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I know that animals can't make poly-unsaturated fatty acids (PUFA) and so require them from dietary sources. For eg.Omega -3 and Omega 6 fatty acids.

My questions : Can animals synthesize other unsaturated fatty acids from scratch? If not, does the unsaturated fatty acids in the phospholipids of plasma membrane of animals come only from dietary sources?

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At least fish should be able to do this - most Omega-3 and Omega-6 fatty acids used for food supplementation are extracted from fish, namely salmon or cod. –  Chris Apr 28 at 12:32
    
Fish don't make it themselves either, they get it from algae through the food chain. –  jarlemag Apr 28 at 12:35

2 Answers 2

Yes, animals can make their own unsaturated fatty acids.

Mammalian fatty acyl desaturases can introduce double bonds at the Δ5, Δ6 and Δ9 positions (i.e. numbering from the functional group). As shown in the diagram below, this means that we cannot introduce double bonds at the ω3 or ω6 positions (i.e. numbering from the methyl end of the molecule) in fatty acids of a reasonable length (see any Biochemistry textbook for confirmation).

I've illustrated the impossibility of us making α-linolenic acid as an example.

enter image description here

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So can the plasma membrane unsaturated phospholipids be made from scratch ? –  biogirl Apr 28 at 14:06
    
@biogirl Yes, the unsaturated fatty acids will be incorporated into new phospholipids at the endoplasmic reticulum then transported in vesicles to the plasma membrane. –  Alan Boyd Apr 28 at 15:01

Only linoleic acid ($\omega 6$) and $\alpha$-linolenic acid ($\omega 3$) are essential. Other PUFAs are synthesized from these by desaturases (as Alan Boyd pointed out).

I haven't come across any study that reports de-novo synthesis of $\alpha$-linolenic acid in animals. However, this study reports that mites can synthesize linoleic acid.

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