C1.2 Cell respiration

The question describes oxidation of a 3-carbon intermediate during glycolysis, coupled with the reduction of NAD⁺ to NADH. This is a redox reaction — one molecule loses electrons (is oxidised) and another gains them (is reduced).

NAD⁺ accepts hydrogen (electrons + H⁺) from the intermediate, becoming NADH. NADH acts as a loaded carrier — it stores energy in the form of high-energy electrons that will be passed to the electron transport chain (ETC).

At the ETC on the inner mitochondrial membrane, NADH donates its electrons. The energy released pumps protons across the membrane, and these protons flow back through ATP synthase, driving the synthesis of approximately 26–28 ATP per glucose via chemiosmosis.

The correct answer is: Energy released from oxidation of the 3-carbon compound reduces NAD⁺ to NADH, which carries electrons to the ETC where ATP is produced. This accurately describes the two-step link between substrate oxidation and oxidative phosphorylation.

The question asks how oxidation of a glycolytic intermediate and reduction of NAD⁺ contributes to ATP production later in respiration. The key concept is the role of NADH as an energy shuttle.

Option A states 'NAD⁺ directly synthesises ATP in the cytoplasm.' NAD⁺ is a coenzyme and cannot directly synthesise ATP — it is an electron carrier, not an enzyme or phosphate donor. Eliminate A.

Option C claims 'NAD⁺ is converted to NADH by absorbing energy from CO₂.' CO₂ is a waste product released by decarboxylation — it does not donate energy to NAD⁺. This is a reversal of the actual process. Eliminate C.

Option D says the oxidised intermediate 'transfers a phosphate group directly to the inner mitochondrial membrane to drive ATP synthase.' Substrate-level phosphorylation occurs in the cytoplasm and matrix, not on the membrane. ATP synthase is driven by proton flow, not direct phosphate transfer from intermediates. Eliminate D.

Option B correctly states that energy from oxidation reduces NAD⁺ to NADH, which carries electrons to the ETC for ATP synthesis. This is the established mechanism linking glycolysis to oxidative phosphorylation.

The experiment collects gas (CO₂) produced by yeast during anaerobic respiration. The volume of gas is measured in a graduated syringe over a set period of time.

Rate of respiration = $\frac{\text{volume of gas produced}}{\text{time}}$. Volume is measured in cm³ (or mL) and time in minutes, giving a unit of cm³ min⁻¹.

Since yeast produces CO₂ during alcoholic fermentation, the unit should specify this gas: cm³ CO₂ min⁻¹. This unit is standard in IB Biology for gas exchange and respiration rate measurements.

The most appropriate unit is cm³ CO₂ min⁻¹ because it expresses the volume of the measured gas per unit time, directly reflecting the rate of the process being investigated.

The question asks for the best unit to express the rate of gas production in a yeast respiration experiment. Rate = quantity per unit time.

'mg of glucose per hour' measures substrate consumption, not gas production. Since the experiment measures gas collected in a syringe, this unit does not match what is being measured. Eliminate this option.

'°C per minute' is a unit for temperature change, not for a volume of gas. Temperature is a controlled variable in this experiment, not the dependent variable. Eliminate this option.

'Number of yeast cells per mL' is a measure of cell density, not respiration rate. Cell counting would require a haemocytometer, not a syringe. Eliminate this option.

cm³ CO₂ min⁻¹ is the only option that expresses a volume of the measured gas (CO₂) per unit time (minutes), making it the correct unit for respiration rate in this context.