Respiration in Plants · TOPIC LESSON
Track carbon, electrons, protons and ATP through plant-cell respiration without mixing their accounts.
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BEFORE YOU START
Original educational lesson. Work through the examples before revealing the practice answers. Check the current official syllabus separately; historical question frequency does not establish eligibility or predict future questions.
CONNECT THE CONCEPT TO THE EXAM
These selected examples are separate from the original practice below. Each identifies its exam, year and wording. They do not represent every paper or every selected year.
No source-verified exam questions have been published for this lesson yet. The worked examples and practice below are original teaching material, not past-exam questions.
01 / UNDERSTAND & APPLY
Cellular respiration transfers part of the free energy released by oxidation of organic substrates into usable forms, particularly ATP. It is not identical to breathing or external gas exchange. Plants respire in roots, stems, leaves and other living tissues during both day and night. A photosynthesising leaf may take up carbon dioxide overall in daylight because photosynthetic uptake exceeds respiratory release; the net gas measurement does not show that respiration stopped.
For complete aerobic oxidation of one glucose, six carbons ultimately leave as six carbon dioxide molecules and oxygen serves as the final electron acceptor. Water is produced at the terminal respiratory reaction, although water also participates in other pathway steps. The net equation is an overall balance, not a description of one explosive reaction in the cell. Many enzyme-catalysed transfers release energy in manageable stages.
Keep four accounts separate. Carbon atoms move through organic intermediates and carbon dioxide. Electrons move to reduced carriers and finally an acceptor. Protons are redistributed across a membrane and contribute to an electrochemical gradient. ATP is made by particular reactions. A carbon count cannot by itself determine ATP yield, and movement of electrons down a chain is not the same as movement of protons through ATP synthase.
02 / UNDERSTAND & APPLY
In a typical aerobic eukaryotic cell, glycolysis occurs in the cytosol. Pyruvate enters mitochondria and is converted to acetyl-CoA in the matrix. Most citric acid cycle reactions are matrix reactions, while succinate dehydrogenase is embedded in the inner mitochondrial membrane and also functions as respiratory complex II. The electron-transport chain and ATP synthase are associated with this inner membrane.
The inner membrane separates the matrix from the intermembrane space. Its selective permeability permits a proton electrochemical gradient to persist. Cristae increase membrane area available for respiratory machinery. Simply having a large outer membrane is insufficient: the energy-conserving barrier and the correctly oriented proteins are essential. Prokaryotic respiratory chains use their plasma membrane rather than mitochondria, so respiration does not universally require that organelle.
Compartmentation also affects accounting. Cytosolic NADH does not simply diffuse across the intact inner mitochondrial membrane. Shuttle systems transfer its reducing equivalents into mitochondrial metabolism, and different routes can lead to different ATP returns. Transport of pyruvate, phosphate, ADP and ATP has energetic consequences. A theoretical total that ignores transport must be identified as a convention, not an invariant measured yield.
03 / UNDERSTAND & APPLY
Glycolysis converts a six-carbon glucose to two three-carbon pyruvate molecules through ten reactions. Two ATP are invested before the main cleavage. Glucose is phosphorylated, rearranged and phosphorylated again to form fructose-1,6-bisphosphate. The six-carbon compound is split into two triose phosphates, and their interconversion means two glyceraldehyde-3-phosphate molecules proceed through the payoff sequence per glucose.
Each triose is oxidised while NAD+ is reduced to NADH, and inorganic phosphate is incorporated into an intermediate. Two subsequent phosphate-transfer steps each produce one ATP per triose. Because the payoff sequence happens twice, four ATP are produced gross. Subtracting the two invested gives a net gain of two ATP per glucose, alongside two NADH and two pyruvate. There is no carbon dioxide release in ordinary glycolysis.
The two ATP-producing donor steps are 1,3-bisphosphoglycerate to 3-phosphoglycerate, catalysed by phosphoglycerate kinase, and phosphoenolpyruvate (PEP) to pyruvate, catalysed by pyruvate kinase. Each transfers phosphate to ADP. The preceding glyceraldehyde-3-phosphate dehydrogenase step is different: glyceraldehyde-3-phosphate is oxidised, NAD+ is reduced, and free inorganic phosphate is added to form 1,3-bisphosphoglycerate. That oxidation/Pi-addition step does not itself produce ATP or require a new ATP phosphate donation.
The direct transfer of phosphate from a metabolic intermediate to ADP is substrate-level phosphorylation. It does not require an electron-transport chain or a proton gradient. Glycolysis itself does not consume oxygen, but continued flux requires NAD+ to be regenerated from NADH. Therefore a pathway can be independent of direct oxygen use yet become indirectly limited when the cell loses its usual means of oxidising reduced carriers.
High ATP availability can restrain glycolytic flux, while greater demand and changes in energy-charge signals can favour it. Regulation is distributed and depends on organism, tissue and conditions; do not infer that one enzyme alone determines every rate. For exam reasoning, the decisive bookkeeping is two ATP invested, four produced gross, two NADH made and six carbons retained in two pyruvates.
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04 / UNDERSTAND & APPLY
Pyruvate oxidation converts a three-carbon pyruvate into a two-carbon acetyl group attached to coenzyme A. One carbon leaves as carbon dioxide and one NAD+ is reduced to NADH per pyruvate. Coenzyme A is an acyl-group carrier, not a second name for the electron carrier NAD+. This oxidative decarboxylation is catalysed by the pyruvate dehydrogenase complex.
One glucose supplies two pyruvates, so the link reactions together produce two acetyl-CoA, two carbon dioxide and two NADH. They do not directly generate ATP by substrate-level phosphorylation. If a question asks for products before entry into the citric acid cycle, do not include the cycle outputs prematurely.
Acetyl-CoA is a metabolic junction. Fatty acid oxidation also supplies acetyl units, while carbon skeletons from amino acids can enter at several points after appropriate processing. The glucose-specific totals are therefore not universal totals for every respiratory substrate. State the starting molecule before calculating products.
05 / UNDERSTAND & APPLY
The citric acid cycle, also called the Krebs or tricarboxylic acid cycle, begins when a two-carbon acetyl group combines with four-carbon oxaloacetate to form six-carbon citrate. Rearrangements and two oxidative decarboxylations lead through a five-carbon intermediate to four-carbon compounds. Subsequent reactions regenerate oxaloacetate, which can accept another acetyl group. The acceptor is regenerated rather than consumed as a net product of each turn.
The conventional major sequence is citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate and oxaloacetate. Per acetyl-CoA, three NADH are formed, one FAD-linked reduction occurs at succinate dehydrogenase, two carbon dioxide are released and one ATP or GTP equivalent is made by substrate-level phosphorylation. Depending on the enzyme system, the nucleotide product can differ; count one ATP equivalent for this step under the stated convention.
Two turns are needed for the two acetyl groups derived from one glucose. Thus the cycle contributes six NADH, two FADH2 equivalents, two substrate-level ATP equivalents and four carbon dioxide per glucose. Add the link reaction separately when a question asks for complete oxidation of both pyruvates. The first turn does not simply release the two incoming acetyl carbons as its two carbon dioxide molecules; carbon identities rearrange and are lost over subsequent turns.
The cycle does not directly consume molecular oxygen at its individual decarboxylation steps, but under usual aerobic conditions it depends on respiratory reoxidation of NADH and reduced flavins. Without adequate regeneration of oxidised carriers, flux falls. Intermediates also support biosynthesis, so replenishing reactions may be needed when intermediates are withdrawn. A pathway that contributes to both breakdown and synthesis is described as amphibolic.
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06 / UNDERSTAND & APPLY
NAD+ accepts a hydride equivalent, carrying two electrons and one proton as NADH; another proton may be released to the solution depending on the balanced reaction. NADH is therefore a reduced carrier, not an ATP molecule. Reoxidation returns NAD+ for further substrate oxidation. The NAD pool can cycle repeatedly, so two NADH formed per glucose does not mean two permanently consumed NAD molecules.
Flavins such as FAD are often enzyme-bound. The succinate dehydrogenase flavin accepts reducing equivalents during succinate oxidation and transfers electrons onward through the enzyme to the ubiquinone pool. Textbook bookkeeping calls this one FADH2 per cycle, but a free FADH2 molecule is not released to swim between complexes. This distinction matters when comparing a carrier balance with a structural diagram.
Ubiquinone, also called coenzyme Q, is a lipid-soluble mobile carrier in the inner membrane. Its reduced form ubiquinol can carry two electrons and protons. Cytochrome c is a small mobile protein on the intermembrane-space side and transfers one electron at a time between complexes III and IV. Iron-sulfur centres and cytochromes within complexes relay electrons through changes in oxidation state. The proteins remain in the system; their metal centres cycle rather than being consumed.
NADPH is central to many reductive biosynthetic and photosynthetic processes, while NADH has the principal bookkeeping role in the respiratory scheme here. Similar names do not justify replacing one with the other in a pathway balance. Determine which reaction and enzyme system the question specifies.
07 / UNDERSTAND & APPLY
In the standard mitochondrial pathway, electrons from matrix NADH enter complex I and pass to ubiquinone. Electrons from succinate enter through complex II and also reach ubiquinone. From the reduced quinone pool they pass through complex III, cytochrome c and complex IV, where oxygen is reduced to water. Oxygen is not converted to carbon dioxide at complex IV; respiratory carbon dioxide arose from substrate decarboxylations earlier.
Complexes I, III and IV contribute to proton translocation from the matrix towards the intermembrane space. Complex II does not pump protons in the standard scheme. Electrons entering at complex II therefore bypass one energy-conserving step, which helps explain the lower ATP return associated with FAD-linked entry compared with matrix NADH. The electron route and the proton route must be drawn separately.
The resulting proton-motive force combines a concentration difference and an electrical potential difference. Protons can return through ATP synthase, whose membrane component provides a coupled route and whose catalytic head forms ATP from ADP and inorganic phosphate on the matrix side. The enzyme couples favourable proton movement to ATP synthesis. It does not make ATP merely because oxygen touches it, nor does it pump electrons through the membrane.
Oxidative phosphorylation names the coupling between substrate oxidation through respiratory electron transfer and phosphorylation of ADP. The chain establishes the gradient; ATP synthase uses it. An intact low-leak membrane, appropriate electron donors and acceptor, ADP and phosphate are all needed for sustained coupled ATP production. A chain without a retained gradient may still consume oxygen while making much less ATP.
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08 / UNDERSTAND & APPLY
When oxygen-supported respiratory oxidation cannot meet demand, fermentation can regenerate NAD+ in the cytosol. In alcoholic fermentation, pyruvate first loses carbon dioxide to form acetaldehyde, which is reduced to ethanol by NADH. In lactate fermentation, pyruvate is reduced directly to lactate without that decarboxylation. The fermentation reactions themselves provide no additional ATP beyond the net two from glycolysis in this standard glucose pathway.
Fermentation is not synonymous with all anaerobic respiration. Anaerobic respiration in suitable microorganisms uses an electron-transport chain with a terminal acceptor other than oxygen. In fermentation, an organic metabolite derived from the substrate accepts the electrons without the same respiratory-chain mechanism. For plant tissues under oxygen shortage, do not infer that a large aerobic ATP yield persists just because glucose is still consumed.
Plants can possess alternative respiratory routes, including alternative oxidase. These routes change how electron flow is coupled to proton pumping and can reduce ATP conservation per electron pair. Their existence is a reason to identify the standard cytochrome pathway as a model rather than assert that every plant mitochondrion has exactly the same ATP yield in every condition. The detailed alternative-pathway biochemistry is beyond the present accounting model.
Respiratory quotient, RQ, is carbon dioxide released divided by oxygen consumed, using equal molar or volume bases under comparable conditions. Complete carbohydrate oxidation gives RQ near one; fats generally give a lower ratio because more oxygen is required relative to carbon dioxide output. An RQ calculation needs both rates and a defined process. If no oxygen is consumed, dividing by zero does not yield an ordinary finite aerobic RQ.
09 / UNDERSTAND & APPLY
Complete oxidation of one glucose through the standard pathway produces ten NADH in total: two glycolytic, two from pyruvate oxidation and six from the cycle. There are two FAD-linked reduction equivalents, two net glycolytic ATP and two cycle ATP equivalents. The direct substrate-level total is therefore four. The remaining ATP depends on coupling and transport rather than being attached automatically to every carrier symbol.
A modern illustrative accounting uses approximately 2.5 ATP per matrix NADH and 1.5 per FAD-linked electron pair. Treating all ten NADH as equivalent gives 10 × 2.5 + 2 × 1.5 + 4 = 32 ATP, whereas a lower-return route for the two cytosolic NADH can give about 30. Older textbook idealisations use 3 and 2 and produce a theoretical 38. These conventions must not be mixed, and none is an unconditional measured plant-cell yield.
An electron-transfer inhibitor blocks a specified chain step, often leaving upstream carriers more reduced and limiting proton pumping and oxygen consumption. An uncoupler increases proton leak, weakening ATP synthesis without necessarily stopping electron transfer; oxidation can continue or accelerate if substrates and oxygen are available. An ATP-synthase inhibitor blocks the main proton-return route, initially increasing back-pressure on further pumping. These predictions assume an otherwise intact, initially functioning system and are not treatment recommendations.
10 / UNDERSTAND & APPLY
Original cases deliberately separate carbon, carrier and ATP statements.
ORIGINAL WORKED EXAMPLE
Three glucose molecules complete glycolysis. Find gross ATP, ATP invested, net ATP, pyruvate and NADH.
Answer 12 gross ATP, 6 invested, 6 net ATP, 6 pyruvate and 6 NADH.
ORIGINAL WORKED EXAMPLE
Two pyruvates are converted to acetyl-CoA before either acetyl group enters the cycle. What has been produced?
Answer 2 acetyl-CoA, 2 CO2 and 2 NADH; no direct ATP from the link reaction.
ORIGINAL WORKED EXAMPLE
A question reports six NADH, two FAD-linked reductions and two ATP equivalents from the cycle. How many turns occurred?
Answer Two turns, releasing four CO2 in the cycle.
ORIGINAL WORKED EXAMPLE
Two electron pairs enter via complex I and two enter via complex II. Does each pair drive the same proton-pumping route?
Answer No. Complex-II entry bypasses one pumping step and has a lower illustrative ATP return.
ORIGINAL WORKED EXAMPLE
Use 2.5 ATP per NADH, 1.5 per FAD-linked pair and four direct ATP equivalents, assuming ten NADH have matrix-equivalent entry. Calculate the total.
Answer 32 ATP equivalents under this model, not a universal cellular yield.
ORIGINAL WORKED EXAMPLE
An oxygen-limited tissue still has glucose but its NAD+ pool is becoming reduced. What immediate role can alcoholic fermentation serve?
Answer Carrier regeneration, allowing net two ATP per fermented glucose through glycolysis; fermentation does not add a large oxidative yield.
ORIGINAL WORKED EXAMPLE
A preparation consumes oxygen but ATP production falls after membrane proton permeability increases. Is complete chain blockage the best explanation?
Answer Uncoupling fits better than complete electron-chain blockade under the stated observations.
ORIGINAL WORKED EXAMPLE
A tissue releases 18 micromoles CO2 while consuming 24 micromoles O2 over the same interval. Calculate RQ and assess the inference.
Answer RQ = 0.75; substrate identity is not uniquely established.
ORIGINAL WORKED EXAMPLE
A three-carbon intermediate is oxidised while NAD+ becomes NADH and free Pi is added. The resulting compound then transfers phosphate to ADP. Which event makes ATP?
Answer The phosphate-transfer event makes ATP; the preceding oxidation/Pi-addition event makes NADH, not ATP.
11 / UNDERSTAND & APPLY
State the pathway stage before using a memorised yield.
ORIGINAL PRACTICE · 1
No.
Photosynthetic uptake can exceed simultaneous respiratory release.
ORIGINAL PRACTICE · 2
Zero.
The six carbons remain in two three-carbon pyruvates.
ORIGINAL PRACTICE · 3
Two ATP net.
Gross and net are different accounts.
ORIGINAL PRACTICE · 4
Failure to regenerate NAD+ can limit its oxidation step.
An indirect dependency is different from direct oxygen consumption.
ORIGINAL PRACTICE · 5
Succinate dehydrogenase.
It also functions as respiratory complex II.
ORIGINAL PRACTICE · 6
Oxaloacetate.
Regeneration permits another acetyl group to enter; it is not net consumption of the acceptor.
ORIGINAL PRACTICE · 7
6 NADH, 2 FAD-linked reduction equivalents, 2 ATP/GTP equivalents and 4 CO2.
Double the per-turn ledger and do not add pyruvate oxidation again.
ORIGINAL PRACTICE · 8
No.
The enzyme-bound flavin transfers electrons through complex II to ubiquinone; textbook FADH2 counts are reducing-equivalent bookkeeping.
ORIGINAL PRACTICE · 9
I, III and IV; cytochrome c joins III to IV.
Complex II is an entry route without proton pumping.
ORIGINAL PRACTICE · 10
From the intermembrane-space side into the matrix.
This return uses the gradient built by outward pumping.
ORIGINAL PRACTICE · 11
No; it is reduced to water.
CO2 is released by decarboxylation of carbon-containing substrates.
ORIGINAL PRACTICE · 12
Eight ATP.
Each glucose yields two net ATP through glycolysis, with fermentation recycling NAD+.
ORIGINAL PRACTICE · 13
They may use different carrier-to-ATP conventions and transport assumptions.
Using older 3/2 and modern approximate 2.5/1.5 values in one sum is inconsistent.
ORIGINAL PRACTICE · 14
It stays the same.
The ratio has both numerator and denominator multiplied by two.
12 / UNDERSTAND & APPLY
Trace glucose carbon through pyruvate, acetyl-CoA and carbon dioxide. In parallel, track oxidised versus reduced carriers, then locate proton pumping and return. Only then add direct and oxidative ATP using one explicit convention. A correct answer explains why a count or prediction follows; it does not merely repeat a pathway name.
KEEP THE BIG IDEAS
READ WITH CONTEXT
The teaching notes, worked explanations and original practice are written for this lesson. Separately labeled selected exam questions identify their exam, year, source and whether wording is paraphrased or an exact excerpt. Educational references do not certify current exam coverage.
Factual cross-check only; explanations, cases and figures are original.
Factual cross-check only; explanations, cases and figures are original.
Factual cross-check only; explanations, cases and figures are original.
Factual cross-check only; explanations, cases and figures are original.
Analytics use a separately reviewed, incomplete historical corpus. Opening a topic does not change its counts or denominator.