ما هي معادلة التنفس الخلوي الهوائي؟ What is the balanced equation for aerobic respiration?

معادلة التنفس الخلوي الهوائي هي: C₆H₁₂O₆ + 6O₂ ➔ 6CO₂ + 6H₂O + ATP. وتمر بـ 4 مراحل: انشطار الجلوكوز، تفاعل التحول، دورة كريبس، وسلسلة نقل الإلكترون. [الصيغة الرمزية الموزونة: C₆H₁₂O₆ + 6O₂ ➔ 6CO₂ + 6H₂O + 36-38 ATP]

C1.2Theme C — Interaction and interdependence

Cell respiration — IB Biology HL Revision Notes & Topical Questions

10 min read 12 parts Lesson 3 of 11 3 quizzes

📚 IB Biology HL Hub / Topical Revision

Curriculum Equation Key معادلة التنفس الخلوي الهوائي الموزونة - Balanced Cellular Respiration EquationFast Exam Solution

C₆H₁₂O₆ + 6O₂ ➔ 6CO₂ + 6H₂O + 36-38 ATP

Word Equation & Conditions: Glucose + Oxygen -> Carbon Dioxide + Water + Energy (36-38 ATP) | جلوكوز + أكسجين ← ثاني أكسيد كربون + ماء + طاقة

Location & Specifications: الميتوكوندريا (Mitochondria) والسيتوسول.

0%read

Lesson summary

C1.2 Cell respiration — IB BIOLOGY HL notes: Explain ATP as the energy currency and describe aerobic and anaerobic respiration, glycolysis, the link reaction, the Krebs cycle, oxidative phosphorylation and respiratory substrates.Role — cell respiration releases energy in small st…

Universal energy currency
Not stored
Energy and ATP are different
Hydrolysis
Controlled release
Uses of the energy

Part 1 of 10ATP as the energy currency

ATP hydrolysis transfers energy to cellular work; respiration supplies energy for ATP regeneration.

ATP supplies energy for active transport across cell membranes.

ATP as the energy currency

Role

Role cell respiration releases energy in small steps and transfers it to ATP, and heat lost at each step warms endotherms.

Structure

ATP is a phosphorylated nucleotide of ribose, adenine and three phosphate groups.

Universal energy currency

Universal energy currency it is used in all organisms and reused countless times.

Feature	Benefit
Small, sufficient energy release	Drives important reactions with little waste
Stable molecule	Does not break down without the ATPase enzyme
Recyclable	Reformed from ADP and Pi
Quick, easy hydrolysis	Meets sudden energy demand
Soluble and mobile	Carries energy to any part of the cell, by facilitated diffusion
Forms phosphorylated intermediates	Makes metabolites more reactive and lowers activation energy

ATP as the energy currency

Processes using ATP

Anabolic reactions making macromolecules.
Active transport against concentration gradients.
Movement of the whole cell.
Movement of cell components such as chromosomes.

Not stored

ATP is too reactive to store, so glucose and fatty acids are short-term stores, and glycogen, starch and triglycerides are long-term stores.

Energy and ATP are different

Energy and ATP are different energy is the capacity to do work and ATP is a molecule that carries it.

Part 2 of 10ATP–ADP cycle and cell respiration and respiratory substrates

1Hydrolysis

ATP is hydrolysed to ADP and Pi, releasing free energy (about 30.5 kJ mol⁻¹ per phosphate removed first and second, and 14.2 kJ mol⁻¹ for the third).

2Synthesis

ADP and Pi combine to form ATP in an energy-requiring condensation reaction that releases water.

3Turnover

Turnover humans use over 50 kg of ATP a day yet hold only about 200 g at any time, so cells must make ATP as needed.

1Respiration

Respiration the controlled release of energy from organic compounds to produce ATP, a catabolic process in every cell.

Glucose enters glycolysis directly, so it is the main fuel.
Lipids and proteins need several changes first, and proteins are used only when glucose and lipids are not available.

2Controlled release

Controlled release enzymes release energy through a pathway rather than in one step, which would damage cells.

3Uses of the energy

Uses of the energy anabolic processes, muscle contraction, active transport, moving molecules and heat for body temperature.

Respiration is a chemical process, while gas exchange is diffusion of oxygen and carbon dioxide.

Part 3 of 10Aerobic and anaerobic respiration

Feature	Aerobic	Anaerobic
Oxidation of glucose	Complete	Incomplete
Oxygen	Required	Not required
ATP yield	High (about 30–36 per glucose)	Low (2 per glucose)
Products in humans	CO₂ and H₂O	Lactate
Location	Cytoplasm and mitochondria	Cytoplasm only

Aerobic

Aerobic water is a by-product and helps desert animals, and carbon dioxide is excreted.
- (used in photosynthesis by plants)

Anaerobic

Anaerobic only glycolysis makes ATP, giving a short discharge of energy when oxygen runs out.
It occurs in vigorous exercise or waterlogged soil.
Plants and yeast make ethanol and carbon dioxide, and animals make lactate.

Word equations

Aerobicglucose + oxygen → carbon dioxide + water + ATP.
Anaerobic in humansglucose → lactate + ATP.

Part 4 of 10Factors affecting the rate of respiration and measuring respiration with a respirometer

Cell activity

Cell activity muscle cells respire faster than adipose cells.

Organism size

Organism size smaller organisms have a larger SA:V and respire faster to replace heat loss.

Oxygen supply

Oxygen supply low oxygen makes cells respire anaerobically.

Substrate supply

Substrate supply less glucose gives a lower rate.

Temperature

Temperature the rate rises to the enzyme optimum and then falls as enzymes denature.

pH carbon dioxide lowers cell pH and can denature respiratory enzymes.

Apparatus

Apparatus a sealed container with live organisms such as germinating seeds, an alkali such as potassium hydroxide to absorb CO₂,
- and a capillary tube manometer with a scale.
- (preferable to animals)

Principle

Factors affecting the rate of respiration and measuring respiration with a respirometer

The organisms take up oxygen.

The alkali absorbs the CO₂ they release.

Pressure inside falls and the manometer fluid moves towards the organisms.

Factors affecting the rate of respiration and measuring respiration with a respirometer

Controls

Controls use a thermostatic water bath, a control tube with inert material of equal volume, and repeat readings for a mean.

Volume of oxygen consumed

πr²h, in mm³,

where r is the radius of the capillary lumen and h the distance moved by the fluid;
- divide by time for mm³ min⁻¹.

Technology

Technology oxygen sensors, CO₂ monitors and dataloggers give real-time data without hazards such as strong alkali,
- and respiration rate differs from breathing rate.

Part 5 of 10Oxidation and reduction

Reducing and non-reducing sugar structure and test — section 4

Redox reactions occur together and transfer electrons.

Oxidation	Reduction
Loss of electrons	Gain of electrons
Loss of hydrogen	Gain of hydrogen
Gain of oxygen	Loss of oxygen
Exergonic	Endergonic

Reducing agents tend to donate electrons, and oxidising agents tend to accept them.

Electron carriers

NAD and FAD are coenzymes that act as oxidising agents.
NAD⁺ + 2e⁻ + 2H⁺ → NADH + H⁺
FAD + 2e⁻ + 2H⁺ → FADH₂
NAD⁺ is the oxidised form and NADH the reduced form, and they release electrons to return to the original form.

Part 6 of 10Glycolysis

Site and result
- in the cytoplasm, one glucose gives two pyruvate (3C), a net gain of two ATP and two reduced NAD.
  - (four made, two used)

Phosphorylation: two ATP phosphorylate glucose (6C) to fructose-1,6-bisphosphate, which is less stable and more reactive.

Lysis: fructose-1,6-bisphosphate splits into two triose phosphate (3C).

Oxidation: dehydrogenase removes hydrogen from each triose phosphate to reduce NAD, forming glycerate-3-phosphate.

ATP formation: phosphates pass to ADP by substrate-linked phosphorylation to make four ATP and two pyruvate.

Each step is catalysed by a different enzyme.

Part 7 of 10Anaerobic respiration pathways

Purpose
- reduced NAD is reoxidised so glycolysis continues with a net yield of about two ATP.

Feature	Lactate fermentation	Alcoholic fermentation
Organisms	Mammalian muscle and some microorganisms	Yeast and plants
Pathway	Pyruvate is reduced to lactate by lactate dehydrogenase, with pyruvate as hydrogen acceptor	Pyruvate is decarboxylated to ethanal, releasing CO₂, then ethanal is reduced to ethanol by alcohol dehydrogenase
Products	Lactate	Ethanol and carbon dioxide
Fate	Oxidised back to pyruvate for the Krebs cycle, or converted to glycogen in the liver	Ethanol is a waste product

1Oxygen debt

Oxygen debt the extra oxygen needed to oxidise lactate explains deeper, faster breathing after exercise.

Yeast in bread

Anaerobic respiration pathways

Flour starch is hydrolysed to maltose and glucose, which yeast respires aerobically at first.

When oxygen is used up, anaerobic respiration makes CO₂ bubbles that make the dough rise.

Baking kills the yeast and evaporates the ethanol, leaving a fluffy texture.

Yeast is also used for ethanol in beer and wine.

Part 8 of 10Link reaction and Krebs cycle

Link reaction
- in the mitochondrial matrix, pyruvate undergoes oxidative decarboxylation.

Link reaction and Krebs cycle

Carbon dioxide is removed to leave a 2C molecule.

It is oxidised, reducing NAD to NADH, to give an acetyl compound.

Acetyl combines with coenzyme A to form acetyl CoA.

Per pyruvate it gives acetyl CoA, CO₂ and reduced NAD, and it happens twice per glucose.

Krebs cycle
- in the matrix, a cycle of enzyme-controlled reactions.

Acetyl CoA (2C) joins oxaloacetate (4C) to form citrate (6C), releasing coenzyme A.

Citrate is decarboxylated, releasing two CO₂.

Citrate is oxidised, reducing 3 NAD and 1 FAD.

Substrate-level phosphorylation forms one ATP.

Oxaloacetate is regenerated.

Yield per glucose
- 4 CO₂, 2 ATP, 6 reduced NAD and 2 reduced FAD.

Part 9 of 10Oxidative phosphorylation

Electron transport chain
- carriers in the inner mitochondrial membrane form a series of redox reactions.

Oxidative phosphorylation

Electrons from reduced NAD and FAD pass from carrier to carrier, regenerating NAD⁺ and FAD.

Energy released is used to pump protons across the membrane into the intermembrane space.

A proton gradient builds up because the membrane is impermeable to protons.

Chemiosmosis

Oxidative phosphorylation

Protons flow back into the matrix through ATP synthase by facilitated diffusion.

ATP synthase turns like a water wheel and phosphorylates ADP to ATP.

Oxygen

Oxygen the final electron acceptor, reduced with protons from the matrix to form metabolic water.
Without oxygen, NADH and FADH₂ are not reoxidised, the chain stops and ATP is no longer made.

Yield

Yield about 2.5–3 ATP per reduced NAD, giving about 30–32 ATP per glucose in total.

Part 10 of 10Lipids and carbohydrates as substrates

Feature	Lipids	Carbohydrates
Energy per gram	More than twice as much	Less
Metabolic water	More	Less
Solubility	Insoluble, so no osmotic effect	Soluble, so osmotic effect
Breakdown	Hydrolysed less easily, so energy is transferred more slowly	Hydrolysed more easily, so energy is transferred faster

Reason for high yield

Reason for high yield lipids have fewer oxygen atoms, so their carbon and hydrogen atoms are more oxidisable.

Roles

Roles fat stores help animals survive harsh periods and plants store oil in seeds and fruits,
while desert animals use lipids as a source of metabolic water.

Entry to respiration

Glycolysis and anaerobic respiration need carbohydrate.
Lipids are broken into fatty acids and then 2C acetyl groups that combine with coenzyme A to give acetyl CoA for the Krebs cycle.

Key terms

Term	Meaning
ATP	adenosine triphosphate, the molecule that carries energy for cell work.
Glycolysis	splitting of glucose in the cytoplasm into two pyruvate with net two ATP.
Link reaction	conversion of pyruvate to acetyl CoA in the matrix.
Krebs cycle	a cycle in the matrix that oxidises acetyl CoA and reduces NAD and FAD.
Chemiosmosis	ATP synthesis driven by protons flowing down a gradient through ATP synthase.
Electron carrier	a molecule such as NAD or FAD that accepts and donates electrons.
Oxygen debt	extra oxygen needed after anaerobic exercise to clear lactate.
Respirometer	apparatus for measuring oxygen consumption.

Avoid these mistakes

ATP is stored in large amounts — it is made as needed and cannot be stored.

Anaerobic respiration makes no ATP — glycolysis still yields a net two ATP.

Ethanal and ethanol are the same — ethanal is reduced to ethanol in yeast.

Oxygen is used in the Krebs cycle directly — oxygen is the final electron acceptor in the electron transport chain.

Last updated: 26 October 2026

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.