Introduction to Cellular Respiration
- Cellular Respiration: The mechanism of breakdown of food materials within the cell to release energy and trap it for ATP synthesis.
- Respiratory substrates: The compounds oxidised during respiration. Carbohydrates are the most common, but proteins, fats, and organic acids can also be used in certain conditions.
- The energy released is not dissipated as heat but trapped in chemical bonds of ATP (Energy Currency).
- Gas Exchange in Plants: Plants lack specialized respiratory organs but exchange gases via stomata and lenticels. Plant parts respire at lower rates than animals, and during the day, O2 is internally provided by photosynthesis.
Glycolysis (EMP Pathway)
- The scheme was given by Gustav Embden, Otto Meyerhof, and J. Parnas, hence referred to as the EMP pathway.
- Site: Occurs in the cytoplasm of the cell and is present in all living organisms AIPMT 2000 NEET 2019.
- Mechanism: The partial, step-wise oxidation of one molecule of Glucose (6C) to form two molecules of Pyruvic acid (3C) AIPMT 1997 2010.
- Crucial detail: Glycolysis is the only process of respiration in anaerobic organisms AIPMT 2004.
- In plants, glucose is derived from sucrose (end product of photosynthesis), which is converted into glucose and fructose by the enzyme Invertase NEET 2020. Both monosaccharides then enter the glycolytic pathway.
Key Reaction Steps & Enzymes
- Glucose is phosphorylated to Glucose-6-phosphate by Hexokinase (consumes 1 ATP) AIPMT 2000 NEET 2020.
- Fructose-6-phosphate is phosphorylated to Fructose 1,6-bisphosphate (consumes 1 ATP).
- Fructose 1,6-bisphosphate splits into two 3C compounds: PGAL and DHAP.
- Oxidation of PGAL to 1,3-bisphosphoglyceric acid (BPGA) yields NADH + H+.
- Conversion of BPGA to 3-Phosphoglyceric acid (3-PGA) yields ATP via Substrate-Level Phosphorylation.
- Conversion of 2-Phosphoglycerate to Phosphoenolpyruvate (PEP) involves the removal of a water molecule NEET 2021.
- Conversion of PEP to Pyruvic acid by Pyruvate kinase yields ATP.
- Net Yield: 2 Pyruvic acid + 2 ATP (4 produced - 2 consumed) + 2 NADH + H+ AIPMT 1998 2007.
Fermentation (Anaerobic Respiration)
- Key Term: Incomplete oxidation of glucose under anaerobic conditions where pyruvic acid is converted to CO2 and ethanol, or lactic acid.
- Alcoholic Fermentation: Occurs in yeast. Pyruvic acid is converted to Ethanol and CO2 AIPMT 1997 2003 NEET 2017.
- Catalyzed by the enzymes Pyruvic acid decarboxylase and Alcohol dehydrogenase AIPMT 2014 NEET 2018.
- Crucial detail: Yeasts poison themselves to death when the concentration of alcohol reaches about 13% NEET 2020 2021.
- Lactic Acid Fermentation: Occurs in some bacteria and animal muscle cells during strenuous exercise. Pyruvic acid is reduced to lactic acid by Lactate dehydrogenase.
- Exception: Lactic acid fermentation does not release CO2 AIPMT 2014.
- Energy Yield: In both types of fermentation, less than 7% of the energy in glucose is released, yielding a net of only 2 ATP. NADH + H+ is re-oxidised to NAD+ in both processes.
Aerobic Respiration: Link Reaction
- Occurs entirely within the Mitochondrial matrix AIPMT 2010.
- Pyruvic acid enters the mitochondrion and undergoes oxidative decarboxylation catalyzed by the enzyme Pyruvate dehydrogenase NEET 2018.
- The reaction requires multiple coenzymes: NAD+, Coenzyme A (CoA), and Mg2+ ions AIPMT 2014 NEET 2020.
- Reaction: Pyruvic acid + CoA + NAD+ → Acetyl CoA + CO2 + NADH + H+ (catalyzed by Mg2+, Pyruvate dehydrogenase).
- Acetyl CoA serves as the vital connecting link between glycolysis and the Krebs cycle AIPMT 1990.
Tricarboxylic Acid (TCA) Cycle / Krebs Cycle
- Discovered by Hans Krebs. Occurs in the Mitochondrial matrix AIPMT 1996 2006.
- Step 1 (Condensation): Acetyl CoA (2C) condenses with Oxaloacetic acid (OAA, 4C) and water to yield Citric acid (6C), releasing CoA. Catalyzed by Citrate synthase AIPMT 2015 NEET 2020.
- Decarboxylation: Two successive steps of decarboxylation lead to α-ketoglutaric acid (5C) and then Succinyl CoA (4C) AIPMT 2004 2008.
- Substrate-Level Phosphorylation (SLP): The conversion of Succinyl CoA to Succinic acid is coupled with the synthesis of 1 molecule of GTP (which subsequently transfers a phosphate to form ATP) AIPMT 2011 NEET 2020 2023.
- FAD Reduction: The oxidation of Succinic acid to Fumaric acid generates FADH2 NEET 2017 2019.
- Yield per turn: 3 NADH + H+, 1 FADH2, 1 ATP (from GTP), and 2 CO2. (These values are doubled for every one molecule of glucose).
Electron Transport System (ETS) and Oxidative Phosphorylation
- Located exclusively in the Inner mitochondrial membrane AIPMT 2011 NEET 2018.
- Terminal Acceptor: Oxygen (O2) acts as the final hydrogen and electron acceptor, combining with protons to form metabolic water AIPMT 1997 2008 NEET 2017 2019.
- Crucial detail: Although oxygen's role is limited to the terminal stage of the process, its presence is vital because it acts as a magnet that pulls electrons down the whole system NEET 2019.
| ETS Complex | Enzyme/Component | Key Function & NEET Facts |
|---|---|---|
| Complex I | NADH Dehydrogenase | Oxidizes NADH + H+, transfers electrons to Ubiquinone (UQ). |
| Complex II | Succinate Dehydrogenase | Oxidizes FADH2, transfers electrons to Ubiquinone. |
| Complex III | Cytochrome bc1 complex | Transfers electrons from Ubiquinol to Cytochrome c. |
| Cytochrome c | Mobile electron carrier | A small protein attached to the outer surface of the inner membrane; moves electrons between Complex III and IV AIPMT 2010 NEET 2019. |
| Complex IV | Cytochrome c Oxidase | Contains cytochromes a and a3, and possesses two Copper (Cu) centers AIPMT 2000 NEET 2022. |
| Complex V | ATP Synthase (F0-F1) | Synthesizes ATP using the proton gradient. F1 is the peripheral membrane protein complex (site of ATP synthesis); F0 is the integral membrane proton channel. |
- Energy Equivalent: Oxidation of 1 NADH produces 3 ATP, while 1 FADH2 produces 2 ATP AIPMT 1995.
The Respiratory Balance Sheet
- Net gain of ATP is calculated based on theoretical assumptions (e.g., sequential, orderly functioning, NADH transport to mitochondria).
- Net Yield: A net gain of 38 ATP (or 36 ATP depending on shuttle system efficiency in eukaryotes) per glucose molecule during aerobic respiration.
Amphibolic Pathway
- Respiratory pathway is historically considered catabolic, but intermediates are withdrawn for the synthesis of other compounds, making it an Amphibolic pathway (involving both breakdown and synthesis) AIPMT 2009 NEET 2022.
- Acetyl CoA acts as the crucial common link across multiple macromolecule metabolic pathways AIPMT 1990 1997 2014 NEET 2021 2023:
- Carbohydrates: Converted to Pyruvate → Acetyl CoA.
- Fats: Broken down into fatty acids and glycerol. Fatty acids are directly degraded into Acetyl CoA before entering the Krebs cycle. If the body needs to synthesize fats, Acetyl CoA is withdrawn from the pathway.
- Proteins: Degraded by proteases into amino acids, which after deamination enter the pathway at various stages (Pyruvate, Acetyl CoA, or Krebs cycle intermediates).
Respiratory Quotient (RQ)
- Definition: The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration. (RQ = CO2/O2)
- Carbohydrates: RQ is exactly 1.0 because equal volumes of CO2 are evolved and O2 are consumed AIPMT 1992 NEET 2018.
- Fats: RQ is always less than 1. For example, the fatty acid Tripalmitin has an RQ of 0.7 AIPMT 2010 NEET 2019 2022.
- Proteins: RQ is approximately 0.9.
- Crucial detail: In living organisms, respiratory substrates are often mixed; pure proteins or fats are never used exclusively as respiratory substrates.