Unit 3: Lipid and nucleic acid metabolism
Biochemical Metabolism notes · PTU syllabus (BMLS202-18)
On this page
Unit summary
Lipids store energy and build membranes, and nucleic acids and energy carriers drive the cell. This unit covers lipid classification, digestion and absorption of fats, lipoproteins, fatty acid synthesis and oxidation, the functions of nucleic acids and energy carriers.
After this unit you can
- Classify lipids and describe fat digestion and absorption
- Describe lipoproteins and their clinical importance
- Explain fatty acid synthesis and β-oxidation
- Describe nucleic acids and energy carriers
PTU syllabus topics
- Lipid classification
- digestion and absorption of fats
- lipoproteins
- fatty acid biosynthesis and oxidation
- nucleic acid introduction and functions
- energy carriers
Chylomicrons
Dietary fat from the gut
Largest, least dense
VLDL
Triglycerides from the liver
Raised in hypertriglyceridaemia
LDL
Cholesterol to tissues
'Bad' cholesterol
HDL
Cholesterol back to the liver
'Good' cholesterol
Topic 1
Classification of lipids
Simple
Fats and oils (triacylglycerols), waxes
Compound
Phospholipids (lecithin), glycolipids, lipoproteins
Derived
Fatty acids, glycerol, steroids (cholesterol), fat-soluble vitamins
Fatty acids
Saturated (palmitic), unsaturated (oleic); essential — linoleic and α-linolenic
Topic 2
Digestion and absorption of fats
- 1
Bile salts emulsify fat into small droplets
- 2
Pancreatic lipase (with colipase) → 2-monoacylglycerol and free fatty acids
- 3
Micelles carry products to the mucosa
- 4
Inside cells, triacylglycerols are re-formed
- 5
Packed into chylomicrons
- 6
Enter lymph (lacteals) and then blood
- Steatorrhoea: fatty stools from bile or pancreatic deficiency (obstructive jaundice, chronic pancreatitis).
Topic 3
Lipoproteins
Chylomicrons
Dietary triglycerides
Carry dietary fat from gut to tissues
VLDL
Endogenous triglycerides
From liver to tissues
LDL
Cholesterol
Delivers cholesterol to tissues — "bad" cholesterol; atherosclerosis
HDL
Phospholipids, cholesterol
Reverse cholesterol transport to the liver — "good" cholesterol
Friedewald LDL (mg/dL)
LDL = total cholesterol − HDL − (triglycerides ÷ 5), valid if TG below 400
Desirable values
Total cholesterol below 200; LDL below 100; HDL above 40 (men) or 50 (women); TG below 150
Example
TC 220, HDL 45, TG 150: LDL = 220 − 45 − 30 = 145 mg/dL (borderline high).
Topic 4
Fatty acid oxidation and synthesis
Site
Mitochondria
Cytoplasm
Transport
Carnitine shuttle carries fatty acyl groups in
Citrate carries acetyl units out
Key enzyme
Carnitine palmitoyl transferase I
Acetyl-CoA carboxylase (biotin) and fatty acid synthase
Coenzymes
FAD, NAD⁺ produced as FADH₂, NADH
NADPH used (from the pentose phosphate pathway)
Product
Acetyl-CoA, 2 carbons per cycle
Palmitate (16 C)
Hormones
Glucagon, adrenaline
Insulin
Example
Palmitate (16 C): 7 cycles of β-oxidation → 8 acetyl-CoA, 7 NADH, 7 FADH₂ → net about 106 ATP.
- Ketone bodies (acetoacetate, β-hydroxybutyrate, acetone) form in the liver when fat breakdown is high — fasting and uncontrolled diabetes (ketoacidosis). Rothera's test detects urinary ketones.
Topic 5
Nucleic acids: introduction and functions
- DNA stores and transmits genetic information; RNA — mRNA (message), tRNA (adapter carrying amino acids), rRNA (ribosome structure and catalysis). Nucleotides also form coenzymes (NAD, FAD, CoA) and second messengers (cAMP).
- Purine breakdown produces uric acid; excess causes gout (normal serum uric acid about 3.5–7.2 mg/dL in men).
Topic 6
Energy carriers
- ATP
- Energy currency; hydrolysis of the terminal phosphate releases about 7.3 kcal/mol
- GTP
- Protein synthesis, signalling
- NADH and FADH₂
- Carry electrons to the electron transport chain
- NADPH
- Reducing power for synthesis
- Creatine phosphate
- Rapid ATP regeneration in muscle
- Oxidative phosphorylation: electrons flow through complexes I–IV to O₂; the proton gradient drives ATP synthase. Each NADH gives about 2.5 ATP and each FADH₂ about 1.5 ATP.
Key terms
- Lipoprotein
- Particle transporting lipids in blood
- β-Oxidation
- Mitochondrial breakdown of fatty acids two carbons at a time
- Carnitine shuttle
- Transports fatty acids into mitochondria
- Ketone bodies
- Fuels from fatty acid breakdown in the liver
- ATP synthase
- Enzyme making ATP using the proton gradient
Quick revision
- Simple, compound, derived lipids; essential fatty acids.
- Bile salts, lipase, micelles, chylomicrons; steatorrhoea.
- Chylomicrons, VLDL, LDL, HDL; lipid profile; Friedewald formula.
- β-Oxidation vs synthesis; ketone bodies.
- DNA, RNA roles; uric acid; ATP, NADH, FADH₂, NADPH; oxidative phosphorylation.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Name the essential fatty acids.
- Q2.What is the role of bile salts in fat digestion?
- Q3.Why is HDL called good cholesterol?
- Q4.Calculate LDL for TC 200, HDL 50, TG 100.
- Q5.Where does β-oxidation occur?
- Q6.How many ATP does one NADH yield?
Long-answer questions
- Q1.Classify lipids and describe their digestion and absorption.
- Q2.Describe lipoproteins and the lipid profile.
- Q3.Compare β-oxidation and fatty acid synthesis.
- Q4.Describe energy carriers and oxidative phosphorylation.
Stuck on this unit?
Message SBS on WhatsApp for help with Biochemical Metabolism, or to ask about studying B.Sc MLS at Synetic.