Lecture 20 - Overview of metabolic pathways

Why is photosynthesis important to the food chain?

• Converts CO₂ + H₂O → C(H₂O) + O₂ using sunlight.

• Produces carbohydrates that serve as the primary energy source.

• Supports herbivores, carnivores, omnivores.

• Releases oxygen for aerobic respiration.

• Without photosynthesis, no sustainable food chain or atmospheric oxygen.

Define catabolism and anabolism. Why are both important for health?

• Catabolism: Breakdown of complex molecules → energy (ATP, NADH, FADH₂). Oxidative, degradative, exergonic.

• Anabolism: Building complex molecules → requires energy (ATP, NADPH). Reductive, biosynthetic, endergonic.

• Importance: Catabolism provides energy/building blocks; anabolism uses them for growth, repair, storage. Balance is essential for health.

Compare metabolism in starvation and obesity.

• Starvation: Catabolism dominates; glycogen, fat, protein broken down; maintains ATP/glucose; leads to muscle wasting.

• Obesity: Anabolism/storage dominates; excess food stored as fat; energy intake > expenditure; metabolic dysregulation (insulin resistance, fatty liver, cardiovascular risk).

How can energetically unfavourable (+ΔG) reactions occur?

• Increase temperature.

• Increase substrate concentration.

• Couple with favourable (–ΔG) reactions.

• Example: ATP hydrolysis (ΔG°’ ≈ –30.7 kJ/mol) drives biosynthesis.

Why is ATP so important in metabolism?

• Universal energy currency.

• Hydrolysis releases large free energy.

• Drives mechanical work, active transport, biosynthesis.

• Links catabolism (energy production) with anabolism (energy consumption).

• Other nucleotides: GTP (protein metabolism), CTP (lipid biosynthesis), UTP (carbohydrate metabolism).

Name three high-energy thioesters and their roles.

• Acetyl CoA – enters citric acid cycle.

• Succinyl CoA – intermediate in TCA cycle, heme biosynthesis.

• Fatty acyl CoA – activated fatty acids for β-oxidation/lipid synthesis.

• Hydrolysis of thioester bonds releases large negative ΔG (≈ –34 kJ/mol).

What roles do NAD⁺ and FAD⁺ play in metabolism?

• NAD⁺ → NADH: electron carrier in catabolism (glycolysis, TCA cycle).

• NADP⁺ → NADPH: reducing power for anabolism (fatty acid synthesis, detoxification).

• FAD → FADH₂: redox reactions (succinate dehydrogenase).

• Stepwise electron transfer to oxygen for controlled energy release.

How is metabolism controlled?

• Negative feedback (end product inhibits pathway).

• Feed forward (substrate accumulation stimulates pathway).

• Rate-limiting step (slowest step controls flux).

• Committed step (first irreversible step unique to pathway).

• Isoenzymes (tissue-specific regulation).

• Multi-enzyme complexes (substrate channeling).

• Compartmentalisation (different organelles).

• Reciprocal regulation (catabolism vs. anabolism controlled oppositely).