Respiration in Plants · Lesson 9 of 9
Chapter Summary and Practice
“Integrated respiratory pathway revision with source exercises, diagrams, ATP and RQ calculations.”
• Integrate gas exchange, glycolysis, fermentation and mitochondrial respiration. • Trace carbon and electron carriers through each pathway. • Compare ATP balances using explicitly stated assumptions. • Interpret amphibolic metabolism and respiratory quotients. • Answer all twelve chapter exercises with appropriate scientific reasoning.
How respiration links carbon, electrons and ATP
Plants manufacture carbohydrates through photosynthesis in illuminated green tissues, but all living tissues need ATP for maintenance, biosynthesis and active transport. Cellular respiration transfers chemical energy stored in organic molecules into ATP and reduced electron carriers through many regulated reactions. Stomata and lenticels enable gas exchange, while roots normally receive oxygen from soil air spaces. Gas diffusion is not itself the same as cellular respiration: it delivers or removes gases used or produced by internal biochemical reactions.
A reliable way to analyse respiratory pathways is to follow three things independently: carbon atoms from glucose into pyruvate, acetyl-CoA and ultimately CO₂; electrons removed from substrates and transferred into NADH or FAD-linked cofactors; and energy conserved as direct ATP/GTP or in a mitochondrial proton gradient. This explains why a single overall combustion equation cannot show the complete cellular mechanism.
A compartment-by-compartment summary
| Pathway | Location | Output per glucose |
|---|---|---|
| Glycolysis | Cytosol | 2 pyruvate, net 2 ATP, 2 NADH |
| Pyruvate oxidation | Mitochondrial matrix | 2 acetyl-CoA, 2 CO₂, 2 NADH |
| Citric acid cycle | Mostly matrix; succinate dehydrogenase membrane-bound | 4 CO₂, 6 NADH, 2 FADH₂, 2 GTP/ATP |
| Respiratory ETS | Inner mitochondrial membrane | Reduced carriers oxidised, O₂ reduced to H₂O, H⁺ gradient |
| Oxidative phosphorylation | Inner membrane; F₁ faces matrix | ATP synthesised using the H⁺ gradient |
Glycolysis: the ten-step EMP route
The ordered intermediates are glucose → glucose-6-phosphate → fructose-6-phosphate → fructose-1,6-bisphosphate → glyceraldehyde-3-phosphate (GAP) plus dihydroxyacetone phosphate → two GAP → two 1,3-bisphosphoglycerate → two 3-phosphoglycerate → two 2-phosphoglycerate → two phosphoenolpyruvate → two pyruvate. ATP is invested at the hexokinase and phosphofructokinase steps, and made at the phosphoglycerate kinase and pyruvate kinase steps. NADH forms when GAP is oxidised. Two ATP invested and four formed give net two ATP and two NADH; all six carbons remain in two pyruvates without glycolytic CO₂ release.
Fermentation and the need to regenerate NAD⁺
When respiratory electron transport cannot reoxidise NADH adequately, fermentative reactions recycle NADH to NAD⁺ and allow glycolysis to continue. Lactate dehydrogenase converts pyruvate to lactate without releasing CO₂ during that conversion. In yeast ethanol fermentation, pyruvate decarboxylase produces acetaldehyde and CO₂, followed by alcohol dehydrogenase producing ethanol and regenerating NAD⁺. Classical fermentation adds no ATP after glycolysis, leaving a net of two ATP per glucose. Some anaerobic microbes use alternative electron acceptors in true anaerobic respiration, which is distinct from fermentation.
Pyruvate oxidation and the TCA cycle
Pyruvate dehydrogenase converts pyruvate (3C) into acetyl-CoA (2C), NADH and CO₂. Two pyruvates give 2 NADH and 2 CO₂ per glucose. Acetyl-CoA combines with oxaloacetate (4C) to form citrate (6C). Through isocitrate, α-ketoglutarate (5C), succinyl-CoA (4C), succinate, fumarate and malate, the cycle regenerates oxaloacetate. Each turn makes 3 NADH, 1 FADH₂-equivalent, 1 GTP/ATP and 2 CO₂; two turns per glucose double these numbers.
Two successive oxidative decarboxylations occur when isocitrate becomes α-ketoglutarate and when α-ketoglutarate becomes succinyl-CoA. Succinyl-CoA conversion produces GTP or ATP through substrate-level phosphorylation; succinate dehydrogenase transfers electrons through enzyme-bound FAD; malate dehydrogenase restores oxaloacetate with NADH formation. Although O₂ is not a reactant in every TCA step, oxygen-dependent electron transport maintains the oxidised carrier supply required for sustained cycling.
Electron transport and chemiosmosis
NADH transfers electrons to complex I; succinate-derived FAD-linked electrons reach complex II. Ubiquinone carries electrons to complex III, cytochrome c to complex IV and molecular oxygen accepts terminal electrons to form water. Complexes I, III and IV pump protons from matrix to intermembrane space, but complex II does not. F₀ of ATP synthase permits downhill proton return to matrix; the matrix-facing F₁ catalytic sector forms ATP from ADP and inorganic phosphate.
The source's ATP balance and its assumptions
| Energy carrier or pathway | Historical school calculation | Modern simplified model |
|---|---|---|
| 10 NADH | 10 × 3 = 30 ATP | 10 × 2.5 ≈ 25 ATP |
| 2 FADH₂ | 2 × 2 = 4 ATP | 2 × 1.5 ≈ 3 ATP |
| Direct ATP/GTP equivalents | 4 | 4 |
| Ideal result | 38 ATP | ~32 ATP, potentially ~30 with alternative cytosolic shuttles |
The source's theoretical 38 ATP requires orderly sequential pathways, effective transfer of glycolytic NADH reducing equivalents into mitochondrial phosphorylation, no diversion of intermediates for synthesis and only glucose as a substrate. Real pathways operate simultaneously and exchange compounds, so the balance is an idealisation. Modern estimates frequently give about 30–32 ATP for many eukaryotic models, and plant values can vary further according to respiratory coupling and alternative pathways. Never mix the 3/2 and 2.5/1.5 carrier conventions in one calculation.
Why the pathway is amphibolic
Central respiratory metabolites are connected with both catabolism and anabolism. Hydrolysed carbohydrate carbon enters glucose and glycolytic intermediates; glycerol can enter the triose-phosphate pathway, fatty acids can undergo β-oxidation to acetyl-CoA, and amino-acid carbon skeletons may become pyruvate, acetyl-CoA or TCA intermediates. Conversely, acetyl-CoA and other intermediates may be withdrawn to build lipids, amino acids and other substances. Replenishing depleted TCA intermediates is important for continued oxidation.
Respiratory quotient
Respiratory quotient (RQ) is the volume of CO₂ evolved divided by volume of O₂ consumed under comparable conditions. Complete glucose oxidation has RQ = 6/6 = 1. The source's tripalmitin equation is 2 C₅₁H₉₈O₆ + 145 O₂ → 102 CO₂ + 98 H₂O, giving RQ = 102/145 ≈ 0.7. Protein has a source-approximate RQ near 0.9. Malic-acid oxidation provides a supplementary case with RQ 4/3 ≈ 1.33. Mixed fuels, photosynthesis and oxygen limitation complicate measured ratios; with zero O₂ uptake ordinary aerobic RQ is not defined as a finite quotient.
Quick Quiz
Quiz
What is the most common respiratory substrate?
Where does glycolysis occur?
How many pyruvates per glucose?
How many net ATP does glycolysis yield?
Which reaction reduces NAD⁺ during glycolysis?
What is the main purpose of classical fermentation?
Which pathway releases CO₂ while forming ethanol?
Where is pyruvate converted to acetyl-CoA in a eukaryote?
Which compound is regenerated by TCA?
How many NADH form per TCA turn?
Which TCA-related enzyme is complex II?
Which respiratory complexes pump protons?
Which molecule is final electron acceptor?
Where is the ATP synthase F₁ head?
What historical ATP estimate is in the source?
Which modern factors are often used for NADH and FADH₂?
Why is the respiratory pathway amphibolic?
Which is the usual route for fatty acid carbon entering TCA?
What is the RQ definition?
What is RQ for complete glucose oxidation?
What is RQ for source tripalmitin?
Why can sunlit leaf RQ measurements be misleading?
Worked responses to all source exercises
Practice Problems
- 1. Differentiate (a) respiration and combustion, (b) glycolysis and Krebs cycle, (c) aerobic respiration and fermentation.
- 2. What are respiratory substrates? Give the common one.
- 3. Give the schematic representation of glycolysis.
- 4. What are the main steps in aerobic respiration and their sites?
- 5. Give a schematic overview of Krebs cycle.
- 6. Explain the electron transport system.
- 7. Distinguish (a) aerobic and anaerobic respiration, (b) glycolysis and fermentation, (c) glycolysis and citric acid cycle.
- 8. List assumptions for net ATP calculation.
- 9. Explain why respiration is amphibolic.
- 10. Define RQ and state its value for fats.
- 11. What is oxidative phosphorylation?
- 12. Why is stepwise energy release significant?
- Compute historical versus modern ATP per glucose.
- Why does fermentation yield only two ATP per glucose?
- A root is flooded and oxygen becomes limiting. Predict consequences.
- Why is complex II linked to less ATP synthesis?
- Compare substrate-level and oxidative phosphorylation.
- Calculate RQ when CO₂ output is 28 mL and O₂ input 40 mL.
- Why does leaf net gas exchange during daytime not directly equal respiratory gas exchange?
- How can TCA intermediates support anabolism without ending the cycle permanently?
(a) Cellular respiration uses controlled enzyme reactions and conserves some energy in carriers and ATP, whereas combustion is rapid oxidation releasing much energy directly as heat. (b) Glycolysis is cytosolic, makes two pyruvates and net 2 ATP + 2 NADH without CO₂ release. TCA is mainly mitochondrial, regenerates oxaloacetate, releases CO₂ and yields 3 NADH, 1 FADH₂ and 1 GTP per acetyl-CoA. (c) Aerobic respiration uses O₂ as terminal electron acceptor and extracts much more ATP, while fermentation regenerates NAD⁺ with organic acceptors and yields only net 2 ATP per glucose in the standard routes.
Final integration
The key is to combine compartment, stoichiometry and mechanism: glycolysis transfers glucose carbon to pyruvate; fermentation recycles NAD⁺ under appropriate conditions; pyruvate oxidation and TCA release CO₂ and generate carriers; electron transport uses O₂ and pumps protons; ATP synthase generates ATP from the gradient. Alternative respiratory substrates and withdrawal of intermediates for building molecules explain amphibolic metabolism. RQ measures substrate-dependent gas stoichiometry, but real tissues often oxidise mixtures.
Key Takeaways
• Plants respire continuously in living tissues; stomata, lenticels and root surfaces support gas exchange. • Glycolysis converts glucose to two pyruvates, producing net 2 ATP and 2 NADH. • Fermentation regenerates NAD⁺ and retains glycolysis as an ATP source when respiratory oxidation is limited. • Pyruvate oxidation yields acetyl-CoA, NADH and CO₂ before the TCA cycle. • Each TCA turn yields 3 NADH, 1 FADH₂, 1 GTP/ATP and 2 CO₂ while regenerating oxaloacetate. • Complexes I, III and IV pump H⁺; complex II transfers electrons without proton pumping. • ATP synthase uses proton return from intermembrane space to matrix to produce ATP. • Historical idealised 38 ATP and modern approximate 30–32 ATP calculations rely on different coupling assumptions. • Central respiratory metabolism is amphibolic because breakdown and synthesis share intermediates. • RQ is CO₂ evolved / O₂ consumed; glucose RQ=1, fat around 0.7 and source protein approximately 0.9. • Source exercises are solved by tracing carbon, electron carriers, compartments and biochemical assumptions.
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Respiratory Quotient and Substrate Identification
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