Respiration in Plants

You breathe roughly twelve times a minute, drawing oxygen into your lungs and pushing carbon dioxide out, without ever thinking about it. A banyan tree standing beside you does nothing that looks remotely like that. It has no lungs, no diaphragm, no rib cage that rises and falls. Yet every living cell in that tree, from the tip of its deepest root to the leaf swaying at its crown, is doing something that at the chemical level is exactly what your own cells are doing every second: breaking the carbon-carbon bonds of food, step by careful step, and trapping the released energy as ATP. This chapter asks the question directly: do plants breathe, and if the answer is not a simple yes, what exactly are they doing instead? The story runs from the outside of the plant to the inside of a single mitochondrion. Plants get by without specialised breathing organs because every cell is close enough to air, through stomata, lenticels and loosely packed tissue, to manage its own gas exchange. Inside each cell, glucose is dismantled in stages rather than burned in one violent step, so that the energy can be captured in small, usable packets instead of escaping as heat. The first stage, glycolysis, happens in the cytoplasm of every organism on Earth, aerobic or not. What happens next branches: without oxygen, cells settle for a wasteful shortcut called fermentation; with oxygen, the pyruvate produced is fed into the mitochondrion for a complete overhaul through the Krebs cycle and the electron transport system, extracting nearly twenty times more energy from the very same glucose molecule. By the end of this chapter you will be able to trace a single glucose molecule through glycolysis, the Krebs cycle and oxidative phosphorylation, tally the ATP, NADH and FADH2 produced at each stage, explain why the respiratory pathway is as much about building molecules as breaking them down, and calculate a respiratory quotient from nothing more than a balanced chemical equation.

Key takeaways
  • Plants have no dedicated respiratory organs. Gas exchange happens by diffusion through stomata (leaves) and lenticels (woody stems), since almost every living plant cell has part of its surface in contact with air.
  • Respiration is controlled, stepwise oxidation, not combustion. Both release the same total energy from glucose (C6H12O6+6O2→6CO2+6H2O+EnergyC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}), but combustion releases it as one burst of heat, while respiration releases it in small steps captured as ATP, the cell's energy currency.
  • Glycolysis (the EMP pathway) splits glucose (6C) into two pyruvic acid molecules (3C) in the cytoplasm, a ten-reaction sequence common to every organism, netting 2 ATP and 2 NADH + H⁺.
  • Fermentation, pyruvate's anaerobic fate, gives ethanol (alcoholic, yeast) or lactic acid (lactic acid, some bacteria and exercising muscle), merely reoxidising NADH to NAD+ and releasing under 7 percent of glucose's energy, with no net ATP beyond glycolysis's own 2.
  • Aerobic respiration transports pyruvate into the mitochondrion as acetyl CoA (releasing CO2, NADH + H⁺), entering the tricarboxylic acid (Krebs) cycle in the matrix, fully oxidising the carbon skeleton while generating NADH + H⁺, FADH2 and ATP (via GTP) each turn.
  • The electron transport system (ETS), on the inner mitochondrial membrane, oxidises NADH and FADH2 through Complexes I to IV, pumping protons that drive ATP synthase via oxidative phosphorylation, with O2 as the final acceptor, forming water.
  • Under standard assumptions, complete aerobic oxidation of one glucose yields a net gain of about 38 ATP, against only 2 ATP from fermentation.
  • Intermediates such as acetyl CoA and alpha-ketoglutaric acid are withdrawn for biosynthesis and rejoin the pathway on breakdown, so it is better called amphibolic (catabolic and anabolic) than purely catabolic.
  • The respiratory quotient (RQ), CO2 evolved over O2 consumed, equals 1.0 for carbohydrate, about 0.7 for fats such as tripalmitin, and about 0.9 for proteins.
  • Key idea: glucose is oxidised in controlled steps, first to pyruvate in cytoplasmic glycolysis, then, given oxygen, completely to CO2 and water through the mitochondrial Krebs cycle and electron transport system, netting about 38 ATP per glucose versus 2 ATP from fermentation, with shared intermediates serving both breakdown and synthesis, and the respiratory quotient revealing which substrate is being burned.
The core insight

Respiration oxidises a substrate, usually glucose, in a controlled sequence, glycolysis to pyruvate in the cytoplasm and, given oxygen, pyruvate oxidation and the Krebs cycle in the mitochondrial matrix, whose NADH and FADH2 drive the electron transport system to synthesise a net of about 38 ATP per glucose through oxidative phosphorylation, an amphibolic pathway whose intermediates also feed biosynthesis, and whose respiratory quotient (CO2 evolved over O2 consumed) betrays whether the substrate being burned is carbohydrate, fat, or protein.

What this chapter covers

This chapter builds cellular respiration from the ground up, in the same order as the NCERT text.

  • Do plants breathe (12.1): why plants manage without lungs or gills, using stomata, lenticels and short diffusion distances instead, and why combustion and respiration release the same total energy from glucose in very different ways.
  • Glycolysis (12.2): the ten-reaction EMP pathway converting one glucose molecule into two pyruvic acid molecules in the cytoplasm, with ATP investment, ATP payoff and NADH formation traced step by step.
  • Fermentation (12.3): pyruvate's fate when oxygen is unavailable, alcoholic and lactic acid fermentation, and why both are an energetic dead end compared with aerobic respiration.
  • Aerobic respiration (12.4): pyruvate's journey into the mitochondrion, its oxidative decarboxylation to acetyl CoA, and the Krebs cycle that completely oxidises the carbon skeleton (12.4.1), then the electron transport system and oxidative phosphorylation that manufacture most of the ATP (12.4.2).
  • The respiratory balance sheet (12.5): the assumptions behind counting a net gain of 38 ATP per glucose, and why real cells never quite match this textbook arithmetic.
  • Amphibolic pathway (12.6): why respiration is not purely a breakdown pathway, since its intermediates are drawn on for synthesis of fats, proteins and other biomolecules just as often as they are produced from them.
  • Respiratory quotient (12.7): the ratio of CO2 evolved to O2 consumed, and how its value (1.0 carbohydrate, about 0.7 fat, about 0.9 protein) reveals the substrate being used.

The core ideas

Why plants do not breathe the way animals do

Plants clearly need respiration: they consume O2 and give out CO2. What they lack is a dedicated organ for it. Unlike animals, plants have no lungs, gills or tracheae, yet they never run short of oxygen where it matters, for three reasons. First, each plant part manages its own gas exchange; there is very little gas transport between organs. Second, plants simply do not demand much gas exchange: roots, stems and leaves respire far more slowly than animal tissue, and a photosynthesising leaf produces its own O2 internally. Third, the diffusion distance stays short even in a massive tree. Leaves exchange gases through stomata into loosely packed mesophyll riddled with interconnected air spaces; stems ventilate their thin layer of living cells, just inside and beneath the bark, through lenticels, small openings in the bark, while the dead woody interior needs no oxygen at all. Almost every living cell of a plant has part of its surface in contact with air, making specialised respiratory organs unnecessary. It helps to be precise about what respiration actually is. Complete combustion of glucose, C6H12O6+6O2→6CO2+6H2O+EnergyC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}, releases the same total energy whether as a bonfire or inside a cell; a bonfire releases it all at once as heat, unusable for biological work. Respiration oxidises the same molecule in a long chain of small, enzyme-controlled steps, each sized to release just enough energy to be coupled to ATP synthesis. The oxidised substances are respiratory substrates; carbohydrates are favoured, but proteins, fats and organic acids can also serve under certain conditions. Because many organisms, past and present, live where oxygen is scarce, every living cell retains the machinery to partially oxidise glucose without it. That anaerobic first stage, glycolysis, is where every account of respiration must begin.

Glycolysis: glucose to pyruvate in the cytoplasm

Glycolysis (Greek glycos, sugar, and lysis, splitting) was worked out by Gustav Embden, Otto Meyerhof and J. Parnas, so it is also called the EMP pathway. It occurs in the cytoplasm of every living cell and is the only respiratory process available to strict anaerobes. In plants, the glucose entering glycolysis usually comes from the breakdown of sucrose, the end product of photosynthesis, or from stored carbohydrates; the enzyme invertase splits sucrose into glucose and fructose, both of which enter the pathway. From there, a chain of ten enzyme-catalysed reactions converts one glucose molecule (6 carbons) into two molecules of pyruvic acid (3 carbons each). The pathway opens by spending energy rather than making it: two ATP are invested, first phosphorylating glucose to glucose-6-phosphate (enzyme hexokinase) and then fructose-6-phosphate to fructose-1,6-bisphosphate, before this six-carbon molecule splits into two three-carbon units that both proceed as glyceraldehyde-3-phosphate (PGAL). From here the payoff phase runs twice, once for each PGAL: oxidation of PGAL to 1,3-bisphosphoglyceric acid (BPGA) is the single step where NAD+ is reduced to NADH + H⁺, and the two steps that follow, BPGA to 3-phosphoglyceric acid and, after isomerisation and dehydration through phosphoenolpyruvate, PEP to pyruvic acid, each generate one ATP by substrate-level phosphorylation. The first derivation below walks through this ATP and NADH bookkeeping in full.

Figure
Flowchart of glycolysis showing glucose converted step by step through phosphorylated intermediates into two molecules of pyruvic acid with ATP invested at two steps ATP produced at two steps and NADH formed at one step
Glycolysis — glucose (6C) to two pyruvic acid (3C) in the cytoplasm Glucose (6C) ATP → ADP (used) Glucose-6-phosphate (6C) Fructose-6-phosphate (6C) ATP → ADP (used) Fructose-1,6-bisphosphate (6C) splits into two 3-carbon units 2 × Triose phosphate — PGAL and DHAP (3C each) 2 NAD⁺ → 2 NADH + H⁺ 2 × 1,3-bisphosphoglyceric acid — BPGA (3C) 2 ADP → 2 ATP (produced) 2 × 3-phosphoglyceric acid — PGA (3C) isomerise, then dehydrate (lose H2O) 2 × phosphoenolpyruvate — PEP (3C) 2 ADP → 2 ATP (produced) 2 × Pyruvic acid (3C) Net yield per glucose molecule 4 ATP formed minus 2 ATP invested = net 2 ATP 2 NADH + H⁺ formed, 2 pyruvic acid molecules formed All ten reactions occur in the cytoplasm and require no oxygen — the fate of pyruvate is decided next

Pyruvic acid is the key product of glycolysis, and its fate depends on what the cell needs and what is available: lactic acid fermentation, alcoholic fermentation, or aerobic respiration through the Krebs cycle, which alone achieves complete oxidation to CO2 and water and needs an O2 supply.

The fork in pyruvate's road: fermentation

When oxygen is unavailable, cells such as yeast oxidise glucose only partially, through fermentation. In alcoholic fermentation, pyruvic acid decarboxylase removes CO2 from pyruvic acid to give acetaldehyde, and alcohol dehydrogenase then reduces acetaldehyde to ethanol, consuming the NADH + H⁺ from glycolysis and regenerating NAD+. In lactic acid fermentation, used by some bacteria and by animal muscle cells during vigorous exercise, lactate dehydrogenase reduces pyruvic acid directly to lactic acid, again regenerating NAD+. Neither pathway adds a single ATP-generating step beyond glycolysis; their whole purpose is reoxidising NADH to NAD+ so glycolysis can keep running. Less than seven percent of glucose's energy is released this way, and both routes are somewhat hazardous, one accumulating acid, the other a toxin: yeasts poison themselves once ambient alcohol reaches about 13 percent, which is why naturally fermented beverages cap out near that strength unless concentrated afterwards by distillation or fortified with added alcohol.

Aerobic respiration: pyruvate oxidation and the Krebs cycle

Where oxygen is available, pyruvate is transported from the cytoplasm into the mitochondrion, and the story changes completely. Two crucial events define aerobic respiration: the complete oxidation of pyruvate by stepwise removal of every hydrogen atom, leaving three molecules of CO2 per pyruvate, and the passing of the removed electrons to molecular O2, with simultaneous ATP synthesis. The first event happens in the mitochondrial matrix; the second is located on the inner mitochondrial membrane. Pyruvate first undergoes oxidative decarboxylation, catalysed by the multienzyme complex pyruvic dehydrogenase, requiring NAD+, coenzyme A and Mg²⁺ as participants:

Pyruvic acid+CoA+NAD+→Pyruvate dehydrogenase, Mg2+Acetyl CoA+CO2+NADH+H+\text{Pyruvic acid} + \text{CoA} + \text{NAD}^+ \xrightarrow{\text{Pyruvate dehydrogenase, Mg}^{2+}} \text{Acetyl CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+

Since one glucose molecule yields two pyruvate, this reaction produces two NADH + H⁺ and two CO2 per glucose. The resulting acetyl CoA then enters the tricarboxylic acid cycle, more commonly the Krebs cycle, named for Hans Krebs, who first worked it out. The cycle opens with citrate synthase condensing the acetyl group with oxaloacetic acid (OAA) and water to form citric acid, releasing CoA. Citrate isomerises to isocitrate, then undergoes two successive decarboxylations, first to alpha-ketoglutaric acid and then, via succinyl-CoA, to succinic acid, a substrate-level phosphorylation step producing one GTP, converted to ATP in a coupled reaction. Succinic acid is oxidised, through fumaric and malic acid, back to oxaloacetic acid, regenerating the starting molecule so the cycle can turn again. Across one full turn, three points reduce NAD+ to NADH + H⁺ and one point reduces FAD+ to FADH2.

Figure
Circular diagram of the citric acid cycle showing acetyl coenzyme A combining with oxaloacetic acid to form citric acid which is processed through alpha ketoglutaric acid and succinic acid and malic acid back to oxaloacetic acid with carbon dioxide NADH and FADH2 released at labelled points and one ATP formed via GTP
The citric acid (Krebs) cycle in the mitochondrial matrix Pyruvate (3C) NAD⁺ → NADH+H⁺, CoA, releases CO2 Acetyl CoA (2C) enters cycle, releases CoA Oxaloacetic acid (4C) Citric acid (6C) CO2 NAD⁺ → NADH+H⁺ α-ketoglutaric acid (5C) CO2, NAD⁺→NADH+H⁺ via succinyl-CoA: GDP→GTP→ATP Succinic acid (4C) FAD⁺ → FADH2 Malic acid (4C) NAD⁺ → NADH+H⁺ regenerates OAA One turn per acetyl CoA: 2 CO2, 3 NADH + H⁺, 1 FADH2, 1 ATP (via GTP) Oxaloacetic acid is regenerated every turn, so the cycle keeps running as long as acetyl CoA arrives Fumaric acid, the intermediate between succinic and malic acid, is omitted here for clarity

Summed over the mitochondrial matrix reactions, the oxidation of one pyruvate molecule (through pyruvic dehydrogenase and one turn of the Krebs cycle) can be written as a single balance:

Pyruvic acid+4NAD++FAD++2H2O+ADP+Pi→3CO2+4NADH+4H++FADH2+ATP\text{Pyruvic acid} + 4\text{NAD}^+ + \text{FAD}^+ + 2\text{H}_2\text{O} + \text{ADP} + P_i \rightarrow 3\text{CO}_2 + 4\text{NADH} + 4\text{H}^+ + \text{FADH}_2 + \text{ATP}

Doubling this for the two pyruvate molecules from one glucose gives, before any ETS activity: 6 CO2 released, 8 NADH + H⁺, 2 FADH2 and 2 ATP produced. Neither oxygen nor the large ATP payoff respiration promises has appeared yet — those depend entirely on what NADH and FADH2 do next.

The electron transport system and oxidative phosphorylation

NADH + H⁺ and FADH2 are carriers of reducing power, energy-rich electrons still to be spent. That spending happens on the inner mitochondrial membrane, through the electron transport system (ETS). Electrons from NADH pass through NADH dehydrogenase (Complex I) to the mobile carrier ubiquinone; FADH2 feeds the same carrier via Complex II, bypassing Complex I. Reduced ubiquinone is oxidised, handing electrons via the cytochrome bc1 complex (Complex III) to cytochrome c, which ferries them to Complex IV (cytochrome c oxidase), where they finally combine with O2 and protons to form water: oxygen is the terminal electron acceptor, and it is O2's continuous removal of electrons that drives the whole chain forward. As electrons move through Complexes I to IV, protons are pumped from the matrix into the intermembrane space, building an electrochemical gradient. ATP synthase (Complex V) puts this to work: its F0 component forms a proton channel and its F1 headpiece synthesises ATP from ADP and inorganic phosphate as protons flow back into the matrix, four protons per ATP, the chemiosmotic mechanism already met for photophosphorylation. Because oxidation-reduction energy, not light, drives this proton gradient, the process is called oxidative phosphorylation. The ATP yield differs by carrier: about 3 ATP per NADH, and about 2 ATP per FADH2, which enters later and bypasses Complex I.

Figure
Diagram of the electron transport chain on the inner mitochondrial membrane showing NADH feeding Complex one and FADH2 feeding Complex two with electrons passing through ubiquinone Complex three cytochrome c and Complex four to reduce oxygen to water while protons are pumped across the membrane to drive ATP synthase
Electron transport system and oxidative phosphorylation inner mitochondrial membrane intermembrane space mitochondrial matrix Complex I NADH deh. NADH → NAD⁺ Complex II succinate deh. FADH2 → FAD⁺ Ubiquinone Complex III cytochrome bc1 cyt c Complex IV cyt oxidase O2 + 4H⁺ + 4e⁻ → 2H2O H⁺ pumped into intermembrane space at Complexes I, III, IV, building a proton gradient ATP synthase (Complex V) F0 channel + F1 head 4H⁺ per ATP H⁺ flows back down gradient ADP + Pi → ATP Oxidative phosphorylation: about 3 ATP per NADH, about 2 ATP per FADH2 Chemiosmotic hypothesis: electron transport pumps protons; their controlled return drives ATP synthesis

The respiratory balance sheet

Adding up glycolysis, pyruvate oxidation, the Krebs cycle and the ETS gives a theoretical accounting of ATP per glucose. This arithmetic rests on assumptions that rarely hold exactly in a living cell: a strict, orderly sequential pathway; NADH from glycolysis fully shuttled into the mitochondrion; no intermediate diverted elsewhere; and only glucose, no other substrate, being respired. Real cells violate all of these constantly, running pathways simultaneously and switching substrates on demand. Even so, the bookkeeping is worth doing: it yields a net gain of about 38 ATP per glucose completely oxidised aerobically, against fermentation's net gain of only 2, and it shows aerobic respiration completely degrading glucose to CO2 and water rather than stopping at a partial breakdown.

The amphibolic pathway

Respiration is conventionally filed under catabolism, the breakdown of complex molecules. That label is only half true. Glucose is the favoured substrate, and any other carbohydrate is usually converted to glucose before it is respired, but fats, proteins and their components can also be respired, entering the pathway at specific points rather than at the beginning. Fats must first be split into glycerol and fatty acids; glycerol enters after conversion to PGAL, while fatty acids are degraded to acetyl CoA before joining the Krebs cycle. Proteins are broken down by proteases into amino acids, which, after deamination, enter at various points, sometimes within the Krebs cycle itself, sometimes as pyruvate or acetyl CoA, depending on the amino acid's structure. The crucial insight is that these same entry points are also exit points. When a plant needs to synthesise fatty acids, it withdraws acetyl CoA from the respiratory pathway rather than producing it purely to burn it; the same intermediates that link breakdown also link biosynthesis. Because the respiratory pathway participates in both the breakdown (catabolism) and the synthesis (anabolism) of major biomolecules, it is best described as an amphibolic pathway, not simply a catabolic one.

The respiratory quotient

The respiratory quotient (RQ), or respiratory ratio, is the ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration:

RQ=volume of CO2 evolvedvolume of O2 consumedRQ = \frac{\text{volume of CO}_2 \text{ evolved}}{\text{volume of O}_2 \text{ consumed}}

Its value depends entirely on which respiratory substrate is being oxidised. For carbohydrates, complete oxidation consumes and releases equal volumes of gas, since C6H12O6+6O2→6CO2+6H2O+EnergyC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy} gives RQ=6CO2/6O2=1.0RQ = 6CO_2 / 6O_2 = 1.0. Fats, being more reduced (richer in hydrogen relative to oxygen) than carbohydrates, require more O2 to oxidise fully than they release as CO2, giving an RQ less than 1; for the fatty acid tripalmitin, 2(C51H98O6)+145O2→102CO2+98H2O+Energy2(C_{51}H_{98}O_6) + 145O_2 \rightarrow 102CO_2 + 98H_2O + \text{Energy}, so RQ=102/145≈0.7RQ = 102/145 \approx 0.7. Proteins, intermediate in composition, give an RQ of about 0.9. In practice, plant cells rarely respire a single pure substrate at any moment; the measured RQ of a living tissue is usually somewhere between these reference values, reflecting a mixture of substrates being oxidised together.

Derivation ATracing carbon ATP and NADH through glycolysis

The goal is to account for exactly where every ATP is spent, where every ATP is regained, and at which single step NADH is produced as one glucose molecule (6 carbons) is converted into two molecules of pyruvic acid (3 carbons each) in the cytoplasm, so the textbook claim of a net gain of 2 ATP and 2 NADH + H⁺ per glucose becomes a checked calculation rather than a number to memorise.

Step 1. Count the investment phase. Hexokinase phosphorylates glucose to glucose-6-phosphate, spending one ATP; this isomerises to fructose-6-phosphate, then is phosphorylated again to fructose-1,6-bisphosphate, spending a second ATP. Running total: 2 ATP spent, still one 6-carbon molecule.

Step 2. Split the six-carbon sugar into two three-carbon units. Fructose-1,6-bisphosphate cleaves into glyceraldehyde-3-phosphate (PGAL) and dihydroxyacetone phosphate, which isomerises to a second PGAL. From here every reaction happens twice per original glucose, so all further counts are doubled.

Step 3. Count the single NADH-producing step, doubled. Each PGAL is oxidised to 1,3-bisphosphoglyceric acid (BPGA), handing two hydrogen atoms to NAD+ to form NADH + H⁺; with two PGAL, that is 2 NADH + H⁺ per glucose. Running total: 2 ATP spent, 2 NADH + H⁺ produced.

Step 4. Count the payoff phase, doubled. Each BPGA converts to 3-phosphoglyceric acid, trapping one ATP by substrate-level phosphorylation (2 ATP for both); each resulting molecule is then reshuffled through phosphoenolpyruvate to pyruvic acid, trapping a second ATP each (another 2 ATP). Payoff phase total: 4 ATP.

Step 5. Subtract investment from payoff. Net ATP =4−2=2= 4 - 2 = 2 ATP per glucose molecule, alongside 2 NADH + H⁺ and 2 molecules of pyruvic acid — the fixed cytoplasmic contribution every later pathway, fermentation or aerobic respiration, builds upon.

Go deeper · Derivation B · 1 minBuilding the respiratory balance sheet for one glucose molecule
Derivation BBuilding the respiratory balance sheet for one glucose molecule

The goal is to add up every ATP-yielding step across glycolysis, pyruvate oxidation, the Krebs cycle and the electron transport system for one complete glucose molecule, under the standard textbook assumptions, to arrive at the net gain of about 38 ATP the NCERT respiratory balance sheet reports.

Step 1. Bring forward the glycolysis result: net 2 ATP, 2 NADH + H⁺ and 2 pyruvic acid, entirely in the cytoplasm.

Step 2. Add pyruvate oxidation. Both pyruvate are oxidatively decarboxylated in the matrix: Pyruvic acid + CoA + NAD+ → Acetyl CoA + CO2 + NADH + H⁺. For two pyruvate, this adds 2 more NADH + H⁺ and releases 2 CO2. Running total: 2 ATP, 4 NADH + H⁺.

Step 3. Add two turns of the Krebs cycle. Each turn (per acetyl CoA) contributes 3 NADH + H⁺, 1 FADH2 and 1 ATP via GTP; with two turns: 6 NADH + H⁺, 2 FADH2, 2 more ATP. Running total: substrate-level ATP 2+2=42+2=4; NADH + H⁺ 4+6=104+6=10; FADH2 =2=2.

Step 4. Convert NADH and FADH2 to ATP through the ETS, at 3 ATP per NADH + H⁺ and 2 ATP per FADH2: 10×3=3010 \times 3 = 30 ATP plus 2×2=42 \times 2 = 4 ATP, giving 34 ATP from the ETS.

Step 5. Add substrate-level and oxidative phosphorylation: 4+34=384 + 34 = 38 ATP net per glucose, alongside 6 CO2 released, matching all 6 carbons of the original glucose — the balance sheet figure, valid only under the stated assumptions.

Go deeper · Derivation C · 1 minCalculating the Respiratory Quotient for different respiratory substrates
Derivation CCalculating the Respiratory Quotient for different respiratory substrates

The goal is to show the respiratory quotient is not an arbitrary measured number but falls directly out of the balanced combustion equation of whichever substrate is being oxidised, comparing the ratio of CO2 produced to O2 consumed for a carbohydrate and a fat, and seeing why a hydrogen-rich, oxygen-poor molecule like fat must give an RQ below 1.

Step 1. Write glucose's balanced combustion equation: C6H12O6+6O2→6CO2+6H2O+EnergyC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}. Reading the coefficients, exactly 6 O2 are consumed for exactly 6 CO2 produced.

Step 2. Divide to get the carbohydrate RQ: RQ=6CO2/6O2=1.0RQ = 6CO_2 / 6O_2 = 1.0, because glucose already carries roughly the oxygen proportion needed to fully oxidise it.

Step 3. Write a fat's balanced equation, using tripalmitin: 2(C51H98O6)+145O2→102CO2+98H2O+Energy2(C_{51}H_{98}O_6) + 145O_2 \rightarrow 102CO_2 + 98H_2O + \text{Energy}. Fats carry far less oxygen relative to carbon and hydrogen than carbohydrates, so oxidising all that hydrogen to water demands disproportionately more O2.

Step 4. Divide to get the fat RQ: RQ=102/145≈0.7RQ = 102/145 \approx 0.7, below 1 since O2 consumed exceeds CO2 released. Proteins, intermediate in composition, give an intermediate RQ of about 0.9 by the same logic: the closer a substrate's own oxygen content is to what full oxidation requires, the closer its RQ sits to 1.

Worked examples

Board level - the net ATP gain from fermentation of one glucose molecule

One molecule of glucose is fermented anaerobically by yeast to ethanol and CO2. Using the fact that glycolysis alone nets 2 ATP and 2 NADH + H⁺ per glucose, and that fermentation adds no further ATP-generating step, calculate the net ATP gain from the complete fermentation of one glucose molecule.

Step 1. Identify what glycolysis alone contributes. Glycolysis converts glucose to 2 pyruvic acid, netting 2 ATP (4 produced, 2 invested) and 2 NADH + H⁺, entirely in the cytoplasm, whether or not oxygen is present afterwards.

Step 2. Identify what the fermentation steps add. Pyruvic acid decarboxylase and alcohol dehydrogenase (alcoholic fermentation) or lactate dehydrogenase (lactic acid fermentation) merely consume the NADH + H⁺ from glycolysis to regenerate NAD+; neither step involves substrate-level phosphorylation, so fermentation contributes zero additional ATP.

Step 3. Add the two contributions. Net ATP =2 (glycolysis)+0 (fermentation)=2= 2 \text{ (glycolysis)} + 0 \text{ (fermentation)} = 2 ATP per glucose. Answer: fermentation of one glucose molecule, to ethanol or to lactic acid, nets only 2 ATP, the ATP gain of glycolysis alone, since fermentation's sole role is regenerating NAD+, not making more ATP.

Board level - why naturally fermented beverages cap out near 13 percent alcohol

Yeast cells poison themselves once the surrounding alcohol concentration reaches about 13 percent. Reason out what this implies for the maximum alcohol content of a beverage produced purely by fermentation, and how beverages with a higher alcohol content are actually obtained.

Step 1. Identify the limiting factor. Alcoholic fermentation is carried out by living yeast cells, and the ethanol they produce is toxic to those cells once its concentration rises high enough; near 13 percent alcohol the yeast population dies off faster than it can keep fermenting.

Step 2. Reason to the ceiling this sets. Since fermentation stops the moment the yeast dies, a beverage produced by fermentation alone cannot exceed roughly this concentration, regardless of how much sugar is available to ferment.

Step 3. Explain how stronger beverages are obtained. Spirits stronger than 13 percent, such as whisky or rum, are made from a fermented liquid at or below that limit, then distilled, a physical process that concentrates the more volatile ethanol away from water, or fortified by directly adding extra alcohol. Answer: naturally fermented beverages cannot exceed about 13 percent alcohol, since the fermenting yeast is poisoned beyond that; stronger beverages are obtained afterwards by distillation or fortification, not by fermentation alone.

Competitive level (NEET) - scaling the respiratory balance sheet to 5 moles of glucose

Assuming the standard respiratory balance sheet (net 38 ATP, 6 CO2, 10 NADH + H⁺ and 2 FADH2 per glucose molecule aerobically respired), calculate the total ATP yield, total CO2 evolved, and total NADH + H⁺ and FADH2 generated when 5 moles of glucose are completely oxidised aerobically.

Step 1. Fix the per-glucose values from the balance sheet. Per glucose molecule: net ATP =38= 38; CO2 released =6= 6 (matching all 6 carbons of glucose); NADH + H⁺ generated =10= 10 (2 from glycolysis, 2 from pyruvate oxidation, 6 from two turns of the Krebs cycle); FADH2 generated =2= 2 (one per turn of the Krebs cycle, two turns per glucose).

Step 2. Scale each quantity by the number of moles. For 5 moles of glucose, multiply every per-glucose value by 5: ATP =38×5=190= 38 \times 5 = 190; CO2 =6×5=30= 6 \times 5 = 30 moles; NADH + H⁺ =10×5=50= 10 \times 5 = 50; FADH2 =2×5=10= 2 \times 5 = 10.

Step 3. Sanity-check the ATP total against the NADH and FADH2 route. Oxidative phosphorylation from 50 NADH + H⁺ gives 50×3=15050 \times 3 = 150 ATP; from 10 FADH2 gives 10×2=2010 \times 2 = 20 ATP; substrate-level phosphorylation across glycolysis and the Krebs cycle gives 4×5=204 \times 5 = 20 ATP. Adding these routes: 150+20+20=190150 + 20 + 20 = 190 ATP, matching Step 2 exactly. Answer: 5 moles of glucose aerobically respired yield 190 ATP, 30 moles of CO2, 50 NADH + H⁺ and 10 FADH2. Timing note: about two minutes. The whole problem is pure scaling once the per-glucose balance sheet (38 ATP, 6 CO2, 10 NADH + H⁺, 2 FADH2) is fixed in memory; do not re-derive glycolysis and the Krebs cycle from scratch under exam pressure. The cross-check in Step 3, rebuilding the ATP total from NADH and FADH2 contributions independently, is the fastest way to catch an arithmetic slip, and NEET questions often test exactly this consistency rather than the multiplication itself.

Common traps

  • Equating breathing with respiration. Breathing (gas exchange) is only the physical movement of gases; respiration is the biochemical oxidation of food inside cells that breathing supports. Plants exchange gases without lungs, and every living cell respires whether or not a gas exchange organ is nearby.
  • Assuming combustion and respiration release different total energy. Both release exactly the same total energy from the same quantity of glucose; the difference is only in how it is released, all at once as heat in combustion, in small trapped packets as ATP in respiration.
  • Forgetting glycolysis needs no oxygen and happens in every organism. Only what happens to pyruvate afterwards depends on oxygen availability.
  • Mixing up where each stage occurs. Glycolysis is cytoplasmic; pyruvate oxidation and the Krebs cycle occur in the mitochondrial matrix; the ETS and oxidative phosphorylation sit on the inner mitochondrial membrane.
  • Treating fermentation as a weaker aerobic respiration. It is a completely different fate for pyruvate that never enters the mitochondrion, adds no ATP beyond glycolysis, and exists purely to regenerate NAD+.
  • Assuming RQ is always exactly 1. RQ is 1 only for carbohydrate; it is below 1 for fats and around 0.9 for proteins, with real tissues giving intermediate values.

You should be able to…

  • Explain why plants manage without specialised respiratory organs, citing stomata, lenticels, short diffusion distances and low gas-exchange demand.
  • State the balanced combustion equation for glucose and explain why respiration captures far more of its energy usefully than combustion.
  • Trace glycolysis step by step: where ATP is invested and regained, where NADH + H⁺ forms, and the net yield of 2 ATP, 2 NADH + H⁺ and 2 pyruvic acid per glucose.
  • Distinguish alcoholic from lactic acid fermentation by enzyme and end product, and explain why neither adds net ATP beyond glycolysis.
  • Describe pyruvate's oxidative decarboxylation to acetyl CoA, and locate this step and the Krebs cycle in the mitochondrial matrix.
  • Outline one turn of the Krebs cycle, naming its key intermediates and its per-turn yield of CO2, NADH + H⁺, FADH2 and ATP.
  • Explain the ETS and oxidative phosphorylation, including Complexes I to IV, ATP synthase, chemiosmosis, and O2 as the final electron acceptor.
  • Reconstruct the balance sheet's net gain of 38 ATP per glucose from its components, and state why the figure is only approximate.
  • Explain what "amphibolic pathway" means, with a concrete example of an intermediate used in both breakdown and synthesis.
  • Define the respiratory quotient and calculate its value for carbohydrate, fat and protein from their combustion equations.

Ask the tutor: Want to work through the full ATP balance sheet for a different starting quantity of glucose, or trace exactly where a fatty acid enters the respiratory pathway together?

If plants exchange gases through stomata and lenticels rather than lungs, does that mean they breathe more slowly or less efficiently than animals

Not really, because plants simply do not need the same rate of gas exchange animals do. Roots, stems and leaves respire far more slowly than animal tissue, and there is very little demand to transport gases between different plant organs, since each part manages its own small requirement locally. Add to that the very short diffusion distance from any living plant cell to the nearest air space, thanks to loosely packed parenchyma tissue riddled with interconnected air spaces, and there is simply no pressure to evolve specialised organs. It is not a cruder version of animal breathing, it is a system properly scaled to a much lower and more evenly distributed demand.

Why does fermentation bother reducing pyruvic acid to lactic acid or ethanol at all, if it does not generate any extra ATP

Because glycolysis cannot continue without a fresh supply of NAD+, and under anaerobic conditions there is no electron transport system available to reoxidise the NADH + H⁺ that glycolysis itself produces. Reducing pyruvic acid to lactic acid, or to ethanol via acetaldehyde, consumes that NADH + H⁺ and regenerates NAD+, which glycolysis needs as an electron acceptor at its one NADH-producing step. Without this regeneration, glycolysis would grind to a halt within a few reaction cycles and even the modest 2 ATP net gain would be lost. Fermentation is best understood not as an ATP-generating pathway but as a NAD+-recycling pathway that keeps the only ATP-generating step still available, glycolysis, running.

Why is the NADH produced during glycolysis treated differently from the NADH produced inside the mitochondrion when counting the balance sheet

Glycolysis happens in the cytoplasm, but the electron transport system that turns NADH into ATP sits on the inner mitochondrial membrane, so glycolytic NADH must be shuttled into the mitochondrion before use, unlike the NADH made directly in the matrix by pyruvate oxidation and the Krebs cycle. This chapter's balance sheet simply assumes that shuttled NADH is fully oxidised too, one of its explicitly stated assumptions behind the 38 ATP figure; other textbooks apply a small efficiency penalty to it and arrive at a slightly lower total, which is why you may see numbers other than 38 quoted elsewhere.

Why does fat give a lower respiratory quotient than carbohydrate if both are eventually oxidised completely to CO2 and water

Because a fat molecule starts out containing much less oxygen, relative to its carbon and hydrogen atoms, than a carbohydrate molecule does. Both substrates are indeed oxidised all the way to CO2 and water, but oxidising the abundant hydrogen atoms in a fat into water requires drawing in a disproportionately large amount of O2 from outside, whereas a carbohydrate already carries roughly the right proportion of oxygen internally to balance its own combustion. The tripalmitin equation in this chapter makes this concrete: 145 molecules of O2 are consumed to produce only 102 molecules of CO2, giving an RQ of about 0.7, safely below the carbohydrate value of exactly 1.0.

Is the respiratory pathway really useful for building molecules, or is that just a technicality

It is genuinely useful, not a technicality. Every time a plant cell needs to synthesise a fatty acid, it withdraws acetyl CoA directly from the respiratory pathway rather than manufacturing it from scratch through some separate route; the same is true for several amino acids drawn from Krebs cycle intermediates like alpha-ketoglutaric acid. This means the respiratory pathway is constantly running in both directions depending on the cell's needs at that moment, sometimes stripping fatty acids down to acetyl CoA for energy, sometimes pulling acetyl CoA back out to build new fatty acids. That dual role, supplying both energy through catabolism and raw material through anabolism, is exactly why the chapter insists on calling it amphibolic rather than simply catabolic.

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