Course: BIO 101, Intro to Biology | Source: Cellular Respiration lecture notes
Difficulty: Introductory | Prerequisites: Basic chemistry (atoms, bonds, energy), chapter on enzymes and metabolism
Cellular respiration is how your cells extract usable energy from the food you eat. It sits at the centre of metabolism: every cell in your body runs on ATP, and this is the set of pathways that produces most of it. You should already be comfortable with basic chemistry (what atoms and bonds are, what "exergonic" means) and have a working sense of how enzymes lower activation energy. This first part covers the foundational ideas, energy coupling, redox chemistry, and glycolysis, before Part 2 moves into the citric acid cycle, oxidative phosphorylation, and fermentation.
Cells break down glucose through a series of redox reactions, shuffling electrons to carrier molecules (NAD+ and FAD) that feed the electron transport chain. Glycolysis is the first stage: it splits one glucose into two pyruvate molecules, nets 2 ATP and 2 NADH, and happens in the cytoplasm with no oxygen required. Everything after glycolysis depends on what the cell does with those pyruvates.
Cellular respiration
The catabolic (breakdown) pathways in which chemical bond energy in organic molecules is released and trapped to build ATP.
In simple terms, this is how your cells "burn" food to make energy, without actual flames.
Redox reaction (oxidation–reduction reaction)
A chemical reaction in which an electron is transferred from one substance to another. The substance that loses electrons is oxidised; the one that gains them is reduced.
Think of it as a handoff: one molecule gives up electrons (gets oxidised), another catches them (gets reduced).
Oxidising agent
The molecule that accepts (gains) electrons in a redox reaction. It gets reduced in the process.
In simple terms, the oxidising agent is the electron "taker."
Reducing agent
The molecule that donates (loses) electrons in a redox reaction. It gets oxidised in the process.
In simple terms, the reducing agent is the electron "giver."
NAD+ (nicotinamide adenine dinucleotide)
A coenzyme carrier that serves as an electron shuttle. It picks up electrons (plus H+) during glycolysis and the Krebs cycle, becoming NADH, and is later oxidised back to NAD+ in the electron transport chain or during fermentation.
Think of NAD+ as an empty taxi that picks up electron passengers and drops them off at the electron transport chain.
NADH
The reduced form of NAD+. It carries high-energy electrons to the electron transport chain, where those electrons are used to drive ATP production.
Glycolysis
A pathway of ten cytosolic reactions that split glucose and oxidise it to form two molecules of pyruvate, producing a small amount of ATP and reduced NADH. The first major stage of cellular respiration.
In simple terms, glycolysis is the opening act: it chops glucose in half and harvests a little energy.
Substrate-level phosphorylation
The production of ATP that occurs when an enzyme transfers a phosphate group from another organic substrate directly onto ADP.
Think of it as a direct, hands-on way of making ATP, no electron transport chain needed.
Phosphofructokinase (PFK)
A key regulatory enzyme in glycolysis. It is activated by AMP (signalling low energy) and inhibited by high concentrations of ATP and end products.
Dehydrogenase
A class of enzymes that catalyse the removal of hydrogen atoms (electrons + H+) from substrates, facilitating redox reactions. These are the enzymes that load electrons onto NAD+.
Exergonic reaction
A reaction that releases free energy. Cellular respiration overall is exergonic: the products (CO₂ and H₂O) hold less energy than the reactants (glucose and O₂).
Energy coupling
The use of energy released by an exergonic reaction to drive an endergonic (energy-requiring) reaction. In cells, the energy from breaking down glucose is coupled to the synthesis of ATP.
Energy enters biological systems as light. Plants capture it through photosynthesis and store it in organic molecules (glucose, among others). Animals and other heterotrophs then break those molecules down to release usable energy in the form of ATP.
The key idea is energy coupling: cells pair exergonic reactions (ones that release energy) with endergonic reactions (ones that require it). The ATP/ADP cycle is the main coupling mechanism. ATP is synthesised when energy is available and hydrolysed when work needs doing.
Enzymes make this practical. They lower the activation energy of exergonic reactions so that energy-coupled reactions happen on a timescale the cell can work with, not over geological time.
Cellular respiration is, at bottom, a long chain of redox reactions. Electrons are stripped from glucose and passed, step by step, to oxygen.
Oxidation means losing electrons. The molecule that loses them is oxidised.
Reduction means gaining electrons. The molecule that gains them is reduced.
The electron donor is called the reducing agent (it gets oxidised).
The electron acceptor is called the oxidising agent (it gets reduced).
A useful mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).
Electrons carry energy. When they move from a less electronegative molecule (like glucose) to a more electronegative one (like oxygen), energy is released. Cells capture that energy in stages rather than all at once, which would generate too much heat.
NAD+ is the primary electron shuttle. Dehydrogenase enzymes strip electrons (along with hydrogen ions) from substrates and load them onto NAD+, reducing it to NADH. NADH then carries those high-energy electrons to the electron transport chain.
FAD is a second electron carrier that works on the same principle, becoming FADH₂ when reduced. It plays its main role in the citric acid cycle (covered in Part 2).
Think of hydrogen atoms as having two components that matter here: the electron (e⁻) and the proton (H+). When the notes say "hydrogen is transferred," they mean electrons are moving, and protons often come along for the ride.
Glycolysis takes place in the cytoplasm and does not require oxygen. It splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules through a sequence of ten enzyme-catalysed reactions.
The cell spends 2 ATP to phosphorylate glucose, making it reactive enough to be split. This is the "investment": you have to put energy in before you get energy out.
The phosphorylated fragments are oxidised. Electrons are transferred to NAD+, producing NADH. ATP is generated by substrate-level phosphorylation (enzymes transfer phosphate groups directly onto ADP).
Net yield per glucose molecule:
2 ATP (4 produced minus 2 invested)
2 NADH (carrying electrons to be used later)
2 pyruvate (the starting material for the next stage)
Glycolysis is tightly regulated by feedback mechanisms.
Phosphofructokinase (PFK) is the main control point. It catalyses an early, irreversible step.
High ATP concentration inhibits PFK, slowing glycolysis when energy is abundant.
High AMP concentration activates PFK, speeding glycolysis when the cell needs more ATP.
High concentrations of downstream end products also down-regulate the pathway.
This is a straightforward supply-and-demand system: the cell makes ATP when it needs it and slows down when it has enough.
Pyruvate's fate depends on oxygen availability.
With oxygen: pyruvate enters the mitochondria for pyruvate oxidation and then the citric acid cycle (Part 2).
Without oxygen: pyruvate is diverted to fermentation, which regenerates NAD+ so glycolysis can continue (also Part 2).
Overall equation for cellular respiration:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + heat)
In this equation, glucose is oxidised (it loses electrons, its hydrogens end up as water) and oxygen is reduced (it gains electrons, becoming water).
Glycolysis summary:
Glucose (6C) + 2 NAD+ + 2 ADP + 2 Pᵢ → 2 Pyruvate (3C) + 2 NADH + 2 ATP + 2 H₂O
Redox shorthand:
Oxidation: molecule loses e⁻ (or loses H, or gains O)
Reduction: molecule gains e⁻ (or gains H, or loses O)
Glycolysis is one of the most ancient metabolic pathways, shared by nearly every organism on Earth. It evolved before oxygen was abundant in the atmosphere, which is why it works without it.
When you sprint, your muscles burn through ATP faster than oxygen can be delivered. Glycolysis ramps up to meet demand, and the resulting pyruvate gets shunted into lactic acid fermentation to keep NAD+ available. That burn you feel during intense exercise is partly the result of this shift.
Students often confuse which molecule is oxidised and which is reduced. Remember: the oxidising agent is the one that gets reduced (it accepts electrons). The reducing agent is the one that gets oxidised (it donates electrons). The agent is named for what it does to the other molecule, not what happens to itself.
Glycolysis does not require oxygen, but that does not make it "anaerobic respiration." Glycolysis is simply the first stage. Anaerobic respiration is a separate process that uses an electron transport chain with a final electron acceptor other than oxygen.
Students sometimes think glycolysis produces a large amount of ATP. It does not. The net gain is only 2 ATP per glucose. The real payoff comes later, in oxidative phosphorylation.
"Oxidation always involves oxygen" is wrong. Oxidation means losing electrons. It was named after oxygen historically, but many oxidation reactions have nothing to do with oxygen gas.
⚠️ Be able to write and explain the overall equation for cellular respiration. Know which side is oxidised and which is reduced.
⚠️ Know the net ATP and NADH yield of glycolysis (2 ATP, 2 NADH per glucose), not just the gross output.
⚠️ Understand the difference between substrate-level phosphorylation (direct transfer, happens in glycolysis) and oxidative phosphorylation (driven by the proton gradient, happens at the electron transport chain).
⚠️ Be ready to explain how PFK regulates glycolysis. What activates it? What inhibits it? This is a common exam question on metabolic regulation.
⚠️ Know that glycolysis happens in the cytoplasm, not in the mitochondria. Location questions come up frequently.
True or false: Glycolysis requires oxygen to proceed.
Fill in the blank: The net ATP yield of glycolysis is ______ ATP per glucose molecule.
True or false: The oxidising agent in a redox reaction is the molecule that loses electrons.
Fill in the blank: The enzyme ______ is the main regulatory control point in glycolysis.
True or false: NAD+ is the oxidised form of the electron carrier; NADH is the reduced form.
Answers: 1. False. 2. 2. 3. False (it gains electrons; it is the one that gets reduced). 4. Phosphofructokinase (PFK). 5. True.
Q: What is the overall equation for cellular respiration, and which molecule is oxidised?
A: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy. Glucose is oxidised (it loses electrons/hydrogens). Oxygen is reduced (it gains electrons, forming water).
Q: Explain the difference between the investment phase and the payoff phase of glycolysis.
A: In the investment phase, 2 ATP are consumed to phosphorylate glucose, making it unstable enough to split. In the payoff phase, the two 3-carbon fragments are oxidised, producing 4 ATP (by substrate-level phosphorylation) and 2 NADH. The net gain is 2 ATP.
Q: Why is NAD+ essential for glycolysis to continue?
A: NAD+ is the electron acceptor. Without it, the oxidation steps in glycolysis cannot proceed because there is nothing to accept the electrons. This is why fermentation exists: it regenerates NAD+ when the electron transport chain is unavailable.
Q: How does PFK regulate glycolysis?
A: PFK is allosterically regulated. High ATP inhibits it (the cell has enough energy, so glycolysis slows). High AMP activates it (the cell is low on energy, so glycolysis speeds up). This creates a feedback loop that matches ATP production to the cell's needs.
Q: A molecule donates electrons in a chemical reaction. Is it oxidised or reduced? Is it the oxidising agent or the reducing agent?
A: It is oxidised (it lost electrons). It is the reducing agent (it caused the other molecule to be reduced).
This material connects directly to photosynthesis (the reverse process: photosynthesis stores energy in glucose; cellular respiration releases it). If you understand redox reactions here, the light reactions of photosynthesis will make far more sense.
Glycolysis also links to metabolism more broadly. Amino acids and fats can feed into the same pathways at different entry points, so these are not glucose-only pathways.
The regulation of PFK is an example of allosteric regulation and feedback inhibition, concepts covered in the enzymes chapter. If those terms are not solid for you yet, revisit that material.
cellular respiration, glycolysis, redox reactions, oxidation reduction, NAD+ NADH, electron carrier, electron shuttle, ATP production, substrate-level phosphorylation, energy coupling, ATP ADP cycle, exergonic reaction, phosphofructokinase PFK, dehydrogenase, activation energy, enzymes metabolism, glucose breakdown, pyruvate, cytoplasm, OIL RIG mnemonic, BIO 101, intro biology