Lipid tricks that make membranes click

Saturated: all single C-C bonds, no kinks, pack tightly → solid at room temp (butter). Unsaturated: C=C double bonds → kinks → can't pack → liquid (olive oil). Trans fats: artificially hydrogenated, behave like saturated.

Each phospholipid: polar head (faces water) + two nonpolar fatty acid tails (face inward). Bilayers form spontaneously. Fluid mosaic model: proteins float in this bilayer.

At high temperatures: restrains phospholipid movement → prevents excess fluidity. At low temperatures: prevents tight packing → prevents solidifying. A Goldilocks molecule.

Steroids are derived from cholesterol. Lipid-soluble → diffuse through membrane → bind intracellular receptors → directly regulate gene expression. Peptide hormones (insulin, glucagon) can't cross → bind surface receptors.

Fatty acids are activated to acyl-CoA → enter mitochondria → each cycle removes 2 carbons as acetyl-CoA + produces FADH₂ + NADH. Acetyl-CoA enters Krebs cycle. Fat produces more ATP per gram than carbohydrates.

Glycerol backbone: position 1 and 2 — fatty acids (ester bonds). Position 3 — phosphate + head group (choline → phosphatidylcholine, serine → phosphatidylserine, ethanolamine, inositol). Head group determines charge and interactions. Phosphatidylserine: negatively charged, faces cytoplasm, flips to outer leaflet during apoptosis.

Sphingosine: 18-carbon amino alcohol backbone. Ceramide: sphingosine + fatty acid — signaling molecule linked to apoptosis. Sphingomyelin: ceramide + phosphocholine — major component of myelin sheath (nerve insulation). Glycosphingolipids: ceramide + sugar — cell recognition, blood group antigens (ABO system).

Arachidonic acid (20-carbon PUFA) released from membrane phospholipids by phospholipase A₂. COX (cyclooxygenase) pathway: prostaglandins (inflammation, fever, pain) and thromboxanes (platelet aggregation, vasoconstriction). Lipoxygenase pathway: leukotrienes (bronchoconstriction, allergic response). Aspirin inhibits COX (cyclooxygenase) → reduces prostaglandins → anti-inflammatory.

Lipoproteins: lipid + protein transport particles. VLDL (Very Low Density Lipoprotein): liver exports triglycerides. IDL (Intermediate Density Lipoprotein): intermediate product. LDL (Low Density Lipoprotein): delivers cholesterol to tissues — 'bad' because excess deposits in arterial walls. HDL: reverse cholesterol transport — removes cholesterol from tissues back to liver — 'good.' Apolipoproteins determine receptor binding.

Omega (ω) position: carbon counting from the methyl end (omega end). Omega-3: double bond at 3rd carbon — EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) found in fish oil (anti-inflammatory, cardiovascular protection). Omega-6: double bond at 6th carbon — linoleic acid (pro-inflammatory in excess). Omega-9: oleic acid in olive oil (monounsaturated, neutral).

Wax: ester of long-chain fatty acid + long-chain alcohol. Very hydrophobic — excellent waterproofing. Plant cuticle wax: prevents water loss from leaves. Animal uses: earwax (cerumen), bee wax (honeycomb), spermaceti (whale), lanolin (sheep wool — water resistance). Hard solid at room temperature.

Free radicals (ROS): highly reactive molecules with unpaired electrons. Attack polyunsaturated fatty acids (PUFAs — fats with multiple double bonds) in membranes → lipid radical → chain reaction propagates. Damages membrane proteins and DNA. Antioxidants (vitamin E in membranes, vitamin C, glutathione) donate electrons to quench radicals. Linked to aging and atherosclerosis.