Plasma transport of triacylglycerol versus free fatty acids
Triacylglycerol (also called triglyceride) is hydrophobic (water-hating), so it travels in lipoproteins, whereas free fatty acids, which are not attached to glycerol, travel on albumin.
Remember
Because triacylglycerol is hydrophobic, it is packaged into lipoproteins to travel in plasma, whereas free fatty acids travel bound to albumin.
Memory aid
albumin is a taxi for single fatty acids, while lipoproteins are lorries for packaged fat
Albumin is the taxi for single fatty acids, and lipoproteins are the lorries for packaged fat.
Plasma, the watery fluid part of blood, is mostly water. Triacylglycerol is hydrophobic, meaning it repels water, so it cannot dissolve freely there.
A triacylglycerol (TAG, also called triglyceride) is one glycerol molecule joined to three long hydrophobic fatty acid chains. The glycerol is water-loving but small, so the three long chains dominate and the whole molecule is hydrophobic.
The body therefore packs it into lipoprotein particles, each with a fatty core wrapped in a water-loving shell of phospholipid and protein. Triacylglycerol from the diet travels in chylomicrons (particles that carry dietary fat from the small intestine into the blood). Triacylglycerol made in the liver is exported in VLDL (very low-density lipoprotein) particles. At the capillary walls in muscle and adipose (fat) tissue, an enzyme called lipoprotein lipase removes fatty acids from the particle for the cells to use or store.
Free fatty acids, also called non-esterified fatty acids, are single fatty acids not attached to glycerol. They are released from fat stores when glucose is low, and they travel bound to albumin, a protein in plasma. The word free means that they are not attached to glycerol, not that they travel unaccompanied: they still need albumin as a carrier.
So triacylglycerol travels in lipoproteins, whereas free fatty acids travel on albumin.
Key facts
- Structure
- triacylglycerol is one glycerol joined to three hydrophobic fatty acid chains.
- The problem
- triacylglycerol is hydrophobic, so it cannot dissolve in plasma, which is mostly water.
- Fat from the diet
- triacylglycerol travels in chylomicrons.
- Fat made in the liver
- triacylglycerol is exported in VLDL.
- Free fatty acids
- single fatty acids not attached to glycerol, released from fat stores, travel bound to albumin.
- Meaning of free
- not attached to glycerol, which does not mean carrier-free.
Exam traps
- Albumin does not carry intact triacylglycerol: albumin binds free fatty acids, whereas triacylglycerol is too bulky and insoluble for albumin and travels inside lipoproteins.
- A water-soluble glycerol backbone does not let triacylglycerol dissolve in plasma, because the three long hydrophobic fatty acid chains dominate and the whole molecule is hydrophobic.
- Chylomicrons are not the carrier of free fatty acids released from fat stores, because they carry dietary fat from the small intestine, whereas those fatty acids travel on albumin.
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