Respiration in Plants

🎯 Key Points

• Aerobic respiration: Glycolysis (cytoplasm) → Krebs cycle (mitochondrial matrix) → Electron Transport Chain (inner mitochondrial membrane); net ~36-38 ATP per glucose
• Glycolysis itself yields only 2 ATP (net) and 2 NADH per glucose — the vast majority of ATP comes later, from the ETC
• Anaerobic fermentation: pyruvate → ethanol + CO2 (yeast) or lactic acid (muscle/some bacteria) — regenerates NAD+ without using O2, but yields far less energy
• Respiratory Quotient (RQ) = CO2 released/O2 absorbed: RQ=1 for carbohydrates, RQ<1 for fats (need more O2 relative to CO2 released), RQ>1 for organic acids
• Respiration is described as an "amphibolic" pathway — it both breaks down (catabolic) and supplies intermediates for biosynthesis (anabolic), not purely a one-way energy-release process

Glycolysis

• Occurs in the cytoplasm; the EMP (Embden-Meyerhof-Parnas) pathway; common to both aerobic and anaerobic respiration
• One glucose (6-carbon) is split into 2 molecules of pyruvic acid (3-carbon) through a 10-step enzyme-catalysed sequence
• Net yield per glucose: 2 ATP (4 produced, 2 consumed in early steps) and 2 NADH
• The fate of pyruvate depends on oxygen availability: aerobic conditions send it into the mitochondria; anaerobic conditions send it into fermentation

Krebs Cycle (Citric Acid Cycle / TCA Cycle)

• Occurs in the mitochondrial matrix; requires aerobic conditions
• Pyruvate is first converted to Acetyl CoA (releasing CO2) before entering the cycle, combining with oxaloacetate to form citric acid
• One full turn of the cycle (per acetyl CoA) releases 2 CO2 and generates 3 NADH, 1 FADH2, and 1 ATP (via substrate-level phosphorylation, e.g. through GTP in some texts)
• Since each glucose yields 2 pyruvate (hence 2 acetyl CoA), the cycle turns twice per glucose

Electron Transport Chain (ETC) and Oxidative Phosphorylation

• Located in the inner mitochondrial membrane; this is where the vast majority of ATP is generated
• NADH and FADH2 (collected from glycolysis and the Krebs cycle) donate electrons to a chain of complexes (NADH dehydrogenase, cytochromes, etc.), ultimately reducing O2 to water
• Electron flow pumps protons across the inner membrane, creating a gradient; ATP synthase uses this gradient to make ATP (chemiosmosis) — the same fundamental mechanism as in photosynthesis's light reaction, just running in the opposite organelle
• Each NADH yields roughly 2.5-3 ATP, each FADH2 roughly 1.5-2 ATP via the ETC, giving the commonly cited total of ~36-38 ATP per glucose for complete aerobic respiration

Anaerobic Respiration (Fermentation)

• Occurs when O2 is unavailable; pyruvate is NOT sent to the mitochondria, avoiding the need for O2 as a final electron acceptor
• Alcoholic fermentation (yeast): pyruvate → ethanol + CO2
• Lactic acid fermentation (muscle cells under oxygen debt, some bacteria): pyruvate → lactic acid
• Both pathways exist primarily to regenerate NAD+ from NADH so glycolysis can continue — without this regeneration, glycolysis would stall
• Much less efficient than aerobic respiration: only the 2 ATP from glycolysis are gained, since no Krebs cycle or ETC operates

Respiratory Quotient (RQ)

• RQ = volume of CO2 released / volume of O2 consumed
• RQ = 1 for carbohydrate respiration (CO2 released exactly equals O2 consumed, since carbohydrates are already partially oxidised)
• RQ < 1 for fat respiration (fats are more reduced/less oxidised, so MORE O2 is needed relative to CO2 produced)
• RQ > 1 for organic acid/protein respiration (these substrates are already more oxidised, releasing relatively more CO2 per O2 consumed)

Cellular Respiration: An Overview

• Respiration is the enzyme-controlled breakdown of C-C bonds of respiratory substrates by oxidation, releasing energy that is trapped as ATP and used for cellular work; the process happens in every living cell
• Respiratory substrates: carbohydrates (mainly glucose) are the commonest substrate, but fats, proteins, and organic acids can also be oxidised when needed
• Unlike burning (combustion), where all energy is released in one uncontrolled burst as heat and light, cellular respiration releases energy in small, stepwise amounts across many enzyme-catalysed reactions, allowing it to be captured efficiently as ATP
• Plants lack specialised respiratory or transport organs; each plant part takes care of its own gas exchange, and O2/CO2 diffuse directly through stomata and lenticels — so plants have far lower gas-exchange demands than animals

Link Reaction (Pyruvate Oxidation)

• The connecting step between glycolysis and the Krebs cycle; pyruvate (3-carbon) formed in the cytoplasm enters the mitochondrial matrix
• Pyruvate undergoes oxidative decarboxylation: it loses one CO2 and is oxidised (producing 1 NADH), and the remaining 2-carbon acetyl group is joined to coenzyme A to form acetyl CoA
• Catalysed by the pyruvate dehydrogenase complex; requires coenzymes such as NAD+ and CoA
• Since each glucose yields 2 pyruvate, the link reaction runs twice per glucose, producing 2 CO2 and 2 NADH before the Krebs cycle even begins

Respiratory Balance Sheet (ATP Yield)

• A theoretical tally of ATP per glucose in aerobic respiration, based on simplifying assumptions: an orderly sequential pathway, glucose as the sole substrate, NADH from glycolysis fully transferred into mitochondria, and nothing withdrawn for biosynthesis
• Per glucose: glycolysis gives 2 ATP + 2 NADH; the link reaction gives 2 NADH; the Krebs cycle gives 2 ATP (GTP) + 6 NADH + 2 FADH2 — a total of 10 NADH and 2 FADH2 feeding the ETC
• Counting each NADH as 3 ATP and each FADH2 as 2 ATP via oxidative phosphorylation gives 30 + 4 = 34 ATP, plus the 4 ATP from substrate-level phosphorylation, totalling the classic 38 ATP per glucose
• Real yields are lower (~36 or fewer) because these assumptions rarely hold — intermediates are continuously withdrawn for synthesis, and shuttling glycolytic NADH into the mitochondria has an energy cost

Amphibolic Pathway

• The respiratory pathway is amphibolic: it is not purely catabolic (breakdown) but also supplies carbon skeletons for anabolism (biosynthesis)
• Intermediates are constantly withdrawn as raw material: acetyl CoA for fatty acid and steroid synthesis, alpha-ketoglutarate and oxaloacetate for amino acid synthesis
• Because these intermediates are drained off for building molecules, they must be continually replenished, so respiration links directly to the synthesis of fats, proteins, and other biomolecules
• This dual role is why respiration is best described as amphibolic rather than simply catabolic — a point NEET specifically tests

🚀 NEET Advanced Edge

Why respiration is called an "amphibolic" pathway: Glycolysis and the Krebs cycle don't just break glucose down for energy — their intermediates (e.g. pyruvate, acetyl CoA, oxaloacetate, α-ketoglutarate) are also starting points for synthesising fatty acids, amino acids, and other biomolecules. This dual catabolic-and-anabolic role is why respiration is amphibolic rather than purely catabolic, a distinction NEET specifically tests.

RQ as a diagnostic in respiration experiments: A germinating seed using only stored fat as its respiratory substrate shows RQ<1 (more O2 consumed than CO2 released, since fats need more oxidation); pure carbohydrate respiration gives exactly RQ=1 — exam questions often give an RQ value from a real experimental setup (like germinating gram vs castor seeds) and ask which substrate is dominant.

Worked problem: In an experiment, a respiring tissue absorbs 500 mL of O2 and releases 400 mL of CO2 in the same period. Calculate the RQ and identify the likely respiratory substrate. Approach: RQ = CO2 released/O2 absorbed = 400/500 = 0.8. Since RQ < 1, the substrate is predominantly fat (a value of exactly 1 would indicate pure carbohydrate; the partial value of 0.8 suggests a mix, leaning toward fat).