Fatty acids from cell membranes digestible?

[WillyEngland]

Vegetables are considered to contain almost no fat.

But all cell membranes consist of fatty acids, simplified.

Why do they not contribute to the energy content?

What is it, that we cannot digest them?

[John Cuthber]

Plant cell walls are thin and partly made of sterols.

[StringJunky]

Plant and mammalian organisms require different metabolic pathways, enzymes to directly break them down for respiration that is suitable for each system. Mammalian metabolism requires extra steps for plant-based fatty acids to be useful in respiration. 

Edited June 21 by StringJunky

[joigus]

Sorry. I was editing and it came out by mistake. I'm working on a proper answer.

Ok. Here's my answer. As @StringJunky said. You need enzymes, if you want to digest anything. If you have molecules that constitute cellular walls it makes a lot of sense that they be stable under a wide range of conditions. Eukaryotes use phospholipids as cellular walls, with the phosphoric group pointing out, and the fatty acid pointing in.

I think there are good evolutionary reasons why no eukaryot would 'want' to evolve an enzyme that digests cellular membrane, as it is shared by all eukaryotes in the form of a double layer, like this:

It would be like a suicide mechanism for all eukaryotes. Why do it?

It's not because nature can't do it. Nature can, if it sets its non-mind to it. Think about N₂. A sturdy molecule if there is one. Yet organisms have developed enzymes to break it. But why break down something that's the first chemical step to make you?

Edited June 21 by joigus
correction

[CharonY]

Eh, I think we are talking about different scopes here. Lipid contents of cell membranes are quite minuscule. When we talk about dietary fats, we really mean large-ish fat deposits relative to the residual mass. A complete dissolution on the cellular level would likely take more time than the residence time in your gut. Remember, food is moved through your gut and gets showered by e.g. bile salts and enzymes, but you only extract a portion out of it before it moves further and ultimately excrete it.

[John Cuthber]

On 6/21/2024 at 1:49 PM, StringJunky said:

Mammalian metabolism requires extra steps for plant-based fatty acids to be useful in respiration. 

How?
What's the difference between oleic acid from lamb and oleic acid from an olive?
 

Edited June 22 by John Cuthber

[StringJunky]

3 hours ago, John Cuthber said:

How?
What's the difference between oleic acid from lamb and oleic acid from an olive?
 

What has that got to do with the question? There is zero difference between them, but how they are used for respiration, they have different processes to achieve the same goal i.e. energy production. 

Edited June 23 by StringJunky

[WillyEngland]

On 6/21/2024 at 3:44 PM, joigus said:

It would be like a suicide mechanism for all eukaryotes. Why do it?

Hey, that makes sense!

Thank you.