Anaerobic Respiration and Required Practical Revision Notes | A Level AQA Biology

1. Why Anaerobic Respiration Exists

Your muscles are sprinting, your lungs cannot supply oxygen fast enough — so your cells switch strategy. Anaerobic respiration is not a backup plan; it is a precisely regulated metabolic response to oxygen shortage, and understanding it starts with one critical bottleneck: NAD.

Recall from glycolysis that glucose is split into two pyruvate molecules, generating a net yield of 2 ATP and, crucially, reducing two molecules of NAD to NADH (reduced NAD). The problem is that NAD is present in cells in limited quantities. If all the NAD becomes reduced and cannot be reoxidised, glycolysis grinds to a halt — no NAD, no glycolysis, no ATP.

Under aerobic conditions, the electron transport chain reoxidises NADH back to NAD. Under anaerobic conditions, that route is closed. Anaerobic respiration solves this by using pyruvate itself as the hydrogen acceptor, regenerating the oxidised NAD that glycolysis needs to keep running.

2. The Two Pathways: Lactate and Alcoholic Fermentation

There are two distinct anaerobic routes, and which one a cell uses depends on the organism. Both share the same purpose — regenerating oxidised NAD — but they produce different end products and occur in different cell types.

The reaction is a single-step reduction: pyruvate + reduced NAD → lactate + NAD. Because lactate is not a gas, it accumulates in the muscle tissue and blood, contributing to the metabolic acidosis associated with intense exercise. Lactate is not a waste product in the long term — once oxygen becomes available, it is transported to the liver and converted back to glucose via gluconeogenesis (the Cori cycle), but that detail sits beyond the immediate spec requirement.

Alcoholic fermentation is a two-step process: pyruvate → ethanal + CO₂, then ethanal + reduced NAD → ethanol + NAD. The carbon dioxide released is what causes bread dough to rise and beer to carbonate. Unlike lactate, ethanol cannot be converted back to pyruvate in yeast — it is an irreversible end product that becomes toxic to the yeast at concentrations above roughly 12–15%, which is why wine and beer have a natural alcohol ceiling.

3. Tracing the Hydrogen: A Step-by-Step Walkthrough

The logic of anaerobic respiration becomes much clearer when you trace exactly where the hydrogen atoms go. Here is a guided walkthrough for alcoholic fermentation — the pathway most commonly tested in AQA exam questions.

Worked example: Tracing Hydrogen Through Alcoholic Fermentation

Explain, step by step, how one molecule of glucose leads to ethanol production and how NAD is regenerated during anaerobic respiration in yeast.

1. Glucose (6C) → 2 Pyruvate (3C) + 2 ATP (net) + 2 reduced NAD — During glycolysis, NAD accepts hydrogen from triose phosphate, becoming reduced NAD (NADH). This is the step that creates the NAD bottleneck under anaerobic conditions.
2. Pyruvate (3C) → Ethanal (2C) + CO₂ [catalysed by pyruvate decarboxylase] — A carboxyl group is removed as carbon dioxide. The carbon skeleton shrinks from 3C to 2C. No NAD is involved at this stage.
3. Ethanal + reduced NAD → Ethanol + NAD [catalysed by ethanol dehydrogenase] — The hydrogen carried by reduced NAD is transferred to ethanal, reducing it to ethanol. This reoxidises NADH back to NAD. The regenerated NAD can now re-enter glycolysis at step 1.
4. 1 glucose → 2 ethanol + 2 CO₂ + 2 ATP (net) — The only ATP gain is from glycolysis. The fermentation steps themselves do not produce ATP — their sole function is to regenerate NAD so glycolysis can continue.

Answer: Per glucose: 2 ethanol + 2 CO₂ produced; 2 ATP net yield; 2 NAD regenerated for re-use in glycolysis.

4. Required Practical 9 — Investigating Respiration Rate in Single-Celled Organisms

AQA Required Practical 9 asks you to investigate how a named variable affects the rate of respiration in cultures of single-celled organisms such as yeast. The most common approach uses a redox indicator — typically methylene blue or resazurin — that changes colour as it is reduced by the reducing agents (e.g. reduced NAD) produced during respiration, giving a measurable proxy for respiration rate.

An alternative approach uses a respirometer to measure oxygen consumption (aerobic) or carbon dioxide production (anaerobic). For anaerobic investigations with yeast, measuring the volume of CO₂ produced per unit time is particularly appropriate because alcoholic fermentation releases CO₂ as a direct product, as you traced in the step-by-step walkthrough above.

Notice how the practical links directly back to the biochemistry: the CO₂ measured is produced specifically during the decarboxylation of pyruvate to ethanal — the first step of alcoholic fermentation. If you were investigating aerobic respiration instead, CO₂ would also be produced in the Krebs cycle, making it harder to isolate the anaerobic pathway. The paraffin oil layer is therefore a critical control, not an optional extra.

5. Evaluating the Investigation: Reliability, Validity, and Improvements

AQA exam questions on Required Practical 9 frequently ask you to evaluate the method rather than describe it. You need to distinguish between reliability (consistency of results across repeats) and validity (whether the measurement actually reflects respiration rate).

• Reliability issue: Yeast cultures are biological systems with natural variation between batches. Using the same batch of yeast throughout and taking at least three repeats at each temperature improves reliability.
• Validity issue: CO₂ could dissolve in the yeast suspension rather than being collected, leading to an underestimate of respiration rate. Acidifying the medium slightly reduces CO₂ solubility and improves validity.
• Validity issue: If the paraffin oil layer is too thin or disturbed, some aerobic respiration may occur alongside anaerobic respiration, confounding the results. Checking the oil layer before each run controls for this.
• Validity issue: Temperature fluctuations during the experiment alter enzyme activity independently of the variable being tested. Monitoring temperature with a thermometer throughout each run and discarding data from runs where temperature deviated by more than ±1°C improves validity.
• Precision improvement: Using a gas syringe instead of an inverted measuring cylinder allows smaller volumes of CO₂ to be measured more precisely, reducing random error.

6. Exam Technique and Common Mistakes

Anaerobic respiration questions are reliable mark-winners once you know the language the mark scheme expects. The most common reason students lose marks is imprecision — writing 'NAD is recycled' instead of specifying that NAD is reoxidised (oxidised NAD is regenerated), or writing 'pyruvate is broken down' without naming the product or the enzyme.

For practical evaluation questions, the command word 'evaluate' requires you to weigh strengths against limitations and reach a conclusion. A common error is listing limitations only, without acknowledging what the method does well. Structure your answer as: strength → limitation → how to address the limitation → overall judgement.

Bringing the lesson together: anaerobic respiration exists to solve a single biochemical problem — the depletion of oxidised NAD under oxygen-limited conditions. Whether the cell produces lactate or ethanol depends on the organism, but the outcome is identical in both cases: NAD is regenerated, glycolysis continues, and ATP production is sustained. The required practical gives you the tools to quantify this process and critically evaluate the evidence — both of which are skills the AQA examiner will reward directly.