Revision: Class 11 >> Respiration in Plants NEET (UG) Respiration in Plants

The breakdown of glucose in the absence of oxygen to produce alcohol or lactic acid and a small amount of energy is called anaerobic respiration.

The breakdown of glucose in the presence of oxygen to produce carbon dioxide, water, and energy is called aerobic respiration.

It is a process of release of energy from food substances such as glucose and fats under the control of enzymes, to carry out life processes, by the living organisms.

The process occurring in the cytoplasm where one glucose molecule is stepwise oxidized to form two molecules each of pyruvic acid, ATP, NADH₂, and water is called glycolysis.

The cyclic series of reactions occurring in the mitochondria, where acetyl-CoA is completely oxidized to produce CO₂, H₂O, NADH₂, and FADH₂, is called the tricarboxylic acid cycle or Krebs cycle.

The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration is called Respiratory Quotient (RQ) or respiratory ratio.

Respiratory quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a period of time.

\[\mathrm{RQ=\frac{Volume~ofCO_{2}~evolved}{Volume~ofO_{2}~consumed}}\]

• Respiration breaks down food in cells to release energy for all life processes.
• All food energy originally comes from photosynthesis by green plants and cyanobacteria.
• Only chloroplast-containing (green) cells photosynthesise; other plant parts get food via transport.
• Respiration is a step-wise, enzyme-controlled oxidation of food that traps energy as ATP.
• ATP is the cell's energy currency, used to power all cellular activities and biosynthesis.

• Plants exchange gases through stomata (leaves/stems) and lenticels (thick stems), not specialised organs.
• Each plant part manages its own gas exchange; demand is much lower than in animals.
• O₂ produced during photosynthesis is directly used for respiration within the same cell.
• Loosely packed parenchyma cells form air spaces, allowing easy gas diffusion across short distances.
• Glucose is broken down in multiple steps to minimise heat loss and trap energy efficiently as ATP.

• Respiration is the breaking of C-C bonds of complex compounds (respiratory substrates) inside cells to release energy, stored as ATP.
• Aerobic respiration occurs in the presence of O₂; glucose is completely oxidised into CO₂ + H₂O, releasing 36 ATP (in cytoplasm + mitochondria).
• Anaerobic respiration occurs without O₂; glucose is partially oxidised into ethanol/lactic acid + CO₂, releasing only 2 ATP (only in cytoplasm).
• Glycolysis (cytoplasm) is the first and common step in both types — glucose breaks down into pyruvate, releasing some energy.
• In aerobic respiration, pyruvate enters the mitochondria and goes through the Krebs cycle and ETS (electron transport system) to release a large amount of energy as ATP.

• Glycolysis (EMP pathway) breaks one glucose (6C) into two pyruvic acid (3C) molecules in the cytoplasm; common to both aerobic and anaerobic respiration.
• It involves 10 enzyme-controlled reactions in two phases — Preparatory and Pay-off.
• Preparatory Phase — Glucose is phosphorylated using 2 ATP and split into two 3C molecules (PGAL + DHAP; DHAP converts to PGAL).
• Pay-off Phase — PGAL is oxidised, NADH₂ is formed, and ATP is produced via substrate-level phosphorylation.
• Net gain = 2 ATP (4 produced − 2 consumed); PEP → Pyruvic acid is the final energy-yielding step.
• Fate of pyruvate — with O₂: enters the Krebs cycle; without O₂: forms lactic acid (muscles) or ethanol + CO₂ (yeast).
• In plants, glucose comes from sucrose (a photosynthesis product), split by invertase into glucose and fructose before entering glycolysis.

• Fermentation is the incomplete oxidation of pyruvic acid under anaerobic conditions, found in bacteria, yeast, and muscle cells.
• Alcoholic fermentation (yeast) - pyruvic acid → ethanol + CO₂; enzymes: pyruvic acid decarboxylase and alcohol dehydrogenase.
• Lactic acid fermentation (bacteria/muscles) - pyruvic acid → lactic acid; enzyme: lactate dehydrogenase; causes muscle stiffness.
• In both types, NADH + H⁺ is reoxidised to NAD⁺, which is reused in glycolysis.
• Both release less than 7% of glucose energy; products (alcohol/lactic acid) are hazardous in nature.
• Yeast dies when alcohol concentration reaches 13%; higher alcohol beverages are made by distillation.
• Aerobic respiration fully oxidises glucose in mitochondria using O₂, releasing far more energy than fermentation.

• It requires oxygen and completely breaks down substrates like glucose to CO₂ and H₂O.
• Pyruvic acid produced in glycolysis is transported into the mitochondrial matrix.
• There, pyruvic acid is converted to acetyl Co‑A by the pyruvate dehydrogenase complex in a reaction called the link reaction.
• In this reaction, pyruvic acid combines with coenzyme A and NAD⁺ to form acetyl Co‑A, CO₂ and NADH + H⁺.
• Acetyl Co‑A serves as the connecting link between glycolysis and the citric acid (Krebs) cycle, entering the cycle for further oxidation.

• It is a common oxidative pathway where acetyl Co‑A (from pyruvic acid via link reaction) is completely oxidised to CO₂.
• The cycle also supplies intermediates (e.g., α‑ketoglutarate, oxaloacetate) for synthesis of amino acids such as glutamate and aspartate.
• Per pyruvic acid, the cycle produces 3 CO₂, 4 NADH + 4H⁺, 1 FADH₂ and 1 ATP (or GTP) in the mitochondrial matrix.
• For each glucose (2 pyruvates), Krebs cycle output is 6 CO₂, 8 NADH + 8H⁺, 2 FADH₂ and 2 ATP molecules.
• Considering the whole respiratory pathway, glucose breakdown yields CO₂, 8 NADH + H⁺, 2 FADH₂ and 2 ATP at the Krebs‑cycle level.
• Because its intermediates are used both for breakdown (catabolism) and for biosynthesis (anabolism), the respiratory pathway is termed an amphibolic pathway.

• ETS is located in the inner mitochondrial membrane; it oxidises NADH and FADH₂ to release stored energy.
• Electrons from NADH enter via Complex I; from FADH₂ via Complex II - both pass to Ubiquinone (UQ).
• Electrons move: UQ → Complex III → Cytochrome c → Complex IV → finally to O₂ (final acceptor), forming water.
• Movement of electrons pumps H⁺ ions from the matrix to the intermembrane space, creating an electrochemical proton gradient.
• Protons flow back through F₀ into the matrix via ATP synthase (Complex V); energy is used by F₁ to synthesise ATP - this is oxidative phosphorylation.
• Energy yield - 1 NADH = 3 ATP; 1 FADH₂ = 2 ATP.
• Oxidative phosphorylation uses energy from redox reactions, unlike photophosphorylation, which uses light energy.

• Theoretical net ATP gain from one glucose in aerobic respiration = 38 ATP, assuming a sequential pathway of glycolysis → TCA cycle → ETS.
• In reality, this value is not fully practical as pathways run simultaneously and enzymatic rates vary.
• NADH from glycolysis is transferred to mitochondria for oxidative phosphorylation; no intermediates are diverted for other uses in this calculation.
• Fermentation = 2 ATP (partial breakdown); Aerobic respiration = 38 ATP (complete oxidation to CO₂ + H₂O).
• NADH oxidation is slow in fermentation but rapid in aerobic respiration, making aerobic respiration far more efficient.

• The respiratory pathway is amphibolic as it involves both catabolism (breakdown for energy) and anabolism (synthesis of complex substances).
• Glucose is the preferred substrate; other carbohydrates are first converted to glucose before entering the pathway.
• Fats break down into glycerol (→ PGAL) and fatty acids (→ Acetyl CoA), entering the pathway at different points.
• Proteins are degraded into amino acids; after deamination, they enter at various stages - pyruvate, Acetyl CoA, or the Krebs cycle.
• Intermediates can be withdrawn for biosynthesis (anabolic use), making the pathway truly amphibolic - not just catabolic.

• Respiratory Quotient (RQ) is the ratio of volume of CO₂ evolved to volume of O₂ consumed during aerobic respiration. Formula: RQ = Volume of CO₂ evolved ÷ Volume of O₂ consumed.
• For carbohydrates - RQ = 1 (equal volumes of CO₂ and O₂); e.g., glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy.
• For fats - RQ is less than 1 (more O₂ needed for oxidation); e.g., Tripalmitin: RQ = 102/145 = 0.7.
• For proteins, RQ is approximately 0.9.
• In living organisms, multiple substrates are respired together (not pure fats or proteins), so RQ is often more than 1; pure fats or proteins are never the sole respiratory substrate.