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Fats are generally used to store energy, so organisms try to maximise how much fat they can store. 

Saturated fats are straight (as they contain no double bonds) and can pack together closely. Unsaturated fats contain bends, so cannot pack together as closely. This means that saturated fats are more efficient, as you can squeeze more in. 

If this was the only thing to take into account, all organisms would probably use saturated fats.

However, organisms also prefer their fats to be liquid at their normal “operating temperature”. Humans, for example, operate at around 37 degrees, so require our fats to be liquid at that temperature. Plants will operate at a lower temperature.

As saturated fats are straight, and pack together well, they solidify at higher temperatures than unsaturated fats. So saturated fats are well suited to organisms who have a higher body temperature, like mammals. 

Unsaturated fats cannot pack together as closely, so remain liquid at lower temperatures. This makes them more suitable for plants. 

Interestingly, this also means that plants that exist in tropical climates have more saturated fats than plants that exist in colder climates. 

Brandon Turner

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Brandon Turner