SN6
SHORT NOTE
5M
★ High-yield
Classify phospholipids and describe their functions. Add a note on lung surfactant.
Definition
Phospholipids = complex lipids containing phosphoric acid, in which one hydroxyl of glycerol (or sphingosine) is esterified to phosphate and a nitrogenous base.
- They are amphipathic — a hydrophilic head and hydrophobic tails — which is why they form bilayers
Classification
| Type | Example | Note |
|---|---|---|
| Glycerophospholipids | Lecithin (phosphatidylcholine) | The most abundant in membranes and in bile |
| — | Cephalin (phosphatidylethanolamine) | Membranes; clotting |
| — | Phosphatidylserine | Normally on the inner leaflet; its appearance outside signals apoptosis and triggers coagulation |
| — | Phosphatidylinositol | Source of IP3 and DAG second messengers |
| — | Cardiolipin | Only in the inner mitochondrial membrane; the antigen in the VDRL test |
| Sphingophospholipid | Sphingomyelin | Rich in myelin; accumulates in Niemann–Pick disease |
Functions
- Structural — the basic framework of all biological membranes
- Lung surfactant — dipalmitoyl phosphatidylcholine is the chief surface-active agent
- Lipoprotein structure — forms the amphipathic surface coat
- Signal transduction — phosphatidylinositol yields IP3 and DAG
- Source of arachidonic acid for eicosanoids
- Bile — lecithin keeps cholesterol in solution, preventing gallstones
- Lipotropic action — needed for VLDL export from the liver
- Blood coagulation — platelet phospholipid (factor 3) provides the surface for the clotting cascade
Lung Surfactant
- Secreted by type II pneumocytes from about the 24th–28th week of gestation; adequate by 35 weeks
- Composition — 90% lipid (chiefly dipalmitoyl phosphatidylcholine), 10% protein (SP-A, B, C, D)
- Reduces alveolar surface tension → prevents collapse, increases compliance, keeps alveoli dry
- Lecithin : sphingomyelin ratio > 2 in amniotic fluid indicates lung maturity
Applied Aspects
- Respiratory distress syndrome of the newborn — surfactant deficiency in prematurity; treated with exogenous surfactant and prevented by antenatal corticosteroids, which accelerate its synthesis
- Niemann–Pick disease — sphingomyelinase deficiency → sphingomyelin accumulates → hepatosplenomegaly and a cherry-red spot
- Antiphospholipid syndrome — antibodies against phospholipid-binding proteins → paradoxical thrombosis with a prolonged APTT; recurrent fetal loss
- Cardiolipin antibodies account for the biological false positive VDRL in SLE
All questions in Lipid Metabolism
- Q1Describe the β-oxidation of fatty acids. Explain the carnitine shuttle, the energetics for palmitate, and the regulation of the pathway. ★
- Q2Describe the synthesis and utilisation of ketone bodies. Explain the causes of ketosis and the biochemical basis of diabetic ketoacidosis. ★
- Q3Describe the de novo synthesis of fatty acids. Explain the citrate shuttle, the role of acetyl-CoA carboxylase, and compare it with β-oxidation. ★
- Q4Describe the biosynthesis and regulation of cholesterol. Give its functions, transport and fate. Add a note on hypercholesterolaemia. ★
- Q5Classify the plasma lipoproteins. Describe their metabolism and the role of apolipoproteins. Add a note on the hyperlipoproteinaemias. ★
- SN1Describe the carnitine shuttle and its regulation. ★
- SN2Define fatty liver. Describe its causes and mechanisms. Add a note on lipotropic factors. ★
- SN3Define essential fatty acids. Give their sources, functions and the features of deficiency. ★
- SN4Describe the synthesis and actions of the eicosanoids. Explain the action of aspirin. ★
- SN5Describe lipoprotein lipase with its site, activator and significance.
- SN6Classify phospholipids and describe their functions. Add a note on lung surfactant. ★
- SN7Describe the biochemical basis of atherosclerosis and the lipid risk factors. ★