Cellular Respiration and Energy Harvest

Core Concepts

Oxidation and reduction (redox reactions)

Cellular respiration involves the transfer of electrons from one molecule to another — called oxidation-reduction (redox) reactions.

• Oxidation: Loss of electrons. A molecule that loses electrons is oxidized.

• Reduction: Gain of electrons. A molecule that gains electrons is reduced.

A useful memory aid: OIL RIG — Oxidation Is Loss, Reduction Is Gain of electrons.

In respiration, glucose is oxidized (it loses electrons) and oxygen is reduced (it gains electrons, forming water). The electrons do not jump directly from glucose to oxygen; they are passed through a series of electron carriers, mainly NAD+ and FAD.

• NAD+ accepts two electrons and one proton to become NADH. NADH carries high-energy electrons to the electron transport chain.

• FAD accepts two electrons and two protons to become FADH2. FADH2 also delivers electrons to the electron transport chain, but at a later entry point than NADH, resulting in fewer ATP molecules produced per electron pair.

An overview of cellular respiration

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP + heat). The energy is not released in one step — it is captured in small increments, primarily as ATP.

Stage 1: Glycolysis

Location: Cytoplasm (cytosol)

Oxygen requirement: None (glycolysis occurs with or without O2)

Glycolysis ("sugar splitting") breaks one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each). The process consists of ten enzyme-catalyzed reactions and has two phases:

• Energy investment phase: 2 ATP are consumed to phosphorylate glucose, making it more reactive.

• Energy payoff phase: 4 ATP and 2 NADH are produced.

Net yield per glucose: 2 ATP (by substrate-level phosphorylation), 2 NADH, 2 pyruvate.

Substrate-level phosphorylation is the direct transfer of a phosphate group from a substrate molecule to ADP, forming ATP. This is distinct from oxidative phosphorylation, which uses the electron transport chain and chemiosmosis.

Stage 2: Pyruvate oxidation

Location: Mitochondrial matrix (in eukaryotes)

Oxygen requirement: Indirect (the process itself does not consume O2, but the NADH produced requires O2 for recycling via the electron transport chain)

Each pyruvate (3 carbons) is converted to acetyl-CoA (2 carbons). One carbon is released as CO2. NAD+ is reduced to NADH.

Per glucose (2 pyruvate): 2 acetyl-CoA, 2 CO2, 2 NADH.

Stage 3: The citric acid cycle (Krebs cycle)

Location: Mitochondrial matrix

Oxygen requirement: Indirect (as above)

The citric acid cycle is a series of eight enzyme-catalyzed reactions that completes the oxidation of acetyl-CoA. Each turn of the cycle:

1. Acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons).

2. Through a series of reactions, two carbons are released as CO2.

3. Oxaloacetate is regenerated, ready for another turn.

Per turn of the cycle: 1 ATP (by substrate-level phosphorylation, as GTP), 3 NADH, 1 FADH2, 2 CO2.

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

Stage 4: Oxidative phosphorylation

Location: Inner mitochondrial membrane

This stage has two components: the electron transport chain and chemiosmosis. Together, they account for most of the ATP produced in cellular respiration.

The electron transport chain (ETC)

• A series of protein complexes (I, II, III, IV) and mobile electron carriers embedded in the inner mitochondrial membrane.

• NADH donates electrons to Complex I; FADH2 donates electrons to Complex II.

• Electrons pass through the chain in a series of redox reactions, each acceptor being more electronegative than the previous one.

• At the end of the chain, electrons are transferred to O2, which combines with H+ to form H2O. Oxygen is the final electron acceptor.

Chemiosmosis

• As electrons pass through the ETC, the protein complexes pump H+ (protons) from the mitochondrial matrix into the intermembrane space, creating a proton gradient (higher [H+] in the intermembrane space, lower in the matrix).

• This gradient represents potential energy — protons are driven to flow back into the matrix.

• ATP synthase is a protein complex that provides a channel for protons to flow back into the matrix. The flow of protons through ATP synthase (like water turning a turbine) drives the synthesis of ATP from ADP + Pi.

• This coupling of electron transport to ATP synthesis is called oxidative phosphorylation.

ATP yield (approximate): Glycolysis (cytoplasm): 2 ATP, 2 NADH. Pyruvate oxidation (matrix): 2 NADH, 2 CO2. Citric acid cycle (matrix): 2 ATP, 6 NADH, 2 FADH2, 4 CO2. Oxidative phosphorylation (inner membrane): ~26-28 ATP. Total: ~30-32 ATP per glucose. Most ATP comes from oxidative phosphorylation, not glycolysis.

Fermentation

When oxygen is unavailable (or in organisms that lack the ETC), cells use fermentation to regenerate NAD+ from NADH, allowing glycolysis to continue. Fermentation does NOT produce ATP beyond the 2 ATP from glycolysis.

Lactic-acid fermentation

• Pyruvate is reduced directly by NADH, forming lactate (lactic acid) and regenerating NAD+.

• Occurs in human muscle cells during strenuous exercise when oxygen delivery cannot keep pace with ATP demand. Also occurs in some bacteria and fungi (used in yogurt and cheese production).

• The lactate can be recycled back to pyruvate when oxygen becomes available.

Alcohol fermentation

• Pyruvate is converted to acetaldehyde (releasing CO2), then reduced by NADH to ethanol, regenerating NAD+.

• Occurs in yeast and some bacteria. Used in brewing, winemaking, and bread baking (CO2 makes dough rise; ethanol evaporates during baking).

Comparison: Aerobic respiration: O2 required, ~30-32 ATP, ETC regenerates NAD+, final acceptor O2. Fermentation: no O2, 2 ATP, organic molecule regenerates NAD+, final acceptor pyruvate/acetaldehyde. CO2: yes (aerobic and alcohol fermentation); no (lactic acid).

Using other fuels

Cells can oxidize molecules other than glucose:

• Other carbohydrates: Hydrolyzed to monosaccharides and fed into glycolysis.

• Proteins: Broken down to amino acids, which are deaminated (amino group removed). The carbon skeletons enter respiration at various points (pyruvate, acetyl-CoA, or citric acid cycle intermediates).

• Fats: Broken down to glycerol (enters glycolysis) and fatty acids (converted to acetyl-CoA via beta-oxidation). Fats yield more ATP per gram than carbohydrates because they are more reduced (more electrons to donate).

Regulation of respiration

Cellular respiration is tightly regulated by feedback inhibition. Key control points:

• Phosphofructokinase (PFK): The main control point in glycolysis. Inhibited by ATP and citrate; activated by AMP and ADP. When ATP is abundant, glycolysis slows.

• Pyruvate dehydrogenase: Inhibited by NADH and acetyl-CoA. When these products accumulate, pyruvate oxidation slows.

ELI Example

A campfire vs. a power plant. Both burn the same log (glucose). The campfire releases all energy at once (useless for work). The power plant captures energy in controlled stages to spin turbines (ATP). Respiration is the power plant — controlled, efficient, multi-step. CO2 and H2O are the exhaust.

Do Not Confuse

Lab Link

Respiration and fermentation laboratories commonly involve: (1) measuring O2 consumption using a respirometer (germinating seeds are a classic subject); (2) measuring CO2 production using a pH indicator (phenol red or bromothymol blue); (3) demonstrating fermentation by capturing CO2 from yeast cultures with different sugar substrates; (4) measuring metabolic rate in small organisms or isolated mitochondria. Controls and careful measurement of rates (not just presence/absence) are essential for meaningful results.

High-Yield Memory Anchors

• Glycolysis: cytoplasm, 2 ATP, 2 NADH, 2 pyruvate. Anaerobic capable.

• Citric acid cycle: matrix, spins twice per glucose, produces NADH + FADH2.

• ETC: inner membrane. Electrons flow → H+ pumped → ATP synthase spins → ATP.

• O2 = final electron acceptor. No O2 = chain backs up = no oxidative phosphorylation.

• Fermentation = NAD+ recycling. Does NOT make ATP beyond glycolysis.

Quick Check

Q1 (Foundational): List the four stages of cellular respiration in order. For each stage, state where it occurs in a eukaryotic cell.

Q2 (Application): Cyanide binds to and inhibits cytochrome c oxidase (Complex IV of the electron transport chain). Explain why cyanide exposure is rapidly fatal at the cellular level. What specifically stops working?

Q3 (Comparison/Reasoning): A yeast cell is placed in a sealed container with glucose but no oxygen. Later, the container is opened to air. Compare what happens to ATP production in the yeast cell under these two conditions. Account for differences in ATP yield and the metabolic pathways involved.

Quick Check Answers

A1: (1) Glycolysis — cytoplasm. (2) Pyruvate oxidation — mitochondrial matrix. (3) Citric acid cycle (Krebs cycle) — mitochondrial matrix. (4) Oxidative phosphorylation (ETC + chemiosmosis) — inner mitochondrial membrane.

A2: Cyanide blocks the transfer of electrons to oxygen at Complex IV, the final step of the electron transport chain. When electron flow stops: (1) Proton pumping ceases — no H+ gradient is established. (2) ATP synthase cannot produce ATP — oxidative phosphorylation halts. (3) NADH and FADH2 cannot be re-oxidized — the entire upstream metabolism (glycolysis, pyruvate oxidation, citric acid cycle) eventually stalls because NAD+ and FAD are not regenerated. Without ATP, active transport, biosynthesis, and all other energy-requiring processes cease. Cells — particularly neurons and heart muscle, which have extremely high ATP demands — die rapidly.

A3: Without oxygen: The yeast performs alcohol fermentation. Glycolysis produces 2 ATP per glucose, and pyruvate is converted to ethanol, regenerating NAD+ so glycolysis can continue. Total yield: 2 ATP per glucose. CO2 is released. With oxygen: The yeast switches to aerobic respiration. Glycolysis (2 ATP), pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation all operate. Total yield: ~30–32 ATP per glucose. The presence of oxygen allows the full respiratory pathway to operate, producing roughly 15 times more ATP per glucose than fermentation alone. This is an example of the Pasteur effect — the observation that yeast consume less sugar under aerobic conditions because respiration is far more efficient than fermentation.

Chapter Summary

Respiration: glycolysis (cytoplasm) → pyruvate oxidation → citric acid cycle (matrix) → oxidative phosphorylation (inner membrane). NADH/FADH2 carry electrons to the ETC; proton gradient drives ATP synthase; O2 is the final acceptor. Fermentation regenerates NAD+, yielding 2 ATP. Cells also oxidize fats and proteins.

Common Mistakes

Mistake: "The purpose of respiration is to produce CO2."

Reality: CO2 is a waste product. The purpose of respiration is to produce ATP. CO2 is released because the carbon atoms of glucose must be removed as the molecule is oxidized.

Mistake: "Oxygen is directly used to make ATP."

Reality: Oxygen serves as the final electron acceptor at the end of the electron transport chain. Without O2 to accept electrons, the chain backs up, proton pumping stops, and oxidative phosphorylation ceases. Oxygen does not directly participate in ATP synthesis.

Mistake: "Fermentation produces ATP."

Reality: Fermentation produces ZERO ATP beyond glycolysis. Its purpose is to regenerate NAD+ so that glycolysis — which nets 2 ATP — can continue. All the ATP in fermentation comes from glycolysis.

Mistake: "Plants only perform photosynthesis, not cellular respiration."

Reality: Plants perform cellular respiration just like animals. They use photosynthesis to produce glucose (and O2) and respiration to break down glucose (consuming O2) to produce ATP. At night, when photosynthesis stops, plants are net consumers of O2 and producers of CO2 — just like animals.