Class 11 Biology · NCERT chapter 12 · 6 topics · free to read
Respiration Overview
Cellular respiration is the stepwise oxidation of organic molecules (mainly glucose) to release energy as ATP. It occurs in all living cells 24/7 — unlike photosynthesis. Aerobic respiration (with O₂) yields 36-38 ATP; anaerobic respiration (without O₂) yields only 2 ATP. The pathway is amphibolic — it both breaks down substrates (catabolic) and provides intermediates for biosynthesis (anabolic).
Cellular respiration is the enzymatic oxidation of organic compounds to CO₂ and H₂O with the release of energy trapped as ATP. It is NOT the same as breathing (which is just gas exchange).
If you burn glucose in a test tube, all energy is released as heat and light in one shot — useless for a cell. Cellular respiration does the same oxidation, but in small controlled steps, each catalyzed by a specific enzyme. Energy is captured in the phosphate bonds of ATP — the universal energy currency of the cell.
Respiration ≠ Breathing — Breathing is gas exchange (physical); respiration is oxidation of food (biochemical)
Occurs in every living cell — plant, animal, fungal, bacterial — 24 hours a day
Glucose is the preferred substrate, but fats and proteins can also be respired
Energy is released in a stepwise manner through enzyme-catalyzed reactions, NOT all at once
Final products: CO₂ + H₂O + ATP (in aerobic respiration)
Aerobic vs Anaerobic
Aerobic respiration requires O₂ and yields 36-38 ATP per glucose. Anaerobic respiration (fermentation) works without O₂ but yields only 2 ATP — incomplete oxidation.
Requires O₂ as terminal electron acceptor
Does NOT require O₂
Complete oxidation of glucose
Incomplete oxidation of glucose
End products: CO₂ + H₂O
End products: Ethanol + CO₂ OR Lactic acid
Yields 36-38 ATP per glucose
Yields only 2 ATP per glucose
Occurs in cytoplasm + mitochondria
Occurs only in cytoplasm
Most plants and animals
Yeast, some bacteria, muscle cells (temporarily)
Even in aerobic organisms, the first step (glycolysis) is always anaerobic — it doesn't need O₂. Oxygen enters the picture only at the ETC. This is why glycolysis is considered the most ancient metabolic pathway — it evolved before O₂ appeared in Earth's atmosphere.
The Four Stages
Aerobic respiration has four sequential stages: glycolysis (cytoplasm), pyruvate oxidation (mitochondrial matrix), Krebs cycle (matrix), and ETC + oxidative phosphorylation (inner mitochondrial membrane).
Glycolysis — Glucose (6C) → 2 Pyruvate (3C) in the cytoplasm. Net gain: 2 ATP + 2 NADH. No O₂ needed.
Pyruvate Oxidation (Link Reaction) — Each pyruvate → Acetyl CoA (2C) + CO₂ + NADH in the mitochondrial matrix.
Krebs Cycle (TCA Cycle) — Acetyl CoA enters the cycle in the matrix. Per turn: 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂.
Electron Transport Chain + Oxidative Phosphorylation — NADH and FADH₂ donate electrons to complexes in the inner mitochondrial membrane. O₂ is the final electron acceptor. Produces 32-34 ATP via chemiosmosis.
Think of it as a relay race: glycolysis hands pyruvate to the link reaction, which hands acetyl CoA to Krebs, which hands NADH/FADH₂ to the ETC. Each stage passes the baton to the next. The bulk of ATP (about 90%) comes from the last stage — the ETC.
Only glycolysis occurs in the cytoplasm — all other steps are in mitochondria
Glycolysis is the only stage that occurs in both aerobic and anaerobic pathways
The link reaction and Krebs cycle occur in the mitochondrial matrix
ETC and oxidative phosphorylation occur on the inner mitochondrial membrane
Total ATP yield from one glucose: 36-38 ATP (theoretical maximum)
NCERT specifically says organisms that lack mitochondria (like prokaryotes) perform the ETC on the plasma membrane. The principle is the same — it just happens on a different membrane.
Respiration vs Photosynthesis
Photosynthesis stores energy by reducing CO₂ to glucose; respiration releases energy by oxidizing glucose to CO₂. They share intermediates but use different pathways, enzymes, and compartments.
Anabolic (builds glucose)
Catabolic (breaks glucose)
Stores energy in organic molecules
Releases energy as ATP
Uses CO₂ + H₂O → Glucose + O₂
Uses Glucose + O₂ → CO₂ + H₂O
Occurs only in light
Occurs 24/7 (light and dark)
Only in chloroplast-containing cells
In ALL living cells
O₂ is released
O₂ is consumed
NADPH is the electron carrier
NADH / FADH₂ are the electron carriers
They look like mirror images on paper, but they are NOT simply the reverse of each other. Photosynthesis uses the Calvin cycle and light reactions with different enzymes. Respiration uses glycolysis, Krebs cycle, and the ETC with entirely different enzymes. They share some intermediates (like G3P) but follow distinct biochemical routes.
Amphibolic Nature
Respiratory intermediates serve as raw materials for biosynthesis of fats, amino acids, and other molecules — making the pathway amphibolic (both catabolic and anabolic).
Respiration is not just about breaking down glucose. The intermediates generated at various steps are siphoned off to build new molecules. Acetyl CoA makes fatty acids. Krebs cycle intermediates like OAA (oxaloacetate) and α-ketoglutarate are converted into amino acids. This dual role — breakdown AND synthesis — makes it an amphibolic pathway.
Acetyl CoA → used for fatty acid synthesis, cholesterol synthesis
OAA (oxaloacetate) → converted to aspartate (amino acid)
α-Ketoglutarate → converted to glutamate (amino acid)
When intermediates are pulled out for biosynthesis, they must be replenished (anaplerotic reactions) to keep the cycle running
NCERT uses the Krebs cycle as the prime example of an amphibolic pathway
Quick recall
Why is the respiratory pathway called amphibolic?
Because it serves a dual purpose: it breaks down substrates to release energy (catabolic) AND provides intermediates for the synthesis of new molecules like amino acids and fatty acids (anabolic). NCERT specifically highlights the Krebs cycle as amphibolic.
Glycolysis
Glycolysis (EMP pathway) is the first stage of respiration where one glucose molecule (6C) is broken into two molecules of pyruvate (3C) in the cytoplasm. It requires no oxygen, produces a net gain of 2 ATP and 2 NADH, and is common to both aerobic and anaerobic organisms. Named after Gustav Embden, Otto Meyerhof, and Jakub Parnas.
Glycolysis converts one molecule of glucose (6C) into two molecules of pyruvate (3C) through 10 sequential enzymatic reactions in the cytoplasm. It is the most ancient and universal metabolic pathway.
Glycolysis literally means 'sugar splitting' (glyco = sugar, lysis = splitting). It was discovered by Embden, Meyerhof, and Parnas — hence also called the EMP pathway. It occurs in the cytoplasm (cytosol) of every living cell, from bacteria to human neurons.
Occurs in the cytoplasm — no membrane-bound organelle needed
10 enzymatic steps — all catalyzed by soluble cytoplasmic enzymes
Input: 1 glucose (6C) → Output: 2 pyruvate (3C)
Does NOT require oxygen — this is why it's the common step in both aerobic and anaerobic respiration
Found in all living organisms — the most evolutionarily ancient metabolic pathway
Also called EMP pathway (Embden-Meyerhof-Parnas)
Two Phases
Glycolysis has an energy-investment phase (uses 2 ATP to phosphorylate glucose) and an energy-payoff phase (generates 4 ATP + 2 NADH). Net gain: 2 ATP + 2 NADH per glucose.
Glycolysis is a two-act play. In Act 1 (steps 1-5), the cell spends 2 ATP to activate and split glucose into two 3-carbon fragments. In Act 2 (steps 6-10), the cell earns back 4 ATP + 2 NADH from those fragments. Net profit: 2 ATP + 2 NADH per glucose.
Phase 1 — Energy Investment (Steps 1-5): Glucose (6C) is phosphorylated twice (costs 2 ATP), rearranged, and split into 2 molecules of G3P (glyceraldehyde-3-phosphate, 3C each).
Step 10: PEP → Pyruvate (enzyme: pyruvate kinase — produces 1 ATP per G3P = another substrate-level phosphorylation)
Key Regulatory Enzymes
Phosphofructokinase (PFK) is the main regulatory enzyme and rate-limiting step of glycolysis. Hexokinase and pyruvate kinase are also important control points.
Not all 10 steps are equally important for regulation. Three enzymes catalyze irreversible reactions and act as control points. Of these, phosphofructokinase (PFK-1) at step 3 is the master regulator — it decides how fast the entire pathway runs.
Hexokinase (Step 1) — inhibited by its own product glucose-6-phosphate (product inhibition)
Phosphofructokinase / PFK (Step 3) — rate-limiting enzyme of glycolysis. Activated by AMP/ADP (low energy signal). Inhibited by ATP, citrate (high energy signal)
Pyruvate kinase (Step 10) — activated by fructose-1,6-bisphosphate (feedforward activation). Inhibited by ATP
All three catalyze irreversible reactions — these are the control points
When the cell has plenty of ATP → PFK is inhibited → glycolysis slows down. Smart energy management!
Fate of Pyruvate
Pyruvate, the end product of glycolysis, can follow three paths: (1) aerobic oxidation to acetyl CoA, (2) alcohol fermentation, or (3) lactic acid fermentation — depending on the organism and oxygen availability.
Pyruvate stands at a metabolic crossroads. What happens next depends entirely on whether the cell has oxygen and what type of organism it is. NCERT describes three fates:
Path 1 — Aerobic (Oxidative Decarboxylation): Pyruvate enters the mitochondrial matrix → Acetyl CoA + CO₂ + NADH. Enzyme: pyruvate dehydrogenase complex. This feeds into the Krebs cycle.
Path 2 — Alcohol Fermentation: Pyruvate → Acetaldehyde + CO₂ → Ethanol + NADH is oxidized back to NAD⁺. Occurs in yeast and some plants under anaerobic conditions.
Path 3 — Lactic Acid Fermentation: Pyruvate → Lactate (using NADH → NAD⁺). Occurs in animal muscle cells during intense exercise when O₂ supply is insufficient. Also in some bacteria (e.g., Lactobacillus).
In the presence of O₂: Pyruvate → Acetyl CoA (enters Krebs cycle) — this is the aerobic path
In the absence of O₂: Pyruvate → Ethanol or Lactate — these are the anaerobic/fermentation paths
Pyruvate dehydrogenase is a multi-enzyme complex requiring 5 coenzymes: TPP, lipoic acid, CoA, FAD, NAD⁺
The conversion of pyruvate to acetyl CoA is called oxidative decarboxylation (oxidation + loss of CO₂)
This link reaction produces 1 NADH per pyruvate (so 2 NADH per glucose)
Fermentation
Fermentation is anaerobic respiration where pyruvate is incompletely oxidized to either ethanol + CO₂ (alcohol fermentation in yeast) or lactic acid (lactic acid fermentation in muscles/bacteria). Net ATP yield is only 2 per glucose — all from glycolysis. NADH produced in glycolysis is re-oxidized to NAD⁺ to keep glycolysis running, since there is no ETC to accept electrons.
Pyruvate no O_2 Ethanol + CO_2 OR Lactate
Alcohol Fermentation
In the absence of O₂, yeast converts pyruvate to ethanol and CO₂ using pyruvate decarboxylase and alcohol dehydrogenase. This is the basis of brewing and baking.
Occurs in yeast (Saccharomyces cerevisiae) and some plants under anaerobic conditions
The CO₂ released is what makes bread rise and beer fizzy
Ethanol is toxic to yeast above ~13% concentration — that's why wine can't naturally exceed ~13% alcohol
Net ATP = 2 (only from glycolysis; fermentation itself produces zero ATP)
The ENTIRE point of converting pyruvate → ethanol is to regenerate NAD⁺ so glycolysis can continue
Lactic Acid Fermentation
In the absence of sufficient O₂, animal muscle cells and some bacteria convert pyruvate directly to lactate using lactate dehydrogenase. Net ATP = 2. No CO₂ is released.
Unlike alcohol fermentation, lactic acid fermentation is a single-step reaction. Pyruvate is directly reduced to lactate by the enzyme lactate dehydrogenase (LDH). No CO₂ is released — the 3-carbon skeleton stays intact.
Occurs in skeletal muscle cells during vigorous exercise (oxygen debt)
Also occurs in Lactobacillus bacteria — used to make curd, yogurt, cheese
No CO₂ released (unlike alcohol fermentation which releases CO₂)
Net ATP = 2 per glucose (from glycolysis only)
Lactate buildup causes muscle fatigue and cramps — it's cleared by the liver (Cori cycle) once O₂ returns
Enzyme: Lactate dehydrogenase (LDH)
Pyruvate → Ethanol + CO₂
Pyruvate → Lactate
Two-step process
Single-step process
CO₂ is released
No CO₂ released
Yeast, some plants
Muscle cells, Lactobacillus
Irreversible
Reversible (lactate → pyruvate when O₂ returns)
Net 2 ATP
Net 2 ATP
Incomplete Oxidation
Fermentation is called incomplete oxidation because ethanol or lactate still contain a large amount of chemical energy that was never extracted. Only 2 ATP out of a potential 36-38 ATP are captured.
In aerobic respiration, glucose is completely oxidized to CO₂ and H₂O — all the energy is extracted (36-38 ATP). In fermentation, the end products (ethanol or lactate) are still energy-rich organic molecules. Most of the chemical energy in glucose remains locked in these products, unused.
Incomplete oxidation (fermentation): Glucose → Ethanol/Lactate → only 2 ATP extracted
Ethanol can still burn (it's used as fuel!) — proving it still contains lots of energy
Fermentation captures only about 5% of the total energy in glucose
The remaining ~95% of energy stays trapped in ethanol or lactate
Quick recall
Why is fermentation called incomplete oxidation?
Because the end products (ethanol or lactate) are still partially reduced organic molecules containing significant chemical energy. Only 2 out of ~38 potential ATP molecules are extracted — the rest of the energy remains trapped in the products.
Krebs Cycle (TCA Cycle)
The Krebs cycle (also called the Citric Acid Cycle or TCA cycle) occurs in the mitochondrial matrix. Acetyl CoA (2C) enters by combining with OAA (4C) to form citrate (6C), which is then systematically oxidized back to OAA in 8 enzyme-catalyzed steps. Each turn produces 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂. Since each glucose gives 2 acetyl CoA, the cycle turns twice per glucose. Discovered by Hans Krebs (Nobel Prize 1953).
Acetyl CoA + 3NAD^+ + FAD + GDP + P_i arrow 2CO_2 + 3NADH + FADH_2 + GTP + CoA
Acetyl CoA Entry
Acetyl CoA (2C) condenses with oxaloacetate/OAA (4C) to form citrate (6C), catalyzed by citrate synthase. This is the entry point and the reaction that gives the cycle its name — Citric Acid Cycle.
Acetyl CoA (2C) + OAA (4C) Citrate synthase Citrate (6C) + CoA
This is the gateway reaction of the Krebs cycle. Acetyl CoA carries a 2-carbon acetyl group that joins with oxaloacetate (4C) to form the 6-carbon molecule citrate (citric acid). This is why it's also called the Citric Acid Cycle — citrate is the first product formed.
Citrate synthase catalyzes the condensation reaction — irreversible and highly regulated
Citrate (6C) is the first compound formed in the cycle — hence 'Citric Acid Cycle'
OAA (oxaloacetate) is regenerated at the end of each turn — that's what makes it a CYCLE
One glucose → 2 pyruvate → 2 acetyl CoA → the cycle turns TWICE per glucose
Acetyl CoA can also come from fatty acid β-oxidation and amino acid catabolism — not just glucose
The 8 Steps
The Krebs cycle has 8 enzymatic steps: citrate is converted through isocitrate, α-ketoglutarate, succinyl CoA, succinate, fumarate, malate, and back to oxaloacetate. Two decarboxylations release 2 CO₂.
2 CO₂ released per turn (steps 3 and 4) — these are the carbons from acetyl CoA
3 NADH produced per turn (steps 3, 4, 8)
1 FADH₂ produced per turn (step 6)
1 GTP produced per turn (step 5) — equivalent to 1 ATP (substrate-level phosphorylation)
Isocitrate dehydrogenase (step 3) is the rate-limiting enzyme — regulated by ADP (activator) and ATP (inhibitor)
Succinate dehydrogenase (step 6) is unique — it's the only TCA enzyme in the inner mitochondrial membrane (also Complex II of the ETC!)
Energy Harvest Per Turn
Each turn of the Krebs cycle produces 3 NADH, 1 FADH₂, and 1 GTP. Since one glucose produces 2 acetyl CoA, the total Krebs output per glucose is 6 NADH, 2 FADH₂, and 2 GTP.
The Krebs cycle doesn't directly make much ATP — only 1 GTP per turn (equivalent to 1 ATP). Its real job is loading up electron carriers: NADH and FADH₂. These are the hydrogen bombs that will explode with energy at the ETC to produce the bulk of ATP.
GTP → ATP conversion is spontaneous (GTP + ADP → GDP + ATP) — so 1 GTP ≈ 1 ATP
The 6 NADH and 2 FADH₂ will later produce ~22 ATP at the ETC via oxidative phosphorylation
Krebs cycle itself directly produces only 2 ATP (as GTP) per glucose — the rest is indirect via electron carriers
Quick recall
What are the products of one complete turn of the Krebs cycle?
3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂ + CoA (released when citrate synthase accepts a new acetyl CoA). Per glucose (2 turns): 6 NADH + 2 FADH₂ + 2 GTP + 4 CO₂.
Hans Krebs received the Nobel Prize in Physiology or Medicine in 1953 for discovering this cycle. He was initially rejected when he submitted the paper — the journal said they had no room! He published elsewhere and won the Nobel within 16 years.
Electron Transport Chain & Oxidative Phosphorylation
The ETC is the final stage of aerobic respiration, located on the inner mitochondrial membrane. NADH and FADH₂ donate electrons to a chain of protein complexes (I-IV), which pump H⁺ ions into the intermembrane space. The resulting proton gradient drives ATP synthase (Complex V) to produce 32-34 ATP via chemiosmosis. O₂ is the terminal electron acceptor, forming H₂O. This stage produces ~90% of all ATP from glucose oxidation.
NADH arrow Complex I arrow CoQ arrow Complex III arrow Cyt c arrow Complex IV arrow O_2 arrow H_2O
The Four Complexes
Electrons from NADH enter at Complex I and from FADH₂ at Complex II. They pass through ubiquinone (CoQ), Complex III, cytochrome c, and finally Complex IV, where they reduce O₂ to H₂O. Each complex pumps H⁺ ions.
Complex I (NADH dehydrogenase): NADH → NAD⁺ + 2e⁻ + H⁺. Electrons enter here. Pumps 4 H⁺ across the membrane. Contains FMN and Fe-S clusters.
Complex II (Succinate dehydrogenase): FADH₂ → FAD + 2e⁻ + 2H⁺. Electrons from FADH₂ enter here (bypassing Complex I). Does NOT pump H⁺ — that's why FADH₂ makes fewer ATP than NADH.
Ubiquinone (Coenzyme Q): Mobile electron carrier in the lipid bilayer. Collects electrons from BOTH Complex I and II, shuttles them to Complex III.
Complex III (Cytochrome bc₁): Receives electrons from CoQ. Pumps 4 H⁺ across the membrane. Contains cytochrome b, Fe-S clusters, and cytochrome c₁.
Cytochrome c: Small mobile protein on the outer surface of the inner membrane. Shuttles electrons from Complex III to Complex IV.
Complex IV (Cytochrome c oxidase): The final complex. Transfers electrons to O₂ (terminal electron acceptor). Pumps 2 H⁺. O₂ + 4e⁻ + 4H⁺ → 2H₂O.
NADH donates electrons at Complex I → ~2.5 ATP per NADH (passes through 3 proton-pumping complexes: I, III, IV)
FADH₂ donates electrons at Complex II → ~1.5 ATP per FADH₂ (bypasses Complex I, passes through only III, IV)
Complex II = Succinate dehydrogenase — the SAME enzyme from Krebs cycle step 6! It's the only enzyme shared between TCA and ETC
O₂ is the terminal electron acceptor — without O₂, the entire chain backs up and stops
Total H⁺ pumped per NADH: 4 (Complex I) + 4 (Complex III) + 2 (Complex IV) = 10 H⁺
Total H⁺ pumped per FADH₂: 0 (Complex II) + 4 (Complex III) + 2 (Complex IV) = 6 H⁺
Chemiosmosis
As the ETC pumps H⁺ ions into the intermembrane space, a proton gradient (electrochemical gradient) builds up. H⁺ ions flow back through ATP synthase (Complex V), driving the synthesis of ATP from ADP + Pi. This is Peter Mitchell's chemiosmotic hypothesis (Nobel Prize 1978).
The ETC does NOT make ATP directly. It pumps protons (H⁺) from the matrix to the intermembrane space, creating a steep concentration gradient — high H⁺ outside, low H⁺ inside. This gradient stores potential energy (like water behind a dam). The H⁺ ions can only flow back into the matrix through the special channel of ATP synthase — and as they flow through, ATP synthase spins and makes ATP.
Proton gradient (H⁺ gradient) across the inner mitochondrial membrane provides the driving force
ATP synthase (Complex V / F₀F₁ particle): F₀ = the transmembrane channel (proton pore); F₁ = the catalytic headpiece that makes ATP
H⁺ flow through F₀ makes the F₁ head physically rotate — a molecular motor! Each 360° rotation makes ~3 ATP
This mechanism is called oxidative phosphorylation — ATP synthesis driven by electron transport (oxidation)
Peter Mitchell proposed the chemiosmotic hypothesis in 1961, won the Nobel Prize in 1978
The inner mitochondrial membrane must be intact and impermeable to H⁺ for chemiosmosis to work — any holes would collapse the gradient
Total ATP Yield
The theoretical maximum ATP yield from one glucose is 36-38 ATP. The uncertainty comes from the shuttle systems that transport cytoplasmic NADH into mitochondria. The NCERT-stated figure is 36 ATP per glucose.
Time for the final ATP accounting. Every NADH and FADH₂ made during glycolysis, the link reaction, and the Krebs cycle now cashes in at the ETC. But the exact total depends on which shuttle system transports the 2 NADH from glycolysis (which are stuck in the cytoplasm) into the mitochondria.
Glycolysis: 2 ATP (substrate) + 2 NADH (× 2.5 ATP each = 5 ATP) → 7 ATP
Link reaction (× 2): 2 NADH (× 2.5 = 5 ATP) → 5 ATP
Theoretical maximum: 7 + 5 + 20 = 32 ATP (using newer values of 2.5 per NADH, 1.5 per FADH₂)
NCERT uses older values: 3 ATP per NADH, 2 ATP per FADH₂ → total 38 ATP (or 36 if glycolytic NADH uses the glycerol-phosphate shuttle giving 2 ATP instead of 3 per NADH)
The shuttle problem: The 2 NADH from glycolysis are in the cytoplasm but the ETC is in the mitochondria. NADH can't cross the inner membrane directly. Two shuttle systems exist: (1) Malate-aspartate shuttle → delivers electrons as mitochondrial NADH (3 ATP each). (2) Glycerol-3-phosphate shuttle → delivers electrons as FADH₂ (2 ATP each). This is why the answer is either 36 or 38.
Respiratory Quotient (RQ)
The respiratory quotient (RQ) is the ratio of CO₂ evolved to O₂ consumed during respiration. It reveals which substrate is being respired: carbohydrates (RQ = 1), fats (RQ < 1, ~0.7), proteins (RQ ~0.8), and organic acids (RQ > 1). RQ is a powerful diagnostic tool in both plant physiology and clinical nutrition.
RQ = Volume of CO_2 evolved Volume of O_2 consumed
RQ Definition
The respiratory quotient is defined as the ratio of the volume of CO₂ released to the volume of O₂ consumed during cellular respiration. Its value depends on the respiratory substrate.
RQ = Volume of CO_2 evolved Volume of O_2 consumed
The RQ is a dimensionless number that acts as a metabolic fingerprint. By measuring how much CO₂ comes out and how much O₂ goes in, you can deduce what type of molecule the organism is burning for energy. Different substrates have different carbon-to-hydrogen ratios, requiring different amounts of O₂ and releasing different amounts of CO₂.
RQ is a ratio — it has no units
Measured using a respirometer (like Ganong's respirometer)
RQ depends on the substrate being respired, not the organism itself
If an organism respires a mix of substrates, the RQ will be an intermediate value
RQ can help determine whether a seed contains starch (RQ=1) or oil (RQ<1) — useful for agricultural assessments
RQ of Carbohydrates
When glucose (or any carbohydrate) is the sole respiratory substrate, RQ = 1. This is because carbohydrates have the formula (CH₂O)ₙ — their carbon is already partially oxidized, so CO₂ evolved equals O₂ consumed.
C_6H_ 12 O_6 + 6O_2 arrow 6CO_2 + 6H_2O
Count the molecules: 6 O₂ consumed, 6 CO₂ released. So RQ = 6/6 = 1.0. This is because glucose has the empirical formula (CH₂O)₆ — the H and O are already in a 2:1 ratio (like water). Every carbon atom needs exactly 1 O₂ molecule to become CO₂.
RQ = 1 for ALL carbohydrates (glucose, sucrose, starch, etc.)
Germinating starchy seeds (wheat, rice) show RQ = 1
This is the reference value — other substrates are compared to this
If a question says 'RQ = 1' → the substrate is a carbohydrate
RQ of Fats
Fats have a very low oxygen content relative to carbon and hydrogen. They need more O₂ for complete oxidation, so CO₂ produced < O₂ consumed, giving RQ < 1 (typically ~0.7).
2C_ 51 H_ 98 O_6 + 145O_2 arrow 102CO_2 + 98H_2O
For tripalmitin (a typical fat): 145 O₂ consumed, 102 CO₂ released. RQ = 102/145 = 0.7. Fats are highly reduced — lots of C-H bonds, very little oxygen in the molecule. Burning all those hydrogens requires extra O₂ that doesn't produce CO₂ (it produces H₂O instead). Hence O₂ consumed > CO₂ released.
RQ ≈ 0.7 for typical fats (tripalmitin, triolein, etc.)
Germinating oily seeds (groundnut, mustard, castor) show RQ < 1
Fats have the highest energy per gram (~9 kcal/g vs ~4 kcal/g for carbs) because they're so reduced
More O₂ is needed for fat oxidation → RQ < 1
RQ of Organic Acids & Proteins
Organic acids (like malic acid, oxalic acid) are already partially oxidized and need less O₂, giving RQ > 1. Proteins have an intermediate composition, giving RQ ~0.8.
Organic acids like malic acid, citric acid, and oxalic acid have more oxygen atoms relative to carbon. They're already partially oxidized, so they need less O₂ for complete combustion. The result: CO₂ released > O₂ consumed → RQ > 1.
Organic acids: RQ > 1 — they have excess oxygen in their structure (already partially oxidized)
Succulents (like cacti) that accumulate malic acid at night may show RQ > 1 when respiring these acids
Proteins: RQ ≈ 0.8 — intermediate value because proteins contain C, H, O, N, S
Protein RQ is approximate because the nitrogen portion is not fully oxidized — it's excreted as urea
Mixed diet in humans typically gives RQ ≈ 0.85 (mix of carbs, fats, proteins)
Carbohydrates (glucose, starch)
1.0
Fats (tripalmitin)
~0.7
Proteins
~0.8
Organic acids (malic acid)
> 1 (1.33)
Organic acids (oxalic acid)
> 1 (4.0)
Mixed diet (humans)
~0.85
Quick recall
Why is the RQ of organic acids greater than 1?
Organic acids already contain a relatively high proportion of oxygen in their molecular formula. They need less external O₂ for complete oxidation than the CO₂ they release — so CO₂ evolved > O₂ consumed → RQ > 1.