⚡ Tier: A-level / IB Biology & Early Undergraduate
Watch glucose enter the cell, split in half during glycolysis, lose its carbons one by one in the Krebs cycle, and finally power the H⁺ turbine that makes ~30 ATP — the chemistry that keeps every breath you take alive.
1 · Interactive Simulation
🔬 Cell Overview
⚗ Glycolysis
🌀 Krebs Cycle
⚡ ETC + ATP Synthase
📊 ATP Yield
Step 1 — Glucose enters the cell
Glucose is transported into the cytoplasm, ready to begin glycolysis.
Stage 1 / 4
Current stage
Glycolysis
Glucose used
0
ATP made
0
NADH made
0
FADH₂ made
0
CO₂ released
0
Bio-time
0 s
▶ Playback
⚙ Preset Condition
🎚 Cellular Parameters
🎛 Display Toggles
Step labels
Show molecules
Show enzymes
Show electrons
Show H⁺ gradient
Show text labels
📚 Key fact
Total yield (aerobic): 1 glucose → ~30–32 ATP (modern textbook value). Old value 36–38 ATP did not account for the cost of transporting NADH into mitochondria.
2 · The Idea, Step by Step
Start here: food is fuel, ATP is the battery
Eat a snack, then run around — the food becomes the energy that moves you. But a cell can't spend a glucose molecule directly any more than a phone can run straight off crude oil. A car burns its fuel in one quick flash; a cell does the opposite, taking glucose apart in many tiny, careful steps so the energy comes out in small usable packets instead of one burst of heat. Those packets are stored in a tiny rechargeable battery called ATP. Everything you do — thinking, moving, even staying warm — is paid for by spending ATP.
Build it up: one trade, written down
Three players matter: glucose (the fuel), oxygen (what finishes the job), and ATP (the battery the cell actually spends). In plain words, one glucose plus six oxygen turns into six carbon dioxide, six water, and a big stack of recharged batteries:
The one number to remember: one glucose recharges about 30 ATP. Burn glucose in open air and all 2870 kJ/mol comes out as heat; the cell instead saves roughly a third of it as ATP and lets the rest warm you.
Go deeper: where the 30 actually come from
That ATP arrives in four stages. Glycolysis splits glucose into two pyruvate in the cytoplasm (net 2 ATP, no oxygen needed). The link reaction and Krebs cycle in the mitochondrial matrix strip the carbons off as CO$_2$ and load up the electron-carriers $\mathrm{NADH}$ and $\mathrm{FADH_2}$. Finally, oxidative phosphorylation sends those electrons down the chain in the inner membrane; the energy pumps $\mathrm{H^+}$ across the membrane, and the protons flowing back through ATP synthase — a real molecular turbine — make most of the ATP. The exact tally is $10\,\mathrm{NADH}\times2.5 + 2\,\mathrm{FADH_2}\times1.5 + 4\ \text{direct} = 32$. Oxygen's only job is to catch the spent electrons at the very end (forming water) — pull it away and the whole line jams. The sliders map onto this: O₂ availability is the electron drain, glucose supply is the fuel rate, and temperature sets enzyme speed ($Q_{10}\approx2$).
Try this in the sim above
Drag O₂ to 0% and watch the mitochondrion fall silent within seconds — the yield collapses to just 2 ATP as pyruvate ferments to lactate. Then pick the Cyanide poisoning preset: Complex IV is blocked, so the chain stalls even with oxygen present — the reason cyanide kills in minutes. Finally, push Temperature from 37 °C up toward 45 °C and watch the enzymes slow as they denature, then back down to 5 °C to see them turn sluggish.
3 · Biological & Mathematical Analysis
The Process — Cellular Respiration
Cellular Respiration — Embden & Meyerhof (1930s) elucidated glycolysis; Hans Krebs (1937) discovered the citric acid cycle, Nobel Prize 1953; Peter Mitchell (1961) proposed the chemiosmotic theory, Nobel Prize 1978; Paul Boyer & John Walker (1997) Nobel Prize for ATP synthase mechanism.
Cellular respiration is the process by which cells extract energy from food molecules (mainly glucose) and store it in the form of ATP — the universal energy currency of life. Every heartbeat, every thought, every muscle twitch is paid for by this molecular machinery.
Step-by-step explanation (visualize first, then quantify)
Why do cells need respiration? Glucose is a stable molecule — it doesn't spontaneously power your muscles. The cell must break glucose down in small, controlled steps so that the released energy is captured as ATP, not lost as heat. Doing this in one step would release 2870 kJ at once (a fireball); doing it in dozens of small steps lets the cell save most of that energy as chemical bonds in ATP.
Glycolysis (cytoplasm). Glucose (6C) is split into two pyruvate molecules (3C) through 10 enzyme-catalysed steps. First, 2 ATP are invested to phosphorylate the sugar (so it can't leave the cell). Then 4 ATP are produced by substrate-level phosphorylation. Net: 2 ATP + 2 NADH. No oxygen needed.
Link reaction (mitochondrial matrix). Each pyruvate (3C) is decarboxylated — losing one carbon as CO₂ — and joined to coenzyme A to form acetyl-CoA (2C). Per pyruvate: 1 NADH + 1 CO₂. Per glucose (×2): 2 NADH + 2 CO₂.
Krebs cycle (mitochondrial matrix). Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Citrate is then oxidised through 8 steps, losing 2 CO₂ and regenerating oxaloacetate. Per cycle turn: 1 ATP + 3 NADH + 1 FADH₂ + 2 CO₂. The cycle runs twice per glucose (once per acetyl-CoA), so per glucose: 2 ATP + 6 NADH + 2 FADH₂ + 4 CO₂.
Oxidative phosphorylation — Electron Transport Chain (inner membrane). NADH and FADH₂ donate their electrons to a chain of 4 protein complexes (I, II, III, IV). As electrons pass down the chain (losing energy), the released energy pumps H⁺ from the matrix into the intermembrane space, building a proton gradient (chemiosmosis).
Oxygen is the final electron acceptor. At Complex IV, two electrons combine with ½ O₂ and 2 H⁺ to form water. Without O₂, the chain backs up — NADH cannot be re-oxidised, NAD⁺ runs out, and the Krebs cycle and glycolysis halt. This is why oxygen is essential.
ATP synthesis by chemiosmosis. H⁺ flows back from the intermembrane space into the matrix through ATP synthase, a rotating molecular turbine. The mechanical rotation drives the condensation of ADP + Pᵢ → ATP. About 3 H⁺ are needed per ATP, plus 1 H⁺ to import Pᵢ — so roughly 4 H⁺ per ATP.
ATP yield accounting. Each NADH produces ~2.5 ATP (modern P/O ratio); each FADH₂ produces ~1.5 ATP (enters at Complex II, bypasses Complex I, so fewer H⁺ pumped). Per glucose: glycolysis 2 ATP + 2 NADH (×2.5 in heart/liver via the malate-aspartate shuttle, ×1.5 in skeletal muscle/brain via the glycerol-3-phosphate shuttle); link 2 NADH; Krebs 2 ATP + 6 NADH + 2 FADH₂; ETC total: ~28 ATP. Grand total ≈ 30–32 ATP per glucose.
Anaerobic alternatives. Without O₂, the ETC stops, so NAD⁺ is regenerated by fermenting pyruvate. In animals: pyruvate + NADH → lactate + NAD⁺ (lactic acid fermentation). In yeast: pyruvate → acetaldehyde + CO₂; acetaldehyde + NADH → ethanol + NAD⁺ (alcoholic fermentation). Both yield only 2 ATP per glucose — 15× less than aerobic.
The Mathematics — ATP Accounting & Efficiency
The energy released per mole of glucose under standard conditions is:
ATP synthesis stores about 30.5 kJ mol⁻¹ per ATP. So the maximum theoretical ATP yield, if all energy were captured, would be 2870 / 30.5 ≈ 94 ATP. The cell achieves about 30 ATP — so the efficiency is:
Fats (lipids): RQ ≈ 0.7 (less O in molecule → more O₂ needed)
Temperature Coefficient — Q₁₀
The rate of respiration (like any enzyme-catalysed process) roughly doubles for every 10°C rise in temperature, up to the optimum:
$$ Q_{10} = \dfrac{R_{T+10}}{R_T} \approx 2 $$
What the simulation shows
The default Cell Overview tab gives the big-picture map: glucose enters the cytoplasm where glycolysis happens, pyruvate moves into the mitochondrion for the link reaction and Krebs cycle, and the ETC operates across the inner membrane. Each stage's ATP, NADH, FADH₂ and CO₂ contributions are tracked live in the readout cards.
The Glycolysis tab shows the 10-step pathway as a vertical sequence: glucose phosphorylated to G6P → F6P → F1,6BP (ATP invested) → split into two G3P molecules → each oxidised with NAD⁺ → ultimately to pyruvate (ATP produced). The Krebs Cycle tab shows the 8-step circular pathway with CO₂ released at decarboxylation steps and electron carriers (NADH/FADH₂) leaving at each oxidation. The ETC + ATP Synthase tab shows electrons flowing through Complexes I → III → IV (or II → III → IV for FADH₂), H⁺ being pumped, and the ATP synthase rotating as protons flow back.
Set the O₂ availability slider to 0 and watch the entire ETC halt — pyruvate then ferments to lactate (or ethanol in yeast preset) and only 2 ATP/glucose are produced. The Cyanide poisoning preset blocks Complex IV, demonstrating why cyanide kills within minutes (cells can no longer make ATP, even with oxygen present).
Worked numerical example
Example — ATP yield calculation
For one molecule of glucose oxidised completely under aerobic conditions:
Stage
ATP (direct)
NADH
FADH₂
CO₂
Glycolysis
+2 (net)
+2
0
0
Link reaction (×2)
0
+2
0
+2
Krebs cycle (×2)
+2
+6
+2
+4
Subtotal
4 ATP
10 NADH
2 FADH₂
6 CO₂
From ETC (using modern P/O ratios — 2.5 ATP per NADH, 1.5 ATP per FADH₂):
(The 2 glycolytic NADH must be shuttled into the mitochondrion. The malate-aspartate shuttle delivers them as NADH (yielding 2.5 ATP each), but the glycerol-3-phosphate shuttle delivers them as FADH₂ (1.5 ATP each), giving 30 ATP. Hence the textbook range "30–32 ATP".)
References
Campbell, N. A. & Reece, J. B. — Biology, 12th ed., Pearson, 2021, Chapter 9: "Cellular Respiration and Fermentation".
Alberts, B. et al. — Molecular Biology of the Cell, 7th ed., W. W. Norton, 2022, Chapter 14: "Energy Conversion: Mitochondria and Chloroplasts".
Berg, J. M., Tymoczko, J. L. & Stryer, L. — Biochemistry, 9th ed., Freeman, 2019, Chapters 16–18 (Glycolysis, Krebs, Oxidative phosphorylation).
Jones, M., Fosbery, R., Taylor, J. & Gregory, J. — Cambridge International AS & A Level Biology Coursebook, 5th ed., Hodder/CUP, 2022, Chapter 12: "Respiration".
Allott, A. & Mindorff, D. — Biology for the IB Diploma, 3rd ed., OUP, 2023, Topic 8.2.
Mitchell, P. — "Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism", Nature, 191: 144–148, 1961.
Krebs, H. A. & Johnson, W. A. — "The role of citric acid in intermediate metabolism in animal tissues", Enzymologia, 4: 148–156, 1937.
4 · Frequently Asked Questions
⚡ ConceptualWhy does the cell need four separate stages instead of one big reaction?▼
Burning glucose in a single reaction releases 2870 kJ/mol — that would literally cook the cell. By breaking glucose down in 30+ small enzyme-controlled steps, the cell captures most of that energy in small, manageable amounts (each step releasing about 30 kJ, which is exactly enough to phosphorylate one ADP to ATP). Different stages also evolved at different times — glycolysis is so ancient that even bacteria use it (and don't need oxygen), while the Krebs cycle and ETC evolved later with the rise of oxygen on Earth. The compartmentalisation (cytoplasm vs matrix vs inner membrane) lets each stage have its own optimal pH and enzyme concentrations.
Key takeaway: Many small controlled steps = high energy capture + low heat loss + evolutionary modularity.
🌍 AppliedWhere does this appear in medicine and exercise science?▼
Cellular respiration is the target of dozens of drugs and the explanation of many diseases. Cyanide and carbon monoxide kill by blocking Complex IV — your cells can't use the oxygen in your blood, so you suffocate at the cellular level even when breathing fresh air. Metformin (the world's most-prescribed diabetes drug) partially inhibits Complex I, mildly lowering ATP production and reducing glucose use by the liver. Mitochondrial diseases (Leber's hereditary optic neuropathy, MELAS syndrome) are caused by mutations in mitochondrial DNA and primarily affect high-energy tissues — brain, muscle, heart. In exercise physiology, the switch from aerobic (steady jog) to anaerobic respiration (sprint) shows up as lactate accumulation, the "burn" you feel — measured as the lactate threshold, a key predictor of endurance performance. Brown adipose tissue in infants and hibernating mammals uses an uncoupling protein (UCP1) to generate heat instead of ATP — the biological basis of non-shivering thermogenesis.
Key takeaway: From cyanide to metformin to your sprint training, every intervention works by tweaking some step you just animated.
🔬 SimulationWhat exactly is the simulation showing?▼
The Cell Overview tab is a simplified eukaryotic cell with the cytoplasm (outer region) and a mitochondrion (inner region). Glucose enters from the left, is split into two pyruvates in the cytoplasm (glycolysis), the pyruvates cross into the mitochondrion (link reaction), and go through the Krebs cycle in the matrix. CO₂ bubbles leave at the decarboxylation steps. Blue particles are electrons being carried by NADH/FADH₂ to the inner membrane, where they fall through the Electron Transport Chain. Red particles are H⁺ being pumped into the intermembrane space — and you can watch ATP synthase rotate as they flow back through. Set O₂ to 0% and watch the entire mitochondrion go silent within seconds.
Key takeaway: This is the actual energy economy of every cell in your body, running roughly 10⁹ times per second.
💡 Non-obviousWhy is oxygen so essential if it's "just" the final electron acceptor?▼
Oxygen isn't directly needed to make ATP — Complex IV makes water by combining electrons, protons, and O₂, but ATP itself is made by ATP synthase, which only needs the H⁺ gradient. So why do we die without oxygen in 4 minutes? Because without an electron acceptor at the end of the chain, electrons back up. NADH cannot offload its electrons, so NAD⁺ runs out. Without NAD⁺, glycolysis and the Krebs cycle can't proceed (they both require NAD⁺ to accept electrons in their oxidation steps). The whole metabolic chain freezes. Oxygen is essential not as a fuel but as the electron drain that keeps the whole pipeline flowing. Think of it like a sink: water only flows out of the tap if the drain is open.
Key takeaway: O₂ is the drain, not the fuel. Block the drain, and the entire metabolic plumbing backs up.
📐 QuantitativeWhy is the ATP yield now "30–32" instead of the old "36–38"?▼
Older textbooks assumed each NADH made exactly 3 ATP and each FADH₂ made exactly 2 ATP, giving 38 total per glucose. Modern measurements show the real ratios are about 2.5 ATP per NADH and 1.5 ATP per FADH₂. Why the difference? Because ATP synthase needs ~3 H⁺ per ATP made, plus an extra H⁺ to import Pᵢ — about 4 H⁺ per ATP. Complex I pumps 4 H⁺, Complex III pumps 4 H⁺, Complex IV pumps 2 H⁺ (per 2 electrons) — that's 10 H⁺ per NADH, giving 10/4 = 2.5 ATP. FADH₂ enters at Complex II, skipping Complex I, so only 6 H⁺ pumped, giving 6/4 = 1.5 ATP. Additionally, cytoplasmic NADH (from glycolysis) must be shuttled into mitochondria, and one shuttle (glycerol-3-phosphate) delivers them as FADH₂ — losing 1 ATP per NADH. Total: 30–32 ATP, depending on which shuttle a tissue uses.
Key takeaway: The "38" was a round number from incomplete chemistry; modern P/O ratios give 30–32. Both are accepted in A-level exams but always justify your number.
🎓 DeepWhat happens when respiration goes wrong?▼
Almost every disease has a mitochondrial angle. Cancer cells often switch to aerobic glycolysis (the Warburg effect) — they make ATP from glycolysis alone even when O₂ is plentiful, supporting fast biosynthesis for rapid division. PET scans exploit this: cancer cells take up the glucose tracer ¹⁸F-FDG voraciously. Heart attack and stroke are essentially respiration failures — blood flow stops, O₂ stops, ATP plummets in seconds, Na⁺/K⁺ pumps fail, cells swell and die within minutes. Reactive oxygen species (ROS) leak from the ETC when Complex I or III misbehaves — these damage DNA and proteins and underlie much of aging. Parkinson's disease involves Complex I dysfunction in dopaminergic neurons; certain pesticides (rotenone, MPTP) cause parkinsonism by blocking Complex I in animal models. Sepsis involves mitochondrial dysfunction in many tissues, causing multi-organ failure even when blood pressure is restored.
Key takeaway: Mitochondrial dysfunction is the molecular common denominator of most modern chronic diseases.
⚡ ConceptualIs "anaerobic respiration" the same as "fermentation"?▼
In A-level biology, the two terms are often used interchangeably, but strictly they're different. Fermentation (in animals → lactate; in yeast → ethanol) uses no electron transport chain — pyruvate is reduced by NADH to regenerate NAD⁺, full stop. Yield: 2 ATP/glucose. True anaerobic respiration, used by some bacteria and archaea, uses an electron transport chain with a non-O₂ final electron acceptor — sulfate (SO₄²⁻), nitrate (NO₃⁻), CO₂ (in methanogens), or even Fe³⁺. These still pump H⁺ and make ATP via chemiosmosis, often yielding more than 2 ATP per glucose. So in human biology, "anaerobic respiration" almost always means lactate fermentation; in microbiology it means something more sophisticated.
Key takeaway: Fermentation = no ETC. True anaerobic respiration = ETC with a non-oxygen acceptor. A-level usually only asks about fermentation.
❌ Misconception: "Respiration is the same thing as breathing."
✅ Correction: Breathing (ventilation) is the mechanical movement of air in and out of the lungs. Cellular respiration is the biochemical process inside every cell that breaks down glucose to make ATP. They are connected — breathing delivers O₂ to and removes CO₂ from cells — but they are not the same. Plants do cellular respiration in every cell, 24 hours a day, even though they have no lungs. Bacteria do cellular respiration without any breathing apparatus at all.
📖 Campbell & Reece, Biology, 12th ed., Ch. 9, Concept 9.1; Jones et al., Ch. 12.
❌ Misconception: "The Krebs cycle runs only once per glucose molecule."
✅ Correction: The Krebs cycle runs twice per glucose, because glycolysis produces two pyruvates, which are converted to two acetyl-CoA molecules — each of which enters the cycle once. So per glucose: 2 cycles × (1 ATP + 3 NADH + 1 FADH₂ + 2 CO₂) = 2 ATP + 6 NADH + 2 FADH₂ + 4 CO₂. Forgetting the ×2 is one of the most common ATP-tally mistakes in exam papers.
✅ Correction: ATP is produced by ATP synthase, powered by the H⁺ gradient across the inner mitochondrial membrane. Oxygen's role is to accept electrons at Complex IV (forming water), keeping the electron transport chain flowing. Without oxygen the chain stops, the H⁺ gradient collapses, and ATP synthesis halts — but oxygen itself never becomes part of ATP. Knock out ATP synthase and oxygen consumption continues briefly but no ATP is made; knock out oxygen and everything stops.
📖 Alberts et al., Molecular Biology of the Cell, 7th ed., Ch. 14.3.
B · Common exam & calculation errors
❌ Error: Writing the ATP yield as 38 ATP per glucose without justification.
✅ Correct approach: Modern values are 30–32 ATP per glucose (2.5 ATP/NADH, 1.5 ATP/FADH₂). The "38 ATP" is a legacy number assuming 3 ATP/NADH and 2 ATP/FADH₂. Both numbers are still accepted in many exam mark schemes, but you must justify whichever you use by stating the ATP-per-coenzyme ratio you assumed. Cambridge International, AQA, and IB all explicitly accept either provided the working is shown.
🔍 Students memorise "38" without realising it's an idealised maximum and the textbook may have changed.
❌ Error: Drawing the inner mitochondrial membrane as smooth instead of folded.
✅ Correct approach: The inner membrane is highly folded into cristae, which dramatically increase the surface area available for the ETC and ATP synthase. A typical liver mitochondrion has cristae adding 5× more membrane area than a flat inner membrane would. Highly active tissues (heart, brown fat, flight muscle) have densely packed cristae; less active tissues (sperm, plant cells) have fewer. Examiners explicitly award marks for showing cristae on a labelled diagram of a mitochondrion.
🔍 Students confuse the inner and outer membranes — only the inner has cristae and houses the ETC.
❌ Error: Saying "glycolysis produces 4 ATP" without mentioning the 2 ATP invested.
✅ Correct approach: Always state glycolysis produces 4 ATP gross minus 2 ATP invested = 2 ATP net. The investment step (phosphorylation of glucose and fructose-6-phosphate) traps the sugar inside the cell and primes it for splitting. Forgetting the investment causes students to overcount the total ATP yield by 2.
🔍 The "4 ATP" figure dominates summaries; the "−2 invested" detail is in step diagrams and often skipped.
References
Campbell, N. A. & Reece, J. B. — Biology, 12th ed., Pearson, 2021, Ch. 9.
Alberts, B. et al. — Molecular Biology of the Cell, 7th ed., W. W. Norton, 2022, Ch. 14.
Jones, M. et al. — Cambridge International AS & A Level Biology Coursebook, 5th ed., 2022, Ch. 12.
Cambridge International Examiners' Reports (9700 Biology Paper 4) — recurring confusion about ATP yields and cycle stoichiometry.
AQA A-level Biology 7402 Examiners' Report — Topic 5.2 "Respiration": common errors in ATP accounting and mitochondrial structure.