

• Suggesting to "use more accurate equipment" without specifying what (e.g., a more accurate thermometer instead of specifying a water bath to control temperature).
• Forgetting to control the concentration or volume of the enzyme or substrate.
• Suggesting to measure foam depth more accurately (e.g., using a ruler with smaller mm markings) when the method itself (foam depth) is the problem.
• Confusing reliability (repeats, controlling variables) with accuracy (smaller intervals, better apparatus).

Things to Be Careful About

The question asks for ways to make the results more reliable. The mark scheme gives a list of points. Candidates should provide at least four clear, distinct points. Ensure that "repeat and find a mean" is included if not already awarded in (c)(i), as it is a fundamental reliability improvement. Also, controlling the volume/concentration of both substrate and enzyme is crucial.