6 min readConcept sheetA 10-mark or 5-mark answer

Lipoproteins: classification and function

Lipoproteins are protein-coated fat carriers in blood. Chylomicrons, VLDL, IDL, LDL and HDL differ in density and job; high LDL harms, high HDL protects the heart.

Why neededFats do not mix with water (blood)

Density orderChylomicron < VLDL < IDL < LDL < HDL

'Bad' vs 'good'LDL takes cholesterol out; HDL brings it back

Key apoproteinsB-48, B-100, C-II, E, A-I

The five lipoproteins (lightest to densest)

Lipoprotein	Made in	Main lipid	Main job
Chylomicron	Intestine	Dietary triglycerides	Carries fat from food to tissues; remnants go to the liver
VLDL	Liver	Triglycerides made in the liver	Carries liver-made fat to muscle and adipose tissue
IDL	Blood, from VLDL	Triglycerides and cholesterol esters	Short-lived; becomes LDL or is taken up by the liver
LDL	Blood, from IDL	Cholesterol (mostly as esters)	Delivers cholesterol to tissues ('bad')
HDL	Liver and intestine	Phospholipids and cholesterol	Brings extra cholesterol back to the liver ('good')

Structure and density

Each lipoprotein is a tiny ball. The core holds triglycerides and cholesterol esters, which do not mix with water. The shell has phospholipids, free cholesterol and proteins called apoproteins. More fat means lower density: chylomicrons are the largest and lightest, and HDL is the smallest and densest, with the most protein. On electrophoresis, VLDL runs as pre-β, LDL as β and HDL as α; chylomicrons stay at the origin.

Apoproteins in brief

Apo B-48: on chylomicrons; made in the intestine.
Apo B-100: on VLDL, IDL and LDL; binds the LDL receptor.
Apo C-II: switches on lipoprotein lipase, the enzyme on capillary walls that unloads triglycerides.
Apo E: helps the liver take up remnant particles.
Apo A-I: main HDL protein; activates LCAT (lecithin-cholesterol acyltransferase), which turns cholesterol into esters inside HDL.

LDL vs HDL

LDL, 'bad' cholesterolCarries cholesterol to the tissues. Excess LDL enters artery walls, gets oxidised and is swallowed by macrophages, forming foam cells and then plaque. This is atherosclerosis, which narrows arteries and leads to heart attack and stroke.

HDL, 'good' cholesterolCollects extra cholesterol from the tissues and carries it back to the liver for removal (reverse cholesterol transport). Higher HDL means lower risk of heart disease.

Diet link: saturated and trans fats raise LDL; soluble fibre and swapping SFA for MUFA or PUFA lower it; exercise raises HDL.

Nutrition link

To keep LDL low, limit ghee, vanaspati, butter, fried snacks and fatty meat (saturated and trans fats). Choose mixed vegetable oils and eat soluble fibre from oats, barley, legumes and methi seeds. Regular exercise raises HDL, and ω-3 fats from fish and flaxseed lower triglycerides.

Exam angle

10 marks: “Classify lipoproteins and explain the function of each.” Give the table (origin, main lipid, function, density order), a line on apoproteins, and the LDL–HDL link to atherosclerosis.

5 marks: “How does LDL lead to atherosclerosis?” Excess LDL enters artery walls, is oxidised and taken up by macrophages as foam cells, forming plaques that narrow arteries.

2 marks: “Why is HDL called 'good cholesterol'?” It carries extra cholesterol from tissues back to the liver (reverse cholesterol transport), lowering heart disease risk.

Remember it

HDL is Healthy, LDL is Lousy. Density order, lightest first: Chylomicron, VLDL, IDL, LDL, HDL.