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Chapter 14

Energy Conversion: Mitochondria and Chloroplasts

Molecular Biology · End-of-chapter questions below · Part 1 of 2 · 10 questions per part
Part 1 of 2
Mitochondria and Oxidative Phosphorylation
Every time you breathe, mitochondria harness the energy in NADH and FADH₂ to pump protons across a membrane, then use the resulting gradient to synthesize ATP—the universal cellular energy currency. This chemiosmotic mechanism is one of biology's most elegant thermodynamic machines.

In Part 1, you will explore:

  • Mitochondrial structure: outer membrane, inner membrane, cristae, and matrix
  • The electron transport chain (Complexes I–IV) and NADH/FADH₂ oxidation
  • Proton gradient generation and ATP synthase (Complex V)
  • Chemiosmosis and oxidative phosphorylation
  • Reactive oxygen species (ROS) and their management

14.1 Mitochondrial Structure

Mitochondria have a distinctive double-membrane architecture. The smooth outer mitochondrial membrane is permeable to small molecules via voltage-dependent anion channels (VDAC/porins). The highly folded inner mitochondrial membrane forms deep invaginations called cristae that greatly increase surface area for the electron transport chain and ATP synthase. The compartment between the two membranes is the intermembrane space; the central compartment enclosed by the inner membrane is the matrix, which contains mitochondrial DNA, ribosomes, and the enzymes of the TCA (Krebs) cycle.

Key term
Cristae

Folds of the inner mitochondrial membrane that greatly increase its surface area; they are the site of the electron transport chain complexes and ATP synthase, maximizing the capacity for oxidative phosphorylation.

14.2 The Electron Transport Chain

The electron transport chain (ETC) consists of four multi-protein complexes embedded in the inner mitochondrial membrane. NADH donates electrons to Complex I (NADH dehydrogenase); FADH₂ donates electrons to Complex II (succinate dehydrogenase). Both feed electrons to ubiquinone (CoQ), which carries them to Complex III (cytochrome bc₁ complex), then via cytochrome c to Complex IV (cytochrome c oxidase), which reduces O₂ to water.

As electrons flow "downhill" in the thermodynamic sense, Complexes I, III, and IV use the released energy to pump protons (H⁺) from the matrix into the intermembrane space, creating the proton-motive force—a combination of a chemical gradient (ΔpH) and an electrical gradient (ΔΨ).

✎
Pause & Recall
Why does FADH₂ generate fewer ATP molecules than NADH per molecule oxidized?
FADH₂ donates electrons directly to CoQ via Complex II, bypassing Complex I. Since Complex I pumps 4 protons per electron pair, bypassing it means fewer protons are pumped per FADH₂ (~6 vs. ~10 for NADH), yielding ~1.5 ATP per FADH₂ compared to ~2.5 per NADH.

14.3 ATP Synthase and Chemiosmosis

ATP synthase (Complex V / F₁F₀-ATPase) harnesses the proton-motive force to synthesize ATP by the process of chemiosmosis. The F₀ subunit is embedded in the inner membrane and forms a proton channel; proton flow back into the matrix through F₀ drives rotation of the central γ-subunit stalk. This rotation causes sequential conformational changes in the three β-subunits of F₁ that convert ADP + Pᵢ to ATP. Each 360° rotation produces approximately three ATP molecules. The binding-change mechanism, worked out by Paul Boyer, explains how mechanical rotation is coupled to chemical synthesis.

Key term
Chemiosmosis

The process by which ATP synthase uses the electrochemical proton gradient (proton-motive force) across the inner mitochondrial membrane to drive ATP synthesis; proton flow through the F₀ subunit powers rotation of the catalytic F₁ subunit.

14.4 Reactive Oxygen Species

A small fraction (~1–2%) of electrons leak from the ETC, particularly at Complexes I and III, and react with O₂ to produce reactive oxygen species (ROS) such as superoxide (O₂·⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (·OH). ROS can damage proteins, lipids, and DNA. Cells counter ROS with antioxidant enzymes: superoxide dismutase (SOD) converts O₂·⁻ to H₂O₂, and catalase or glutathione peroxidase converts H₂O₂ to water. Excessive ROS production is implicated in aging, neurodegenerative diseases, and ischemia-reperfusion injury.

✎
Pause & Recall
What is the substrate and product of superoxide dismutase, and why is its activity not sufficient alone to eliminate ROS?
SOD converts superoxide (O₂·⁻) to H₂O₂ and O₂. But H₂O₂ is itself reactive and can generate the highly damaging hydroxyl radical (·OH) via the Fenton reaction with Fe²⁺. A second enzyme—catalase or glutathione peroxidase—is needed to convert H₂O₂ to water, completing ROS detoxification.
Practice questions — Part 1Score: 0 / 10

1. What is the name of the first complex in the electron transport chain that accepts electrons from NADH?

2. Which ETC complex reduces molecular oxygen (O₂) to water?

3. The folds of the inner mitochondrial membrane that house the ETC are called:

4. In the ATP synthase binding-change mechanism, what directly drives rotation of the γ-subunit?

5. Which mobile carrier shuttles electrons between Complex I/II and Complex III?

6. Uncoupling proteins (like UCP1 in brown adipose tissue) dissipate the proton gradient without making ATP. What is the physiological consequence?

7. Which enzyme converts superoxide radicals (O₂·⁻) to hydrogen peroxide?

8. Where in the mitochondrion are the enzymes of the TCA (Krebs) cycle located?

9. The proton-motive force driving ATP synthesis has two components. Which pair correctly describes them?

10. Cyanide is a potent respiratory poison. Which ETC complex does it inhibit?

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Part 1 complete

Part 1 → 2

Mitochondria harvest energy from organic molecules and O₂ via the ETC and ATP synthase. Part 2 turns to chloroplasts—where light energy drives the reverse: producing organic molecules from CO₂. We also cover C₃/C₄ plant strategies, mitochondrial DNA, and the endosymbiotic evidence for the bacterial origin of both organelles.

Part 2 of 2
Chloroplasts, Photosynthesis, and Organelle Evolution

14.5 Chloroplast Structure

Chloroplasts are bounded by a double-membrane envelope (outer and inner envelope membranes). Inside lies the stroma—an aqueous phase containing the enzymes of the Calvin cycle, chloroplast DNA, and ribosomes. Suspended in the stroma is a third internal membrane system: the thylakoids, flattened membrane sacs stacked into grana and connected by stroma lamellae. Photosystems I and II, the cytochrome b₆f complex, and ATP synthase reside in the thylakoid membranes.

14.6 Light Reactions: Photosystems I and II and the Z-scheme

The light reactions convert solar energy to chemical energy (ATP and NADPH) in the thylakoid membranes. Photosystem II (PSII) absorbs light at 680 nm, energizing a special chlorophyll pair (P680). The energized electron is passed to plastoquinone; the "electron hole" in P680 is filled by splitting water (photolysis): 2 H₂O → O₂ + 4 H⁺ + 4 e⁻. Electrons travel through the cytochrome b₆f complex (pumping H⁺ into the thylakoid lumen) to plastocyanin, then to Photosystem I (PSI), which absorbs light at 700 nm (P700). PSI re-energizes the electrons, which reduce ferredoxin and ultimately NADP⁺ to NADPH via ferredoxin-NADP⁺ reductase.

The overall electron path—PSII → PQ → Cytb₆f → PC → PSI → Fd → NADPH—forms a "Z" shape when plotted on a redox potential diagram, hence the Z-scheme. Proton accumulation in the thylakoid lumen drives chloroplast ATP synthase (CF₁CF₀) to produce ATP.

Key term
Z-scheme

The diagram describing the light reactions of photosynthesis; electrons flow from water (via PSII) through a series of carriers to NADP⁺ (via PSI), with two "uphill" photochemical boosts, tracing a Z-shape on a scale of redox potential.

✎
Pause & Recall
Which molecule captures light in Photosystem II and what happens to it after excitation?
The special chlorophyll a dimer P680 absorbs a photon at 680 nm. The excited electron is transferred to pheophytin (the primary acceptor) and then to plastoquinone. The resulting P680⁺ is a very strong oxidant that oxidizes water (via the oxygen-evolving complex) to replace the lost electron, releasing O₂ as a byproduct.

14.7 The Calvin Cycle and Carbon Fixation

The Calvin cycle (light-independent reactions) uses the ATP and NADPH from the light reactions to reduce CO₂ to carbohydrate in the stroma. In the carboxylation step, the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) adds CO₂ to the 5-carbon ribulose-1,5-bisphosphate (RuBP), producing two molecules of 3-phosphoglycerate (3-PGA). In the reduction step, 3-PGA is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH. In the regeneration step, most G3P is used to regenerate RuBP using additional ATP.

In C₃ plants (wheat, rice, soybeans), the first stable product of carbon fixation is the 3-carbon 3-PGA. In hot, dry conditions, photorespiration wastes fixed carbon because RuBisCO oxygenates RuBP instead of carboxylating it. C₄ plants (maize, sugarcane, sorghum) pre-concentrate CO₂ in mesophyll cells as oxaloacetate/malate and release it around RuBisCO in bundle sheath cells, suppressing photorespiration and improving efficiency in warm climates.

14.8 Mitochondrial DNA and the Endosymbiotic Origin of Organelles

Both mitochondria and chloroplasts contain their own circular DNA, 70S ribosomes, and divide by binary fission—features consistent with a bacterial origin. The endosymbiotic theory (championed by Lynn Margulis) proposes that mitochondria descended from α-proteobacteria engulfed by an ancestral eukaryote, and chloroplasts from cyanobacteria engulfed by the mitochondriate ancestor. Key evidence: (1) organelle ribosomes are inhibited by antibacterial antibiotics (e.g., chloramphenicol) but not cytosolic ones; (2) mitochondrial membrane lipids resemble bacterial lipids; (3) phylogenetic trees place mitochondrial genes within α-proteobacteria; (4) the double membrane reflects the original bacterial outer membrane plus the host endosome membrane.

✎
Pause & Recall
Why does chloramphenicol selectively inhibit mitochondrial but not cytosolic translation in eukaryotes?
Mitochondria retained 70S-type ribosomes from their bacterial ancestor; the 50S subunit of bacterial (and mitochondrial) ribosomes has the binding site for chloramphenicol. Eukaryotic cytosolic ribosomes are 80S (60S + 40S subunits) and structurally different, so they are insensitive to chloramphenicol. This antibiotic selectivity is itself evidence for the endosymbiotic origin of mitochondria.
Practice questions — Part 2Score: 0 / 10

1. Which molecule captures light energy in Photosystem II?

2. What are the products of the water-splitting (photolysis) reaction in PSII?

3. Which enzyme catalyzes the carboxylation step of the Calvin cycle?

4. C₄ plants like maize suppress photorespiration by pre-concentrating CO₂ around RuBisCO. Where does the Calvin cycle then operate in C₄ plants?

5. Which antibiotic selectively inhibits mitochondrial (but not cytosolic) translation, supporting the endosymbiotic theory?

6. Where in the chloroplast does the Calvin cycle take place?

7. According to the endosymbiotic theory, chloroplasts evolved from which type of bacteria?

8. In the Z-scheme, which mobile carrier transfers electrons from the cytochrome b₆f complex to Photosystem I?

9. What is the first stable carbon compound produced when CO₂ is fixed by RuBisCO in the Calvin cycle?

10. Which compartment of the chloroplast accumulates H⁺ during the light reactions to drive ATP synthesis?

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Part 2 complete

Chapter 14 takeaways
  • Mitochondrial cristae house ETC Complexes I–IV; electron flow from NADH/FADH₂ to O₂ pumps H⁺ into the intermembrane space, creating the proton-motive force that drives ATP synthase.
  • Chemiosmosis: proton re-entry through F₀ drives rotation of the γ-subunit of F₁, synthesizing ~3 ATP per full rotation via the binding-change mechanism.
  • ROS escape from Complexes I and III; SOD, catalase, and glutathione peroxidase provide antioxidant defense.
  • Chloroplast light reactions: PSII splits water (releasing O₂), electrons traverse the Z-scheme via cytochrome b₆f and plastocyanin, and PSI reduces NADP⁺ to NADPH.
  • Calvin cycle in the stroma: RuBisCO fixes CO₂ into 3-PGA; ATP and NADPH reduce 3-PGA to G3P; RuBP is regenerated. C₄ plants concentrate CO₂ to minimize photorespiration.
  • Both organelles retain bacterial features (circular DNA, 70S ribosomes, binary fission) consistent with endosymbiotic origin—mitochondria from α-proteobacteria, chloroplasts from cyanobacteria.

End-of-chapter questions

Type your answer, then click Check answer for feedback and a sample answer to compare.

Section B — Recall Questions

B1

Describe the sequence of electron transfer through the four complexes of the ETC and identify which complexes pump protons.

B2

Explain the binding-change mechanism of ATP synthase and how proton flow is mechanically coupled to ATP synthesis.

B3

How are reactive oxygen species generated in mitochondria, and what enzymatic defenses exist against them?

B4

Trace the path of an electron from water to NADPH during the light reactions, naming each carrier in order.

B5

Outline the three stages of the Calvin cycle (carboxylation, reduction, regeneration) and identify the ATP/NADPH requirements per CO₂ fixed.

B6

Explain how the C₄ pathway suppresses photorespiration and why this is advantageous in hot, sunny environments.

B7

List four pieces of evidence that support the endosymbiotic origin of mitochondria.

B8

Why do cells with high energy demands (e.g., cardiac muscle cells) have mitochondria with particularly numerous, densely packed cristae?

B9

Describe the internal membrane organization of chloroplasts (thylakoids, grana, stroma lamellae) and the function of each compartment.

B10

What is oxidative phosphorylation uncoupling and how do uncoupling proteins (UCPs) achieve it physiologically?

Section C — Critical Thinking

C1

Cyanide poisoning is rapidly lethal even though the body contains large stores of glycogen. Explain why blocking Complex IV alone is sufficient to cause death despite ample fuel reserves.

C2

RuBisCO can fix either CO₂ or O₂. Analyze why this oxygenase activity (photorespiration) is considered a "design flaw" and what evolutionary pressures originally made it tolerable.

C3

The proton gradients driving ATP synthesis in mitochondria and chloroplasts are oriented in opposite directions relative to the enzyme. Explain what this means structurally and why both converge on the same mechanistic outcome.

C4

Over evolutionary time, most genes originally in the proto-mitochondrial genome were transferred to the nucleus. What advantages might gene transfer to the nucleus confer, and why might a small core set of genes be retained in the mitochondrion?

C5

In addition to linear electron flow (PSII→PSI→NADPH), chloroplasts can perform cyclic electron flow around PSI. Analyze why both modes are needed to balance ATP and NADPH production for the Calvin cycle.

Section D — Interactive Questions

D1

What is the name of the first complex in the electron transport chain? (e.g. "complex i")

D2

Which molecule captures light in Photosystem II? (one word)

D3

Which enzyme in the Calvin cycle fixes CO₂ onto RuBP? (one word)

D4

What are the folds of the inner mitochondrial membrane called? (one word)

D5

What enzyme converts superoxide radicals to hydrogen peroxide? (three words, abbreviation also accepted: "sod")