01 / 21

Cellular
Respiration

Discover how cells harvest chemical energy from glucose and convert it into ATP: the universal energy currency of life.

AP Biology

What Is Cellular Respiration?

Cellular respiration is a set of catabolic reactions that break down organic molecules such as glucose to release energy. Cells capture that energy by synthesizing ATP (adenosine triphosphate).

This process is essentially the reverse of photosynthesis and occurs in every living cell: plants, animals, fungi, and most prokaryotes.

Why Does ATP Matter?

Energy Currency
ATP powers muscle contraction, nerve impulses, active transport, and biosynthesis.
Constant Demand
Your body recycles its weight in ATP every single day: roughly 40 kg of ATP turned over daily.
Coupled Reactions
ATP hydrolysis (\( \Delta G \approx -30.5 \text{ kJ/mol} \)) drives otherwise unfavorable reactions forward.

Key Molecules to Know

\( \text{NAD}^+ / \text{NADH} \)
Electron carrier: \( \text{NAD}^+ \) is oxidized form, \( \text{NADH} \) is reduced form carrying \( 2e^- + \text{H}^+ \).
\( \text{FAD} / \text{FADH}_2 \)
Second electron carrier; delivers electrons to Complex II of the ETC.
\( \text{ATP} / \text{ADP} \)
Energy currency: \( \text{ATP} \) stores energy; \( \text{ADP} + P_i \) are recycled back to \( \text{ATP} \).

The Three Stages of Aerobic Respiration

Glycolysis Cytoplasm 2 ATP net Krebs Cycle Mitochondrial Matrix 2 ATP
\( 10 \text{ NADH} + 2 \text{ FADH}_2 \)
 ETC + Oxidative Inner Membrane ~28 ATP Glucose (C₆H₁₂O₆) 2 Pyruvate
2 CO₂ lost
2 NADH
2 Acetyl-CoA Electron Transport Chain
\( \text{NADH} \rightarrow \text{NAD}^+ + \text{H}^+ \)
\( \text{FADH}_2 \rightarrow \text{FAD} + \text{H}^+ \)
 H+ gradient → ATP synthase
\( \text{O}_2 \rightarrow \text{H}_2\text{O} \) (final e⁻ acceptor)
 Total: ~30-32 ATP per glucose

Stage 1: Glycolysis

Glycolysis ("sugar splitting") occurs in the cytoplasm and does not require oxygen. One 6-carbon glucose is split into two 3-carbon pyruvate molecules.

1
Energy Investment Phase: 2 ATP are consumed to phosphorylate glucose, making it unstable.
2
Cleavage: Fructose-1,6-bisphosphate is split into two 3-carbon molecules (G3P).
3
Energy Payoff Phase: Each G3P generates 2 ATP + 1 NADH. Two G3Ps yield 4 ATP + 2 NADH total.

The Bridge: Pyruvate Oxidation

Before entering the Krebs cycle, each pyruvate must be oxidized in the mitochondrial matrix. This transition step links glycolysis to the citric acid cycle.

Pyruvate (3C)
\( \text{CO}_2 \) released
\( \text{NAD}^+ \rightarrow \text{NADH} \)
 Acetyl-CoA (2C + CoA) Krebs Cycle

Stage 2: The Krebs Cycle

Also called the citric acid cycle, this stage occurs in the mitochondrial matrix. Each acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C), which is systematically oxidized.

1
\( \text{Acetyl-CoA} + \text{Oxaloacetate} \rightarrow \text{Citrate} \) (6C). CoA is released for reuse.
2
Oxidation steps release \( 2 \text{ CO}_2 \) and reduce \( 3 \text{ NAD}^+ \rightarrow 3 \text{ NADH} \) and \( 1 \text{ FAD} \rightarrow 1 \text{ FADH}_2 \).
3
Substrate-level phosphorylation produces \( 1 \text{ GTP} (\approx 1 \text{ ATP}) \) per turn.
4
Oxaloacetate regenerated: the cycle is ready for the next acetyl-CoA.

Identify why glucose is a model fuel rather than the only respiratory substrate.

Carbohydrates, fatty acids and amino-acid carbon skeletons enter metabolism at different points. Their diverse structures do not have one universal ATP yield.

Alternative entry nodes and compartments
Alternative entry nodes and compartments
Illustrative cell and food contexts accompany distinct entry routes: carbohydrate via glycolysis and pyruvate, fatty acid via beta oxidation, and amino acid carbon at varied nodes. A companion separates cytosol from mitochondrial matrix.
Carbohydrates, fatty acids and amino-acid carbon skeletons enter metabolism at different points. Their diverse structures do not have one universal ATP yield.

Stage 3: Oxidative Phosphorylation

INNER MITOCHONDRIAL MEMBRANE INTERMEMBRANE SPACE (high H+) MATRIX (low H+) I
\( \text{NADH} \)
 II
\( \text{FADH}_2 \)
 III IV
\( \text{O}_2 \rightarrow \text{H}_2\text{O} \)
 e⁻ flow → H+ H+ H+ ATP Synthase Complex V H+ flows back ~28 ATP Chemiosmosis H+ gradient drives ATP synthase (like a turbine)

Inside a mitochondrial crista

A cutaway mitochondrion shows a continuous outer boundary and the folded inner membrane surrounding the matrix.The folded inner mitochondrial membrane separates the matrix from the intermembrane space and provides the surface for oxidative phosphorylation.

A proton gradient powers ATP synthesis

Across the inner membrane, arrows at Complexes I, III and IV point toward the intermembrane space. An arrow through ATP synthase returns toward the matrix, beside ADP plus phosphate and ATP labels. Complex II has no pumping arrow.Complexes I, III and IV pump protons from the matrix into the intermembrane space. Proton return through ATP synthase drives ATP production on the matrix side; Complex II does not pump protons.Open full diagram

A sprouting seed makes invisible oxygen use measurable

Three sealed respirometer chambers share a water bath: pale germinating seeds, dormant seeds with glass beads, and beads only. All samples sit above separated absorbent and connect to capillary water plugs.With carbon dioxide absorbed below a separating barrier, oxygen consumption reduces the enclosed gas volume and moves the capillary indicator toward the seeds. Matched chambers share a constant-temperature bath; the bead control measures physical drift.

Oxygen use changes gas volume

Oxygen use changes gas volume. With carbon dioxide absorbed below a separating barrier, oxygen consumption reduces the enclosed gas volume and moves the capillary indicator toward the seeds. Matched chambers share a constant-temperature bath; the bead control measures physical drift.With carbon dioxide absorbed below a separating barrier, oxygen consumption reduces the enclosed gas volume and moves the capillary indicator toward the seeds. Matched chambers share a constant-temperature bath; the bead control measures physical drift.

ATP Accounting per Glucose

Stage ATP NADH \( \text{FADH}_2 \) \( \text{CO}_2 \)
Glycolysis 2 net 2 0 0
Pyruvate Oxidation 0 2 0 2
Krebs Cycle (x2) 2 6 2 4
Oxidative Phosphorylation ~28 consumed consumed 0
Total ~30-32 6

Aerobic vs Anaerobic Respiration

Feature Aerobic Anaerobic (Fermentation)
Oxygen Required Yes No
ATP Yield per Glucose ~30-32 2 (glycolysis only)
End Products \( \text{CO}_2 + \text{H}_2\text{O} \) \( \text{Ethanol} + \text{CO}_2 \) or Lactate
Location Cytoplasm + Mitochondria Cytoplasm only
ETC Used? Yes No

Two Types of Fermentation

Lactic Acid Fermentation
Pyruvate is reduced directly by NADH → lactate. Occurs in muscle cells during intense exercise and in some bacteria (yogurt production).
Alcoholic Fermentation
Pyruvate is decarboxylated to acetaldehyde, then reduced by NADH → \( \text{ethanol} + \text{CO}_2 \). Used by yeast in baking and brewing.
Bread context and an authored per-glucose sequence show glycolysis to two pyruvate, decarboxylation to acetaldehyde and carbon dioxide, ethanol formation, and oxidation of two NADH to two NAD+.
Yeast fermentation in dough releases CO₂ and regenerates NAD⁺ before baking. The baked crumb records earlier gas formation; living or actively fermenting yeast in finished bread is not implied. Net two ATP per glucose arise in glycolysis.

How to Solve Cellular Respiration Problems

1
Identify the stage: Is the question about glycolysis, Krebs, or the ETC? Location clues help: cytoplasm vs. mitochondria.
2
Track inputs and outputs: Write what goes in (glucose, NAD+, FAD, ADP) and what comes out (pyruvate, NADH, FADH₂, ATP, CO₂).
3
Check oxygen status: If O₂ is present → aerobic (all 3 stages). If O₂ is absent → only glycolysis + fermentation.
4
Count the ATP: Use the NADH/FADH₂ conversion factors (~2.5 and ~1.5 ATP) when asked for oxidative phosphorylation yields.
5
Consider regulation: Allosteric enzymes (PFK, isocitrate dehydrogenase) respond to ATP/ADP ratios via feedback inhibition.

Example 1: NADH Yield

1
Glycolysis: 2 NADH
2
Pyruvate oxidation (x2): 2 NADH
3
Krebs cycle (x2 turns): 6 NADH (3 per turn)
4
Total: \( 2 + 2 + 6 = \) 10 NADH per glucose

Example 2: Cyanide Poisoning

1
ETC halts: Electrons cannot pass to \( \text{O}_2 \), so all complexes become fully reduced and stop accepting electrons.
2
H+ gradient collapses: No protons are pumped, so ATP synthase stops producing ATP via chemiosmosis.
3
NADH/FADH₂ accumulate: They cannot be oxidized back to NAD+/FAD, which stalls the Krebs cycle.
4
Only glycolysis remains: The cell can produce just 2 ATP per glucose via fermentation: insufficient for survival.

Common Mistakes

✕ "Glycolysis needs \( \text{O}_2 \)"
Glycolysis is anaerobic: it occurs in the cytoplasm with or without oxygen.
✕ "Fermentation makes ATP"
Fermentation itself produces zero ATP. It only regenerates NAD+ so glycolysis can continue making 2 ATP.
✕ "\( \text{O}_2 \) is used in Krebs"
\( \text{O}_2 \) is used only at the ETC (Complex IV). The Krebs cycle uses no oxygen directly, though it requires it indirectly.

Question 1 of 5

What is the net ATP yield from glycolysis per glucose molecule?

Question 2 of 5

Where in the cell does the Krebs cycle take place?

Question 3 of 5

What is the primary purpose of fermentation?

Question 4 of 5

Which molecule serves as the final electron acceptor in the electron transport chain?

Question 5 of 5

If a poison blocks ATP synthase (Complex V) but the electron transport chain is still functional, what would you expect?