Week 6 · Chapter

Chapter 6 — Cellular Respiration

Introduction to Biology Free sample edition

The module's primary reading · ~40–50 minutes · Objective 4 (explain how cells obtain and transform energy — the laws of thermodynamics, ATP, and enzyme action with its temperature and pH limits — and trace cellular respiration and photosynthesis as ordered, located overviews, including fermentation and the relationship between the two processes), with Objective 3 (compare prokaryotic and eukaryotic cells, relate each eukaryotic organelle to its function, explain how substances cross the plasma membrane by passive and active transport (including osmosis and tonicity), and use the surface-area-to-volume ratio to explain why cells stay small) and Objective 1 (describe the practice of science in biology — the characteristics of life as a set, the levels of biological organization, the scientific method and a controlled experiment's variables and control, and the difference between a hypothesis and a theory — and explain evolution by natural selection as the unifying framework of biology) carried forward · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

Every minute of your life, your cells take apart sugar and use the energy inside it to recharge their ATP. This week you follow that sugar from start to finish. You will meet cellular respiration (the chemistry inside your cells that turns food energy into ATP), which is a different thing from breathing. You will walk its four stages in order and learn the place in the cell where each one runs, because in biology the order and the place are facts you can be graded on. Finally, you will meet fermentation (the backup a cell uses when oxygen runs out), the same process that makes bread rise.

Why you already care about this

Picture two runs on the same afternoon. In the first, you sprint up a long flight of stairs as fast as you can, and by the top your thighs burn and you are gasping. In the second, you jog slowly around a park, and you could keep going and talk the whole way. Both runs were paid for in ATP. So why does one leave you burning after less than a minute while the other feels as if it could last all afternoon?

The explanation is a pathway inside your cells with 4 stages, a fixed order, and one very important gas at the end. When oxygen arrives fast enough, your muscle cells run the whole pathway and squeeze about 30 to 32 ATP out of each sugar. When the sprint outruns your oxygen supply, the cells fall back on a quick shortcut worth only 2 ATP. You have felt both of these happen in your own legs.

You have seen the same chemistry in a kitchen, too. Bread dough rises because yeast cells are running that shortcut and giving off a gas. And when someone loses a pound of fat, most of it leaves the body by a route you would not guess: out through the lungs, as carbon dioxide breathed out.

By the end of this chapter you will be able to:

tell cellular respiration apart from breathing, and say who does it;
read the overall equation forward and backward and name where each molecule comes from and goes;
label the parts of a mitochondrion and tie each part to its job;
list the four stages in order, each with the place in the cell where it runs;
state what glycolysis makes and why oxygen matters only at the end;
describe the ATP yield per glucose in words, as a range;
compare lactic acid fermentation with alcoholic fermentation;
compute the increments in the yeast balloon experiment, with every step shown.

Carry this line with you: respiration is chemistry in your cells, not air in your lungs.

Where we are

Already covered: Week 1 taught you to read a controlled experiment; Week 2 gave you chemical formulas; Week 3 introduced glucose; Week 4 toured the mitochondrion; Week 5 showed you ATP and enzymes.
This week: cellular respiration, its four stages and their locations, the ATP it makes, and fermentation when oxygen runs out.
How they connect: each stage is a chain of enzyme-run reactions from Week 5, it happens in the cell parts you toured in Week 4, and its fuel is the glucose you met in Week 3.
Where we're going next: Week 7, Photosynthesis, runs a near-mirror image of this pathway, building sugar from carbon dioxide and water with light.

Week 1 gave you the controlled experiment, and you will use it on the yeast balloon lab in Section 10. Week 3 gave you glucose, the six-carbon sugar that fuels the pathway. Week 4 introduced the mitochondrion as the organelle that makes ATP, with its folded inner membrane, the cristae. Week 5 showed you that ATP carries three phosphates when charged and two when spent, and that a cell never makes energy; it captures energy from food and transfers it. This week you see exactly how that capture works.

1. Separate cellular respiration from breathing

Think back to the sprint up the stairs. You breathed hard the whole way. It is natural to call that heavy breathing "respiration," and in everyday speech it is. Biologists use the word for something else that happens deeper inside you.

Cellular respiration is the chemistry inside cells that breaks down glucose with oxygen, releasing carbon dioxide and water and using the released energy to recharge ATP. It is not breathing, and nearly every living thing, plants included, does it day and night.

Breathing and cellular respiration are partners, not twins. Breathing moves gas between the air and your blood: oxygen in, carbon dioxide out. Cellular respiration happens inside each cell: it uses the oxygen that breathing delivered, and it makes the carbon dioxide that breathing carries away. Your lungs are the delivery truck; your cells are the factory.

Two more words describe whether a process uses oxygen:

Aerobic describes a process that uses oxygen; the full pathway of cellular respiration, ending with oxygen at the electron transport chain, is aerobic.
Anaerobic describes a process that runs without oxygen; glycolysis and fermentation are anaerobic.

Worked example — who respires? Decide whether each living thing does cellular respiration.

A runner at the finish line: yes. Every one of the runner's cells respires.
A houseplant on a windowsill at midnight: yes. The plant cannot photosynthesize in the dark, but its cells keep respiring.
The same houseplant at noon: yes. In daylight it photosynthesizes and respires at the same time.
A yeast cell in bread dough: yes. Yeast break down sugar for ATP too, and when oxygen runs low they ferment.

In words: nearly every living thing respires all the time, because every cell needs ATP all the time.

Misconception check. Students often think "cellular respiration is breathing," and a second, related slip is "plants do not respire; they photosynthesize instead." Both are tempting because the everyday word respiration means breathing, and because plants are famous for making oxygen. But cellular respiration is chemistry inside cells, and plants have mitochondria as well as chloroplasts. A houseplant respires around the clock.

Memory hook: lungs deliver; cells respire.

2. Read the overall equation both ways

A recipe card lists what goes in and what comes out before it explains the steps. Cellular respiration has a recipe card too, and you can read the whole process from it.

In words: glucose + oxygen → carbon dioxide + water + energy (ATP). In formulas:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP

Read the left side, the reactants. Glucose, C₆H₁₂O₆, comes from the food you eat. The oxygen, O₂, comes from the air you breathe in. Now read the right side, the products. The carbon dioxide, CO₂, is what you breathe out. The water, H₂O, joins the water already in your body. And the ATP is the point of the whole process: the charged energy currency your cells spend on every job.

Worked example — check the recipe card. Count every atom on both sides, the way the equation in the outline does.

Carbon. The left side has 6 carbons in one glucose. The right side has 6 CO₂ with 1 carbon each, and 6 × 1 = 6.
Hydrogen. The left side has 12 hydrogens in one glucose. The right side has 6 H₂O with 2 hydrogens each, and 6 × 2 = 12.
Oxygen. The left side has 6 in the glucose plus 6 O₂ with 2 each: 6 + (6 × 2) = 18. The right side has 6 CO₂ with 2 each plus 6 H₂O with 1 each: (6 × 2) + (6 × 1) = 18.

In words: every atom that goes in comes out; nothing is created or destroyed, which is Week 5's first law at work on the atoms themselves.

The equation also tells you where a lost pound of fat goes. The carbon atoms in burned fuel leave as CO₂, so most of the mass of a lost pound of fat leaves your body through your lungs, as the carbon dioxide you breathe out.

Misconception check. Students often read the equation as if the ATP were made from nothing, or as if the oxygen became the carbon dioxide's carbon. The equation forbids both. The energy in ATP was already stored in glucose's bonds, and the carbon in CO₂ came from the glucose. Count the atoms, and the equation keeps you honest.

Memory hook: count every atom — nothing appears, nothing vanishes.

3. Picture the mitochondrion: structure to function

Picture a balloon pushed inside a slightly larger balloon, with the inner one crumpled into deep folds. That is the shape of a mitochondrion, and every part of the shape has a job.

Describe it as a labeled figure, from the outside in:

the outer membrane, a smooth wrapper around the whole organelle;
the inner mitochondrial membrane, the folded inner membrane of a mitochondrion; it carries the electron transport chain, and its folds add surface for that job. The folds are the cristae you met in Week 4.
the mitochondrial matrix, the fluid-filled space inside a mitochondrion's inner membrane, where pyruvate oxidation and the citric acid cycle run.

Structure determines function here, as it has every week. The electron transport chain is built into the inner membrane, so more membrane means more room for more chains. Folding the membrane packs a large surface into a small organelle, the same coping trick you saw for the cube cell in Week 4. The matrix, meanwhile, holds the enzymes of the citric acid cycle in one enclosed space.

Worked example — match the part to the job. Read the figure for each job.

Runs the citric acid cycle: the matrix.
Carries the electron transport chain: the inner mitochondrial membrane.
Adds surface for the chain: the folds of the inner membrane, the cristae.

In words: the matrix runs the cycle, and the folded inner membrane carries the chain.

Misconception check. Students often think "plant cells have chloroplasts instead of mitochondria." The idea is tempting because chloroplasts are the plant cell's famous organelle. But plants have both: chloroplasts make sugar in the light, and mitochondria break sugar down for ATP, day and night.

Memory hook: the folds hold the chain; the matrix spins the cycle.

4. Split glucose in the cytoplasm: glycolysis

Picture a long log that is too big for a small stove. Before you can burn it, you split it in half. A cell does the same with glucose, and it does it before the sugar ever reaches a mitochondrion.

Glycolysis is the first stage of cellular respiration, in the cytoplasm, which splits one glucose into two pyruvate molecules and nets 2 ATP and 2 NADH without using oxygen.

Pyruvate is the three-carbon molecule that glycolysis makes, two from each glucose; with oxygen it enters the mitochondrion, and without oxygen it is turned into lactic acid or into ethanol and carbon dioxide.

Glycolysis also loads a carrier. An electron carrier is a molecule that picks up electrons in one step of a pathway and delivers them to a later step; NAD⁺ is an empty carrier, and NADH and FADH₂ are loaded carriers. NADH is the loaded form of the electron carrier NAD⁺; glycolysis and the citric acid cycle fill it, and it delivers its electrons to the electron transport chain. Think of NAD⁺ as an empty delivery van and NADH as the same van, loaded.

Three facts about glycolysis carry the rest of the week. It runs in the cytoplasm. It needs no oxygen, so it is anaerobic. And it runs in nearly every cell, prokaryote or eukaryote, with or without a mitochondrion.

Worked example — scale up glycolysis. An illustrative muscle cell (numbers made up for this chapter) runs glycolysis on 5 glucose molecules.

Pyruvate: each glucose makes 2 pyruvate, so 5 × 2 = 10 pyruvate.
Net ATP: each glucose nets 2 ATP, so 5 × 2 = 10 ATP.
NADH: each glucose loads 2 NADH, so 5 × 2 = 10 NADH.

In words: whatever the number of glucose molecules, glycolysis doubles it into pyruvate, ATP, and NADH, and it does all of it without oxygen.

Misconception check. Students often say "oxygen is used in glycolysis." It is tempting because the whole process is called aerobic respiration. But glycolysis runs without oxygen; oxygen acts only at the very end of the pathway, in the electron transport chain. That is why glycolysis can keep running during a sprint.

Memory hook: glycolysis: split the sugar, no oxygen needed.

5. Carry pyruvate into the mitochondrion: pyruvate oxidation

Picture a relay race. The runner from the first leg has done their part, and now the baton has to cross into the next lane. Pyruvate is that baton, and the next lane is inside the mitochondrion.

Pyruvate oxidation is the second stage of cellular respiration, in which each pyruvate enters the mitochondrion and is converted to acetyl-CoA while a carbon leaves as carbon dioxide.

Acetyl-CoA is the molecule that pyruvate becomes in pyruvate oxidation; it carries the remaining carbon from the glucose into the citric acid cycle.

This short stage does two things at once: it moves the fuel into the mitochondrion, and it releases the first carbon dioxide of the pathway.

Worked example — count the first carbon dioxide. One glucose has made 2 pyruvate in glycolysis.

Pyruvate in: 2 pyruvate enter the mitochondrion.
Carbon dioxide out: each pyruvate gives off 1 CO₂, so 2 × 1 = 2 CO₂.
Acetyl-CoA formed: each pyruvate becomes 1 acetyl-CoA, so 2 acetyl-CoA move on to the cycle.

In words: two of glucose's six carbons leave here as carbon dioxide, and the rest ride into the cycle as acetyl-CoA.

Misconception check. Students often skip this stage and jump straight from glycolysis to the citric acid cycle. The shortcut is tempting because the stage is short. But pyruvate oxidation is a stage in its own right, it is where the fuel crosses into the mitochondrion, and leaving it out puts the pathway out of order.

Memory hook: pyruvate crosses in, and a carbon walks out.

6. Turn the citric acid cycle in the matrix

Picture a merry-go-round at a playground. Riders climb on at one spot, ride around, and step off, and the ride is ready for the next rider. The third stage is a cycle like that: the same set of molecules is rebuilt at the end of every turn.

The citric acid cycle is the third stage of cellular respiration, also called the Krebs cycle, which runs in the mitochondrial matrix, releases carbon dioxide, makes 2 ATP or GTP per glucose, and loads the carriers NADH and FADH₂. It is named the Krebs cycle after Hans Krebs, who worked out its steps (Krebs and Johnson, 1937).

FADH₂ is a second loaded electron carrier, filled in the citric acid cycle, that also delivers electrons to the electron transport chain.

Notice what the cycle makes only a little of: ATP. Its real job is loading carriers. The cycle strips the remaining carbon from the fuel and releases it as CO₂, and it packs the energy into loaded NADH and FADH₂ for the next stage to cash in.

Worked example — count the carbon dioxide. Follow all six of glucose's carbons out of the cell.

Turns per glucose: one glucose makes 2 acetyl-CoA, so the cycle turns twice.
Carbon dioxide in pyruvate oxidation: 2, from Section 5.
Carbon dioxide in the cycle: 4 across the two turns.
Total: 2 + 4 = 6 CO₂, one for each of glucose's 6 carbons.

In words: by the end of the cycle, every carbon atom that entered as glucose has left as carbon dioxide, which is the CO₂ in the overall equation and the gas you breathe out.

Misconception check. Students often place the citric acid cycle in the cytoplasm, right beside glycolysis. It is tempting because the two stages are taught back to back. But the cycle runs in the mitochondrial matrix, inside the inner membrane. Only glycolysis runs in the cytoplasm.

Memory hook: the cycle spins in the matrix and loads the vans.

7. Cash in at the electron transport chain

Picture a row of loaded delivery vans pulling up to a warehouse dock. The vans unload, the goods move down a conveyor, and at the end of the line someone has to take the last box, or the whole line jams. The fourth stage works like that dock.

The electron transport chain is the fourth stage of cellular respiration, a chain of carriers built into the inner mitochondrial membrane that receives electrons from NADH and FADH₂ and makes the most ATP of any stage.

The final electron acceptor is the molecule waiting at the end of the electron transport chain to take the electrons; in cellular respiration it is oxygen, which joins with hydrogen to form water.

This is where the oxygen you breathe finally does its job. Without oxygen at the end of the line, electrons have nowhere to go, the chain stops, and the loaded carriers cannot unload.

Now you have the whole pathway in order, with its places:

glycolysis (cytoplasm) → pyruvate oxidation (mitochondrion) → citric acid cycle (matrix) → electron transport chain (inner membrane)

Worked example — trace one oxygen molecule. Follow the O₂ from your last breath.

Delivered: breathing brings O₂ into your lungs, and your blood carries it to a muscle cell.
Waiting: the O₂ crosses into a mitochondrion and waits at the end of the chain on the inner membrane.
Accepting: electrons arrive at the end of the chain, and oxygen takes them as the final electron acceptor.
Leaving as water: the oxygen joins with hydrogen to form H₂O, the water in the overall equation.

In words: the oxygen you breathe ends up in water, not in carbon dioxide, and it is used at the very end of the pathway, not the beginning.

Misconception check. Students often say "most of the ATP comes from glycolysis." It is tempting because glycolysis is the stage taught first and in the most detail. But glycolysis nets only 2 ATP per glucose. The electron transport chain makes the most ATP by far.

Memory hook: oxygen waits at the end of the line.

8. Count the payoff in words

Picture a receipt for one glucose that has run through all four stages. How much ATP does the receipt show? Biologists have worked hard on that number, and the honest answer is a range, not one digit.

One glucose yields about 30 to 32 ATP in current estimates; older estimates say 36 to 38. The exact total depends on the cell and on details of how the chain runs, so it is an estimate. You learn it in words, as a range, and you are never asked to memorize one number.

What you should hold firmly is the shape of the receipt. Glycolysis nets 2 ATP. The citric acid cycle makes 2 ATP or GTP. The electron transport chain makes the rest, by far the largest share. And this is why the full aerobic pathway matters: about 30 to 32 ATP is a far richer payoff than glycolysis's 2 alone.

Misconception check. Students often treat the exact ATP count as a fact to memorize, and then feel lost when two sources give different numbers. The slip is tempting because a single number feels more certain. Caveat: the count is an estimate that has been revised over time, which is why older sources say 36 to 38 and newer ones say about 30 to 32. Learn the range and the shape: a small amount from glycolysis, the most from the chain.

Memory hook: about thirty, mostly from the chain.

Pause and reflect. Choose one of these two prompts and think it through, or jot a few sentences in your notes.

(a) Two sources you trust give different ATP totals per glucose. In your own words, explain why a scientist might report a range instead of one number, and why that is honest rather than vague.
(b) Think of a number in your own life that is really an estimate, such as a commute time or a phone's battery life. How is the ATP count like that number?

9. Fall back on fermentation when oxygen runs out

Go back to the sprint up the stairs. Halfway up, your muscle cells are burning ATP faster than your blood can deliver oxygen. The electron transport chain slows, the loaded NADH has nowhere to unload, and the empty NAD⁺ that glycolysis needs runs short. Without a fix, even glycolysis would stop.

Fermentation is a low-energy backup for when oxygen runs out: glycolysis plus a side path that turns NADH back into NAD⁺, so glycolysis can keep running and making its 2 ATP per glucose.

There are two side paths, and they differ in what pyruvate becomes:

Lactic acid fermentation is fermentation in which pyruvate and NADH become lactic acid (lactate) and NAD⁺; it runs in hard-working muscle short of oxygen and in red blood cells, which have no mitochondria.
Alcoholic fermentation is fermentation in which pyruvate gives off carbon dioxide and becomes ethanol, regenerating NAD⁺; yeast use it, and its carbon dioxide is the gas that raises bread.

Ethanol is the alcohol that yeast make from pyruvate in alcoholic fermentation, alongside carbon dioxide. In one line: pyruvate → ethanol + CO₂.

Each kind of fermentation yields only glycolysis's 2 ATP per glucose. That is why the sprint cannot last: fermentation keeps the muscle going for a short burst, but at a small fraction of the aerobic payoff. A slow jog stays aerobic, because your oxygen supply keeps up, and the full pathway pays out about 30 to 32 ATP per glucose.

Aerobic respiration compared with the two kinds of fermentation, per glucose
Feature	Aerobic respiration	Lactic acid fermentation	Alcoholic fermentation
Oxygen needed?	Yes, at the end of the chain	No	No
Where pyruvate goes	Into the mitochondrion	Becomes lactic acid	Becomes ethanol and CO₂
ATP per glucose	About 30 to 32	2	2
Example	A slow jog	A sprint; red blood cells	Yeast in bread dough

Worked example — which path? Name the path each cell takes.

A red blood cell: it has no mitochondria, so it can never run the chain. It uses lactic acid fermentation all the time.
A thigh muscle cell, last seconds of a sprint: oxygen is short, so it leans on lactic acid fermentation.
A yeast cell in sealed bread dough: oxygen runs low, so it uses alcoholic fermentation and gives off CO₂, which puffs up the dough.
A thigh muscle cell on a slow jog: oxygen keeps up, so it runs the full aerobic pathway.

In words: fermentation is the backup when oxygen is missing or, in a red blood cell, when there are no mitochondria to use it.

Misconception check. Students often read fermentation as a high-ATP alternative to respiration, as if yeast had found a better deal. It is tempting because fermentation is fast and useful in the kitchen. But fermentation yields only glycolysis's 2 ATP. It is a low-ATP backup that keeps glycolysis running, not a high-ATP alternative.

Memory hook: no oxygen, no chain; ferment to keep glycolysis turning.

10. Read the yeast balloon experiment from start to finish

Now you can read a full fermentation experiment, from the question to the conclusion. This is the example you will see in the lecture, the slides, the tutorial, the quiz, and this week's Biology Lab & Inquiry. The numbers below are an illustrative dataset constructed for this course, not real data; your own bottles at home will land somewhere nearby.

The question: does the amount of sugar change how much gas yeast makes?

The hypothesis: if more sugar is added to the yeast, then the balloon will grow larger, because yeast ferments sugar and gives off CO₂.

The setup: four bottles hold the same amount of yeast in the same warm water, with 0, 1, 2, and 4 teaspoons of sugar; a balloon covers each opening, and you measure its circumference at 30 minutes.

Balloon circumference in centimeters at 30 minutes for four amounts of sugar (illustrative, constructed for this course, not real data)
Sugar	Balloon circumference
0 teaspoons (the control)	0 cm
1 teaspoon	8 cm
2 teaspoons	14 cm
4 teaspoons	18 cm

Worked example — the yeast balloon data. Work through the steps in order: read the control, then subtract each reading from the one after it.

Control. The 0-teaspoon bottle reads 0 cm. No sugar means no fuel, so no gas.
Increments. 8 − 0 = 8 cm; 14 − 8 = 6 cm; 18 − 14 = 4 cm. The gains shrink from 8 to 6 to 4.
Doubling. Going from 2 to 4 teaspoons doubles the sugar, yet 18 − 14 = 4 cm, half of the 8 cm the first teaspoon added.
Check the total. 8 + 6 + 4 = 18, which matches 18 − 0 = 18 cm, the change from the control to the fullest bottle.

In words: the circumference always rises with more sugar, and the gains shrink, which is diminishing returns. The first teaspoon bought 8 cm versus only 4 cm for the last two teaspoons together, a 2-fold drop in payoff for twice the sugar. The trend and the zero control are what matter, never any one centimeter.

Diminishing returns is a pattern in which each added amount of an input produces a smaller gain than the one before, so the output keeps rising but levels off. Here, a fixed amount of yeast, the ethanol building up, the time, and the space in the balloon all cap the output.

Now name the parts, the way Week 1 taught you:

Independent variable: the teaspoons of sugar.
Dependent variable: the balloon circumference at 30 minutes.
Controlled variables: the amount of yeast, the water, the bottle and balloon, and the 30-minute timing.
Control group: the 0-teaspoon bottle.

The gas and the process: the gas is CO₂ from alcoholic fermentation, pyruvate → ethanol + CO₂. The process is anaerobic and yields only glycolysis's 2 ATP per glucose. One glucose splits into 2 pyruvate, and each pyruvate gives off 1 CO₂ on the way to ethanol, so 2 × 1 = 2 CO₂ per glucose fermented.

The conclusion: the evidence supports the hypothesis. It does not prove it; one small experiment supports or fails to support a claim and invites a repeat. Honesty note: yeast packets, water temperature, and balloons vary, so your own circumferences will differ from these engineered numbers. The rising trend, the shrinking gains, and the zero control are what you should expect to see again.

In practice (your tools). In a free spreadsheet such as Google Sheets, type the teaspoons 0, 1, 2, and 4 into cells A2 to A5 and the circumferences into B2 to B5. In cell C3, type =B3-B2 to get the first increment, then copy it down to C5. To sketch the trend, select columns A and B, then choose Insert and then Chart. Always check the spreadsheet's increments against your own hand subtraction.

Misconception check. Students often read the 0 cm control as a failed bottle and leave it out. It is tempting because a zero looks like nothing happened. But the control is the most important bottle: it shows that the gas comes from fermenting the sugar, not from the yeast or the water alone. Without it, you could not say the sugar caused the gas.

Memory hook: no sugar, no gas; more sugar, smaller gains.

Back to the sprint and the jog from that afternoon. The sprint outruns your oxygen, which is why your thighs burn, so your muscle leans on fermentation and its 2 ATP; the jog stays aerobic and runs all four stages.

Key terms

Cellular respiration — The chemistry inside cells that breaks down glucose with oxygen, releasing carbon dioxide and water and using the released energy to recharge ATP. (first used: 1. Separate cellular respiration from breathing)
Aerobic — Describes a process that uses oxygen. (first used: 1. Separate cellular respiration from breathing)
Anaerobic — Describes a process that runs without oxygen, as glycolysis and fermentation do. (first used: 1. Separate cellular respiration from breathing)
Inner mitochondrial membrane — The folded inner membrane of a mitochondrion, which carries the electron transport chain. (first used: 3. Picture the mitochondrion: structure to function)
Mitochondrial matrix — The fluid-filled space inside a mitochondrion's inner membrane, where pyruvate oxidation and the citric acid cycle run. (first used: 3. Picture the mitochondrion: structure to function)
Glycolysis — The first stage, in the cytoplasm, which splits one glucose into two pyruvate and nets 2 ATP and 2 NADH without oxygen. (first used: 4. Split glucose in the cytoplasm: glycolysis)
Pyruvate — The three-carbon molecule glycolysis makes, two from each glucose. (first used: 4. Split glucose in the cytoplasm: glycolysis)
Electron carrier — A molecule that picks up electrons in one step of a pathway and delivers them to a later step. (first used: 4. Split glucose in the cytoplasm: glycolysis)
NADH — The loaded form of the electron carrier NAD⁺, which delivers electrons to the electron transport chain. (first used: 4. Split glucose in the cytoplasm: glycolysis)
Pyruvate oxidation — The second stage, in which each pyruvate enters the mitochondrion and becomes acetyl-CoA while a carbon leaves as carbon dioxide. (first used: 5. Carry pyruvate into the mitochondrion: pyruvate oxidation)
Acetyl-CoA — The molecule pyruvate becomes, which carries the remaining carbon into the citric acid cycle. (first used: 5. Carry pyruvate into the mitochondrion: pyruvate oxidation)
Citric acid cycle — The third stage, also called the Krebs cycle, which runs in the matrix, releases carbon dioxide, makes 2 ATP or GTP per glucose, and loads NADH and FADH₂. (first used: 6. Turn the citric acid cycle in the matrix)
FADH₂ — A second loaded electron carrier, filled in the citric acid cycle. (first used: 6. Turn the citric acid cycle in the matrix)
Electron transport chain — The fourth stage, a chain of carriers in the inner mitochondrial membrane that makes the most ATP of any stage. (first used: 7. Cash in at the electron transport chain)
Final electron acceptor — The molecule that takes the electrons at the end of the chain; in cellular respiration it is oxygen, which forms water. (first used: 7. Cash in at the electron transport chain)
Fermentation — A low-energy backup when oxygen runs out: glycolysis plus a side path that turns NADH back into NAD⁺. (first used: 9. Fall back on fermentation when oxygen runs out)
Lactic acid fermentation — Fermentation in which pyruvate and NADH become lactic acid and NAD⁺, in hard-working muscle and red blood cells. (first used: 9. Fall back on fermentation when oxygen runs out)
Alcoholic fermentation — Fermentation in which pyruvate gives off carbon dioxide and becomes ethanol, as in yeast. (first used: 9. Fall back on fermentation when oxygen runs out)
Ethanol — The alcohol yeast make from pyruvate in alcoholic fermentation, alongside carbon dioxide. (first used: 9. Fall back on fermentation when oxygen runs out)
Diminishing returns — A pattern in which each added amount of an input produces a smaller gain than the one before. (first used: 10. Read the yeast balloon experiment from start to finish)

Summary

Cellular respiration is chemistry inside cells, not breathing.
Nearly every living thing respires, plants included, day and night.
Glucose and oxygen go in; carbon dioxide, water, and ATP come out.
The inner membrane carries the chain; the matrix runs the cycle.
The order: glycolysis, pyruvate oxidation, citric acid cycle, electron transport chain.
Glycolysis runs in the cytoplasm, nets 2 ATP and 2 NADH, and needs no oxygen.
Oxygen is the final electron acceptor and becomes water at the very end.
The chain makes the most ATP; the total is about 30 to 32 per glucose, an estimate.
Fermentation regenerates NAD⁺ so glycolysis keeps running, for only 2 ATP.
Muscle and red blood cells make lactic acid; yeast make ethanol and carbon dioxide.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

Put these four stages in order and give the place each one runs: citric acid cycle, electron transport chain, glycolysis, pyruvate oxidation.
An illustrative cell (made-up numbers) runs glycolysis on 7 glucose molecules. How many pyruvate does it make, and what is its net ATP from glycolysis?
In an illustrative test with made-up numbers, four sealed jars of yeast in warm water get 0, 1, 2, and 3 spoonfuls of honey. In an hour, the jars give off 0, 10, 16, and 19 mL of gas. Find the three increments, describe the pattern, and name the jar that serves as the control.
A friend says, "A red blood cell gets most of its ATP from the electron transport chain, like every other cell." Correct the sentence.

Answers (worked reasoning — read after attempting)

Glycolysis runs first, in the cytoplasm. Pyruvate oxidation is second, as pyruvate enters the mitochondrion. The citric acid cycle is third, in the mitochondrial matrix. The electron transport chain is last, on the inner mitochondrial membrane. The order is a fact, so a list in any other order is wrong even if every location is right.
Each glucose makes 2 pyruvate, so 7 × 2 = 14 pyruvate. Each glucose nets 2 ATP in glycolysis, so 7 × 2 = 14 ATP. Glycolysis needs no oxygen, so this count holds whether or not oxygen is present.
The increments are 10 − 0 = 10 mL, 16 − 10 = 6 mL, and 19 − 16 = 3 mL. The gas keeps rising but the gains shrink from 10 to 6 to 3, which is diminishing returns. The 0-spoonful jar is the control: with no honey it makes no gas, which shows that the gas comes from fermenting the honey's sugar.
A red blood cell has no mitochondria, so it has no electron transport chain at all. It makes its ATP by glycolysis and keeps glycolysis running with lactic acid fermentation, so it gets only 2 ATP per glucose. A better sentence: "A red blood cell gets its ATP from glycolysis, using lactic acid fermentation to regenerate NAD⁺."

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, and the quiz — and next week photosynthesis, the light-driven mechanism that builds the sugar this pathway breaks down, takes the stage.

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