A molecule made up of adenine, ribose sugar, and three phosphate groups, which stores and releases energy by breaking phosphate bonds for cellular activities, is called adenosine triphosphate (ATP).
Definitions [8]
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Aerobic respiration
Cellular respiration taking place in the presence of oxygen is known as aerobic respiration.
Write a definition.
Cellular respiration
Oxidation of glucose and other food components, which takes place inside the cell in the presence or absence of oxygen, is known as cellular respiration.
Definition: ATP
Definition: Glycolysis
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.
Definition: Tricarboxylic Acid Cycle (Citric Acid Cycle or Kreb’s Cycle)
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.
Definition: Electron Transfer Chain Reaction
The process occurring in the mitochondria, where NADH₂ and FADH₂ release electrons to form ATP and water, producing 3 ATP from each NADH₂ and 2 ATP from each FADH₂, is called the electron transfer chain reaction.
Definition: Respiratory Quotient
The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called Respiratory Quotient (RQ) or respiratory ratio.
Define RQ.
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.
Formulae [1]
Formula: Respiratory Quotient
\[\mathrm{RQ=\frac{Volume~ofCO_{2}~evolved}{Volume~ofO_{2}~consumed}}\]
Key Points
Key Points: ATP
- ATP formation is called phosphorylation and occurs in three ways: photophosphorylation, substrate-level phosphorylation, and oxidative phosphorylation.
- Photophosphorylation occurs during photosynthesis, while the other two occur during respiration.
- Substrate-level phosphorylation involves direct transfer of a phosphate group to ADP and occurs in the cytoplasm and mitochondrial matrix.
- Oxidative phosphorylation uses energy from oxidation of NADH and FADH₂ and occurs in the inner mitochondrial membrane.
- ATP is hydrolysed to release energy whenever the cell needs it for metabolic activities.
Key Points: Glycolysis
- 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.
Key Points: Electron Transport System (Ets) and Oxidative Phosphorylation
- 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.
Key Points: Tricarboxylic Acid Cycle (Citric Acid Cycle or Kreb’s Cycle)
- 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.
Key Points: Phases of Respiration: Electron Transport Chain (Electron Transfer System)
- The electron transport chain is located on the inner mitochondrial membrane and contains a series of electron and proton carrier complexes (I–V).
- Complex I (NADH dehydrogenase) accepts electrons from NADH via FMN and Fe‑S centres, passes them to ubiquinone (Co‑Q), and pumps 4 H⁺ into the intermembrane space.
- Complex II (succinate dehydrogenase) receives electrons from succinate through FADH₂ and Fe‑S centres, passes them to Co‑Q, but does not pump protons.
- Complex III (cytochrome‑c reductase) transfers electrons from reduced Co‑Q (UQH₂) to cytochrome‑c and pumps 4 H⁺ into the intermembrane space.
- Complex IV (cytochrome‑c oxidase) passes electrons from cytochrome‑c to oxygen, reducing it to water and pumping additional H⁺ across the membrane.
- Complex V (ATP synthase) uses the proton gradient generated by complexes I, III and IV to synthesise ATP from ADP and Pi; this proton‑driven ATP formation is called chemiosmosis.
- Oxidation of each NADH yields about 3 ATP, while every FADH₂ yields about 2 ATP, though exact yields can vary with conditions and substrate.
Key Points: Fermentation
- 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.
Key Points: Respiratory Balance Sheet
- 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.
Key Points: Amphibolic Pathways
- 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.
Key Points: Utility of Stepwise Oxidation
- Respiration (aerobic and anaerobic) occurs through many small, enzyme‑controlled steps.
- Stepwise oxidation releases energy gradually, so more of it can be trapped in ATP instead of being lost as heat.
- Enzyme control at each step allows the cell to regulate the rate of the pathway according to its energy needs.
- Different steps provide metabolic intermediates that can be used to synthesise various biomolecules.
- Thus, stepwise oxidation makes energy production efficient and integrates respiration with biosynthetic pathways.
Key Points: Respiratory Quotient
- 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.
Concepts [11]
- Production of ATP
- Glycolysis
- Phases of Respiration: Pyruvate Oxidation (Link Reaction)
- Electron Transport System (Ets) and Oxidative Phosphorylation
- Tricarboxylic Acid Cycle (Citric Acid Cycle or Kreb’s Cycle)
- Phases of Respiration: Electron Transport Chain (Electron Transfer System)
- Fermentation
- Respiratory Balance Sheet
- Amphibolic Pathways
- Utility of Stepwise Oxidation
- Respiratory Quotient
