AP Syllabus focus:
‘Fermentation allows glycolysis to continue without oxygen, regenerating NAD+ and producing organic products like alcohol or lactic acid.’
Fermentation is a set of anaerobic reactions that keep cells making a small but vital supply of ATP when oxygen is unavailable by recycling key electron carriers.
Core idea: why fermentation happens
Cells rely on glycolysis to generate ATP quickly. Glycolysis requires a steady supply of NAD+ to accept electrons. When oxygen is absent, the electron transport chain cannot oxidise NADH back to NAD+, so glycolysis would stop unless NAD+ is regenerated another way.
Fermentation: An anaerobic metabolic process that regenerates NAD+ from NADH by transferring electrons to an organic molecule, allowing glycolysis to continue producing ATP.
A crucial AP Biology takeaway is that fermentation does not add extra ATP beyond glycolysis; its main function is maintaining electron flow so glycolysis can keep running.
What fermentation accomplishes
Regenerates NAD+ from NADH (maintains redox balance)
Enables continued ATP production by substrate-level phosphorylation in glycolysis
Produces organic end products (e.g., lactate or ethanol) that store the electrons removed from glucose
Redox logic: keeping NAD+ available
NAD+ acts as an electron acceptor during glycolysis and becomes NADH. Without oxygen, NADH must be oxidised by passing its electrons to an organic acceptor (usually derived from pyruvate). This restores NAD+ so glycolysis can proceed.
The central constraint in anaerobic conditions is not “lack of ATP machinery,” but lack of an external terminal electron acceptor; fermentation substitutes by using internal organic molecules.
Major fermentation pathways (AP-aligned)
Lactic acid fermentation
Common in many bacteria and in animal cells under oxygen limitation (e.g., working muscle). Pyruvate (from glycolysis) is reduced to lactate, and NADH is oxidised to NAD+.
Lactic acid fermentation couples the reduction of pyruvate to lactate with the oxidation of NADH back to NAD+. The diagram emphasizes the redox recycling that maintains a usable NAD+ pool so glycolysis can keep producing ATP by substrate-level phosphorylation when oxygen is limited. Source
= Electron acceptor derived from glycolysis (no unit in this context)
= Reduced electron carrier donating electrons (no unit in this context)
= Oxidised electron carrier regenerated for glycolysis (no unit in this context)
Key outcomes:
NAD+ is regenerated immediately
Lactate is the organic product that retains electrons
Useful for short-term ATP production when oxygen delivery is limited
Alcohol (ethanol) fermentation
Common in yeasts and some plant cells.
Alcohol (ethanol) fermentation proceeds in two main steps: pyruvate is decarboxylated to acetaldehyde (releasing CO2), then acetaldehyde is reduced to ethanol. The figure highlights that NADH is oxidized to NAD+ during the reduction step, which is what keeps glycolysis running in anaerobic conditions. Source
Pyruvate is converted to ethanol and CO₂, with NADH oxidised to NAD+. Although the steps differ from lactic fermentation, the purpose is the same: NAD+ regeneration so glycolysis continues.
Key outcomes:
Produces ethanol as the reduced organic product
Releases carbon dioxide as pyruvate is processed
Maintains glycolysis under anaerobic conditions
What “allows glycolysis to continue” means mechanistically
For glycolysis to keep turning over, cells must:
Maintain a pool of NAD+ for the glyceraldehyde-3-phosphate oxidation step
Prevent buildup of NADH that would stall redox reactions
Continue producing ATP via glycolysis even though yield is low compared with aerobic pathways
Fermentation is therefore best understood as a metabolic backup that preserves ATP production rate (not efficiency) under oxygen limitation.
Biological significance and limits
Advantages
Supports survival in low-oxygen environments
Provides rapid ATP production when oxygen supply cannot meet demand
Enables organisms with anaerobic lifestyles to generate ATP using glycolysis continuously
Costs and constraints
Low ATP yield per glucose (ATP comes only from glycolysis)
Accumulation of organic end products (lactate or ethanol) can affect cellular conditions
Continued operation depends on availability of fermentable substrates (e.g., sugars)
FAQ
If ATP demand briefly exceeds oxygen delivery, fermentation can supply ATP faster per unit time, despite low efficiency.
It depends on enzymes present in the organism/tissue. Different enzyme sets channel pyruvate into different reduction pathways.
Lactate can be transported to other tissues and converted back to useful metabolites when oxygen availability improves.
Production and export of acidic or charged end products can alter ion balance and contribute indirectly to pH changes.
Glycolysis requires NAD$^+$ as an electron acceptor; without recycling NAD$^+$, key oxidation steps halt and ATP output stops.
Practice Questions
State two roles of fermentation in cells when oxygen is absent. (1–3 marks)
Regenerates NAD from NADH (1)
Allows glycolysis to continue producing ATP in the absence of oxygen (1)
Explain how fermentation maintains ATP production without oxygen and compare the organic products formed in lactic acid fermentation and alcohol fermentation. (4–6 marks)
Without oxygen, NADH cannot be oxidised via aerobic pathways (1)
Fermentation oxidises NADH to NAD by transferring electrons to an organic molecule (1)
Regenerated NAD allows glycolysis to continue (1)
ATP is produced only via glycolysis/substrate-level phosphorylation (1)
Lactic acid fermentation produces lactate (1)
Alcohol fermentation produces ethanol (and releases CO) (1)
