Cellular Respiration: Aerobic vs Anaerobic, ATP Simulation | Shikkha Web

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Current stage: Glycolysis
Pathway: Aerobic respiration
ATP made: 0
NADH made: 0
FADH₂ made: 0
CO₂ released: 0
O₂ used: 0
Energy efficiency: 40.4%
End product: —
Glucose completed: 0
Cell clock: 0.0min

How to use this simulation

Start in the Whole cell view with oxygen on: a glucose molecule enters, splits at the glycolysis site into two pyruvate, then heads for the mitochondrion while the ATP pool on the right fills up.
Click through the stage buttons one at a time — Glycolysis, Link reaction, Krebs cycle, Electron transport chain — and watch the clock jump straight to the start of each one so you can study it in isolation.
Switch to Inside a mitochondrion: on the left the link reaction feeds the rotating Krebs cycle, on the right the cristae carry the electron transport chain with a spinning ATP synthase. Watch the H⁺ ions rise.
Turn oxygen off. The mitochondrion fades and stops turning, and back in the Whole cell view pyruvate now heads straight for fermentation instead.
Switch fermentation type: choose Lactic acid to watch it build up quietly at the muscle icon, or Alcohol to watch CO₂ bubbles rise from the yeast vat.
Compare the readings panel with oxygen on and off: ATP, NADH, FADH₂, CO₂, O₂ and the efficiency percentage all change, and by roughly the same ratio every time.

Why you gasp for breath after a sprint

Sprint 100 metres and your chest heaves, your heart pounds, and your legs start to burn. Why? Your muscle cells suddenly need energy far faster than your lungs and blood can deliver oxygen. Your body opens a backup route that makes some energy without oxygen at all. That whole story, the fast route and the slow one, is cellular respiration.

"Respiration" is easy to mix up with breathing, but they are not the same thing. Breathing is the mechanical act of moving air in and out of your lungs. Cellular respiration is the chemistry that happens inside every one of your cells, where food is actually broken down and its energy captured. Breathing supplies the oxygen; cellular respiration is what that oxygen gets used for.

On this page we will follow one glucose molecule step by step as it is taken apart and turned into ATP, see how the story changes completely depending on whether oxygen is available, and look at how the very same chemistry shows up in bread, yoghurt and a sore leg after a run.

What cellular respiration actually means

A definition worth learning cold: cellular respiration is the biochemical process by which a cell oxidises food, mainly glucose, to release chemical energy that is captured as ATP, giving off carbon dioxide and water as by-products. It happens in every living cell, not only animals: plants, fungi and bacteria all respire too, all the time, day and night.

Photosynthesis and respiration are opposites. Photosynthesis builds glucose using light energy; respiration tears glucose back down to release that energy. That makes respiration a catabolic process. The overall equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy, and almost all of that released energy is captured inside ATP molecules, a currency the cell can spend a little at a time exactly when and where it is needed.

Respiration comes in two forms, depending on whether oxygen is available. With oxygen it is aerobic respiration; without it, it is anaerobic respiration, better known as fermentation. Both start with the very same first stage, glycolysis, and only split apart afterwards — which is exactly what the oxygen switch in the simulation controls.

Key terms in cellular respiration

These words appear in almost every question on this topic. Learn them once and the rest follows easily.

Term	What it means
Cellular respiration	Breaking down food to release chemical energy captured as ATP
Aerobic respiration	Respiration that needs O₂; finishes in the mitochondrion; yields far more ATP
Anaerobic respiration (fermentation)	Respiration without O₂; stops in the cytoplasm; yields far less ATP
Glycolysis	Splitting one glucose into two pyruvate in the cytoplasm, making ATP and NADH
Link (oxidative decarboxylation) reaction	Turning pyruvate into acetyl-CoA, releasing CO₂ and making NADH
Krebs cycle (citric acid cycle)	A cyclic reaction in the mitochondrial matrix that makes ATP, NADH, FADH₂ and CO₂
Electron transport chain (ETC)	Protein complexes on the inner membrane that pass electrons to O₂ and pump H⁺
ATP synthase	The turbine-like enzyme that spins as H⁺ flows through it, joining ADP and phosphate into ATP
NADH and FADH₂	Electron carriers that ferry high-energy electrons to the electron transport chain
Mitochondrion	The cell's powerhouse: a double membrane enclosing the matrix and the folded cristae
Fermentation	Turning pyruvate into lactate or ethanol + CO₂ without O₂, regenerating NAD⁺
ATP	The cell's energy currency, adenosine triphosphate; releases energy when broken down
Respiratory quotient (RQ)	The ratio of CO₂ released to O₂ used during respiration

The equation and its mass balance

Written out in full: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (roughly 2,870 kJ per mole of glucose). The carbon in the CO₂ comes from glucose itself; the water forms from the oxygen you breathe in combining with hydrogen released from glucose along the way — the mirror image of photosynthesis, where the released oxygen comes from water rather than carbon dioxide.

Checking the mass balance: on the left, one glucose (180 g) plus six O₂ (6 × 32 = 192 g) makes 372 g. On the right, six CO₂ (6 × 44 = 264 g) plus six H₂O (108 g) also makes 372 g. The two sides match, so mass is conserved — only its form changes.

This equation is the whole basis of the simulation. With oxygen on, a glucose molecule follows it all the way to CO₂ and H₂O. With oxygen off, glucose only gets as far as glycolysis before turning into lactate or ethanol instead — a partial, much less complete breakdown.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energythe full aerobic respiration equation

C₆H₁₂O₆ → 2C₃H₆O₃ (lactic acid) + energylactic acid fermentation, e.g. in a sprinting muscle

C₆H₁₂O₆ → 2C₂H₅OH (ethanol) + 2CO₂ + energyyeast's alcoholic fermentation

Where each stage happens

Respiration's four stages happen in two different places in the cell, and that alone is one of the most commonly tested facts on this topic. It starts in the cytoplasm and, with oxygen, finishes in the mitochondrion; without oxygen the whole thing is stuck in the cytoplasm.

Stage	Where it happens	Needs O₂?
Glycolysis	Cytoplasm	No
Link reaction	Mitochondrial matrix (entry point)	Indirectly yes (only runs on the aerobic path)
Krebs cycle	Mitochondrial matrix	Indirectly yes
Electron transport chain	Inner mitochondrial membrane (cristae)	Directly yes — O₂ is the final electron acceptor
Fermentation (anaerobic)	Cytoplasm	No, not at all

The four stages, in detail

Now let's go stage by stage. Remembering what goes in and what comes out of each one makes the whole picture click.

Glycolysis: taking glucose apart

In the cytoplasm, one six-carbon glucose molecule is broken down, step by step, into two three-carbon pyruvate molecules. 2 ATP are spent early on to activate glucose, and 4 ATP are made later; after subtracting the cost, the net gain is 2 ATP and 2 NADH.

The single most important fact about glycolysis is that it needs no oxygen at all. Both the aerobic and anaerobic pathways pass through this exact same stage first — the fork in the road only comes right afterwards, when pyruvate's fate is decided by whether oxygen is available.

Input: one glucose (C₆H₁₂O₆)
Output: two pyruvate, a net 2 ATP, 2 NADH
Location: cytoplasm, across ten enzyme-catalysed steps

The link reaction: entering the mitochondrion

With oxygen available, both pyruvate molecules cross into the mitochondrion. Each one loses a CO₂ and gains an NADH as it is converted into a two-carbon acetyl-CoA. Across both pyruvate, that is 2 CO₂ and 2 NADH. No ATP is made in this step.

This is the step drawn on the left of the simulation's mitochondrion view, where pyruvate arrives from outside and CO₂ escapes.

Input: two pyruvate
Output: two acetyl-CoA, 2 CO₂, 2 NADH

The Krebs cycle: banking energy carriers

Each acetyl-CoA enters the Krebs cycle once, so for one glucose the cycle turns twice. Each turn makes 1 ATP, 3 NADH, 1 FADH₂ and 2 CO₂; across two turns that is 2 ATP, 6 NADH, 2 FADH₂ and 4 CO₂.

It keeps turning because the same intermediate compound (oxaloacetate) that starts each turn is regenerated at the end of it, ready to accept the next acetyl-CoA — which is exactly why it is drawn as a rotating loop in the simulation.

Input: two acetyl-CoA (two turns per glucose)
Output: 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂
Location: mitochondrial matrix

The electron transport chain: the real ATP factory

All the NADH and FADH₂ banked so far, 10 NADH and 2 FADH₂ in total, are cashed in here. A row of protein complexes on the inner membrane accepts their electrons, and as electrons move from complex to complex, the energy released pumps H⁺ ions out of the matrix, building up a concentration difference across the membrane.

That difference drives H⁺ back in through ATP synthase, a turbine-like enzyme, and the flow spins it, joining ADP and phosphate into ATP. At the end of the chain, electrons and H⁺ combine with O₂ to make water, exactly as the overall equation says.

On the classic accounting, each NADH yields 3 ATP and each FADH₂ yields 2, so this stage alone produces 34 ATP.

Input: 10 NADH, 2 FADH₂, 6 O₂
Output: 34 ATP, 6 H₂O
Location: inner mitochondrial membrane (cristae)

ATP accounting: getting to 38, stage by stage

Let's see each stage's contribution side by side. The electron transport chain itself makes no CO₂ at all — it simply spends the NADH and FADH₂ that the earlier stages banked.

Stage	ATP	NADH	FADH₂	CO₂
Glycolysis	2	2	0	0
Link reaction	0	2	0	2
Krebs cycle	2	6	2	4
Electron transport chain	34	—	—	—
Grand total	38	10	2	6

Classic 38 vs a modern ~32: why the number moves

Older textbooks (and this simulation) assume every NADH is worth 3 ATP and every FADH₂ worth 2, giving 38 in total. Modern measurements show that shuttling cytoplasmic NADH into the mitochondrion, and exporting finished ATP back out, both cost a little energy — so a more realistic figure is about 2.5 ATP per NADH and 1.5 per FADH₂.

At those modern ratios, the total comes to roughly 32.0 ATP, which is 6.0 fewer than the classic 38 — about 15.8% less. Exams still expect the classic figure of 38, so write that in your answer, but it is worth knowing the real number runs a bit lower.

Carrier	Classic ATP/molecule	Modern ATP/molecule
NADH	3	2.5
FADH₂	2	1.5
Grand total (per glucose)	38	32.0

Aerobic vs anaerobic respiration

Whether oxygen is available changes the whole story. Here are the two pathways side by side.

Feature	Aerobic respiration	Anaerobic respiration (fermentation)
Needs O₂?	Yes	No
Location	Cytoplasm + mitochondrion	Cytoplasm only
How much glucose is broken down	Completely, all the way to CO₂ and H₂O	Partially, stopping at lactate or ethanol
ATP per glucose	38	2
Energy efficiency	40.4%	2.13%
Products	CO₂, H₂O	Lactic acid or ethanol + CO₂
Seen in	Humans, animals, most plants	Sprinting muscle (briefly), yeast, some bacteria

Fermentation in everyday life

Anaerobic respiration might sound exotic, but it is happening around you constantly. Two kinds show up most often.

Lactic acid fermentation: the burn after a sprint

When you sprint, your muscle cells' demand for oxygen outruns what your blood can supply. Instead of entering the mitochondrion, pyruvate is converted to lactate right there in the cytoplasm. That step turns NADH back into NAD⁺, so glycolysis can keep running without oxygen. The lactic acid that builds up is what causes that burning, tight feeling in your muscles.

The same reaction is behind yoghurt: bacteria (Lactobacillus) ferment the sugar in milk (lactose) into lactic acid, which curdles the milk into yoghurt.

Alcoholic fermentation: the chemistry of bread and drinks

Without oxygen, yeast (a fungus) converts pyruvate first into acetaldehyde, releasing CO₂, and then into ethanol. In bread dough, it is exactly that CO₂ gas that forms the bubbles that make the dough rise; the alcohol evaporates away in the oven.

The same reaction, run on grape juice or grain instead, is the chemistry behind wine and beer, where the ethanol is what stays behind. The simulation's yeast vat shows exactly this: bubbles of CO₂ rising to the surface.

The respiratory quotient (RQ)

The respiratory quotient tells you which fuel a cell is actually burning. It is defined as the volume of CO₂ released divided by the volume of O₂ used, over the same period.

For glucose, the equation itself shows 6 O₂ used for 6 CO₂ released, so RQ = 6 ÷ 6 = 1.0. Fat molecules carry less oxygen per carbon, so oxidising them needs relatively more O₂; for a typical fat, RQ = 16 ÷ 23 = 0.70. Protein's measured RQ is about 0.8, because some of its nitrogen leaves as urea rather than as CO₂.

Fuel	CO₂ : O₂	RQ
Carbohydrate (glucose)	6 : 6	1.0
Fat	16 : 23	0.70
Protein	—	0.8

Cellular respiration vs photosynthesis

Photosynthesis and respiration are near-exact opposites, two sides of the same coin. Photosynthesis builds food from light; respiration tears that food back down to release energy. Our site also has a dedicated photosynthesis simulation, where you can see almost the mirror image of this page play out inside a chloroplast.

Feature	Photosynthesis	Respiration
Where it happens	Only in the chloroplasts of green cells	Every living cell (cytoplasm and mitochondria)
When	Only in light	Continuously, day and night
Raw materials	CO₂ and water	Glucose and O₂ (aerobic path)
Products	Glucose and O₂	CO₂, water and ATP
Energy direction	Light energy is stored in food	Stored energy in food is released as ATP
Type of metabolism	Anabolic (building)	Catabolic (breaking down)

Try these experiments in the simulation

Press Reset before each one, so the ATP and glucose counts start from zero.

Experiment 1: follow one glucose's whole journey

With oxygen on and the Whole cell view showing, press play from a reset state. Click through the stage buttons in order — Glycolysis, Link reaction, Krebs cycle, Electron transport chain — and watch how the story advances at each one. One full lap takes 12 seconds at 1×.

Experiment 2: cut the oxygen supply

While it's running, turn oxygen off. Switch to the mitochondrion view and you will see it fade and the Krebs cycle stop turning. Back in the Whole cell view, pyruvate now heads straight to fermentation instead.

Experiment 3: compare the two kinds of fermentation

With oxygen still off, switch the fermentation type. Choose Lactic acid and watch it build up quietly at the muscle icon with no bubbles at all. Choose Alcohol and watch bubbles of CO₂ rise from the yeast vat.

Experiment 4: check the ATP numbers add up

Run it for one full minute at 1× and note how much the ATP reading has grown. In one minute, about 5.0 glucose molecules should be completed, so ATP should rise by roughly 190.0. With oxygen off across the same minute, ATP rises by only about 10.0.

Experiment 5: change the speed

Try the speed buttons from 0.25× to 2×. The clock and the ATP count both scale proportionally, because the ATP made per glucose never changes — only how quickly the events happen does.

Solved problems

Every number below is computed straight from the same model the simulation runs, never typed in by hand.

Problem 1: balance the equation by mass

On the left, one glucose (180 g) plus six O₂ (192 g) makes 372 g. On the right, six CO₂ (264 g) plus six H₂O (108 g) also makes 372 g. The two sides match, confirming mass is conserved.

Problem 2: split the 38 ATP by source

Of the 38 total ATP, glycolysis contributes 2 (5.3%), the Krebs cycle contributes 2 (5.3%), and the electron transport chain contributes 34 (89.5%). Adding them: 2 + 2 + 34 = 38, which checks out.

Problem 3: the ETC's ATP from NADH and FADH₂

10 NADH and 2 FADH₂ have been banked. Classic yield: ATP = (10 × 3) + (2 × 2) = 34. Modern yield: ATP = (10 × 2.5) + (2 × 1.5) = 28.0.

Problem 4: the gap between classic and modern totals

Classic total: 38 ATP. Modern total: about 32.0 ATP. The difference is 38 − 32.0 = 6.0 ATP, which is 15.8% of the classic figure.

Problem 5: work out the energy efficiency

One mole of glucose stores about 2,870 kJ. Each ATP stores about 30.5 kJ. Aerobic efficiency = (38 × 30.5) ÷ 2,870 × 100 = 40.4%. Anaerobic efficiency = (2 × 30.5) ÷ 2,870 × 100 = 2.13%.

Problem 6: the ATP ratio between the two pathways

Aerobic ATP ÷ anaerobic ATP = 38 ÷ 2 = 19.0. From the very same glucose molecule, aerobic respiration extracts roughly 19.0 times more usable energy than anaerobic respiration.

Problem 7: reading a measured RQ

If someone's measured RQ is 0.85 (between carbohydrate's 1.0 and fat's 0.70), that tells you their cells are burning a mix of carbohydrate and fat at that moment, not pure carbohydrate or pure fat alone.

Common mistakes

Run through this list once before your exam.

Treating "respiration" and "breathing" as the same word. Breathing is the physical act of moving air; respiration is the chemistry inside the cell.
Assuming anaerobic respiration makes zero ATP. Glycolysis alone still makes a net 2 ATP; it is only the much larger downstream stages that shut down.
Thinking NADH "disappears" during fermentation. It hands its electrons back to pyruvate (or a compound made from it), turning back into NAD⁺ so glycolysis can keep running.
Believing the Krebs cycle makes lots of ATP directly. One turn makes just 1 ATP; its real contribution is banking NADH and FADH₂ for the electron transport chain to use later.
Thinking the electron transport chain itself produces CO₂. All the CO₂ comes from the link reaction and the Krebs cycle; the ETC only uses O₂ to make water and ATP.
Treating 38 ATP as the one correct answer everywhere. It is the classic textbook figure; the modern, more realistic number is closer to 30–32.

Cellular respiration in real life

From rising bread dough to the ache in your legs after a run, this same chemistry keeps turning up.

Bread and cake: the CO₂ gas from yeast's alcoholic fermentation is what makes dough rise; the alcohol evaporates in the oven.
Yoghurt: lactic acid bacteria ferment milk sugar (lactose) into lactic acid, which curdles milk into yoghurt.
Sports science: trained athletes have more blood vessels and mitochondria in their muscles, so they stay on the aerobic pathway longer and build up less lactic acid.
High altitude: with less oxygen available, the body relies more on anaerobic respiration, which is part of why altitude sickness causes dizziness and weakness.
Industry: the same fermentation reactions are used to manufacture industrial ethanol, vinegar and many antibiotics.
Cancer research: many cancer cells rely heavily on anaerobic-style glucose breakdown even when oxygen is present (the Warburg effect), a major area of ongoing research.

Exam corner

General exam boards (GCSE, IGCSE, AP Biology and similar syllabuses) ask about cellular respiration in three recurring shapes: define and compare aerobic vs anaerobic respiration, label a diagram of stages and their locations, and calculate or interpret ATP yield and respiratory quotient.

Always state the location alongside the stage: glycolysis (cytoplasm), Krebs cycle (matrix), electron transport chain (inner membrane/cristae).
When asked to compare aerobic and anaerobic respiration, cover all of: oxygen requirement, location, ATP yield, and end products — a partial comparison loses marks.
A calculation question on ATP yield expects you to add glycolysis + Krebs cycle + electron transport chain contributions explicitly, not just quote the final total.
For RQ questions, always show the ratio (CO₂ ÷ O₂) rather than just stating the number, and be ready to say what a value above or below 1 implies.

Revision: the one-screen summary

Read through this the night before your exam.

Definition: oxidising food to store energy in ATP, releasing CO₂ and H₂O as by-products.
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
Glycolysis (cytoplasm): glucose → 2 pyruvate, net 2 ATP, 2 NADH.
Link reaction + Krebs cycle (matrix): 2 ATP, 10 NADH and 2 FADH₂ banked in total, CO₂ released.
Electron transport chain (cristae): NADH and FADH₂ yield 34 ATP; O₂ + H⁺ + e⁻ → H₂O.
Total (classic): 38 ATP aerobic; anaerobic total: 2 ATP.
Anaerobic respiration: lactic acid (muscle, yoghurt) or ethanol + CO₂ (yeast, bread, alcoholic drinks).
RQ = CO₂ released ÷ O₂ used; the RQ of carbohydrate is 1, and of fat is about 0.7.

Frequently asked questions

What is cellular respiration?

Cellular respiration is the biochemical process by which a cell oxidises food, mainly glucose, to release chemical energy captured as ATP, giving off carbon dioxide and water as by-products.

What's the difference between respiration and breathing?

Breathing is the physical, mechanical act of moving air in and out of the lungs. Cellular respiration is the chemical process inside cells that actually uses the oxygen breathing supplies to release energy from food.

What's the main difference between aerobic and anaerobic respiration?

Aerobic respiration needs oxygen, finishes in the mitochondrion, and yields 38 ATP per glucose. Anaerobic respiration needs no oxygen, stays in the cytoplasm, and yields only 2 ATP.

Where does glycolysis happen and what does it produce?

Glycolysis happens in the cytoplasm. It splits one glucose into two pyruvate, producing a net 2 ATP and 2 NADH, and it needs no oxygen.

What is the Krebs cycle and where does it happen?

The Krebs cycle is a cyclic reaction in the mitochondrial matrix that breaks down acetyl-CoA to yield ATP, NADH, FADH₂ and CO₂. It turns twice per glucose, producing 2 ATP, 6 NADH and 2 FADH₂ in total.

Why does the electron transport chain make the most ATP?

Because it cashes in the 10 NADH and 2 FADH₂ banked earlier, using their electrons to pump H⁺ and spin ATP synthase, producing 34 ATP in total — about 89.5% of the classic 38.

Does aerobic respiration really make 38 ATP?

That is the classic textbook figure most syllabuses still teach. Because of energy costs in shuttling molecules across the mitochondrial membrane, a more realistic modern estimate is about 32.0 ATP, but exams generally still expect 38.

What is fermentation?

Fermentation is anaerobic respiration: converting pyruvate into lactate or into ethanol plus CO₂ without oxygen, which regenerates NAD⁺ so glycolysis can keep running.

Why do muscles feel sore or cramp after intense exercise?

When exercise is intense, muscle cells' oxygen demand outruns supply, so they switch to lactic acid fermentation. The lactic acid that builds up causes that burning, tight feeling.

What does the respiratory quotient (RQ) tell you?

RQ = CO₂ released ÷ O₂ used. Carbohydrate's RQ is 1.0, fat's is about 0.70, and protein's is about 0.8. It reveals which fuel a cell is actually burning.

They are near-opposites: photosynthesis uses light to build glucose and release O₂, while respiration breaks that glucose down to make ATP and release CO₂. The product of one is the raw material of the other.