Step by Step
Cat
Catabolism — energy-releasing breakdown
Catabolism breaks down large molecules into smaller ones, releasing energy stored in chemical bonds. Examples: glycolysis, beta-oxidation, the Krebs cycle, protein digestion. Products: ATP, NADH, FADH₂, CO₂, H₂O, small building blocks (acetyl-CoA, amino acids, monosaccharides).
Memory trick: Catabolism = CAT eats things (breakdown). Releases energy.
Ana
Anabolism — energy-requiring synthesis
Anabolism builds large molecules from smaller ones, requiring energy input. Examples: fatty acid synthesis, gluconeogenesis, glycogen synthesis, protein synthesis, cholesterol synthesis. Requires ATP and reducing equivalents (NADPH — note: NADPH, not NADH).
Memory trick: Anabolism = ANAbol steroids BUILD muscle. Building requires energy.
NADPH
NADPH vs NADH — the key distinction
NADH: produced in catabolism (Krebs, glycolysis, beta-oxidation) → goes to ETC → ATP. NADPH: produced in the pentose phosphate pathway → used in anabolism (fatty acid synthesis, cholesterol synthesis, antioxidant defense via glutathione). This is a critical distinction — NADPH and NADH are not interchangeable.
Memory trick: NADH = catabolic (H for harvest). NADPH = anabolic (P for production/building).
Rec
Reciprocal regulation — can't do both at once
Anabolism and catabolism are reciprocally regulated — cells don't waste energy doing both simultaneously. Key regulators: AMP/ADP/ATP ratio, NADH/NAD⁺ ratio, malonyl-CoA (high = fatty acid synthesis active = beta-oxidation inhibited). Insulin favors anabolism; glucagon/epinephrine favor catabolism.
Applied Walkthrough
1
After a meal: insulin rises → promotes glycogen synthesis (anabolism), fatty acid synthesis (anabolism), protein synthesis (anabolism). Catabolism slows.
2
During fasting: glucagon rises → promotes glycogenolysis (catabolism), gluconeogenesis, beta-oxidation. Anabolism slows.
3
During exercise: AMP rises (ATP is consumed faster than it's made) → activates AMPK → stimulates catabolism (glucose uptake, beta-oxidation) → inhibits anabolism (fatty acid synthesis, protein synthesis).
4
Malonyl-CoA links the two arms: when fatty acid synthesis is active (fed state), malonyl-CoA rises → inhibits carnitine palmitoyl transferase I (CPT-I) → prevents fatty acids from entering mitochondria for beta-oxidation. Elegant reciprocal control.
Exam Application
Exams test the distinction between catabolism and anabolism, which pathways belong to each, the NADH vs NADPH distinction (NADPH for biosynthesis, from pentose phosphate pathway), and hormonal regulation (insulin = anabolism, glucagon/epinephrine = catabolism). AMPK as the cellular energy sensor is frequently tested.
⚠ Common Trap
Students confuse NADH and NADPH — they are NOT interchangeable. NADH donates electrons to the ETC for ATP. NADPH is the reducing power for biosynthesis and antioxidant defense. Using NADH for fatty acid synthesis or NADPH for the ETC would be wrong. The pentose phosphate pathway is the main source of NADPH.
✓ Quick Self-Check
1. What is the difference between catabolism and anabolism?
Catabolism breaks large molecules down, releasing energy (ATP). Anabolism builds large molecules up, requiring energy input.
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2. Which electron carrier is used in anabolism?
NADPH — produced mainly by the pentose phosphate pathway. Not NADH (which goes to the ETC for ATP).
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3. Name three catabolic pathways.
Any three of: glycolysis, beta-oxidation, Krebs cycle, protein digestion, glycogenolysis.
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4. Name three anabolic pathways.
Any three of: fatty acid synthesis, gluconeogenesis, glycogen synthesis, protein synthesis, cholesterol synthesis.
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5. How does insulin affect metabolism?
Insulin promotes anabolism — stimulating glycogen synthesis, fatty acid synthesis, and protein synthesis while inhibiting catabolic pathways.
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