Cellular Respiration and Fermentation
Summary
Cellular respiration extracts the chemical energy stored in glucose and converts it into ATP, the currency cells actually spend. This chapter follows the complete oxidative pathway: glycolysis in the cytoplasm produces pyruvate and a small ATP yield; pyruvate oxidation and the Krebs cycle in the mitochondrial matrix generate NADH and FADH₂ electron carriers; and the mitochondrial electron transport chain couples electron flow to a proton gradient that ATP synthase harvests via chemiosmosis. The chapter quantifies the total ATP yield per glucose and closes with anaerobic fermentation — lactic acid and alcoholic pathways that regenerate NAD⁺ when oxygen is unavailable.
Concepts Covered
This chapter covers the following 14 concepts from the learning graph:
- Cellular Respiration Overview
- Glycolysis
- Substrate-Level Phosphorylation
- Pyruvate Oxidation
- Acetyl-CoA
- Krebs Cycle
- Electron Transport Chain (Mitochondria)
- Oxidative Phosphorylation
- Chemiosmosis
- NADH and FADH₂
- ATP Yield of Respiration
- Fermentation
- Lactic Acid Fermentation
- Alcoholic Fermentation
Gregor Welcomes You!
Welcome, investigators! In Chapter 7 we saw how chloroplasts build glucose from sunlight and \(\ce{CO2}\). Now we flip the script. Cellular respiration dismantles glucose, extracting its stored chemical energy and repackaging it as ATP — the molecule every living cell uses to do work. Let's investigate where all that energy goes!
Every time your muscles contract, your neurons fire, or your ribosomes assemble a protein, ATP is consumed. Cellular respiration is the process that continuously replenishes this supply. The overall reaction is deceptively simple:
But the path from glucose to those 32 ATP molecules is a three-stage metabolic journey spanning the cytoplasm and two mitochondrial compartments. Understanding each stage — its location, its inputs and outputs, and its contribution to the ATP total — is central to AP Biology and to understanding life itself.
Overview: Three Stages, Two Locations
Cellular respiration is divided into three sequential stages:
- Glycolysis — occurs in the cytoplasm; splits glucose into two pyruvate molecules; net yield of 2 ATP and 2 NADH
- Pyruvate oxidation + Krebs cycle — occurs in the mitochondrial matrix; completely oxidizes carbon skeletons to \(\ce{CO2}\); produces electron carriers (NADH and \(\ce{FADH2}\))
- Electron transport chain + oxidative phosphorylation — occurs on the inner mitochondrial membrane; uses electrons from NADH and \(\ce{FADH2}\) to power a proton gradient that drives ATP synthesis
The overall process is aerobic — it requires molecular oxygen (\(\ce{O2}\)) as the final electron acceptor. When oxygen is unavailable, cells resort to anaerobic fermentation to regenerate the NAD⁺ needed for glycolysis to continue.
The diagram above shows the mitochondrion's internal architecture — the double membrane, matrix, and cristae — and the locations of the electron transport chain complexes. Keep this structure in mind as you trace the respiration pathway through each stage.
Stage 1: Glycolysis
Glycolysis ("glucose splitting") takes place in the cytoplasm and does not require oxygen. It is the most ancient metabolic pathway, shared by virtually all living organisms. Ten enzyme-catalyzed reactions convert one glucose molecule into two molecules of pyruvate (3-carbon).
Energy Investment and Payoff
Glycolysis is divided into two phases:
Energy-investment phase (reactions 1–5): Two ATP are consumed to phosphorylate glucose, making it more reactive. The resulting fructose-1,6-bisphosphate is cleaved into two interconvertible 3-carbon sugars (glyceraldehyde-3-phosphate, G3P).
Energy-payoff phase (reactions 6–10): Each G3P is oxidized and converted to pyruvate. Four ATP are produced by substrate-level phosphorylation — direct transfer of a phosphate group from a substrate to ADP. Two NADH are also produced per glucose.
The net yield of glycolysis per glucose molecule:
Gregor's Tip
On the AP exam, questions often ask whether glycolysis requires oxygen. It does not — it occurs in both aerobic and anaerobic conditions. The 2 NADH produced in glycolysis can only be used by the ETC if oxygen is present; in anaerobic conditions, fermentation regenerates NAD⁺ so glycolysis can continue.
Substrate-level phosphorylation is the mechanism operating in both glycolysis and the Krebs cycle. Unlike oxidative phosphorylation (which uses a proton gradient), substrate-level phosphorylation couples phosphate transfer directly to a spontaneous chemical reaction. It is less efficient but oxygen-independent.
Diagram: Glycolysis Pathway Simulator
Glycolysis Pathway Simulator — Specification
Type: MicroSim (p5.js)
sim-id: glycolysis-simulator
Library: p5.js
Status: Specified
Learning objective: Students will be able to describe (Bloom's L2: Understand) the inputs, outputs, and net energy yield of glycolysis, distinguishing the investment phase from the payoff phase.
Canvas: 760 × 420 px, responsive.
Layout: Three columns: left (energy investment phase, reactions 1–5 shaded red/pink), center divider showing glucose cleavage, right (energy payoff phase, reactions 6–10 shaded green).
Visual elements: - Hexagon icon for glucose at top-left; arrow flow downward through 10 numbered steps - Each step shows: substrate name → enzyme name → product name - ATP consumed shown as red "−ATP" badges at steps 1 and 3 - ATP produced shown as green "+ATP" badges at steps 7 and 10 (×2 each because 2 G3P) - NADH produced shown as blue "+NADH" badge at step 6 (×2) - Net tally panel at bottom: "Net ATP: +2", "Net NADH: +2", "Net Pyruvate: 2"
Interaction: - Click any numbered step to expand a tooltip panel showing the full reaction equation, enzyme name, and cofactor requirement - Toggle button: "Show Investment Phase" / "Show Payoff Phase" highlights the relevant half of the pathway - "Reset" button returns all steps to default state
Color coding: Investment phase background light coral (#FFCCCC); payoff phase background light green (#CCFFCC); NADH badges blue (#4A90D9); ATP badges gold (#F5A623)
Stage 2: Pyruvate Oxidation and the Krebs Cycle
Pyruvate Oxidation
Before entering the Krebs cycle, each pyruvate molecule produced by glycolysis must be converted to acetyl-CoA. This occurs in the mitochondrial matrix and is catalyzed by the pyruvate dehydrogenase complex — a large, multi-enzyme assembly.
The reaction for each pyruvate:
Per glucose (two pyruvates): two acetyl-CoA, two CO₂ released, and two NADH produced. The carbon atoms lost as \(\ce{CO2}\) will not be recaptured — this is one of the six carbons fully oxidized during respiration.
Acetyl-CoA (acetyl coenzyme A) is a 2-carbon acetyl group attached to coenzyme A via a high-energy thioester bond. It is the entry point for the Krebs cycle, and it also links lipid and protein catabolism to respiration — fatty acids and certain amino acids are also converted to acetyl-CoA.
You've Got This!
The Krebs cycle has eight reactions and a lot of intermediate names. You do not need to memorize every intermediate for the AP exam! Focus on the inputs and outputs: what goes in (acetyl-CoA, NAD⁺, FAD, ADP), what comes out (CO₂, NADH, FADH₂, ATP), and how many turns per glucose. The intermediate names are context — the accounting is what matters.
The Krebs Cycle (Citric Acid Cycle)
The Krebs cycle (also called the citric acid cycle or TCA cycle) runs in the mitochondrial matrix. Each turn of the cycle accepts one 2-carbon acetyl group from acetyl-CoA, combines it with a 4-carbon oxaloacetate to form 6-carbon citrate, and then oxidizes and releases carbon as \(\ce{CO2}\), regenerating oxaloacetate.
Per one turn of the Krebs cycle (one acetyl-CoA):
- 2 \(\ce{CO2}\) released (complete oxidation of the 2-carbon acetyl group)
- 3 NADH produced
- 1 \(\ce{FADH2}\) produced
- 1 ATP (or GTP) produced by substrate-level phosphorylation
Per glucose (two turns):
- 4 \(\ce{CO2}\) released
- 6 NADH produced
- 2 \(\ce{FADH2}\) produced
- 2 ATP produced
The Krebs cycle's primary function is not to produce ATP directly but to extract electrons from organic molecules and load them onto NAD⁺ and FAD, producing NADH and \(\ce{FADH2}\). These electron carriers then deliver their electrons to the ETC, where most of the ATP is generated.
Diagram: Krebs Cycle Explorer
Krebs Cycle Explorer — Specification
Type: MicroSim (p5.js)
sim-id: krebs-cycle-explorer
Library: p5.js
Status: Specified
Learning objective: Students will be able to explain (Bloom's L2: Understand) the inputs, outputs, and overall purpose of the Krebs cycle, and identify where NADH and FADH₂ are produced.
Canvas: 760 × 480 px, responsive.
Layout: Circular pathway with 8 labeled intermediates arranged clockwise. Each intermediate shown as a rounded rectangle with carbon count (e.g., "Citrate (6C)"). Arrows between intermediates indicate reactions.
Molecule nodes: - Oxaloacetate (4C) at top (entry/exit point, highlighted in gold) - Citrate (6C), Isocitrate (6C), α-Ketoglutarate (5C), Succinyl-CoA (4C), Succinate (4C), Fumarate (4C), Malate (4C) - Acetyl-CoA input arrow entering at top-left junction
Product badges on arrows: - CO₂ released: red badge on isocitrate→α-ketoglutarate and α-ketoglutarate→succinyl-CoA steps - NADH produced: blue badge on three dehydrogenase steps - FADH₂ produced: orange badge on succinate→fumarate step - ATP produced: green badge on succinyl-CoA→succinate step
Running tally panel (right side): Live count of CO₂, NADH, FADH₂, ATP accumulated per turn as user clicks through steps.
Interaction: - "Step Forward" button advances through the cycle one reaction at a time - Click any intermediate node to show its name, carbon count, and role - "Run Full Cycle" animates all 8 steps automatically - "×2 for glucose" toggle doubles all product counts and adds label "per glucose"
Responsive design: Cycle radius scales with container width. All text scales proportionally.
Stage 3: Electron Transport Chain and Oxidative Phosphorylation
The Electron Transport Chain
The electron transport chain (ETC) is embedded in the inner mitochondrial membrane — the highly folded membrane (cristae) whose surface area is maximized to accommodate thousands of ETC protein complexes per cell. The ETC consists of four large protein complexes (I–IV) plus two mobile electron carriers (ubiquinone/coenzyme Q and cytochrome c).
Key Insight
Notice that the ETC in mitochondria runs in the opposite direction from the photosynthetic ETC in chloroplasts. In chloroplasts, light energy inputs electrons at a high-energy state. In mitochondria, electrons fall to a lower energy state, releasing energy that pumps protons. Two organelles, two ETCs, one elegant thermodynamic principle — energy flows downhill.
Electrons from NADH enter at Complex I (NADH dehydrogenase); electrons from \(\ce{FADH2}\) enter at Complex II (succinate dehydrogenase). Both converge on ubiquinone (Q), which ferries electrons to Complex III (cytochrome bc₁ complex), then to cytochrome c, and finally to Complex IV (cytochrome c oxidase).
At Complex IV, electrons are transferred to molecular oxygen — the final electron acceptor:
This reduction of oxygen to water is why aerobic respiration requires \(\ce{O2}\). Without oxygen, electrons cannot flow through the chain, the proton gradient collapses, and oxidative phosphorylation halts.
Proton pumping: As electrons move through Complexes I, III, and IV, the energy released is used to pump \(\ce{H+}\) ions (protons) from the mitochondrial matrix into the intermembrane space (IMS). This creates:
- A concentration gradient (more \(\ce{H+}\) in IMS than matrix)
- An electrical gradient (IMS is positively charged relative to matrix)
Together these constitute the proton-motive force (PMF), also called the electrochemical gradient.
Chemiosmosis and ATP Synthesis
The proton-motive force drives protons back into the matrix through ATP synthase (Complex V) — a remarkable molecular motor that uses the energy of proton flow to synthesize ATP from ADP and inorganic phosphate. This process is called chemiosmosis (proposed by Peter Mitchell, Nobel Prize 1978).
ATP synthase has two structural domains: - F₀ subunit — embedded in the inner membrane; the proton channel and rotary motor - F₁ subunit — protrudes into the matrix; contains the catalytic sites where ATP is synthesized
Approximately 2.5 ATP are synthesized per NADH oxidized and 1.5 ATP per \(\ce{FADH2}\) oxidized. These values reflect the actual coupling efficiency; textbooks historically used 3 and 2 (based on theoretical P/O ratios), but modern measurements favor the lower figures.
Oxidative phosphorylation refers to the combined process: electrons flow through the ETC (oxidation), driving proton pumping and subsequent ATP synthesis (phosphorylation) via chemiosmosis.
Diagram: ATP Yield Calculator
ATP Yield Calculator — Specification
Type: MicroSim (p5.js)
sim-id: atp-yield-calculator
Library: p5.js
Status: Specified
Learning objective: Students will be able to calculate (Bloom's L3: Apply) the total ATP yield per glucose from cellular respiration, accounting for each stage's contribution.
Canvas: 760 × 450 px, responsive.
Layout: Vertical table-style visualization with one row per metabolic stage.
Rows: 1. Glycolysis: 2 ATP (substrate-level), 2 NADH × 2.5 = 5 ATP (oxidative) = 7 ATP total 2. Pyruvate Oxidation: 0 substrate-level, 2 NADH × 2.5 = 5 ATP (oxidative) = 5 ATP total 3. Krebs Cycle: 2 ATP (substrate-level), 6 NADH × 2.5 = 15 ATP + 2 FADH₂ × 1.5 = 3 ATP (oxidative) = 20 ATP total 4. Grand Total: ~32 ATP per glucose
Columns: Stage | Substrate-level ATP | NADH → ATP | FADH₂ → ATP | Stage Total
Visual elements: - Color-coded rows: glycolysis (yellow), pyruvate oxidation (orange), Krebs cycle (red/maroon), ETC/oxidative phosphorylation (purple) - Running total shown in bold bottom row - "Modern estimate (~32)" vs "Classic estimate (~38)" toggle button that updates NADH and FADH₂ conversion factors and re-totals - Pie chart (right side) showing proportion of ATP from each stage
Interaction: - Slider for NADH → ATP conversion factor (1.5–3 range) with live recalculation - Slider for FADH₂ → ATP conversion factor (1.0–2.0 range) with live recalculation - Hover over any row for a brief explanation of where that stage occurs
Responsive design: Column widths scale proportionally. Pie chart radius scales with available space.
ATP Yield Summary
| Stage | Location | Substrate-level ATP | NADH produced | FADH₂ produced | ATP from oxidative phosphorylation | Stage total |
|---|---|---|---|---|---|---|
| Glycolysis | Cytoplasm | 2 | 2 | 0 | ~5 | ~7 |
| Pyruvate oxidation | Matrix | 0 | 2 | 0 | ~5 | ~5 |
| Krebs cycle | Matrix | 2 | 6 | 2 | ~18 | ~20 |
| Total per glucose | 4 | 10 | 2 | ~28 | ~32 |
Common Mistake
Many textbooks and older AP materials list the total ATP yield as 36 or 38 ATP per glucose. This is the theoretical maximum based on idealized P/O ratios. Modern biochemistry measures the actual yield at ~30–32 ATP due to membrane leakage, the energy cost of importing cytoplasmic NADH into the mitochondria, and variable coupling efficiency. The AP exam accepts both estimates but be ready to explain why the actual yield is lower than the theoretical maximum.
Fermentation: Anaerobic Energy Production
When oxygen is absent or insufficient, the ETC cannot function — there is no final electron acceptor. NADH accumulates, and because NAD⁺ cannot be regenerated by the ETC, glycolysis halts (it requires NAD⁺ as an electron acceptor at step 6). Fermentation solves this problem by transferring electrons from NADH to organic molecules, regenerating NAD⁺ without producing more ATP.
Fermentation's sole purpose is NAD⁺ regeneration so that glycolysis can continue producing the 2 ATP that sustain cell survival. The organic molecules that accept electrons become the fermentation waste products.
There are two principal fermentation pathways:
Lactic Acid Fermentation
In lactic acid fermentation, pyruvate is the electron acceptor. NADH donates its electrons directly to pyruvate, reducing it to lactate (lactic acid):
This reaction is catalyzed by lactate dehydrogenase (LDH). No CO₂ is released; the carbon atoms in pyruvate are simply rearranged into lactate.
Where it occurs: - Skeletal muscle cells during intense exercise (when \(\ce{O2}\) delivery cannot keep pace with ATP demand) - Red blood cells (which lack mitochondria entirely) - Lactobacillus bacteria (used in yogurt, cheese, and sauerkraut fermentation)
The accumulation of lactate (and accompanying \(\ce{H+}\) ions) is associated with muscle fatigue. When oxygen becomes available, lactate is converted back to pyruvate in the liver (Cori cycle) and re-enters aerobic respiration.
Alcoholic Fermentation
In alcoholic fermentation, pyruvate is first decarboxylated to acetaldehyde by pyruvate decarboxylase, releasing \(\ce{CO2}\):
Acetaldehyde then accepts electrons from NADH, producing ethanol:
Where it occurs: - Yeast (Saccharomyces cerevisiae) — the basis of bread baking (CO₂ makes dough rise) and alcoholic beverage production - Some plant cells under waterlogged (anaerobic) conditions
MicroSim: Brewing Beer Process Explorer
Even though beer brewing is beyond the scope of the AP Biology exam, it's a fun, memorable way to see glycolysis and alcoholic fermentation unfold step by step right inside a real-world fermenter.
Diagram: Fermentation Pathways Comparison
Fermentation Pathways Comparison — Specification
Type: MicroSim (p5.js)
sim-id: fermentation-pathways
Library: p5.js
Status: Specified
Learning objective: Students will be able to compare (Bloom's L4: Analyze) lactic acid and alcoholic fermentation in terms of reactants, products, and organisms, and explain why fermentation is necessary when oxygen is absent.
Canvas: 760 × 440 px, responsive.
Layout: Two parallel flow diagrams side by side, separated by a central divider column.
Left panel (Lactic Acid Fermentation): - Glucose → (Glycolysis) → 2 Pyruvate + 2 NADH + 2 ATP - Arrow from pyruvate → lactate (with NADH → NAD⁺ shown as curved arrow) - Waste product label: "Lactate (lactic acid)" - Organism examples: muscle cells, red blood cells, Lactobacillus (with small icons)
Right panel (Alcoholic Fermentation): - Glucose → (Glycolysis) → 2 Pyruvate + 2 NADH + 2 ATP - Arrow: pyruvate → acetaldehyde + CO₂ (pyruvate decarboxylase) - Arrow: acetaldehyde → ethanol (NADH → NAD⁺ curved arrow) - Waste products: "Ethanol + CO₂" - Organism examples: yeast, waterlogged plant cells
Central column: Shows "NAD⁺ Regenerated!" badge with a circular arrow graphic; label "Purpose: Allow glycolysis to continue"
Running ATP tally: Both panels show "Net ATP = 2 per glucose" with a note "No extra ATP from fermentation"
Interaction: - Click "Animate" to trace molecule flow through either pathway with color-coded NADH → NAD⁺ conversion - Click organism icons to show a fun-fact pop-up (e.g., "Yeast produce CO₂ that makes bread rise!") - Toggle between "Aerobic" (greyed out ETC shown as disabled) and "Anaerobic" (ETC marked with ✗) modes to show context
Responsive design: Panel widths scale with container width. Molecule labels scale proportionally.
| Feature | Lactic Acid Fermentation | Alcoholic Fermentation |
|---|---|---|
| Organism examples | Muscle cells, red blood cells, Lactobacillus | Yeast (Saccharomyces), some plants |
| Electron acceptor | Pyruvate | Acetaldehyde |
| Waste products | Lactate (\(\ce{C3H6O3}\)) | Ethanol (\(\ce{C2H5OH}\)) + \(\ce{CO2}\) |
| CO₂ released? | No | Yes |
| Net ATP per glucose | 2 (from glycolysis only) | 2 (from glycolysis only) |
| Key purpose | NAD⁺ regeneration | NAD⁺ regeneration |
Connecting the Stages: Carbon and Electron Accounting
Every carbon atom in glucose is fully oxidized to \(\ce{CO2}\) by the end of the Krebs cycle: - 2 carbons released as \(\ce{CO2}\) during pyruvate oxidation (one per pyruvate × 2) - 4 carbons released as \(\ce{CO2}\) during two Krebs cycle turns (two per turn × 2) - Total: 6 CO₂ per glucose — matching the stoichiometry in the overall equation
Every hydrogen atom (proton + electron pair) stripped from glucose is captured by NAD⁺ or FAD: - 10 NADH + 2 \(\ce{FADH2}\) per glucose pass their electrons to the ETC - The ETC transfers these electrons to \(\ce{O2}\), producing 6 \(\ce{H2O}\)
This electron flow — from the high-energy electrons in glucose through the ETC to the electronegative \(\ce{O2}\) — is what drives the proton-motive force and, through chemiosmosis, the synthesis of ~28 ATP by oxidative phosphorylation.
Excellent Work!
You've traced the complete journey of a glucose molecule — from the cytoplasm through glycolysis, across the mitochondrial membrane for pyruvate oxidation, around the Krebs cycle, and through the electron transport chain to water and ATP. That's the entire machinery of aerobic life in one chapter. The investigators have officially mastered cellular energy metabolism. Let's investigate the next frontier!
Summary
Cellular respiration extracts the energy stored in glucose through three coordinated stages:
- Glycolysis (cytoplasm): glucose → 2 pyruvate; net +2 ATP, +2 NADH; aerobic or anaerobic
- Pyruvate oxidation (mitochondrial matrix): 2 pyruvate → 2 acetyl-CoA; +2 NADH, −2 CO₂
- Krebs cycle (mitochondrial matrix): 2 acetyl-CoA fully oxidized; +6 NADH, +2 \(\ce{FADH2}\), +2 ATP, −4 CO₂
- ETC + oxidative phosphorylation (inner mitochondrial membrane): electrons from NADH and \(\ce{FADH2}\) reduce \(\ce{O2}\); proton gradient drives ATP synthase; ~28 ATP produced
- Total yield: ~32 ATP per glucose under physiological conditions
Fermentation (lactic acid or alcoholic) sustains glycolysis under anaerobic conditions by regenerating NAD⁺, but produces no additional ATP beyond the 2 from glycolysis.
Self-Check: Test Your Understanding
Question 1. A cell is treated with a drug that blocks Complex I of the ETC. Which of the following would you expect?
Answer
NADH would accumulate (no longer oxidized by Complex I), the proton gradient would decrease, ATP synthesis by oxidative phosphorylation would decline, and the cell would rely more heavily on substrate-level phosphorylation (glycolysis + Krebs) and possibly fermentation to meet ATP demands.
Question 2. Why does \(\ce{FADH2}\) produce fewer ATP than NADH during oxidative phosphorylation?
Answer
\(\ce{FADH2}\) donates electrons at Complex II (succinate dehydrogenase), which does not pump protons. NADH donates electrons at Complex I, which does pump protons. Because \(\ce{FADH2}\) electrons bypass the first proton pump, fewer protons are translocated per electron pair, resulting in a lower PMF contribution and fewer ATP synthesized (~1.5 vs. ~2.5 per carrier).
Question 3. A runner's muscles are producing lactate. What does this tell you about the oxygen supply to those muscles?
Answer
The muscles are not receiving enough \(\ce{O2}\) to keep up with the rate of NADH production. The ETC cannot process NADH fast enough, so cells switch to lactic acid fermentation to regenerate NAD⁺ and keep glycolysis (and ATP production) running. This is a normal physiological response to intense exercise.
Question 4. Where are the six \(\ce{CO2}\) molecules of the overall respiration equation released?
Answer
Two \(\ce{CO2}\) are released during pyruvate oxidation (one per pyruvate × 2 pyruvates per glucose) and four \(\ce{CO2}\) are released during two turns of the Krebs cycle (two per turn × 2 turns). None are released during glycolysis or the ETC.
Concepts Covered
- Cellular Respiration Overview
- Glycolysis
- Substrate-Level Phosphorylation
- Pyruvate Oxidation
- Acetyl-CoA
- Krebs Cycle
- Electron Transport Chain (Mitochondria)
- Oxidative Phosphorylation
- Chemiosmosis
- NADH and FADH₂
- ATP Yield of Respiration
- Fermentation
- Lactic Acid Fermentation
- Alcoholic Fermentation