Biochemistry · Topic
Lipid chemistry !
Lipids are water-insoluble molecules: fatty acids, triacylglycerols for energy storage, and phospholipids, sphingolipids and steroids for membranes.
Updated 2026-11-01
Key facts
- Fatty acid: a carbon chain with a carboxyl group; saturated (no double bonds, like palmitate C16:0) or unsaturated (oleate C18:1 ω-9).
- Natural double bonds are cis: they kink the chain, lower the melting point and add fluidity. Industrial trans fats are straight and raise cardiovascular risk.
- Essential: linoleic (ω-6) and α-linolenic (ω-3) acids; the body cannot insert double bonds beyond carbon 9.
- Triacylglycerol (TAG): glycerol with three fatty acids in ester bonds; the densest energy store (≈ 9 kcal/g), kept water-free in adipocytes.
- Classes: simple (TAG, waxes), compound (phospholipids, glycolipids, lipoproteins) and derived (steroids, eicosanoids, fat-soluble vitamins).
In detail
- 1. Naming: C18:2 Δ9,12 counts double bonds from the carboxyl end; ω-6 counts from the terminal methyl.
- 2. Longer and more saturated chains melt at higher temperatures: animal fat is solid, plant oils are liquid.
- 3. Arachidonic acid (C20:4 ω-6) is made from linoleic acid and gives rise to prostaglandins, thromboxanes and leukotrienes.
- 4. Hormone-sensitive lipase frees fatty acids from adipocyte TAG during fasting; they travel in blood bound to albumin.
- 5. Amphipathic lipids have a polar head and non-polar tails: in water they form micelles or bilayers, the basis of membranes.
- 6. Saponification: alkali breaks the ester bonds of TAG and releases glycerol and fatty acid salts (soaps).
- Also called
- Fatty acids and triacylglycerols
Clinical notes
- Essential fatty acid deficiency (lipid-free parenteral nutrition, malabsorption) causes scaly dermatitis and poor wound healing.
- Trans and saturated fatty acids raise LDL; ω-3 fatty acids lower triglycerides.
- Very-long-chain fatty acids are oxidised in peroxisomes; they build up in X-linked adrenoleukodystrophy.
Pathways
Related conditions
Related
- Palmitate (palmitic acid)
- Phospholipids
- Sphingolipids and sphingolipidoses
- Cholesterol
- Eicosanoids: prostaglandins, thromboxanes and leukotrienes
- Hormone-sensitive lipase (HSL)
- Lipoprotein lipase (LPL)
Test yourself
Think of the answer, then tap the question to see it.
Why do cis fatty acids make a membrane more fluid?
The cis double bond kinks the chain, which packs poorly against its neighbours and lowers the melting point.
Which fatty acids are essential, and why?
Linoleic (ω-6) and α-linolenic (ω-3): humans cannot insert double bonds beyond carbon 9.
Why is triacylglycerol a better store than glycogen?
It is more reduced (≈ 9 vs 4 kcal/g) and is stored without water, so it holds far more energy per gram.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Fatty acid metabolism (Reactome, 2026); Triglyceride metabolism (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.