Why is cyclohexane more prevalent than cyclopentane in biological systems?

[MathewsMD]

I was just reviewing some organic molecule structures and came across cycloalkenes. Reading through some material, it states that the planar structure of cyclopentane is more stable structurally than cyclohexane. Yet cyclohexane is more common in biological systems as opposed to any other type of structure. It doesn't offer any comparison between the chair or boat structure of cyclohexanes and the regular structure of cyclopentanes, but if there was an evolutionary reason cyclohexane is more predominant than any other cyclic molecule, cyclopentane particularly.

Also, what is the exact reason that only cis bonds are allowed in cyclic molecules with 7 or less central atoms? I am trying to come up with the structure but why 7 exactly? What exactly about the trans orientations in other types of cyclic molecules makes it unstable?

 

[Borek]

I believe in both cases it is about the strain. Cyclopentane molecule is mole "stiff" than the cyclohexane, but it is cyclohexane that is more stable thermodynamically, as all its bonds have their natural angles (strained bonds are more eager to react). Same problem with cis-trans bonds in cyclic molecules. Try to build a model if you can - I doubt you will be able, more like trans double bond will snap.