Notes modified from comments of a biochemist friend:
In Figure 8.2, the legend describes a molecule as propane. This should be correctly described as cyclopropane, since the carbons are all linked in a triangle.
Fig. 8.2 illustrates cyclopropane, which is drawn to illustrate how carbon bonds can form at angles. The tension in this configuration makes cyclopropane much less stable than propane itself, which is a linear compound, C-C-C (3 carbons and eight hydrogens, the hydrogens being attached as follows (periods are added to adjust spacing for web):
….H H H
….| | |
….| | |
…H H H
Slightly below this figure, the point is made that carbon-based compounds are not very stable above 50 C. The carbon-to-carbon bonds are relatively strong and stable, so that high-temperature oils and the compounds found in fossil fuels tolerate high temperatures, as do some plastics. (The whole point of frying food is that the oil reaches temperatures at which other compounds cook or denature.) The most important molecules for living organisms, however, are bent into and held in specific shapes by much weaker interactions among atoms in different parts of the molecule, called hydrogen bonds and Van der Waals bonds. It is these latter that are unstable to heat or changes in acidity or ion content, thus determining the fragility of life.