LAISHA Explains
Plant physiology and botany

Respiration in plants: glycolysis, Krebs cycle and ETC

How plants release energy from sugars, the stages of aerobic respiration and their ATP yield, fermentation, respiratory quotient sums and crop respiration.

Exam essential5 min readUpdated
On this page
  1. The overall reaction
  2. The stages
  3. How much ATP?
  4. Special features of plant respiration
  5. Without oxygen: fermentation
  6. Respiratory quotient
  7. Factors affecting respiration
  8. Key points
  9. What the exams ask
  10. Questions people ask
  11. Sources
  12. Related on Laisha
  13. Discussion

Photosynthesis captures energy from sunlight and stores it in sugars. Respiration releases that energy, in a form cells can use, to build new tissue, take up nutrients, move materials and keep cells alive. Every living plant cell respires, day and night, and crops typically use somewhere around 30 to 60 per cent of the carbon they fix in photosynthesis for respiration.

Respiration links basic biochemistry to agriculture: it governs how fast stored produce deteriorates, why waterlogged roots die, and how much of a crop's photosynthesis ends up as yield. Its stages, energy yield and respiratory quotient are standard exam material.

The overall reaction

For glucose, aerobic respiration can be summarised as:

Aerobic respiration
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (about 2,870 kJ per mole of glucose)

Part of this energy is captured as ATP; the rest is released as heat. Unlike burning, respiration releases energy in small, controlled steps.

The stages

Aerobic respiration of one glucose molecule: glycolysis in the cytosol gives two pyruvate, two ATP and two NADH; in the mitochondrion pyruvate oxidation and the Krebs cycle release six carbon dioxide and produce more NADH, FADH2 and two ATP; the electron transport chain passes electrons to oxygen and drives ATP synthase, for a total of about 30 to 32 ATP. Without oxygen, pyruvate is fermented to ethanol and carbon dioxide or to lactate, giving only two ATP Cytosol Mitochondrion Glucose6 carbons Glycolysis→ 2 pyruvate2 ATP, 2 NADH Pyruvate oxidation→ 2 acetyl-CoA2 NADH, 2 CO₂ Krebs (citric acid) cycle2 ATP, 6 NADH,2 FADH₂, 4 CO₂ Electron transport chainNADH and FADH₂ → O₂ → H₂OATP synthase: about 26–28 ATP Total from one glucose with oxygen: about 30–32 ATP (older textbooks: 36–38) Without oxygen (fermentation): pyruvate → ethanol + CO₂, or lactate; only the 2 ATP of glycolysis.
The stages of aerobic respiration and where they happen. Most ATP comes from the electron transport chain, which needs oxygen.Diagram: Laisha, original.

1. Glycolysis (cytosol)

Glucose (6 carbons) is split into two molecules of pyruvate (3 carbons each). The pathway was worked out mainly by G. Embden, O. Meyerhof and J. K. Parnas, and is often called the EMP pathway. Net yield: 2 ATP and 2 NADH per glucose. Glycolysis needs no oxygen. In plants, sucrose and starch rather than free glucose are the usual starting points.

2. Pyruvate oxidation (mitochondrial matrix)

Each pyruvate enters the mitochondrion and is converted to acetyl-CoA by the pyruvate dehydrogenase complex, releasing one CO2 and producing one NADH. Per glucose: 2 CO2 and 2 NADH.

3. Krebs (citric acid, TCA) cycle (matrix)

Described by Hans Krebs in 1937. Acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons), which is oxidised back to oxaloacetate, releasing 2 CO2 per turn. Per glucose (two turns): 2 ATP, 6 NADH, 2 FADH2 and 4 CO2. The cycle also supplies carbon skeletons for amino acids and other compounds.

4. Electron transport chain and oxidative phosphorylation (inner mitochondrial membrane)

NADH and FADH2 pass electrons along a chain of complexes to oxygen, the final electron acceptor, forming water. The energy pumps protons across the inner membrane, and the protons flow back through ATP synthase, making ATP. This is Peter Mitchell's chemiosmotic theory (1961, Nobel Prize 1978).

How much ATP?

Older textbooks give 36 or 38 ATP per glucose, assuming whole-number yields of 3 ATP per NADH and 2 per FADH2. Measurements of the proton costs show about 2.5 ATP per NADH and 1.5 per FADH2, giving a modern estimate of about 30 to 32 ATP per glucose.

StageDirect ATPNADHFADH2
Glycolysis220
Pyruvate oxidation020
Krebs cycle262
Total4102

ATP = 4 + (10 × 2.5) + (2 × 1.5) = 4 + 25 + 3 = 32, or about 30 when the NADH from glycolysis enters the mitochondrion by a less efficient route.

Special features of plant respiration

  • Alternative oxidase: plant mitochondria have a cyanide-resistant pathway that passes electrons directly to oxygen without pumping protons, releasing energy as heat. It helps balance metabolism under stress, and in some flowers, such as Arum lilies and skunk cabbage, it heats the flower to spread scent.
  • Pentose phosphate pathway: an alternative route for glucose that produces NADPH and five-carbon sugars for nucleic acids and other compounds.
  • Growth and maintenance respiration: part of respiration builds new tissue (growth respiration), part keeps existing tissue alive by repairing proteins and maintaining gradients (maintenance respiration). Maintenance respiration rises with temperature, roughly doubling for each 10 °C (a Q10 of about 2).

Without oxygen: fermentation

When oxygen is lacking, as in waterlogged roots, flooded seeds or sealed stores, cells continue glycolysis and regenerate NAD+ by fermentation:

  • Alcoholic fermentation: pyruvate → ethanol + CO2, the main pathway in plant tissues.
  • Lactic fermentation: pyruvate → lactate, usually briefly at the onset of oxygen shortage.

Fermentation yields only the 2 ATP of glycolysis, so tissues soon run short of energy, and ethanol and other products can damage them. This is why waterlogging injures most crops, and why rice, with air channels (aerenchyma) carrying oxygen to its roots, tolerates flooding.

Respiratory quotient

The respiratory quotient (RQ) is the ratio of CO2 released to O2 consumed. It indicates what is being respired:

Worked example

Calculating respiratory quotients

Carbohydrate (glucose): C6H12O6 + 6 O2 → 6 CO2 + 6 H2O. RQ = 6 ÷ 6 = 1.0.

Fat (tripalmitin): 2 C51H98O6 + 145 O2 → 102 CO2 + 98 H2O. RQ = 102 ÷ 145 ≈ 0.70. Fats are poor in oxygen, so they need more O2 per CO2; germinating oilseeds such as groundnut and sunflower show low RQs.

Organic acid (malic acid): C4H6O5 + 3 O2 → 4 CO2 + 3 H2O. RQ = 4 ÷ 3 ≈ 1.33. Acids are oxygen-rich, so RQ exceeds 1.

Proteins give about 0.8 to 0.9. Anaerobic respiration releases CO2 with no O2 uptake, so RQ is very high or infinite. CAM plants at night take up CO2 into acids, so their RQ can be near zero.

Factors affecting respiration

  • Temperature: the strongest factor; rate rises with temperature up to about 30 to 40 °C, then falls as enzymes are damaged.
  • Oxygen: below about 2 to 5 per cent, aerobic respiration slows and fermentation begins.
  • CO2: high CO2 slows respiration, the basis of controlled-atmosphere storage.
  • Substrate supply: sugar-rich tissues respire faster.
  • Tissue age and activity: young, growing tissues and ripening climacteric fruits respire fastest.
  • Wounding and infection: raise respiration.

These factors matter after harvest. Produce keeps longer when respiration is slowed by cooling, reduced O2 and raised CO2, which is why cold chains and controlled-atmosphere stores extend the life of fruit and vegetables.

Key points

  • Respiration releases energy from sugars as ATP: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O.
  • Glycolysis (cytosol) → pyruvate oxidation and Krebs cycle (matrix) → electron transport chain and ATP synthase (inner membrane).
  • Modern yield: about 30–32 ATP per glucose (older value 36–38).
  • Without oxygen, fermentation gives ethanol and CO2 and only 2 ATP.
  • RQ: carbohydrate 1.0, fat about 0.7, protein about 0.8–0.9, organic acids above 1.
  • Temperature, O2 and CO2 control respiration rate, the basis of storage of produce.

What the exams ask

  • School-leaving and entrance papers: aerobic and anaerobic respiration, the stages and where they occur, and RQ values.
  • Degree and research-fellowship entrances: ATP accounting, chemiosmosis, alternative oxidase, pentose phosphate pathway, RQ calculations, growth and maintenance respiration, and Q10.
  • Professional examinations: respiration in post-harvest storage, waterlogging injury, and respiration in crop carbon budgets.

Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.

Questions people ask

Do plants respire at night only?

No. They respire all the time; at night it is simply easier to measure, because photosynthesis stops and the plant releases CO2.

Why do stored potatoes and onions lose weight?

They keep respiring, burning stored sugars and starch into CO2 and water, and they lose water. Cool, well-ventilated storage slows both.

Why does respiration matter for yield?

Carbon respired is carbon not stored as yield. Crops in hot climates, especially with warm nights, respire more, which is one way heat reduces yield.

Sources

  1. Taiz, L., Zeiger, E., Møller, I. M. and Murphy, A. Plant Physiology and Development, 6th edition. Sinauer Associates, 2015. Chapter 12.
  2. Nelson, D. L. and Cox, M. M. Lehninger Principles of Biochemistry, 8th edition. W. H. Freeman, 2021.
  3. Mitchell, P. "Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism." Nature 191, 144–148 (1961).
  4. Amthor, J. S. "The McCree–de Wit–Penning de Vries–Thornley respiration paradigms: 30 years later." Annals of Botany 86, 1–20 (2000).
  5. Kader, A. A. (ed.) Postharvest Technology of Horticultural Crops, 3rd edition. University of California ANR Publication 3311, 2002.

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Discussion

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