Questions
3-5 questions per university paper
Difficulty
Medium-Hard
Importance
High yield for University Semester Exams and competitive Viva
Overview
Metabolism represents the sum of all biochemical reactions occurring in the living cell to sustain life, divided into catabolic pathways that yield energy and anabolic pathways that synthesize complex molecules. Understanding these pathways is critical for board and university exams, as they form the foundation of biochemistry and clinical diagnostics. Students must prioritize the rate-limiting steps, enzymes, and ATP yield of each major metabolic cycle.
Glycolysis & Krebs Cycle
Glycolysis is the anaerobic breakdown of glucose into pyruvate in the cytoplasm, while the Krebs cycle (TCA cycle) occurs in the mitochondria to oxidize acetyl-CoA into carbon dioxide. These pathways are essential for ATP production through substrate-level phosphorylation and oxidative phosphorylation.
- Glycolysis yields a net gain of 2 ATP and 2 NADH molecules per glucose
- Hexokinase, Phosphofructokinase-1 (PFK-1), and Pyruvate Kinase are the irreversible rate-limiting enzymes
- Krebs cycle occurs in the mitochondrial matrix
- One turn of the Krebs cycle generates 3 NADH, 1 FADH2, and 1 GTP/ATP
- Pyruvate dehydrogenase complex links glycolysis to the Krebs cycle
Lipid Metabolism
Lipid metabolism focuses on the mobilization of fatty acids through beta-oxidation to generate acetyl-CoA for the energy cycle. It is a highly efficient process for long-term energy storage and requires transport mechanisms like the carnitine shuttle to enter the mitochondria.
- Beta-oxidation takes place in the mitochondrial matrix
- The carnitine shuttle is the rate-limiting step for fatty acid entry into mitochondria
- Each cycle of beta-oxidation shortens the fatty acid chain by two carbons
- Produces Acetyl-CoA, NADH, and FADH2
- Ketogenesis occurs in the liver during prolonged fasting or low carbohydrate intake
Protein Metabolism
Protein metabolism involves the degradation of amino acids into carbon skeletons and nitrogenous waste. The body handles toxic ammonia primarily through the Urea cycle, occurring in the liver, to prevent neurotoxicity.
- Transamination transfers an amino group to alpha-ketoglutarate
- Deamination releases free ammonia
- The Urea cycle converts toxic ammonia into urea for excretion
- Glucogenic amino acids can be converted into glucose precursors
- Ketogenic amino acids are converted into acetyl-CoA or acetoacetate
Hormonal Regulation of Metabolism
Hormones act as biochemical signals that coordinate metabolic pathways across different tissues to maintain homeostasis based on nutritional status. Key hormones like insulin and glucagon regulate blood glucose levels through antagonistic effects on liver and adipose tissues.
- Insulin promotes glucose uptake and glycogenesis
- Glucagon stimulates glycogenolysis and gluconeogenesis
- Epinephrine triggers rapid energy mobilization in fight-or-flight scenarios
- Cortisol promotes protein breakdown and gluconeogenesis during chronic stress
- Thyroid hormones regulate the basal metabolic rate
Formula Sheet
Glucose + 2 NAD+ + 2 ADP + 2 Pi -> 2 Pyruvate + 2 NADH + 2 ATP + 2 H2O
Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O -> 2 CO2 + 3 NADH + FADH2 + GTP + 2 H+ + HS-CoA
Exam Tip
Always draw the flowcharts of metabolic pathways and highlight the rate-limiting enzymes in bold to ensure high marks in written exams.
Common Mistakes
- Confusing the location of pathways (e.g., placing Glycolysis in the mitochondria)
- Failing to mention specific rate-limiting enzymes, which is essential for full marks
- Overlooking the difference between ATP production in substrate-level phosphorylation vs. oxidative phosphorylation
More Revision Notes
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