Unit 4: Carbohydrate Metabolism
1. Glycolysis and the Fate of Pyruvate
Glycolysis is the universal metabolic pathway that breaks down glucose into pyruvate to generate energy. It occurs in the cytosol of the cell.
The Process
- Initial Breakdown: One molecule of glucose (6C) is converted into two molecules of pyruvate (3C).
- Energy Yield: A net gain of 2 ATP and 2 NADH molecules per glucose molecule.
Fate of Pyruvate
The metabolic destination of pyruvate depends on oxygen availability:
- Aerobic Conditions: Pyruvate enters the mitochondria for the TCA cycle.
- Anaerobic Conditions: Pyruvate undergoes fermentation to produce lactate (in animals) or ethanol (in plants/yeast).
2. Pentose Phosphate Pathway (PPP)
Also known as the Phosphogluconate Pathway, it serves as an alternative to glycolysis.
- Objective: To generate NADPH for reductive biosynthesis (like fatty acid synthesis) and Pentoses (5-carbon sugars) for nucleic acid synthesis.
- Location: Occurs in the cytoplasm.
3. Gluconeogenesis and Glycogenolysis
These pathways ensure the balance of glucose levels within the organism.
- Gluconeogenesis: The synthesis of glucose from non-carbohydrate precursors like pyruvate, lactate, or certain amino acids.
- Glycogenolysis: The breakdown of glycogen into glucose-1-phosphate and glucose.
4. The TCA (Krebs) Cycle
The Tricarboxylic Acid (TCA) cycle is the central hub of metabolism, occurring in the mitochondrial matrix.
Mechanism
- Entry: Pyruvate is converted to Acetyl-CoA, which then enters the cycle.
- Products: For every turn, the cycle produces 3 NADH, 1 FADH2, and 1 GTP (or ATP).
- Carbon Release: Two molecules of CO2 are released as waste products.
5. Electron Transport Chain and Cyanide Resistance
The Electron Transport Chain (ETC) is located in the inner mitochondrial membrane and is the final stage of aerobic respiration.
- ATP Synthesis: Electrons from NADH and FADH2 are passed through a series of complexes, creating a proton gradient that drives ATP synthesis.
- Cyanide-Resistant Respiration: Some plants possess an "Alternative Oxidase" (AOX) pathway that allows respiration to continue even in the presence of cyanide, which typically inhibits Complex IV.
6. Factors Affecting Respiration
Several environmental and internal factors influence the rate of cellular respiration:
- Temperature: Respiration rate generally increases with temperature up to an optimum point.
- Oxygen Concentration: Essential for aerobic respiration; low levels lead to anaerobic fermentation.
- Water: Proper hydration is necessary for enzymatic activities.
- Light: Indirectly affects respiration by providing the substrates through photosynthesis.
7. Exam Focus: Comparative Study & Tips
Exam Tip: Be prepared to compare Glycolysis and the TCA cycle. Remember that Glycolysis is anaerobic and occurs in the cytoplasm, while the TCA cycle is aerobic and occurs in the mitochondria.
Common Pitfalls
- Mistake: Thinking Glycolysis produces the most ATP. Correction: The majority of ATP is produced during the Electron Transport Chain via oxidative phosphorylation.
- Mistake: Assuming the Pentose Phosphate Pathway is for energy. Correction: Its primary role is providing NADPH and precursors for biosynthesis, not ATP production.
Frequently Asked Questions
Q: What is the significance of the Cyanide-resistant pathway in plants?
A: It allows plants to maintain metabolic flux during stress or when the main cytochrome pathway is restricted, and in some cases, it helps in thermogenesis (heat production).
Q: Why is the TCA cycle called "amphibolic"?
A: Because it involves both catabolism (breaking down molecules for energy) and anabolism (providing intermediates for the synthesis of amino acids and other biomolecules).