Definitions [4]
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.
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: 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: 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: 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 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.
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.
