Cellular Respiration & Fermentation
Trace glucose through glycolysis, the Krebs cycle and the electron transport chain — locations, carriers and ATP yields — then compare with lactic acid and alcoholic fermentation.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Cellular Respiration & Fermentation, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
You already know that ATP is the cell's usable energy currency. But cells don't eat ATP — they eat glucose. Cellular respiration is the controlled, stepwise process that transfers the energy locked in glucose's C–H and C–C bonds into roughly thirty molecules of ATP, releasing carbon dioxide and water along the way.
The trick is that the cell never burns glucose in one shot. It strips electrons off carbon a little at a time, parks them on carrier molecules, and cashes them in at the very end using oxygen. In this lesson you'll trace glucose through three stages — glycolysis, the Krebs cycle, and the electron transport chain — noting where each happens and how much ATP it yields. Then you'll see what happens when oxygen runs out: fermentation, a fast but low-yield backup that keeps glycolysis alive.
The trick is that the cell never burns glucose in one shot. It strips electrons off carbon a little at a time, parks them on carrier molecules, and cashes them in at the very end using oxygen. In this lesson you'll trace glucose through three stages — glycolysis, the Krebs cycle, and the electron transport chain — noting where each happens and how much ATP it yields. Then you'll see what happens when oxygen runs out: fermentation, a fast but low-yield backup that keeps glycolysis alive.
The Big Picture: Oxidizing Glucose in Small Steps
The summary equation for aerobic cellular respiration isNotice that this is essentially the reverse of photosynthesis, but the pathways are completely different — respiration is not photosynthesis run backwards.
What actually happens chemically is a redox process. Glucose is oxidized: it loses electrons (carried as hydrogen atoms) and its carbons end up as . Oxygen is reduced: it gains those electrons and combines with hydrogen ions to form water. Energy is released because electrons move from bonds where they are held loosely (C–H bonds in glucose) to bonds where they are held tightly (O–H bonds in water).
But electrons don't jump straight from glucose to oxygen. If they did, all that energy would come out as a burst of heat, like a marshmallow catching fire. Instead the cell uses electron carriers — mostly , which picks up two electrons and a proton to become NADH, and FAD, which becomes . These carriers are like rechargeable shuttles. They collect electrons during glycolysis and the Krebs cycle and deliver them to the electron transport chain, where the energy is released gradually and harvested as ATP.
A common point of confusion: NADH is not energy currency the way ATP is. The cell cannot spend NADH directly to power a pump or a muscle contraction. NADH is a delivery truck; ATP is the cash. Keeping those two roles separate makes every later step easier to follow.
What actually happens chemically is a redox process. Glucose is oxidized: it loses electrons (carried as hydrogen atoms) and its carbons end up as . Oxygen is reduced: it gains those electrons and combines with hydrogen ions to form water. Energy is released because electrons move from bonds where they are held loosely (C–H bonds in glucose) to bonds where they are held tightly (O–H bonds in water).
But electrons don't jump straight from glucose to oxygen. If they did, all that energy would come out as a burst of heat, like a marshmallow catching fire. Instead the cell uses electron carriers — mostly , which picks up two electrons and a proton to become NADH, and FAD, which becomes . These carriers are like rechargeable shuttles. They collect electrons during glycolysis and the Krebs cycle and deliver them to the electron transport chain, where the energy is released gradually and harvested as ATP.
A common point of confusion: NADH is not energy currency the way ATP is. The cell cannot spend NADH directly to power a pump or a muscle contraction. NADH is a delivery truck; ATP is the cash. Keeping those two roles separate makes every later step easier to follow.
Stage 1: Glycolysis in the Cytoplasm
Glycolysis means "sugar splitting," and that is literally what it does. In the cytosol, a ten-reaction pathway converts one six-carbon glucose into two three-carbon molecules of pyruvate.
Glycolysis has an investment phase and a payoff phase. First the cell spends 2 ATP to phosphorylate and destabilize glucose. Then the payoff phase produces 4 ATP by substrate-level phosphorylation (a phosphate group is handed directly from a substrate to ADP) and reduces 2 to 2 NADH.The most-missed detail is "net" versus "gross." Four ATP are made, but two were spent, so the net is 2. Write out both numbers on your homework so you don't lose track.
Two things make glycolysis special. First, it needs no oxygen and no mitochondria — which is why it happens in essentially every living cell, from bacteria to your neurons, and why biologists think it is one of the most ancient metabolic pathways. Second, it is fast. It can crank out ATP much more quickly than the mitochondrial stages, just in much smaller amounts.
If oxygen is present, pyruvate is transported into the mitochondrion and enters a bridge reaction sometimes called pyruvate oxidation or the link reaction. Each pyruvate loses one carbon as , is oxidized to make 1 NADH, and the remaining two-carbon fragment attaches to coenzyme A, forming acetyl-CoA. Per glucose that is 2 and 2 NADH before the Krebs cycle even begins. Students who forget this step end up two NADH short in their ATP tallies.
Glycolysis has an investment phase and a payoff phase. First the cell spends 2 ATP to phosphorylate and destabilize glucose. Then the payoff phase produces 4 ATP by substrate-level phosphorylation (a phosphate group is handed directly from a substrate to ADP) and reduces 2 to 2 NADH.The most-missed detail is "net" versus "gross." Four ATP are made, but two were spent, so the net is 2. Write out both numbers on your homework so you don't lose track.
Two things make glycolysis special. First, it needs no oxygen and no mitochondria — which is why it happens in essentially every living cell, from bacteria to your neurons, and why biologists think it is one of the most ancient metabolic pathways. Second, it is fast. It can crank out ATP much more quickly than the mitochondrial stages, just in much smaller amounts.
If oxygen is present, pyruvate is transported into the mitochondrion and enters a bridge reaction sometimes called pyruvate oxidation or the link reaction. Each pyruvate loses one carbon as , is oxidized to make 1 NADH, and the remaining two-carbon fragment attaches to coenzyme A, forming acetyl-CoA. Per glucose that is 2 and 2 NADH before the Krebs cycle even begins. Students who forget this step end up two NADH short in their ATP tallies.
Stage 2 and 3: The Krebs Cycle and the Electron Transport Chain
The Krebs cycle (also citric acid cycle) runs in the mitochondrial matrix. Acetyl-CoA donates its two carbons to a four-carbon molecule to form six-carbon citrate. Through a series of oxidations the two carbons leave as , electrons are stripped onto carriers, and the four-carbon starting molecule is regenerated — which is why it's a cycle.
Per turn: 2 , 3 NADH, 1 , and 1 ATP (as GTP in many textbooks). Because one glucose produced two acetyl-CoA, the cycle turns twice, giving 4 , 6 NADH, 2 , 2 ATP. Notice that all six carbons of the original glucose have now left as — the gas you exhale is literally your breakfast.
The electron transport chain (ETC) sits in the inner mitochondrial membrane (the folded cristae). NADH and drop their electrons into a series of protein complexes. As electrons fall from complex to complex, the released energy pumps from the matrix into the intermembrane space, building a steep concentration and charge gradient. Protons then flow back through ATP synthase, and that flow spins the enzyme and drives oxidative phosphorylation. This coupling of a proton gradient to ATP synthesis is called chemiosmosis.
Oxygen's job is narrow but essential: it is the final electron acceptor, combining with spent electrons and to form water. Without oxygen the chain backs up, no protons are pumped, and NADH cannot be recycled to . Roughly 26–28 ATP come from this stage, which is why cyanide — which blocks the chain — is so fast-acting.
Per turn: 2 , 3 NADH, 1 , and 1 ATP (as GTP in many textbooks). Because one glucose produced two acetyl-CoA, the cycle turns twice, giving 4 , 6 NADH, 2 , 2 ATP. Notice that all six carbons of the original glucose have now left as — the gas you exhale is literally your breakfast.
The electron transport chain (ETC) sits in the inner mitochondrial membrane (the folded cristae). NADH and drop their electrons into a series of protein complexes. As electrons fall from complex to complex, the released energy pumps from the matrix into the intermembrane space, building a steep concentration and charge gradient. Protons then flow back through ATP synthase, and that flow spins the enzyme and drives oxidative phosphorylation. This coupling of a proton gradient to ATP synthesis is called chemiosmosis.
Oxygen's job is narrow but essential: it is the final electron acceptor, combining with spent electrons and to form water. Without oxygen the chain backs up, no protons are pumped, and NADH cannot be recycled to . Roughly 26–28 ATP come from this stage, which is why cyanide — which blocks the chain — is so fast-acting.
| Stage | Location | ATP (net) | Carriers made | |
|---|---|---|---|---|
| Glycolysis | Cytoplasm | 2 | 2 NADH | 0 |
| Pyruvate oxidation | Matrix | 0 | 2 NADH | 2 |
| Krebs cycle (×2) | Matrix | 2 | 6 NADH, 2 | 4 |
| ETC / chemiosmosis | Inner membrane | 26–28 | — | 0 |
When Oxygen Runs Out: Fermentation
Fermentation is not an alternative way to make lots of ATP. It is a way to recycle so that glycolysis can keep running. A cell has a limited pool of ; if every molecule is stuck as NADH because the ETC is shut down, glycolysis stops within seconds and ATP production collapses. Fermentation solves this by dumping NADH's electrons onto pyruvate instead of onto the electron transport chain.
There are two pathways you should know. In lactic acid fermentation, pyruvate is directly reduced to lactate, regenerating . This happens in your skeletal muscle during intense exercise and in bacteria that make yogurt and cheese. No is released. In alcoholic fermentation, used by yeast and some plant cells, pyruvate first loses a carbon as and then is reduced to ethanol — the process behind bread rising and brewing.
Both pathways yield only the 2 net ATP from glycolysis, roughly 6 percent of the aerobic yield — glucose leaves fermentation still holding most of its energy, which is exactly why ethanol and lactate can be burned as fuels later.
Two misconceptions worth correcting. First, fermentation produces no ATP of its own; every ATP comes from glycolysis. Second, lactate is not simply a waste toxin — your liver converts it back to pyruvate or glucose, and muscle soreness a day after exercise is not caused by lactate buildup.
There are two pathways you should know. In lactic acid fermentation, pyruvate is directly reduced to lactate, regenerating . This happens in your skeletal muscle during intense exercise and in bacteria that make yogurt and cheese. No is released. In alcoholic fermentation, used by yeast and some plant cells, pyruvate first loses a carbon as and then is reduced to ethanol — the process behind bread rising and brewing.
| Lactic acid | Alcoholic | |
|---|---|---|
| Organisms | Animal muscle, some bacteria | Yeast, some plants |
| Products | Lactate, | Ethanol, , |
| ATP per glucose | 2 (from glycolysis) | 2 (from glycolysis) |
Two misconceptions worth correcting. First, fermentation produces no ATP of its own; every ATP comes from glycolysis. Second, lactate is not simply a waste toxin — your liver converts it back to pyruvate or glucose, and muscle soreness a day after exercise is not caused by lactate buildup.
Putting the Numbers Together Without Memorizing Blindly
Teachers often ask you to reconstruct the ATP tally rather than recite it, so it helps to know where the numbers come from. Each NADH delivered to the chain pumps enough protons for about 2.5 ATP; each enters the chain further downstream, past one pumping complex, so it yields about 1.5 ATP. Multiply and add:That gives 28 ATP from the ETC, plus 4 from substrate-level phosphorylation (2 in glycolysis, 2 in the Krebs cycle), for about 32 ATP per glucose. Older textbooks say 36–38 because they used whole-number ratios of 3 and 2; both are approximations, and the real value shifts because some of the proton gradient is spent shuttling molecules across membranes. This is why the yield is written as "about 30–32" — an honest range, not a fudge.
The reason the answer is a range at all is a genuinely important biological idea: the proton gradient is a shared energy pool, not a fixed number of ATP coupons. Anything else the mitochondrion uses the gradient for, including importing pyruvate and generating heat, reduces the ATP output.
Where students go wrong most often is location. Glycolysis is cytoplasm; pyruvate oxidation and Krebs are the matrix; the ETC is the inner membrane. Prokaryotes have no mitochondria, so they run the equivalent chain in their plasma membrane. If a question describes a cell with the ETC blocked, trace the consequences in order: no oxygen accepting electrons, no proton gradient, NADH accumulates, Krebs stalls, and the cell must ferment or die.
The reason the answer is a range at all is a genuinely important biological idea: the proton gradient is a shared energy pool, not a fixed number of ATP coupons. Anything else the mitochondrion uses the gradient for, including importing pyruvate and generating heat, reduces the ATP output.
Where students go wrong most often is location. Glycolysis is cytoplasm; pyruvate oxidation and Krebs are the matrix; the ETC is the inner membrane. Prokaryotes have no mitochondria, so they run the equivalent chain in their plasma membrane. If a question describes a cell with the ETC blocked, trace the consequences in order: no oxygen accepting electrons, no proton gradient, NADH accumulates, Krebs stalls, and the cell must ferment or die.
Key terms
- Glycolysis.
- A ten-step pathway in the cytoplasm that splits one glucose into two pyruvate, netting 2 ATP and 2 NADH; requires no oxygen or organelles.
- Acetyl-CoA.
- The two-carbon molecule attached to coenzyme A, formed when pyruvate is oxidized and loses a carbon as carbon dioxide; it feeds the Krebs cycle.
- Krebs cycle.
- A cyclic pathway in the mitochondrial matrix that completes the oxidation of glucose carbons to carbon dioxide, yielding per turn 3 NADH, 1 FADH2, and 1 ATP.
- Electron transport chain.
- A series of protein complexes in the inner mitochondrial membrane that passes electrons from NADH and FADH2 to oxygen, using the released energy to pump protons.
- Chemiosmosis.
- The use of a proton gradient across a membrane to drive ATP synthesis as hydrogen ions flow back through ATP synthase.
- Oxidative phosphorylation.
- ATP production powered by the proton gradient created by electron transport; accounts for the large majority of ATP made per glucose.
- Final electron acceptor.
- The molecule that receives spent electrons at the end of the transport chain; in aerobic respiration this is oxygen, which becomes water.
- Fermentation.
- An anaerobic pathway that reoxidizes NADH to NAD+ so glycolysis can continue, producing lactate, or ethanol and carbon dioxide, but no additional ATP.
Worked example
A researcher supplies a yeast culture with glucose and measures products. In flask A the yeast has plenty of oxygen; in flask B the culture is sealed and oxygen is depleted. For each flask, state the pathways operating, the location of each, the ATP yield per glucose, and the carbon-containing products. Then explain why flask B must consume far more glucose to make the same total ATP.
Step 1 — Flask A, pathways and locations. With oxygen present, all stages run. Glycolysis occurs in the cytoplasm; pyruvate oxidation and the Krebs cycle occur in the mitochondrial matrix; the electron transport chain and ATP synthase are in the inner mitochondrial membrane.
Step 2 — Flask A, carrier tally per glucose. Glycolysis gives 2 NADH; pyruvate oxidation gives 2 NADH; two turns of the Krebs cycle give 6 NADH and 2 FADH2. Total: 10 NADH and 2 FADH2.
Step 3 — Flask A, ATP tally. Substrate-level phosphorylation gives 2 ATP in glycolysis plus 2 in the Krebs cycle, so 4 ATP. Oxidative phosphorylation gives about ATP from NADH and ATP from FADH2, so about 28. Total is roughly 32 ATP per glucose.
Step 4 — Flask A, carbon products. All six glucose carbons exit as 6 CO₂ (2 from pyruvate oxidation, 4 from two Krebs turns). Oxygen ends up in water, not in the carbon dioxide.
Step 5 — Flask B, what shuts down. Without oxygen there is no final electron acceptor, so electrons stop flowing, protons stop being pumped, and no gradient forms. ATP synthase stalls. NADH cannot be reoxidized, so the Krebs cycle and pyruvate oxidation also stop for lack of NAD+.
Step 6 — Flask B, what continues. Yeast switches to alcoholic fermentation in the cytoplasm: pyruvate loses a carbon as CO₂ and is reduced to ethanol, regenerating NAD+ so glycolysis keeps turning. Net yield is only the 2 ATP from glycolysis. Carbon products are ethanol and CO₂.
Step 7 — The comparison. Flask B captures 2 ATP per glucose versus about 32 in flask A, roughly one-sixteenth as much. To make equal total ATP the sealed culture must break down about sixteen times as much glucose, and the leftover energy stays locked in ethanol's C–H bonds — which is precisely why ethanol still burns.
Step 2 — Flask A, carrier tally per glucose. Glycolysis gives 2 NADH; pyruvate oxidation gives 2 NADH; two turns of the Krebs cycle give 6 NADH and 2 FADH2. Total: 10 NADH and 2 FADH2.
Step 3 — Flask A, ATP tally. Substrate-level phosphorylation gives 2 ATP in glycolysis plus 2 in the Krebs cycle, so 4 ATP. Oxidative phosphorylation gives about ATP from NADH and ATP from FADH2, so about 28. Total is roughly 32 ATP per glucose.
Step 4 — Flask A, carbon products. All six glucose carbons exit as 6 CO₂ (2 from pyruvate oxidation, 4 from two Krebs turns). Oxygen ends up in water, not in the carbon dioxide.
Step 5 — Flask B, what shuts down. Without oxygen there is no final electron acceptor, so electrons stop flowing, protons stop being pumped, and no gradient forms. ATP synthase stalls. NADH cannot be reoxidized, so the Krebs cycle and pyruvate oxidation also stop for lack of NAD+.
Step 6 — Flask B, what continues. Yeast switches to alcoholic fermentation in the cytoplasm: pyruvate loses a carbon as CO₂ and is reduced to ethanol, regenerating NAD+ so glycolysis keeps turning. Net yield is only the 2 ATP from glycolysis. Carbon products are ethanol and CO₂.
Step 7 — The comparison. Flask B captures 2 ATP per glucose versus about 32 in flask A, roughly one-sixteenth as much. To make equal total ATP the sealed culture must break down about sixteen times as much glucose, and the leftover energy stays locked in ethanol's C–H bonds — which is precisely why ethanol still burns.
Practice questions
A drug blocks ATP synthase in the inner mitochondrial membrane but does not affect the electron transport proteins themselves. What is the most immediate consequence inside the mitochondrion?
- Protons accumulate in the intermembrane space and the gradient becomes unusually steep
- Glycolysis in the cytoplasm stops immediately for lack of glucose
- Oxygen is no longer produced, so the cell suffocates
- The Krebs cycle speeds up to compensate for lost ATP
Answer: Protons accumulate in the intermembrane space and the gradient becomes unusually steep
The chain is still pumping protons out of the matrix, but their only return route — ATP synthase — is blocked, so the gradient builds up and ATP synthesis stops. Glycolysis does not halt for lack of glucose (glucose is still available), and cellular respiration consumes oxygen rather than producing it. The Krebs cycle actually slows, because once the steep gradient stops electron flow, NADH accumulates and NAD+ becomes scarce.
A sprinter's leg muscles rely heavily on lactic acid fermentation during a 200-meter race, even though her mitochondria are perfectly functional. Explain why fermentation is useful here, identify exactly how much ATP it yields per glucose, and explain why the cell must convert pyruvate to lactate at all.
Answer: Oxygen delivery cannot keep up with demand during intense effort, so the electron transport chain cannot process NADH fast enough. Glycolysis, which needs no oxygen, produces ATP very rapidly, netting 2 ATP per glucose. Reducing pyruvate to lactate regenerates NAD+ from NADH; without that recycling, the limited NAD+ pool would be exhausted and glycolysis — and therefore ATP production — would stop within seconds.
The key insight is that fermentation's purpose is NAD+ regeneration, not ATP production. Every ATP still comes from substrate-level phosphorylation in glycolysis; converting pyruvate to lactate yields no ATP by itself. A strong answer also notes the tradeoff: fermentation is fast but inefficient, capturing only about 2 of the roughly 32 ATP available per glucose, so it works for short bursts and not for sustained effort.
Which sequence correctly matches a molecule to the location where it is first produced during aerobic respiration of one glucose molecule?
- Pyruvate in the cytoplasm, acetyl-CoA in the mitochondrial matrix, water at the inner mitochondrial membrane
- Pyruvate in the matrix, acetyl-CoA in the cytoplasm, water in the intermembrane space
- Pyruvate at the inner membrane, carbon dioxide in the cytoplasm, water in the matrix
- Pyruvate in the cytoplasm, carbon dioxide in the cytoplasm, water in the cytoplasm
Answer: Pyruvate in the cytoplasm, acetyl-CoA in the mitochondrial matrix, water at the inner mitochondrial membrane
Glycolysis is cytosolic, so pyruvate appears there. Pyruvate is then imported into the matrix, where it is oxidized to acetyl-CoA. Water forms when oxygen accepts electrons at the end of the chain embedded in the inner membrane. Carbon dioxide is never released during glycolysis — all six carbon dioxide molecules come from pyruvate oxidation and the Krebs cycle in the matrix, which rules out the options placing carbon dioxide in the cytoplasm.
FAQ
- Why is the ATP yield given as about 30 to 32 instead of an exact number?
- Because the proton gradient is a shared energy pool rather than a fixed set of coupons. Each NADH pumps enough protons for roughly 2.5 ATP and each FADH2 for roughly 1.5, but the mitochondrion also spends part of that gradient importing pyruvate and phosphate and shuttling electrons from cytosolic NADH inward. Older textbooks say 36 to 38 because they used whole-number ratios of 3 and 2. Any number in the low thirties, with reasoning shown, is a defensible answer.
- Is cellular respiration just photosynthesis in reverse?
- Only the summary equations look reversed. The actual pathways use different enzymes, different organelles, and different carrier molecules, and photosynthesis stores energy from light while respiration releases energy already stored in glucose. Both do use an electron transport chain and chemiosmosis to make ATP, which is a real similarity worth noting — but the reactions themselves are not the same steps run backwards.
- Do plants perform cellular respiration?
- Yes, constantly, in every cell and around the clock. Plants make glucose by photosynthesis in the light, then break some of it down in their mitochondria to power growth, transport, and repair. During daylight photosynthesis usually outpaces respiration, so plants release net oxygen, but at night only respiration is running.
- Where do the carbon dioxide you exhale and the water you make actually come from?
- Every carbon in exhaled carbon dioxide came from the food you digested — two carbons per glucose leave during pyruvate oxidation and four during the two turns of the Krebs cycle. The oxygen you breathe in does not end up in carbon dioxide; it accepts electrons at the end of the transport chain and combines with hydrogen ions to form water, which is called metabolic water.
Learn this with a teacher, not a page
The Crimsora tutor teaches Cellular Respiration & Fermentation live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.