Biology · Energy, Enzymes & Metabolism

Cellular respiration, fermentation and diverse energy strategies

A germinating bean seed can respire before it has green leaves. A yeast cell can process sugar in a way that produces ethanol. Both obtain usable energy through chemical pathways, but their electron routes and products differ.

Read the Chemistry of Life and Cells & Membranes chapters first if atoms, molecular interactions and membrane transport are unfamiliar. The necessary ideas are also introduced here. You do not need to memorise complete biochemical pathways or calculate thermodynamic quantities to follow the first reading.

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By the end, you can

Distinguish ATP formation in glycolysis, respiration and fermentation, including the role of an electron acceptor.

Start with a shared route, then follow the branch

In the familiar glycolytic pathway, one six-carbon glucose is converted to two three-carbon pyruvate molecules. The net account includes two ATP and two NADH per glucose. ATP is formed by transferring phosphate from a reaction intermediate to ADP: substrate-level phosphorylation. Glycolysis itself does not directly consume oxygen and occurs in the cytosol in eukaryotic cells.

In complete aerobic oxidation, pyruvate processing and the citric acid cycle release carbon dioxide and produce reduced carriers. In a typical eukaryotic cell, these later processes are associated with mitochondria. Most ATP formation in this route is linked to oxidative phosphorylation, rather than produced directly by the citric acid cycle.

Ask where the electrons go after glycolysis

Not an exhaustive metabolic map. Many organisms use other substrates and routes; anaerobic does not mean fermentation.

Read the diagram in words

Glucose proceeds through glycolysis to pyruvate with a net two ATP and two NADH in the familiar pathway. Branches show familiar fermentation restoring NAD+ through organic acceptors, aerobic respiration using oxygen at the end of a respiratory electron-transfer chain, and anaerobic respiration using a different terminal acceptor. The branches compare electron routes, not three mandatory stages in every cell.

Glucose: Six-carbon input
Glycolysis: Net: 2 ATP, 2 NADH, 2 pyruvate per glucose
Familiar fermentation: Organic acceptor restores NAD+; lactate or ethanol route
Aerobic respiration: Respiratory electron transfer ends at O2; supports ATP formation
Anaerobic respiration: Respiratory electron transfer ends at another acceptor

Ask where the electrons go after glycolysis
Description	Process
Glucose → Glycolysis	Stepwise transformation
Glycolysis → Familiar fermentation	Pyruvate and NADH in the familiar examples
Glycolysis → Aerobic respiration	Further oxidation and carrier reoxidation
Glycolysis → Anaerobic respiration	Further transformations depend on the organism

Alternative routes — not an obligatory sequence
In the displayed fermentation examples, the net ATP comes from glycolysis
Respiratory ATP yield depends on the pathway and conditions

A membrane separates pumping from return flow

In mitochondrial aerobic respiration, electron transfer from reduced carriers towards oxygen is coupled to proton pumping from the matrix to the intermembrane space. The intact inner membrane helps maintain an electrochemical gradient: both proton concentration and electrical potential matter. Oxygen is reduced to water at the end of the electron-transfer route.

Protons returning through ATP synthase down that gradient can drive ATP formation from ADP and phosphate on the matrix side. The diagram separates electron transfer, outward pumping and inward return. It does not give a universal proton-to-ATP ratio. ATP yield varies with the organism, pathways, transport requirements, leak and conditions; one fixed yield is not a law of respiration.

Pumping out and returning through ATP synthase are different routes

A schematic eukaryotic mitochondrial example, not a proton-counting model or a scale drawing. Prokaryotic respiratory systems use their own membranes.

Read the diagram in words

The inner mitochondrial membrane separates the matrix below from the intermembrane space above. Respiratory electron transfer supports proton pumping out of the matrix. Proton return through ATP synthase down the electrochemical gradient supports ATP formation from ADP and phosphate on the matrix side. A dashed bypass arrow shows uncoupled return through a leak, which weakens the gradient without directly making ATP.

Intermembrane space: Higher proton concentration and more positive potential in the model
Matrix: Lower proton concentration relative to the intermembrane space

Respiratory proton pumping

Coupled to electron transfer

Inner mitochondrial membrane

ATP synthase

Couples proton return to ATP formation

Inner mitochondrial membrane

Bypass

Proton return without direct ATP formation

Inner mitochondrial membrane

H+ in matrix

Pump input and return destination

Matrix

H+ in intermembrane space

Pump destination and return input

Intermembrane space

ADP + phosphate → ATP

Synthesis on the matrix side

Matrix

Pumping out and returning through ATP synthase are different routes
Description	Process
H+ in matrix → Respiratory proton pumping	Pumping route
Respiratory proton pumping → H+ in intermembrane space	Against the electrochemical gradient
H+ in intermembrane space → ATP synthase	Return route
ATP synthase → H+ in matrix	Down the electrochemical gradient
ATP synthase → ADP + phosphate → ATP	Energy coupling
H+ in intermembrane space → Bypass	Uncoupled return
Bypass → H+ in matrix	Weakens the gradient

Inner mitochondrial membrane
Reduced carriers → respiratory electron transfer → O2 reduced to water
Concentration difference + electrical potential
Outward pumping
Inward return

Fermentation is a distinct way to recycle carriers

In familiar lactate fermentation, pyruvate accepts reducing equivalents from NADH, regenerating NAD+. In alcoholic fermentation, pyruvate is converted through acetaldehyde to ethanol, with carbon dioxide released. These steps allow glycolysis to continue supplying its net ATP. In these examples, the fermentation steps after glycolysis do not add ATP through an electron-transport chain.

Anaerobic respiration instead uses a respiratory electron-transfer system with a final acceptor other than oxygen, such as nitrate in some microbes. Absence of oxygen alone does not identify fermentation. Microorganisms also differ in their electron donors and carbon sources. The three branches are useful comparisons, not an exhaustive catalogue of metabolism.

Worked example

A membrane becomes leaky

Consider a simplified mitochondrion with continuing electron supply, oxygen available and an intact ATP synthase. If protons can return across the inner membrane by a bypass, less of their return flow is coupled to ATP formation. A weakened gradient can therefore reduce the ATP made per amount of respiratory substrate.

The observation 'oxygen is being used' would no longer be enough to infer the original ATP yield. Predicting the actual oxygen-consumption rate would need information about substrate supply and regulation. The qualitative comparison holds those other conditions fixed; it does not claim every leak produces the same response.

Check your understanding

Does a plant cell need photosynthesis to be occurring at that moment in order to respire?
Show reasoning
No. It can respire using available organic substrates, including reserves, in darkness or in nonphotosynthetic tissues.
A microbe uses a respiratory electron-transfer chain with nitrate as its final acceptor. Is this fermentation?
Show reasoning
It is an example of anaerobic respiration. The route and acceptor matter; simply lacking oxygen does not establish fermentation.
What does lactate formation restore that allows glycolysis to continue?
Show reasoning
It reoxidises NADH to NAD+, replenishing the oxidised carrier required by glycolysis. ATP in this familiar account is formed during glycolysis.

What to carry forward

Follow the carrier's final electron route and the mechanism of ATP formation, rather than sorting pathways only by the presence of oxygen.

References

Resources supporting this lesson’s explanations and examples.