Difficulty: Intermediate | Prerequisites: Basic chemistry (bonds, electrons, atoms), Chapter 5 enzyme basics, familiarity with ATP structure.
Cellular respiration is the central energy-harvesting pathway in biology, and Chapter 9 of BIO 311C covers how cells break down glucose to produce ATP. This topic sits at the intersection of thermodynamics, redox chemistry, and metabolic biochemistry. You will need a working understanding of enzyme function, chemical bonds, and the structure of ATP before diving in. Everything here connects forward to photosynthesis, fermentation, and metabolic regulation.
Cells break down glucose through a series of redox reactions (glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation) to produce up to 32 ATP per glucose molecule. Energy flows from high-energy electrons in food molecules down to oxygen, the final electron acceptor, releasing usable energy along the way. About 34% of the energy in glucose ends up as ATP; the rest is lost as heat.
Cellular respiration
The oxygen-dependent process by which cells extract energy from organic molecules, producing ATP, CO2, and H2O. Think of it as the way your cells "burn" food for fuel, except in a slow, controlled cascade rather than a single flame.
Exergonic reaction
A chemical reaction that releases free energy (delta G < 0). In simple terms, the products hold less energy than the reactants, and the leftover energy is available to do work.
Endergonic reaction
A chemical reaction that absorbs free energy (delta G > 0). Think of it as a reaction that needs an energy input to proceed, like pushing a boulder uphill.
Free energy (G)
The portion of a system's energy that is available to do work, formally called Gibbs free energy. In simple terms, it is the usable energy in a molecule or system.
Entropy
A measure of disorder or randomness in a system. Every energy transformation increases entropy, meaning some energy becomes less organised and less available for work.
ATP (adenosine triphosphate)
The primary energy currency of the cell, made of adenosine bonded to three phosphate groups. Think of it as the small, spendable cash the cell uses for mechanical, transport, and chemical work.
NAD+ / NADH
NAD+ is an intermediate electron acceptor (coenzyme). When it picks up two electrons and one hydrogen atom, it becomes NADH, a reduced, energy-carrying form that delivers electrons to the electron transport chain.
FAD / FADH2
FAD is another intermediate electron acceptor. When it accepts two electrons and two protons, it becomes FADH2. Think of NAD+ and FAD as molecular taxis that shuttle high-energy electrons to the ETC.
Oxidation
The loss of electrons from a substance. Remember OILRIG: Oxidation Is Loss.
Reduction
The gain of electrons by a substance. OILRIG: Reduction Is Gain.
Reducing agent
The electron donor in a redox reaction. In cellular respiration, glucose is the primary reducing agent.
Oxidising agent
The electron acceptor in a redox reaction. Oxygen is the final oxidising agent in aerobic respiration.
Substrate-level phosphorylation
Direct transfer of a phosphate group from a substrate molecule to ADP, forming ATP. This happens in glycolysis and the citric acid cycle, but accounts for only a small fraction of total ATP.
Oxidative phosphorylation
The process that generates roughly 90% of ATP during cellular respiration, powered by the electron transport chain and chemiosmosis.
Chemiosmosis
The mechanism by which the flow of H+ ions down their concentration gradient through ATP synthase drives the synthesis of ATP. Think of it as a tiny turbine spun by a waterfall of protons.
Electron transport chain (ETC)
A series of protein complexes embedded in the inner mitochondrial membrane (cristae) that pass electrons in stepwise redox reactions, pumping H+ into the intermembrane space. The ETC itself makes no ATP directly.
Proton-motive force
The H+ concentration gradient across the inner mitochondrial membrane, which stores potential energy used by ATP synthase to produce ATP.
Fermentation
Partial degradation of sugars without oxygen. Regenerates NAD+ so glycolysis can continue, but yields only 2 ATP per glucose.
Glycolysis
The splitting of one glucose molecule into two pyruvate molecules, occurring in the cytosol. An ancient pathway that does not require oxygen.
Citric acid cycle (Krebs cycle / TCA cycle)
A cyclic series of eight enzyme-catalysed reactions in the mitochondrial matrix that fully oxidises acetyl CoA to CO2, producing NADH, FADH2, and GTP (converted to ATP). Also called the tricarboxylic acid cycle.
Acetyl CoA
The two-carbon molecule (acetyl group linked to coenzyme A) that enters the citric acid cycle after pyruvate oxidation. Think of it as the entry ticket to the cycle.
Cytochromes
Proteins in the ETC that contain iron atoms and pass electrons along the chain. They are related in structure to haemoglobin.
Cells couple two classes of reaction to keep themselves alive:
Catabolic (exergonic) reactions break down complex molecules and release energy. Cell respiration and the general breakdown of macromolecules are catabolic.
Anabolic (endergonic) reactions build complex molecules from simpler ones and require energy input. Active transport, cell movement, and biosynthesis are anabolic.
The energy released by catabolic reactions is what powers the anabolic ones. ATP is the coupling molecule that transfers energy between them.
Energy is the capacity to cause change, to do work. Not all energy in a system is usable. Free energy (Gibbs free energy, G) is the portion available to do work under cellular conditions.
The change in free energy (delta G) during a reaction determines whether it releases or absorbs energy:
Delta G < 0: exergonic, energy is released, reaction is spontaneous
Delta G > 0: endergonic, energy is absorbed, reaction is not spontaneous
"Spontaneous" in thermodynamics means energetically favourable. It does not mean fast. Many spontaneous reactions require activation energy to get started, which is where enzymes come in: they lower the activation energy barrier without changing the overall delta G.
First Law: Energy in the universe is constant. It changes forms (chemical to kinetic to heat) but is never created or destroyed.
Second Law: Every energy transformation increases the entropy (disorder) of the system. This is why cells can never capture 100% of the energy in glucose as ATP; some is always lost as heat.
These two laws frame everything in cellular respiration. The cell harvests energy from glucose (first law: the energy goes somewhere) but only captures about 34% as ATP (second law: entropy increases, heat is released).
Cellular respiration is, at its core, a long series of redox reactions. Electrons are stripped from glucose (oxidation) and passed, step by step, to oxygen (reduction).
OILRIG mnemonic:
Oxidation Is Loss (of electrons)
Reduction Is Gain (of electrons)
In the overall equation, glucose is oxidised (loses electrons, releases CO2) and oxygen is reduced (gains electrons, forms H2O).
Organic molecules rich in hydrogen are excellent sources of high-energy electrons. As C-H bonds in glucose are broken, the hydrogen splits into H+ and electrons. Those electrons carry the energy.
Electron carriers (NADH and FADH2) are the shuttle molecules:
NAD+ accepts 2 electrons and 1 H atom, becoming NADH
FAD accepts 2 electrons and 2 protons, becoming FADH2
Both carry electrons to the electron transport chain, where the energy is extracted in controlled steps
Energy is released as electrons move from a less oxidised state (higher free energy) to a more oxidised state (lower free energy). Tracking where the hydrogens go is the key to tracking where the energy goes.
Three types of work powered by ATP: mechanical work (muscle contraction), transport work (pumping ions across membranes), and chemical work (driving endergonic reactions like biosynthesis).
The overall equation:
C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP + heat)}Cellular respiration proceeds through four stages, two of which occur in the cytosol and two inside the mitochondria:
Glycolysis (cytosol): glucose to 2 pyruvate. Yields 2 ATP and 2 NADH. No oxygen required.
Pyruvate oxidation (mitochondrial matrix): each pyruvate loses one carbon as CO2 and joins coenzyme A to form acetyl CoA. Yields 2 NADH total (for both pyruvates).
Citric acid cycle (mitochondrial matrix): acetyl CoA is fully oxidised to CO2. Per cycle: 1 ATP, 3 NADH, 1 FADH2. Two turns per glucose, so totals double.
Oxidative phosphorylation (inner mitochondrial membrane): NADH and FADH2 donate electrons to the ETC, which pumps H+ to build a proton gradient. H+ flows back through ATP synthase, producing about 26 to 28 ATP.
ATP accounting per glucose molecule:
Glycolysis: 2 ATP (substrate-level phosphorylation)
Citric acid cycle: 2 ATP (substrate-level phosphorylation)
Oxidative phosphorylation: approximately 26 to 28 ATP
Total: approximately 30 to 32 ATP
About 34% of the free energy in glucose is captured as ATP. The remaining 66% is released as heat, which is why your body is warm.
The money analogy from the lecture: glucose is like a large banknote (stores a lot of energy but is hard to spend directly). Cellular respiration breaks it into many smaller bills (ATP molecules), each easily spent on cellular work. The average person carries about 250 grams of ATP at any given time, yet turns over roughly their own body weight in ATP every day, meaning each ATP molecule is recycled about 300 times daily.
Five principles governing metabolic pathways:
Metabolic pathways are a series of chemical reactions
Each reaction is catalysed by a specific enzyme
Metabolism is regulated by inhibiting or activating enzymes
In eukaryotes, most metabolic pathways (except glycolysis) occur inside the mitochondria
Glycolysis is conserved across nearly all organisms and occurs outside the mitochondria, in the cytosol
Energy flow in ecosystems: photosynthesis and cellular respiration are linked by glucose and energy flow. Producers (plants, algae) capture light energy and fix it into glucose. Consumers break glucose down through respiration. Matter cycles (C, H, O atoms move between CO2, H2O, and organic molecules); energy flows one way (sunlight to chemical energy to heat).
Overall cellular respiration:
C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP + heat)}Gibbs free energy change:
\Delta G = G_{\text{products}} - G_{\text{reactants}}Delta G < 0: exergonic (spontaneous). Delta G > 0: endergonic (not spontaneous).
Pyruvate oxidation:
\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Acetyl CoA} + \text{NADH} + CO_2Alcohol fermentation:
C_6H_{12}O_6 + 2\text{ADP} + 2P_i \rightarrow 2\text{Ethanol} + 2CO_2 + 2\text{ATP}Lactic acid fermentation:
C_6H_{12}O_6 + 2\text{ADP} + 2P_i \rightarrow 2\text{Lactic acid} + 2\text{ATP}Key diagrams to know for the exam:
The exergonic vs endergonic free-energy diagram (reactants to products, delta G arrow)
The activation energy diagram showing enzyme effect (lower peak, same delta G)
The four-stage overview of cellular respiration (glycolysis in cytosol, pyruvate oxidation, citric acid cycle, and oxidative phosphorylation in the mitochondrion)
ATP synthase structure (rotor, stator, stalk, catalytic knob in the inner membrane)
The chemiosmosis diagram showing H+ flow from intermembrane space through ATP synthase into the matrix
Your body temperature is a direct consequence of the second law of thermodynamics applied to cellular respiration: the 66% of glucose energy not captured as ATP is released as heat. Cyanide poisoning kills by blocking Complex IV of the electron transport chain, halting ATP production even though oxygen is present. Alcohol fermentation by yeast is the basis of brewing and baking. Lactic acid fermentation in your muscles during intense exercise is why you feel the burn, and it is the same process used to make yoghurt and sauerkraut.
Students often think "spontaneous" means "fast." It does not. Spontaneous only means delta G is negative. Many spontaneous reactions are extremely slow without an enzyme or activation energy input.
Students confuse the electron transport chain with ATP production. The ETC itself generates no ATP directly. It pumps H+ ions to create a gradient; ATP synthase uses that gradient to make ATP.
Students mix up oxidation and reduction. Use OILRIG every time. Glucose is oxidised (loses electrons). Oxygen is reduced (gains electrons).
Students assume fermentation is a completely separate pathway. It is not. Fermentation always begins with glycolysis. The fermentation reactions that follow exist solely to regenerate NAD+ so glycolysis can keep running.
⚠️ Know the four stages of cellular respiration, where each occurs, and what each produces (ATP, NADH, FADH2, CO2).
⚠️ Be able to explain why the ETC does not make ATP directly, and how chemiosmosis bridges the gap.
⚠️ Understand the difference between substrate-level phosphorylation and oxidative phosphorylation, including which stages use which.
⚠️ Be ready to trace electrons from glucose through NADH/FADH2 to the ETC to oxygen. Exam questions often ask you to follow the electrons.
⚠️ Know the overall ATP yield (approximately 30 to 32 per glucose) and why it is approximate (the number depends on which shuttle moves cytosolic NADH into the mitochondrion).
⚠️ Understand feedback regulation: high ATP inhibits glycolysis (phosphofructokinase is a key regulatory enzyme).
True or False: The electron transport chain directly produces ATP. (False. The ETC pumps H+ to create a gradient; ATP synthase uses that gradient to make ATP.)
Fill in the blank: The overall equation for cellular respiration is C6H12O6 + 6O2 → ______ + ______ + Energy. (6CO2 + 6H2O)
True or False: Glycolysis requires oxygen to proceed. (False. Glycolysis occurs in the cytosol and does not require oxygen.)
Fill in the blank: NAD+ accepts 2 electrons and 1 H atom to become ______. (NADH)
True or False: A negative delta G means the reaction requires energy input. (False. Negative delta G means the reaction is exergonic and releases energy.)
Q: What is the role of oxygen in cellular respiration?
A: Oxygen is the final electron acceptor in the electron transport chain. It pulls electrons down the chain in an energy-yielding series of redox reactions and combines with H+ to form water.
Q: Why does the cell use NADH and FADH2 rather than releasing all the energy from glucose at once?
A: Releasing all the energy at once would be like lighting food on fire: most energy would be lost as heat. By passing electrons through carriers in small steps, the cell captures energy gradually, maximising ATP yield.
Q: Where does each stage of cellular respiration occur in a eukaryotic cell?
A: Glycolysis occurs in the cytosol. Pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation occur in the mitochondria (the latter two specifically at the inner membrane and matrix).
Q: How does the proton-motive force drive ATP synthesis?
A: The ETC pumps H+ from the mitochondrial matrix into the intermembrane space, creating a concentration gradient. H+ flows back down this gradient through ATP synthase, a membrane protein that uses the energy of that flow to join ADP and inorganic phosphate into ATP.
Q: What is the difference between aerobic respiration, anaerobic respiration, and fermentation?
A: Aerobic respiration uses oxygen as the final electron acceptor. Anaerobic respiration uses a different inorganic molecule (such as nitrate or sulphate) instead of oxygen. Fermentation does not use an electron transport chain at all; it partially degrades glucose via glycolysis and then regenerates NAD+ through either alcohol or lactic acid production, yielding only 2 ATP per glucose.
This material connects directly to photosynthesis (Chapter 10), which runs the reverse overall reaction: CO2 + H2O + light energy → glucose + O2. The two processes are mirror images linked by glucose and oxygen cycling.
The enzyme kinetics and regulation concepts from Chapter 5 underpin the feedback mechanisms that control glycolysis (phosphofructokinase inhibition by ATP and citrate).
Redox chemistry here is the same framework you will see in electron transport in photosynthesis (the thylakoid membrane) and in discussions of anaerobic metabolism in microbiology (alternative electron acceptors like nitrate, sulphate, and iron).
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