SCI/STUDY — Cellular Respiration

01 The Net Equation

The balanced overall equation for aerobic cellular respiration. Tap each term to see exactly where and how it participates.

Aerobic Cellular Respiration (per 1 glucose)

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ~30–32 ATP

Tap a term above to see its role in the process.

ΔG = −686 kcal/mol released · ~40% captured as ATP · ~60% lost as heat

📍 AP Exam Tip

The classic answer is "36–38 ATP" but the modern, updated estimate is 30–32 ATP per glucose due to revised H⁺/ATP ratios and transport costs. AP exams accept either but trend toward the updated values. Know why the actual yield is less than the theoretical maximum.

02 Mitochondria Anatomy

Knowing where each stage occurs inside the mitochondrion is essential for AP Bio. Click any region below.

Click any labeled region to learn about that structure.

Location 1 · Cytoplasm

Glycolysis

Occurs in the cytosol, outside the mitochondrion entirely. Does not require O₂. Converts 1 glucose → 2 pyruvate, netting 2 ATP and 2 NADH.

Location 2 · Mitochondrial Matrix

Pyruvate Oxidation + Krebs

Pyruvate enters the matrix via active transport. Acetyl-CoA and the Krebs Cycle enzymes are all dissolved in the matrix fluid. CO₂ is released here.

Location 3 · Inner Membrane

Electron Transport Chain

Complexes I–IV and ATP synthase are embedded in the inner mitochondrial membrane (cristae). H⁺ is pumped into the IMS; flows back through ATP synthase into the matrix.

Location 4 · Intermembrane Space

Proton Reservoir

H⁺ accumulates here, lowering pH and creating the proton-motive force that drives ATP synthase. Analogous to the thylakoid lumen in photosynthesis.

03 The Metabolic Engine

Step through the complete breakdown of 1 glucose molecule. Watch ATP, NADH, and FADH₂ accumulate live in the HUD.

Net ATP Yield

0

NADH Produced

0

FADH₂ Produced

0

STAGE 0 / SYSTEM IDLE

Cytoplasm Ready

1 Glucose molecule (C₆H₁₂O₆) is present in the cytoplasm. All cellular machinery is primed. Awaiting activation of the metabolic pathway.

⚡ Critical Junction: Oxygen Status

Glycolysis is complete. 2 pyruvate molecules await their fate. Does O₂ exist in this cell?

04 Glycolysis — Step by Step

10 enzyme-catalyzed reactions in the cytoplasm. No O₂ required. Divided into an energy-investment phase (steps 1–5) and an energy-payoff phase (steps 6–10). Step through each reaction below.

Glycolysis Net Equation

Glucose + 2 NAD⁺ + 2 ADP + 2 Pᵢ → 2 Pyruvate + 2 NADH + 2 ATP (net) + 2 H₂O

Gross: 4 ATP made − 2 ATP invested = 2 ATP net

STEP 1 OF 10 · ENERGY INVESTMENT PHASE

Hexokinase

Glucose enters the cell and is immediately phosphorylated by hexokinase, adding a phosphate from ATP to form Glucose-6-phosphate (G6P). This "traps" glucose in the cell (charged molecules cannot cross the membrane) and primes it for further reactions. Cost: 1 ATP.

−1 ATP (invested) G6P produced

1 / 10

Steps 1–5 · Energy Investment

The Warmup

2 ATP are consumed to phosphorylate glucose and split it into two 3-carbon molecules (DHAP + G3P). No energy is released yet — the cell is paying upfront to prime the substrate.

Steps 6–10 · Energy Payoff

The Return

4 ATP are produced (2 per G3P × 2) by substrate-level phosphorylation, and 2 NADH are generated. Net gain: 2 ATP, 2 NADH, 2 pyruvate.

📍 Key Enzymes for AP

Hexokinase (step 1) — phosphorylates glucose, regulated by feedback inhibition from G6P. Phosphofructokinase (PFK) (step 3) — rate-limiting enzyme of glycolysis; inhibited by ATP/citrate, activated by AMP. Pyruvate Kinase (step 10) — final ATP-generating step.

05 Pyruvate Oxidation

The bridge reaction between glycolysis and the Krebs Cycle. Often overlooked, but frequently tested. Occurs in the mitochondrial matrix.

Pyruvate Oxidation (per 1 pyruvate — ×2 per glucose)

Pyruvate (3C) + CoA-SH + NAD⁺ → Acetyl-CoA (2C) + CO₂ + NADH

×2 per glucose → 2 Acetyl-CoA + 2 CO₂ + 2 NADH total

The Pyruvate Dehydrogenase Complex

PDC — Multi-Enzyme

Pyruvate oxidation is catalyzed by the Pyruvate Dehydrogenase Complex (PDC), a massive multi-enzyme cluster that performs three linked reactions simultaneously. It requires cofactors including NAD⁺, CoA, FAD, TPP, and lipoic acid.

Regulation

Feedback Inhibited

PDC is inhibited by its own products: Acetyl-CoA and NADH. When energy is abundant (high ATP ratio, high NADH), PDC slows down. It is activated by AMP, CoA, and NAD⁺ (low energy signals). PDC is also inhibited by phosphorylation.

📍 Why This Matters

Pyruvate oxidation releases the first CO₂ from glucose breakdown. Combined with the Krebs cycle, this is why all 6 carbons of glucose eventually leave as CO₂. Zero ATP is made in this step — only 2 NADH and 2 CO₂ per glucose.

06 The Krebs Cycle (Citric Acid Cycle)

Occurs in the mitochondrial matrix. Runs twice per glucose (once per Acetyl-CoA). Each turn completes the oxidation of carbon, generating energy carriers for the ETC. No O₂ is directly required here.

Krebs Cycle Net (per 1 Acetyl-CoA — ×2 for full glucose)

Acetyl-CoA (2C) + OAA (4C) + 3 NAD⁺ + FAD + ADP → 2 CO₂ + 3 NADH + FADH₂ + 1 ATP (GTP) + OAA regenerated

×2 per glucose: 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂

STEP 1 OF 5 KEY STEPS

Citrate Synthase

Acetyl-CoA (2C) condenses with Oxaloacetate (OAA, 4C) → Citrate (6C). This is the entry point of the cycle and gives it its other name — the Citric Acid Cycle. Citrate synthase is inhibited by high [ATP] and [NADH].

Acetyl-CoA + OAA → Citrate (6C) No energy yet

Full Stoichiometry per Glucose (2 turns)

Per turn (1 Acetyl-CoA): 3 NADH + 1 FADH₂ + 1 ATP/GTP + 2 CO₂

Per glucose (2 turns): 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂

Running total with glycolysis + pyruvate oxidation: 10 NADH + 2 FADH₂ + 4 ATP

Stage	ATP (direct)	NADH	FADH₂	CO₂
Glycolysis	2 (net)	2	0	0
Pyruvate Oxidation	0	2	0	2
Krebs Cycle (×2)	2 (GTP)	6	2	4
Oxidative Phosphorylation	~26–28	−10 (used)	−2 (used)	0
Total	~30–32	0 (net)	0 (net)	6

📍 Key Krebs Enzymes (AP Tested)

Citrate Synthase — entry point, inhibited by ATP/NADH. Isocitrate Dehydrogenase — first NADH + CO₂, rate-limiting. α-Ketoglutarate Dehydrogenase — second NADH + CO₂ + succinyl-CoA. Succinate Dehydrogenase — the only Krebs enzyme embedded in the membrane; makes FADH₂; is also Complex II of the ETC.

07 Oxidative Phosphorylation

The powerhouse of respiration. Located in the inner mitochondrial membrane. The ETC uses electrons from NADH/FADH₂ to pump H⁺ into the IMS, creating a proton-motive force that drives ATP synthase.

Overall Oxidative Phosphorylation

10 NADH + 2 FADH₂ + 6 O₂ → ~26–28 ATP + 6 H₂O + 10 NAD⁺ + 2 FAD

Electron Transport Chain + Chemiosmosis (Inner Mitochondrial Membrane)

Chemiosmosis & ATP Synthase

Proton-Motive Force

The Gradient

The combined effect of the concentration gradient (ΔpH) and electrical gradient (ΔΨ) across the inner membrane. Both drive H⁺ through ATP synthase. Uncouplers (e.g., DNP, thermogenin) dissipate this gradient as heat instead of making ATP.

ATP Yield Calculation

P:O Ratio

NADH: ~2.5 ATP each (pumps ~10 H⁺ via CI+CIII+CIV)
FADH₂: ~1.5 ATP each (pumps ~6 H⁺, bypasses CI)
~4 H⁺ per ATP through ATP synthase
10 NADH × 2.5 = 25 ATP
2 FADH₂ × 1.5 = 3 ATP
Total OxPhos: ~28 ATP

Complete ATP Accounting per Glucose

2 ATP (Glycolysis) + 2 ATP (Krebs/GTP) + ~28 ATP (OxPhos) = ~30–32 ATP total

10 NADH × 2.5 + 2 FADH₂ × 1.5 = 25 + 3 = ~28 ATP from OxPhos

⚠️ Common Misconception

FADH₂ yields fewer ATP than NADH because it donates electrons to Complex II, which does NOT pump H⁺. It therefore contributes to a smaller proton gradient (~6 H⁺ pumped vs ~10 H⁺ for NADH). This is why 1 NADH ≈ 2.5 ATP but 1 FADH₂ ≈ 1.5 ATP.

08 Anaerobic Respiration & Fermentation

When O₂ is absent, the ETC cannot function (no final electron acceptor). Fermentation regenerates NAD⁺ from NADH so glycolysis can continue — the cell survives on just 2 ATP per glucose.

⚠️ Critical distinction

Fermentation does NOT produce additional ATP. It only recycles NAD⁺ to keep glycolysis running. Without NAD⁺ regeneration, glycolysis would halt because NAD⁺ would be depleted. Fermentation is about keeping the lights on, not generating new power.

MUSCLE CELLS, BACTERIA, FUNGI

Lactic Acid Fermentation

Pyruvate → Lactate NADH → NAD⁺ recycled Net: 2 ATP only

Pyruvate is directly reduced to lactate (lactic acid) by lactate dehydrogenase, oxidizing NADH back to NAD⁺. Used by: muscle cells during intense exercise (causing the "burn"), red blood cells (no mitochondria!), and many bacteria (yogurt, cheese). The lactate is exported to the liver, where it can be converted back to glucose via the Cori cycle.

Pyruvate+NADH + H⁺→Lactate+NAD⁺

09 Alternative Substrates

Cells don't only burn glucose. Fats, proteins, and other carbohydrates are funneled into the same pathways at different entry points.

Fats / Lipids

β-Oxidation

Triglycerides are hydrolyzed → glycerol + fatty acids. Fatty acids undergo β-oxidation in the mitochondrial matrix, sequentially cleaving 2-carbon units as Acetyl-CoA, which enters the Krebs Cycle. Also produces NADH and FADH₂ per cleavage. Fats yield more ATP per gram than carbohydrates (~9 kcal/g vs ~4 kcal/g).

Proteins / Amino Acids

Deamination

Amino acids are first deaminated (−NH₂ removed, becomes urea/ammonia). The remaining carbon skeleton enters the pathway at different points: some as pyruvate, some as Acetyl-CoA, some directly as Krebs intermediates (OAA, α-ketoglutarate, fumarate, etc.).

Other Carbohydrates

Funneled to Glucose

Glycogen → glucose-1-phosphate → G6P → glycolysis. Fructose → F6P or DHAP. Galactose → G1P → G6P. All carbohydrates are ultimately converted to molecules that enter glycolysis. This is why carbohydrates are interchangeable as energy sources.

10 Regulation of Cellular Respiration

Respiration is tightly regulated by feedback inhibition and allosteric control — the cell only makes ATP as fast as it is used. The key signals are the ATP:ADP ratio and NADH levels.

Enzyme / Complex	Inhibited By (high energy)	Activated By (low energy)	Location
Hexokinase	G6P (product inhibition)	Low [G6P]	Cytoplasm
Phosphofructokinase (PFK)	ATP, Citrate, H⁺	AMP, ADP, F2,6-BP	Cytoplasm (rate-limiting!)
Pyruvate Kinase	ATP, Acetyl-CoA, NADH	AMP, fructose-1,6-BP	Cytoplasm
Pyruvate Dehydrogenase	Acetyl-CoA, NADH, ATP	AMP, CoA, NAD⁺, Ca²⁺	Matrix
Citrate Synthase	ATP, NADH, Succinyl-CoA	ADP, Ca²⁺	Matrix
Isocitrate Dehydrogenase	ATP, NADH	ADP, AMP, Ca²⁺	Matrix (rate-limiting!)

📍 The Big Picture of Regulation

When ATP is abundant: glycolysis and Krebs slow. When ADP/AMP are high (energy depleted): all pathways speed up. This is energy charge regulation. The cell achieves homeostasis — it never runs out of ATP for long, and never wastes energy making ATP it doesn't need.

11 Flashcard Recall

Tap each card to flip it and test your recall on key definitions and values.

Oxidation vs. Reduction

OIL RIG:
Oxidation Is Loss (of e⁻)
Reduction Is Gain (of e⁻)

In respiration, glucose is oxidized; O₂ is reduced to H₂O.

Net ATP from Glycolysis

2 ATP net
(4 produced − 2 invested)

Also: 2 NADH + 2 pyruvate.
No CO₂ released.

Krebs Cycle Yield (per turn)

Per Acetyl-CoA:
3 NADH
1 FADH₂
1 ATP (or GTP)
2 CO₂ released

×2 per glucose → 6 NADH + 2 FADH₂ + 2 ATP

Substrate-Level vs. Oxidative Phosphorylation

Substrate-level: ATP directly transferred from a phosphorylated substrate (glycolysis, Krebs).

Oxidative: ATP made using the proton gradient (OxPhos — the big yield).

Purpose of Fermentation

Regenerates NAD⁺ from NADH so glycolysis can continue.

Does NOT make additional ATP. The cell survives on 2 ATP/glucose only.

Why FADH₂ < NADH in ATP yield

FADH₂ feeds electrons to Complex II, which does NOT pump H⁺.

NADH → Complex I (pumps 4H⁺).
FADH₂ only gets ~6 H⁺ pumped → ~1.5 ATP vs ~2.5 ATP.

Proton-Motive Force

Combines two gradients across the inner mitochondrial membrane:

1. ΔpH (chemical gradient)
2. ΔΨ (electrical gradient)

Both drive H⁺ through ATP synthase.

Uncouplers (e.g. DNP, Thermogenin)

Make the inner mitochondrial membrane leaky to H⁺.

H⁺ crosses without going through ATP synthase → gradient is dissipated as heat, not ATP.
Used in thermogenesis (brown adipose tissue).

Where is CO₂ released?

Pyruvate Oxidation: 2 CO₂ (1 per pyruvate)

Krebs Cycle: 4 CO₂ (2 per turn × 2 turns)

NOT in glycolysis or the ETC.

Phosphofructokinase (PFK)

The rate-limiting enzyme of glycolysis (step 3).

Converts F6P → F1,6-BP using ATP.

Inhibited by: ATP, citrate, H⁺
Activated by: AMP, ADP, F-2,6-BP

RQ (Respiratory Quotient)

RQ = CO₂ produced / O₂ consumed

Glucose: RQ = 1.0
Fats: RQ ≈ 0.7
Proteins: RQ ≈ 0.8

A low RQ indicates fat oxidation.

O₂ Final Electron Acceptor

O₂ accepts electrons at Complex IV (cytochrome c oxidase):

O₂ + 4 e⁻ + 4 H⁺ → 2 H₂O

Without O₂, electrons back up → ETC halts → no more NADH/FADH₂ oxidized → OxPhos stops.

12 AP Exam Key Concepts

Must Know #1

30–32 ATP total

Modern estimate per glucose: 2 (glycolysis) + 2 (Krebs/GTP) + ~28 (OxPhos). Old answer was 36–38; AP exams now accept 30–32.

Must Know #2

10 NADH + 2 FADH₂

Electron carrier total per glucose: 2 (glycolysis) + 2 (pyruvate oxidation) + 6 (Krebs) = 10 NADH; 2 FADH₂ from Krebs only.

Must Know #3

CO₂ from Krebs only

All 6 carbons of glucose leave as CO₂: 2 CO₂ in pyruvate oxidation + 4 CO₂ in the Krebs cycle. None from glycolysis or the ETC.

Must Know #4

O₂ reduced at Complex IV

O₂ is the final electron acceptor, not a direct reactant in ATP synthesis. It accepts electrons at CIV, forming H₂O. Removing O₂ halts the entire ETC.

Must Know #5

PFK is the gatekeeper

Phosphofructokinase is the key regulatory enzyme of glycolysis. High ATP inhibits it (cell has enough energy). High AMP activates it (cell needs energy). Citrate (Krebs product) also inhibits it.

Must Know #6

Fermentation = NAD⁺ recycling

No additional ATP. Purpose is solely to regenerate NAD⁺ so glycolysis can keep producing 2 ATP. Lactic acid (animals/some bacteria) or ethanol + CO₂ (yeast).

Must Know #7

H⁺ gradient direction

Protons are pumped from matrix → IMS by ETC. They flow back from IMS → matrix through ATP synthase. The IMS becomes acidic (low pH, high [H⁺]).

Must Know #8

Uncouplers = heat not ATP

Protonophores (DNP, thermogenin in brown fat) dissipate the H⁺ gradient as heat. The ETC keeps running but ATP synthase is bypassed. This generates body heat (thermogenesis in hibernating animals).

13 AP Exam Practice

Score: 0 / 12

Select the best answer. Explanations appear after answering.

QUESTION 01 / 12

A cell is treated with a poison that blocks Complex I of the electron transport chain. Which of the following is the most direct consequence?

Complex I (NADH dehydrogenase) oxidizes NADH → NAD⁺ and sends electrons to ubiquinone. If blocked, NADH cannot be oxidized and accumulates. High [NADH] in the matrix allosterically inhibits Krebs cycle enzymes (isocitrate dehydrogenase, citrate synthase). The proton gradient weakens, ATP synthesis falls. FADH₂ (Complex II) is unaffected directly.

QUESTION 02 / 12

During intense anaerobic exercise, lactic acid fermentation occurs. What is the PRIMARY reason muscle cells carry out this process?

Glycolysis requires NAD⁺ to accept electrons at step 6 (glyceraldehyde-3-phosphate dehydrogenase). Without O₂, the ETC cannot reoxidize NADH → NAD⁺. Fermentation solves this by using pyruvate as the electron acceptor, regenerating NAD⁺ so glycolysis can continue producing its 2 ATP. No new ATP is made during fermentation itself.

QUESTION 03 / 12

Where in the cell does pyruvate oxidation (conversion of pyruvate → Acetyl-CoA) take place?

Pyruvate is produced in the cytoplasm (glycolysis), then transported into the mitochondrial matrix via specific pyruvate carrier proteins in the inner membrane. The Pyruvate Dehydrogenase Complex (PDC) that catalyzes pyruvate → Acetyl-CoA is dissolved in the matrix. The inner membrane is where the ETC sits.

QUESTION 04 / 12

How many CO₂ molecules are released per glucose molecule during the Krebs cycle alone?

The Krebs cycle releases 2 CO₂ per turn (at isocitrate dehydrogenase and α-ketoglutarate dehydrogenase steps). Since the cycle runs twice per glucose (once per pyruvate/Acetyl-CoA), the Krebs cycle contributes 4 CO₂ total. The other 2 CO₂ come from pyruvate oxidation. Total = 6 CO₂ per glucose.

QUESTION 05 / 12

A researcher adds an uncoupler that makes the inner mitochondrial membrane freely permeable to H⁺. What is the predicted outcome?

Uncouplers dissipate the proton gradient (H⁺ leaks back without going through ATP synthase). Without a gradient, ATP synthase cannot function and ATP production stops. However, the ETC can still transfer electrons to O₂ (the electron acceptor hasn't changed), so electron transport continues, consuming O₂ and oxidizing NADH/FADH₂. The energy is released as heat. This is how thermogenin works in brown adipose tissue.

QUESTION 06 / 12

Why does FADH₂ generate fewer ATP than NADH during oxidative phosphorylation?

NADH donates electrons to Complex I, which pumps 4 H⁺ into the IMS. FADH₂ donates electrons to Complex II (Succinate Dehydrogenase), which does not pump any H⁺. Therefore, FADH₂ only contributes to the gradient through Complexes III and IV (~6 H⁺ total), while NADH contributes to all three pumping complexes (~10 H⁺). Fewer H⁺ pumped = smaller gradient = fewer ATP (≈1.5 vs ≈2.5).

QUESTION 07 / 12

Phosphofructokinase (PFK) is allosterically inhibited by citrate. What is the physiological significance of this?

Citrate is an intermediate of the Krebs cycle. When citrate accumulates, it signals that the Krebs cycle is backed up (not processing acetyl-CoA fast enough). Citrate exits the mitochondria and inhibits PFK in the cytoplasm, slowing glycolysis and preventing further pyruvate/acetyl-CoA production. This is cross-pathway regulation — the Krebs cycle feedback-inhibits glycolysis to match supply with demand.

QUESTION 08 / 12

A cell that lacks mitochondria (like a red blood cell) can still produce ATP. Which pathway does it rely on?

Red blood cells (erythrocytes) have no mitochondria — they would interfere with oxygen transport. They rely entirely on glycolysis for their 2 ATP per glucose, followed by lactic acid fermentation to regenerate NAD⁺ and keep glycolysis running. Krebs cycle and oxidative phosphorylation require mitochondria. β-oxidation also occurs in mitochondria.

QUESTION 09 / 12

In aerobic respiration, what is the direct source of the oxygen atoms in the water (H₂O) produced?

At Complex IV (cytochrome c oxidase), molecular oxygen (O₂) is the final electron acceptor. Each O₂ accepts 4 electrons from cytochrome c and combines with 4 H⁺ to form 2 H₂O: O₂ + 4e⁻ + 4H⁺ → 2H₂O. So the oxygen atoms in the water product come directly from inhaled O₂, not from glucose or CO₂.

QUESTION 10 / 12

An organism has an RQ (respiratory quotient = CO₂/O₂) of approximately 0.7. What substrate is it most likely oxidizing?

The respiratory quotient reflects the substrate being oxidized: Glucose RQ = 1.0 (6CO₂/6O₂). Fats RQ ≈ 0.7 (fats are more reduced, require more O₂ per carbon to oxidize, produce proportionally less CO₂). Proteins RQ ≈ 0.8. Carbohydrates always have RQ = 1.0. An RQ of 0.7 is a classic indicator of fat/lipid oxidation — seen in hibernating animals or during extended fasting.

QUESTION 11 / 12

Which statement correctly describes the role of NAD⁺ in cellular respiration?

NAD⁺ is an electron carrier. During the oxidation of glucose intermediates (in glycolysis, pyruvate oxidation, and the Krebs cycle), NAD⁺ accepts 2 electrons and 1 H⁺ to become NADH. NADH then carries these high-energy electrons to Complex I of the ETC, where they are passed down the chain, driving H⁺ pumping and ultimately ATP synthesis. O₂ (not NAD⁺) is the final electron acceptor.

QUESTION 12 / 12

If a cell's ATP:ADP ratio is very high, which of the following regulatory responses is most expected?

A high ATP:ADP ratio signals that the cell has sufficient energy. This inhibits key regulatory enzymes: PFK (step 3, glycolysis) is allosterically inhibited by high ATP. Citrate synthase and isocitrate dehydrogenase (Krebs) are inhibited by high NADH and ATP. This feedback loop slows the entire pathway so the cell doesn't waste resources producing ATP it doesn't need. When AMP rises (low energy), these enzymes are activated.