In the bustling world inside our cells, energy is the ultimate currency. Carbohydrate metabolism and biological oxidation stand at the heart of this energetic economy, ensuring that every heartbeat, breath, and thought is powered efficiently. Unit 2 sheds light on how glucose journeys through intricate biochemical pathways, fueling life and linking health to disease.
Download UNIT 2 – Carbohydrate Metabolism and Biological Oxidation — Powering the Engine of Life Notes
Get simplified revision notes for this unit:
Download Unit 2 Notes PDF
Carbohydrate Metabolism: Breaking Down Life’s Fuel
Carbohydrates, particularly glucose, serve as the body’s most reliable energy source. Their metabolism involves several interwoven pathways, each designed to optimize energy extraction.
Glycolysis: The First Step of Energy Harvest
Glycolysis, often called the Embden-Meyerhof-Parnas pathway, breaks one molecule of glucose into two molecules of pyruvate. This ten-step process yields a modest 2 ATP molecules and NADH, which carry reducing power to later stages.
Energetics: Net gain of 2 ATP and 2 NADH per glucose.
Significance: Provides rapid energy, especially crucial for tissues like red blood cells and exercising muscles that rely heavily on anaerobic metabolism.
Citric Acid Cycle: The Metabolic Hub
Once pyruvate enters the mitochondria, it transforms into acetyl-CoA and enters the citric acid cycle (Krebs cycle). This circular pathway generates NADH, FADH₂, and GTP, vital for downstream energy production.
Energetics: About 12 ATP equivalents per acetyl-CoA oxidized.
Significance: Beyond energy, it supplies precursors for amino acids, nucleotides, and heme synthesis.
HMP Shunt and G6PD Deficiency
The hexose monophosphate (HMP) shunt diverts glucose for two major purposes: generating NADPH for biosynthesis and maintaining reduced glutathione to combat oxidative stress. It also produces ribose-5-phosphate for nucleotide synthesis.
A deficiency in glucose-6-phosphate dehydrogenase (G6PD) impairs this pathway, making red blood cells vulnerable to oxidative damage, leading to hemolytic anemia — a condition with global health relevance.
Glycogen Metabolism: Storing and Releasing Energy
Glycogen, the stored form of glucose in liver and muscles, acts as an emergency reserve.
Glycogenesis: The synthesis of glycogen for storage.
Glycogenolysis: The breakdown of glycogen to release glucose when needed.
Disorders: Glycogen storage diseases (GSDs) arise from enzyme defects, leading to energy crises and organ dysfunction.
Gluconeogenesis: Making Sugar from Scratch
When dietary glucose is scarce, the body synthesizes glucose from non-carbohydrate sources like amino acids and lactate. This pathway ensures constant blood glucose, vital for the brain and red blood cells.
Hormonal Regulation and Diabetes
Blood glucose levels are tightly regulated by hormones. Insulin lowers glucose by promoting uptake and storage, while glucagon, epinephrine, and cortisol mobilize energy reserves.
Disruption of this balance causes diabetes mellitus, a metabolic disorder marked by chronic hyperglycemia, with severe complications ranging from cardiovascular disease to neuropathy.
Biological Oxidation: Capturing Energy from Electrons
While metabolism provides reducing equivalents, it is biological oxidation that channels them into usable energy.
Electron Transport Chain (ETC)
The ETC, located in the inner mitochondrial membrane, transfers electrons from NADH and FADH₂ through protein complexes, ultimately reducing oxygen to water. This electron flow creates a proton gradient across the membrane — the foundation of cellular energy production.
Oxidative Phosphorylation: ATP Factory
Protons re-enter the mitochondrial matrix through ATP synthase, driving the synthesis of ATP. This process, known as oxidative phosphorylation, yields the lion’s share of cellular energy — nearly 34 ATP molecules per glucose.
In contrast, substrate-level phosphorylation generates ATP directly in glycolysis and the citric acid cycle without needing a gradient.
Inhibitors and Uncouplers
The system, however, is vulnerable. Substances like cyanide and carbon monoxide inhibit ETC complexes, halting energy production and causing fatal cellular suffocation. Uncouplers, such as dinitrophenol, collapse the proton gradient, dissipating energy as heat instead of capturing it as ATP. While dangerous, this principle is exploited in brown fat to generate heat in newborns and hibernating animals.