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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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.
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Aerobic respiration
Cellular respiration taking place in the presence of oxygen is known as aerobic 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.
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.
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.
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: Aerobic Respiration
- 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.
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: 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.
