Biology 9700/51 — October/November 2013

Background Concept

A hypothesis is a testable statement that predicts the relationship between the independent and dependent variables in an experiment. For this investigation, the independent variable is the type of organism and the dependent variable is the rate of respiration (measured as the rate of oxygen uptake in a respirometer). A good hypothesis must be specific, testable, and falsifiable. In a comparative investigation like this, the hypothesis predicts whether the values for the different organisms will be the same or different, and ideally states which is expected to be highest or lowest.

Understanding the Question

The student is using a respirometer to compare respiration in three very different organisms: germinating seeds (plant), insect larvae (animal) and single-celled green algae (aquatic photosynthetic organisms). Part (a)(i) specifically asks for a hypothesis that the student could test using this apparatus. The mark scheme allows any testable hypothesis provided it is in the context of all three organisms, so the statement must mention comparison between them.

Approach

Frame the hypothesis around the dependent variable (respiration rate / oxygen uptake) and state whether the three organisms are predicted to differ, with an optional prediction of which is fastest/slowest. Avoid vague language; a hypothesis is a precise prediction, not a question.

Step-by-Step Reasoning

The mark scheme accepts two main types of hypothesis:

1. Difference hypothesis: "The rate of respiration of the three organisms will be different from each other." This is the safest, most general answer.
2. Specific prediction: "The insect larvae will have a faster rate of respiration than the germinating seeds and the algae." This adds more detail and is still a valid testable prediction.

Either form must include all three organisms (or imply a comparison between them) and must relate to respiration or oxygen uptake, not to some other feature of the organisms. A statement like "the organisms are different" with no reference to respiration would not earn the mark because it does not predict the dependent variable.

Key Takeaways

• A hypothesis states a predicted relationship between variables.
• For comparative experiments, the hypothesis must address all groups being compared.
• The hypothesis is tested by collecting data on the dependent variable.

Common Mistakes

• Stating a hypothesis that only mentions one organism ("insect larvae respire" – too vague, no comparison).
• Phrasing the hypothesis as a question ("Will the organisms have different rates of respiration?") rather than a statement.
• Failing to mention respiration or oxygen uptake, and instead comparing an irrelevant feature.

Things to Be Careful About

The hypothesis must be a statement, not a question. It should be testable with the apparatus described – i.e. it should relate to a measurable feature of respiration in the respirometer.

Background Concept

In any experiment, the independent variable is what the experimenter deliberately changes between test groups. The dependent variable is what is measured to record the effect of that change. Other variables that might affect the result should be kept constant (controlled variables).

Understanding the Question

The student is comparing the respiration rates of three different organisms using the same respirometer. The question asks for the two variables that define what the experiment is doing: what is being changed (the type of organism) and what is being measured (the respirometer reading over time).

Approach

Look at what the student deliberately alters from one experimental run to the next – that is the independent variable. Then look at what the student records a value for – that is the dependent variable.

Step-by-Step Reasoning

• The student puts different organisms into the respirometer on three separate occasions: germinating seeds, insect larvae, algae. The category being changed is therefore the organism used – this is the independent variable. The mark scheme accepts "(different / named) organisms" or a list of the three names.
• As each organism respires, it absorbs O₂ and (with the CO₂ absorbent present) the gas volume inside the closed container falls. Atmospheric pressure pushes water along the graduated tube towards the container. The student therefore measures the distance the water/air moves along the capillary tube in a set time. The mark scheme requires "distance moved by the water / air (along capillary in a specific time)".
• Common wrong answers: "amount of oxygen used" or "rate of respiration" are rejected because they are processed quantities (volume, rate), not the raw measurement from the apparatus. "Volume" is also rejected for the same reason.

Key Takeaways

• Independent variable = what is deliberately varied (the organism).
• Dependent variable = the raw measurement taken from the apparatus (distance moved in a known time).
• Processed quantities (rate, volume) are derived from the dependent variable, not the dependent variable itself.

Common Mistakes

• Writing "amount of O₂ used" or "rate of respiration" as the dependent variable – these are calculated later from the raw measurement.
• Confusing which is which (e.g. putting "time" as the independent variable).

Things to Be Careful About

The dependent variable must be a raw observation, not a derived value. "Distance moved in a specific time" is correct; "volume of O₂ used per minute" is a calculated quantity and would not earn the mark.

Background Concept

A respirometer measures the rate of oxygen uptake of an organism. The organism is sealed in a small air-filled chamber with a CO₂ absorbent (e.g. soda lime or KOH). As the organism respires aerobically:

The O₂ consumed is not replaced (the CO₂ released is absorbed), so the gas volume in the chamber falls. Atmospheric pressure pushes a drop of coloured water (or air–water meniscus) along the graduated capillary tube towards the chamber. The distance the meniscus moves in a given time is proportional to the volume of O₂ used.

For a fair comparison between organisms, the experiment must control all variables that affect respiration rate (mass, temperature, availability of O₂) except the type of organism.

Understanding the Question

The student must describe, in enough detail that another person could follow, how to use the respirometer in Fig. 1.1 to compare the rate of respiration of germinating seeds, insect larvae and single-celled green algae (which live in water). The mark scheme awards marks across several categories: independent variable handling, dependent variable measurement, controlled variables, safety and reliability.

Approach

Work through the categories the examiner will be looking for, in the order they would naturally appear in a good method:

1. How you will set up and standardise the independent variable (mass of each organism).
2. How you will measure the dependent variable (distance moved in a known time).
3. Controlled variables (dark, temperature, airtight, equilibration, control, absorbent).
4. Reliability (replication and a mean).
5. Safety (one specific hazard and its precaution).

Step-by-Step Reasoning

Independent variable – the organism:

• The mark scheme requires the same mass of each organism (mp 1) so any difference in O₂ uptake is due to the organism, not its size. A stated/known/fixed mass earns the mark; "amount" or "number" alone does not.
• A dark condition (mp 2) is essential because algae photosynthesise in the light. Photosynthesis would release O₂, opposing the O₂ uptake being measured and giving a falsely low reading. Covering the respirometer (or the water bath) is acceptable.

Dependent variable – distance moved:

• Use the scale on the graduated tube (or a ruler laid alongside it) to read the position of the meniscus (mp 3). "Volume" is rejected; "distance moved" is the raw observation.
• Take readings at a specific / known time interval (mp 4) – e.g. 5 minutes – so that rate (distance ÷ time) can be calculated.
• For the aquatic algae, mark scheme mp 5 requires a method of holding them. They live in water but their water must not flood the absorbent or block the tube. A small open container (e.g. a small beaker or muslin bag) inside the chamber keeps the algae in place while still allowing gas exchange with the air in the chamber.

Controlled variables:

• The apparatus must be airtight (mp 6) so no air leaks in. Petroleum jelly or a rubber sleeve on the bung joint is the standard way to achieve this. "Watertight" alone is rejected because the chamber does not contain liquid.
• Constant temperature (mp 7) in the water bath: a stated temperature in the range 15–45 °C is accepted. The temperature affects the rate of respiration (Q10 effect) and the volume of gas in the chamber (Charles's law), so it must be standardised.
• Equilibration (mp 8): before taking readings, the respirometer should be left in the water bath for a few minutes so the air in the chamber reaches the bath temperature. Without this, thermal expansion or contraction of the air would move the meniscus and corrupt the reading.
• The air in the chamber should be replaced between runs (mp 9) so each organism starts with a fresh, O₂-rich atmosphere and the same starting conditions.
• A control with inert material of the same mass (mp 10) – e.g. glass beads, or dead organisms – accounts for any non-respiratory movement of the meniscus (due to temperature or pressure fluctuations). The control reading is subtracted from the experimental reading.
• The same mass (or volume) of CO₂ absorbent should be used (mp 11), and it should be sufficient to absorb all the CO₂ produced. If the absorbent becomes saturated, CO₂ will not be removed and the apparent O₂ uptake will be falsely low.

Safety (mp 12):
One specific hazard and one specific precaution earns the mark. Acceptable answers include:

• CO₂ absorbent (e.g. KOH / soda lime) is caustic/corrosive → wear gloves and eye protection.
• Glass graduated tube can break → handle carefully; hold the bung while attaching the tube.
• Allergic risk from organisms / absorbent → wear gloves/mask.

A generic "be careful" or "hot water" does not earn the mark.

Reliability (mp 13):
A minimum of three readings (the original + two more) for each organism, and calculation of a mean distance moved per unit time. Anomalies should be identified and discarded. The mean of multiple timed intervals from a single run (e.g. distances every minute, averaged) is rejected – the repeats must be independent experimental runs.

Key Takeaways

• A respirometer measures the distance the meniscus moves in a known time, which is proportional to O₂ uptake.
• For a fair comparison, all variables except the organism must be controlled: same mass, same temperature, dark, airtight, equilibration, same absorbent, plus a control with inert material.
• Replication and a mean are essential for reliability.
• One specific safety hazard and one precaution must be stated.
• Aquatic organisms need to be held in a small open container so they remain in water but the absorbent stays dry.

Common Mistakes

• Forgetting the dark condition – this is the only mark specifically for the algae treatment, and it is essential because photosynthesis would otherwise release O₂ and mask respiration.
• Writing "use the same amount" rather than "the same mass" – the mark scheme rejects "amount" or "number".
• Forgetting to control the temperature (or failing to state a value in 15–45 °C).
• Not including a control with inert material of the same mass.
• Vague safety statements ("be careful", "hot water") – these are rejected.
• "Replicate by taking readings every minute and averaging" – this is rejected; replicates must be independent full runs of the experiment.

Things to Be Careful About

• The dependent variable is distance moved in a known time, not volume or rate.
• The CO₂ absorbent must be sufficient to absorb all CO₂ produced, or the reading will be wrong.
• For the algae, the water they live in must not be in contact with the absorbent; they need a small open container inside the chamber.

Background Concept

The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed during respiration:

Because the units ($\text{cm}^3\,\text{g}^{-1}\,\text{s}^{-1}$) are the same in both the numerator and denominator, they cancel, leaving a dimensionless number. The value of the RQ gives information about which respiratory substrate is being metabolised (see part (d)(ii)).

Understanding the Question

Table 1.1 gives the mean rates of O₂ uptake and CO₂ production for each organism (per unit mass per unit time), and the RQ has been pre-calculated for germinating seeds. The question asks the candidate to complete the RQ column for the insect larvae and the single-celled green algae.

Approach

Apply the RQ formula to the two pairs of mean values. The numerator and denominator have the same units, so the result is a pure number; round to two decimal places (matching the precision of the data).

Step-by-Step Reasoning

Insect larvae:

Single-celled green algae:

(The mark scheme accepts 0.97 or 1.00 for the algae.)

Both answers are quoted to two decimal places, consistent with the precision of the input data. The value 0.79 is rejected by the mark scheme – it is a rounding error.

Key Takeaways

• RQ = CO₂ produced / O₂ used.
• The RQ is dimensionless; quote to two decimal places.
• RQ ≈ 1.0 indicates carbohydrate respiration; RQ ≈ 0.7 indicates lipid; RQ ≈ 0.8–0.9 indicates protein.

Common Mistakes

• Inverting the ratio (dividing O₂ by CO₂).
• Rounding 0.797 down to 0.79 – the mark scheme rounds to 0.80.
• Failing to show any working – although the answer alone earns the mark, working prevents arithmetic slips.

Things to Be Careful About

• Both inputs are means per unit mass per unit time, so the per-mass and per-time components cancel and need not be re-stated in the RQ calculation.
• Quote the answer to the same precision as the input data (two decimal places here).

Background Concept

The RQ depends on the chemical composition of the substrate being respired:

• Carbohydrate (e.g. glucose): already highly oxidised; complete oxidation to CO₂ and H₂O gives RQ = 1.00. $$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$$ Six CO₂ produced for every six O₂ used → RQ = 1.0.
• Lipid (e.g. a typical triglyceride / oil): already reduced; much more O₂ is needed to oxidise the long hydrocarbon chains, and less CO₂ is produced per O₂. RQ ≈ 0.7.
  e.g. for tripalmitin: $\text{C}_{51}\text{H}_{98}\text{O}_6 + 72.5\,\text{O}_2 \rightarrow 51\,\text{CO}_2 + 49\,\text{H}_2\text{O}$, giving RQ ≈ 0.7.
• Protein / amino acids: the elements C, H, O, N, S require more O₂ per CO₂ than carbohydrate does, giving RQ ≈ 0.8–0.9.

The RQ therefore acts as a fingerprint of the substrate being metabolised.

Understanding the Question

The completed RQ column gives:

• Seeds: 0.72
• Larvae: 0.80
• Algae: 0.97

The question asks the candidate to use these values to draw conclusions about which substrate each organism is respiring.

Approach

Match each RQ to the closest standard value and state which substrate that RQ implies. For full credit, mention all three organisms and the reason for the low RQ of lipid (it consumes proportionally more O₂ than it releases as CO₂).

Step-by-Step Reasoning

Algae – RQ ≈ 0.97 (≈ 1.0):
The RQ is essentially 1, indicating respiration of a carbohydrate. This is biologically sensible: green algae photosynthesise and store the products as carbohydrate (e.g. starch, glucose); when they respire, the substrate is this stored sugar.

Insect larvae – RQ ≈ 0.80:
The RQ is in the range 0.8–0.9, indicating respiration of protein / amino acids (a mixture of lipid and protein is also acceptable). Many insects metabolise protein-rich diets or mobilised amino acids during development, giving RQ values in this range.

Germinating seeds – RQ = 0.72:
The RQ is ≈ 0.7, indicating respiration of fat / oil. Many seeds (e.g. sunflower, linseed, castor) store their food reserves as lipid rather than starch. The reason this gives a low RQ is that fats are highly reduced (long hydrocarbon chains with little internal O₂), so they require a large amount of additional O₂ from the atmosphere to be fully oxidised, while producing relatively little CO₂.

The mark scheme specifically requires the explanation that fat uses proportionally more oxygen than carbohydrate for respiration (mp 4). This is the underlying chemical reason: a triglyceride molecule has far more H and far less O than a sugar, so its oxidation consumes more O₂ per CO₂ released.

Key Takeaways

• RQ ≈ 1.0 → carbohydrate.
• RQ ≈ 0.7 → lipid / fat.
• RQ ≈ 0.8–0.9 → protein / amino acid (or a lipid + protein mix).
• The lower the RQ, the more reduced the substrate, and the more O₂ must be supplied per CO₂ released.
• Stored reserves of seeds (oils) and animal tissues (proteins) are reflected in the RQ during germination / development.

Common Mistakes

• Confusing the order: stating that the algae are respiring fat and the seeds are respiring carbohydrate.
• Failing to state a specific substrate (e.g. "a different substrate") – the mark scheme requires a named substrate (CHO, protein, lipid/oil/fat).
• Not explaining why fat has a low RQ (the mark scheme specifically requires this point for full marks).
• Saying "lipid" without realising that "fat" or "fatty acid" or "oil" are all accepted equivalents.

Things to Be Careful About

• RQ values are approximate indicators – an RQ of 0.8 could in principle indicate a mix of substrates (e.g. 50% carbohydrate + 50% lipid) rather than pure protein, but the simplest interpretation is correct here.
• The reasoning that fat uses more O₂ per CO₂ is the chemical basis for the low RQ – this is the one explanatory mark in this question and is essential for full marks.
• Always quote the RQ value(s) you are interpreting to make the link explicit.