Biology 9700/53 — October/November 2013

Background Concept

In any controlled investigation the independent variable (IV) is the factor the experimenter deliberately changes, and the dependent variable (DV) is the factor measured to record the effect of that change. Everything else must be held constant. In a respirometer, oxygen is consumed by respiring organisms and carbon dioxide is absorbed by the absorbent, so the gas volume in the closed system falls and the coloured dye is pulled along the capillary tube. The distance the dye moves in a set time is therefore a direct read-out of oxygen uptake.

Understanding the Question

The hypothesis links two quantities: "temperature" and "rate of respiration". The student varies temperature on purpose, so that is the IV. The student must measure something that changes because of temperature, and the respirometer delivers that measurement as the distance the dye moves per unit time.

Approach

Read the hypothesis, identify which quantity is being deliberately changed (the IV) and which is being measured (the DV). Then match the DV to what the apparatus actually reads (dye movement, not "volume of oxygen" — that requires extra calculation).

Step-by-Step Reasoning

• Independent: the wording "rise in temperature" tells us temperature is the factor being altered — temperature.
• Dependent: the apparatus does not directly display a rate; the student can only observe the distance the dye moves along the capillary in a fixed time. This is the raw reading and is therefore the DV.
• Note that "volume of oxygen taken up" or "rate of respiration" is not accepted as the DV here, because those are derived (calculated) quantities, not direct measurements from the apparatus.

Key Takeaways

• IV = what you change; DV = what you measure.
• A derived quantity (volume, rate) is not the same as a directly observed measurement; the DV is what you actually read off the apparatus.

Common Mistakes

• Writing "volume of oxygen" or "rate of respiration" as the DV — the mark scheme ignores these because they are calculated, not measured directly.
• Confusing the IV and DV when the hypothesis is given as a single statement.

Things to Be Careful About

• The DV must be observable on the apparatus itself (dye movement, distance along the capillary).

Background Concept

The hypothesis states that the rate doubles for every 10 °C rise. Doubling repeatedly produces a geometric series, which is plotted as an exponential curve, not a straight line. The shape is gently rising at first and then climbs more and more steeply. Above the optimum temperature, the rate would actually fall because enzymes denature, but the question only asks for the trend if the hypothesis is correct, so the curve keeps rising across the range plotted.

Understanding the Question

The candidate is given a blank set of axes (Fig. 1.2) and asked to sketch the predicted result. The marks are for (1) correct axis orientation and labels and (2) the correct curve shape.

Approach

Label the y-axis with what is being measured (rate of respiration, or a directly-observed proxy such as dye movement per unit time) and the x-axis with the independent variable (temperature). Then draw an exponential/rising line.

Step-by-Step Reasoning

• Mark 1 — axis labels: rate of respiration (or distance moved by dye per unit time) on the y-axis; temperature (with °C acceptable) on the x-axis.
• Mark 2 — curve shape: an exponential curve (concave up), or the simpler straight-line-with-positive-gradient alternative that the mark scheme also accepts. A straight line does not strictly represent a doubling, but the mark scheme allows either "exponential" or "increases with temperature", so a steadily rising curve is the safe option.
• The line does not need to pass through the origin or have any numerical scale — units are not required.

Key Takeaways

• "Doubles every 10 °C" → exponential, not linear.
• Graphs are drawn with the IV on the x-axis and the DV on the y-axis.
• In planning, a sketch graph is judged on shape and labels, not on numerical accuracy.

Common Mistakes

• Drawing a straight line through the origin without thinking — that implies additive growth, not the multiplicative (doubling) growth in the hypothesis.
• Drawing a peak-and-decline (enzyme optimum) curve — the hypothesis gives no reason to expect a fall within the range tested.
• Swapping the axes (temperature on the y-axis, rate on the x-axis) — the convention is always IV on x.

Things to Be Careful About

• A sketch only needs the shape of the curve; no values are needed.
• If the axes are left unlabelled, the mark scheme will assume they are the right way round and award the shape mark if appropriate — but you should still label them.

Background Concept

A respirometer measures oxygen uptake by an organism. Oxygen is consumed by respiration and CO₂ is absorbed by the absorbent, so the gas volume inside the closed system falls. The drop of dye in the capillary tube is pulled towards the syringe, and the distance it moves in a fixed time is proportional to the volume of oxygen absorbed. Temperature affects respiration through the kinetic energy of enzyme–substrate collisions (Q₁₀ effect): most biological reactions roughly double in rate per 10 °C rise until enzymes start to denature.

The planning mark scheme always expects the IV, the DV, controlled variables, a reliability element, and safety, in enough detail that another person could follow the method.

Understanding the Question

The student must write a method detailed enough for another person to follow. The starting material is germinating seeds, the apparatus is the respirometer of Fig. 1.1, and the aim is to test the doubling hypothesis. The question explicitly says the method should be detailed enough for another person to use, so vague statements such as "control the temperature" or "measure the dye" will not score.

Approach

Work through each category the mark scheme rewards, in order, and provide the specific detail each mark point demands: IV (mass of seeds; range and number of temperatures), DV (how the dye is introduced, how the distance is measured, the time interval), controlled variables (airtight seal, temperature maintenance, absorbent handling, control for physical effects, replacement of air), reliability (replicates and mean) and safety (specific hazard + precaution).

Step-by-Step Reasoning

• IV — mass of seeds (mp1). Use a known/stated mass (e.g. 20 g) of germinating seeds so that differences in dye movement reflect respiration, not different amounts of respiring tissue. The mark scheme rejects "amount".
• IV — range and number of temperatures (mp2). The hypothesis talks about a 10 °C rise, so to test it you need at least three such rises — a minimum of four temperatures, e.g. 10, 20, 30 and 40 °C.
• DV — method of measurement (mp3). State how the distance is measured: a ruler held alongside the capillary, or a graduated capillary tube, or callipers. "Use a metre rule first on the list" is rejected (a ruler appropriate to the mm-scale capillary is required).
• DV — fixed time/distance (mp4). Either the time is fixed (e.g. measure distance every 5 minutes) and the distance is measured, or vice versa. One of the two must be fixed and stated.
• DV — dye in capillary / resetting (mp5). Explain how the dye is loaded: drawn in with the syringe, pipetted in, or by dipping the tube into dye. Also say how the dye is reset between readings (push back with the syringe).
• Controlled — airtight (mp6). Vaseline / plasticine / tight connectors. "Watertight" is not credited because the issue is gas-tightness, not liquid.
• Controlled — constant temperature (mp7). Place the respirometer in a thermostatically controlled water bath, incubator or temperature-controlled room. Just "use a thermometer" is rejected (that measures, not controls). Air conditioning is also rejected as imprecise.
• Controlled — equilibration (mp8). Leave the respirometer at the test temperature until the dye movement is steady ("to get a steady rate") before recording — otherwise the first reading is contaminated by the apparatus warming up and the gas expanding.
• Controlled — fresh air (mp9). Open the syringe to refresh the air (or push the dye back) between readings so the seeds are not respiring anaerobically once O₂ runs low.
• Controlled — inert control (mp10). Set up a second identical respirometer with an inert material of the same mass (glass beads, boiled seeds, stones). Its dye movement is due to physical effects (temperature/pressure changes); subtract it from the seeds' reading to correct for these.
• Controlled — absorbent (mp11). Use the same (stated) mass of absorbent and replace it when saturated, so that CO₂ continues to be absorbed and the reading reflects only O₂ uptake.
• Reliability (mp13). Repeat each temperature a minimum of three times and take a mean, allowing anomalies to be identified and excluded.
• Safety (mp12). Identify a specific hazard and a specific precaution. The absorbent is the obvious one (corrosive/irritant → gloves and eye protection). Allergic reaction to seeds is also acceptable. Hot water baths (≥ 70 °C) require tongs.

Key Takeaways

• A planning answer is a structured list: IV, DV, controlled variables, reliability, safety.
• Be specific: named masses, named temperatures, named apparatus, named hazards.
• Always include a control for physical effects (the inert respirometer) — this is the classic CIE mark for respirometer experiments.
• Equilibration is a separate step from temperature control and must be stated explicitly.

Common Mistakes

• Writing "control the temperature" without saying how (water bath, incubator).
• Forgetting to replace the air between trials — the seeds run out of O₂ and the rate falls artificially.
• Forgetting the inert control, or using "dead seeds" without first boiling them (to ensure no respiration) and matching the mass.
• Using the word "amount" instead of "mass" for the seeds — the mark scheme rejects it.
• Vague safety ("be careful") instead of a specific hazard + specific precaution.

Things to Be Careful About

• Airtight ≠ watertight.
• A thermometer measures temperature; a water bath controls it.
• "Repeat and take a mean" is the only way to score the reliability mark; just "repeat" or "take a mean" alone is not enough.

Background Concept

An anomalous result is one that does not fit the pattern of the other replicates. It does not have to be a single strange number; a whole trial can be anomalous if all its values are shifted in the same direction away from the others.

Understanding the Question

The table gives four trials at each temperature. The student has already calculated means excluding or including trial 1, and must justify excluding trial 1 by saying why its results are anomalous.

Approach

Look down each temperature column: trial 1 values (52, 42, 35, 28, 17, 14, 12) are higher than the values in trials 2, 3 and 4 at every temperature. That consistent upward offset is the anomaly.

Step-by-Step Reasoning

• Compare trial 1 to trials 2–4 temperature by temperature.
• At every temperature, trial 1's value is higher than each of the other three trials (e.g. at 5 °C: 52 vs 36, 48, 45; at 25 °C: 17 vs 10, 11, 9).
• Therefore trial 1 is consistently higher than the other trials, so its results are anomalous.

Key Takeaways

• An anomaly is identified relative to the pattern of the other replicates, not in isolation.
• A whole trial can be anomalous if all its points are shifted.

Common Mistakes

• Saying "they are very different" without specifying the direction of the difference (higher/lower).
• Saying "the trend is different" — the mark scheme rejects this; the trend is the same, just shifted up.
• Citing only one temperature (e.g. 20 °C) — the mark scheme ignores that.

Things to Be Careful About

• Do not use the word "trend" to describe a shift; trial 1 follows the same downward trend as the others, just at a higher level.

Background Concept

Resting metabolic rate is the lowest rate of oxygen consumption. Any muscular activity (movement, struggling, shaking) increases oxygen demand because working muscles respire aerobically. Stress hormones (e.g. adrenaline) also raise metabolic rate. A new apparatus is unfamiliar and may cause exploratory behaviour, stress or fear, all of which raise oxygen consumption.

Understanding the Question

The student must give a plausible biological reason why trial 1's results are all higher. The mark scheme accepts movement/activity, stress, and lack of acclimatisation.

Approach

Think of any factor that would raise oxygen consumption at every temperature, not just one. A procedural error with the apparatus is rejected, but a behavioural or acclimatisation cause is credited.

Step-by-Step Reasoning

• Because the offset is systematic (all temperatures higher), the cause must be a constant feature of trial 1, not a temperature-specific one.
• The mammal being more active / not at rest fits: moving muscles consume more O₂, so every reading is higher.
• Stress or fear fits the same way: adrenaline and muscle tension raise metabolic rate.
• Lack of acclimatisation fits: in trial 1 the animal is unfamiliar with the apparatus and reacts; by trials 2–4 it has settled down.

Key Takeaways

• A systematic offset implies a systematic cause, not a one-off mistake.
• Animal physiology experiments require acclimatisation to remove stress and activity artefacts.

Common Mistakes

• "Faulty apparatus" or "human error" — explicitly rejected by the mark scheme.
• "Contamination by microorganisms" — rejected.
• Citing a single specific temperature rather than a general cause.

Things to Be Careful About

• A specific biological cause (activity, stress, acclimatisation) is required, not a generic procedural one.

Background Concept

Mammals are endotherms: they maintain a near-constant body temperature largely by adjusting the rate of aerobic respiration in their tissues. Aerobic respiration is not just an ATP source — most of the energy released eventually appears as heat. When the environment is cold, the gradient for heat loss from the mammal to its surroundings is steeper, so heat is lost faster and the mammal must produce heat faster to compensate. It does this by increasing its rate of respiration (and by shivering, which is respiration in muscles doing work).

Understanding the Question

The data show that oxygen uptake is higher at low environmental temperatures (e.g. mean ≈ 45.3 units at 5 °C, falling to ≈ 11.0 units at 35 °C). The student must explain why a mammal respires faster when it is cold.

Approach

Build a three-step chain: more O₂ uptake → more respiration → more heat released to compensate for greater heat loss in a cold environment.

Step-by-Step Reasoning

• mp1 — link O₂ uptake to respiration rate. A higher oxygen uptake means aerobic respiration is occurring at a higher rate (or simply "more O₂ is being used in respiration").
• mp2 — heat loss at low temperatures. A low environmental temperature means a steeper temperature gradient between the mammal's body and its surroundings, so the mammal loses heat faster.
• mp3 — increased respiration to generate heat. To maintain a constant body temperature the mammal must produce more heat, and it does this by respiring faster, consuming more O₂.

Key Takeaways

• Respiration has two products the body uses: ATP and heat.
• Endotherms modulate their respiration rate to balance heat production against heat loss.
• The Q₁₀ effect and the heat-balance effect are different: this question is about heat balance, not about enzyme kinetics.

Common Mistakes

• Saying the mammal "needs more energy" without specifying heat — the mark scheme ignores unqualified "energy".
• Saying "the mammal is shivering" without linking it to respiration/heat production.
• Treating the answer as if it were about an enzyme-controlled reaction rate rising with temperature (which would be a completely different explanation and is wrong here, because at higher environmental temperatures the rate actually falls in the data).

Things to Be Careful About

• The question asks for an explanation of higher rates at low temperatures; the curve falls from 5 °C to 35 °C — the explanation must go the opposite way to the usual Q₁₀ idea.