state that the respiratory quotient (RQ) is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in, as a result of respiration

Energy – Respiration

Why cells need energy (AO1)

• Active transport of ions and nutrients across membranes.
• Synthesis of macromolecules (proteins, nucleic acids, polysaccharides).
• Cell division, movement and growth.
• Maintenance of ion gradients and membrane potential.

The universal energy‑currency molecule is adenosine‑triphosphate (ATP). ATP stores energy in its two high‑energy phosphate bonds; hydrolysis to ADP + P_i releases ≈ 30 kJ mol⁻¹, which can be coupled to otherwise non‑spontaneous cellular processes.

Overview of Aerobic Respiration (AO1)

Aerobic respiration converts the chemical energy of organic substrates into ATP through four linked stages. The overall reaction for glucose is:

\$\mathrm{C6H{12}O6 + 6\,O2 \;\longrightarrow\; 6\,CO2 + 6\,H2O + \text{~30–38 ATP}}\$

Figure: a simple flow‑chart (cytoplasm → mitochondrion) showing the four stages:

• Glycolysis (cytoplasm)
• Link reaction / pyruvate oxidation (mitochondrial matrix)
• Krebs (citric‑acid) cycle (matrix)
• Oxidative phosphorylation (inner mitochondrial membrane)

Stage 1 – Glycolysis

• Location: cytoplasm.
• Glucose (6‑C) → 2 pyruvate (3‑C each) + 2 ATP (substrate‑level) + 2 NADH.
• When O₂ is limited, pyruvate can be reduced to lactate (animal cells) or ethanol + CO₂ (yeast) – see Fermentation.

Stage 2 – Link Reaction (pyruvate oxidation)

• Location: mitochondrial matrix.
• Each pyruvate + CoA + NAD⁺ → acetyl‑CoA + CO₂ + NADH.
• Produces 2 NADH per glucose.

Stage 3 – Krebs Cycle

• Location: mitochondrial matrix.
• Acetyl‑CoA (2‑C) + oxaloacetate (4‑C) → citrate (6‑C) → series of reactions → oxaloacetate regenerated.
• Per glucose: 2 ATP (or GTP), 6 NADH, 2 FADH₂, and 4 CO₂.

Stage 4 – Oxidative Phosphorylation (AO1)

• Location: inner mitochondrial membrane.
• Electrons from NADH and FADH₂ travel through the electron‑transport chain (ETC), releasing energy that pumps protons (H⁺) from the matrix to the inter‑membrane space, creating a proton‑gradient.
• ATP synthase uses the return flow of H⁺ to synthesise ATP (≈ 2.5 ATP per NADH, ≈ 1.5 ATP per FADH₂).
• O₂ is the final electron acceptor, forming H₂O.

Anaerobic Respiration / Fermentation (AO1)

When O₂ is unavailable, cells regenerate NAD⁺ by converting pyruvate into:

• Lactate fermentation (animal muscle): pyruvate + NADH → lactate + NAD⁺ (RQ ≈ 1.0)
• Alcohol fermentation (yeast, some plants): pyruvate → ethanol + CO₂ + NAD⁺ (RQ ≈ 0.9)

Only a small amount of ATP (2 ATP per glucose) is produced, so the process can sustain activity only briefly.

ATP Synthesis – Chemiosmosis (AO1)

• Proton‑gradient (electrochemical potential) is established by the ETC.
• ATP synthase provides a channel for H⁺ to flow back into the matrix; the energy released drives phosphorylation of ADP to ATP.
• Coupling of electron flow to proton pumping is the basis of the “chemiosmotic theory” (Peter Mitchell).

Respiratory Quotient (RQ) – Definition (AO1)

• Definition: the ratio of the number of molecules (or, at constant temperature and pressure, the volume) of carbon dioxide produced to the number of molecules (or volume) of oxygen consumed during respiration.
• Formula:

  \$\$\text{RQ} = \frac{n{\mathrm{CO2}}}{n{\mathrm{O2}}}

  = \frac{V{\mathrm{CO2}}}{V{\mathrm{O2}}}\$\$

Typical RQ Values for Different Substrates (AO1)

Worked Example – Calculating RQ from a Balanced Equation (AO2)

For any substrate, RQ is simply the ratio of the stoichiometric coefficient of CO₂ to that of O₂.

1. Glucose RQ = 6 CO₂ / 6 O₂ = 1.00
2. Palmitic acid RQ = 16 CO₂ / 23 O₂ ≈ 0.70
3. Average amino acid RQ = 5 CO₂ / 5 O₂ = 0.80

Practice Problem (AO2)

Derive the RQ for the oxidation of glycerol (C₃H₈O₃).

Laboratory Determination of RQ (AO2 & AO3)

Equipment

• Closed metabolic (respirometer) chamber – glass or acrylic, airtight.
• Gas‑collection system (graduated burette, eudiometer, or infrared gas analyser).
• Thermometer and barometer – for temperature (T) and pressure (P) to convert volume to moles.
• Water‑bath or temperature‑controlled incubator – keep T constant (±0.5 °C).
• Data‑logging device or stopwatch.

Choice of Organism (Cambridge examples)

• Germinating seeds (e.g., wheat or beans) – high carbohydrate utilisation.
• Blow‑fly larvae – rapid metabolism, useful for activity‑related RQ changes.
• Yeast culture in a sugar solution – inexpensive, safe for classroom.

Step‑by‑Step Procedure

1. Calibrate the gas‑volume sensor with a known volume of dry air at the experimental T and P.
2. Place the organism (seed, larva, or culture tube) inside the sealed chamber; add a moist cotton plug to maintain humidity.
3. Record the initial volumes of O₂ and CO₂ (or baseline analyser reading).
4. Seal the system and allow respiration for a fixed period (e.g., 30 min). Keep temperature constant.
5. Measure the final volumes of O₂ taken up and CO₂ released.
6. Convert volume changes to moles using the ideal‑gas equation

   \$n = \frac{PV}{RT}\$

   (or use the volume ratio directly if T and P are unchanged).

7. Calculate RQ:

   \$\text{RQ} = \frac{V{\mathrm{CO2}}}{V{\mathrm{O2}}}\$

Checklist for a Successful Investigation (AO3)

• ✅ Temperature recorded and maintained within ±0.5 °C.
• ✅ Pressure recorded for each trial.
• ✅ Chamber leak‑tested (water‑displacement test) before each run.
• ✅ Daily calibration of the gas‑volume sensor.
• ✅ Minimum three repeat trials per treatment.
• ✅ Include an empty‑chamber control to correct for background gas exchange.
• ✅ Record data in a tidy table (see template).

Data‑Table Template

Common Sources of Error & Evaluation (AO3)

• Gas leakage: under‑estimates O₂ uptake / over‑estimates CO₂ output. Mitigate with silicone grease and leak‑tests.
• Temperature fluctuations: change gas volumes; use a water‑bath and continuous monitoring.
• Water vapour pressure: saturated air reduces measured dry‑gas volume; correct by subtracting the vapour pressure at the experimental temperature.
• Incomplete mixing of gases: may give inaccurate readings; stir gently or use a magnetic stirrer.
• Biological variability: use sufficient replicates and report mean ± SD.

Interpretation of RQ Values (AO1)

• RQ ≈ 1.0 – Predominant carbohydrate oxidation (or lactate fermentation).
• RQ ≈ 0.7 – Predominant fat oxidation.
• RQ ≈ 0.8–0.9 – Mixed substrate utilisation; proteins contribute noticeably.
• RQ > 1.0 – May indicate lipogenesis (excess carbohydrate stored as fat) or hyperventilation during intense exercise.

Summary

The respiratory quotient (RQ) provides a simple, dimensionless link between the amounts of CO₂ produced and O₂ consumed during respiration. By relating RQ to the stoichiometry of the substrate’s oxidation, students can identify the main energy source (carbohydrate, fat or protein), estimate metabolic rate, and critically evaluate experimental data from respirometry investigations. Understanding the four stages of aerobic respiration, the role of ATP, and the alternative anaerobic pathways completes the Cambridge AS & A‑Level Biology requirements for the topic “Energy & Respiration”.