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 [5]
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.
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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
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: 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: 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: 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: Respiration in Animals
| Organism | Habitat | Respiratory Organ |
|---|---|---|
| Protists, Sponges, Coelenterates | Aquatic | Plasma membrane |
| Planaria, Earthworm, Leech, Frog | Aquatic/Semiaquatic | Moist skin / Plasma membrane |
| Insects | Terrestrial | Tracheal tubes & Spiracles |
| Spiders, Scorpions (Arachnids) | Terrestrial | Book lungs |
| Limulus (Arthropod) | Aquatic | Book gills |
| Tadpoles, Salamanders, Newts | Aquatic | External gills |
| Fish | Aquatic | Internal gills |
| Reptiles, Birds, Mammals | Terrestrial | Lungs |
| Turtles | Underwater | Cloaca |
| Plants | — | Stomata & Lenticels |
Concepts [11]
- Glycolysis
- Electron Transport System (Ets) and Oxidative Phosphorylation
- Tricarboxylic Acid Cycle (Citric Acid Cycle or Kreb’s Cycle)
- Phases of Respiration: Pyruvate Oxidation (Link Reaction)
- Fermentation
- Production of ATP
- Respiration Vs. Burning (Combustion)
- Respiration in Plant
- Experiments on Respiration in Plants
- Respiration and Photosynthesis
- Respiration in Animals
