Several types of macromolecules are involved in forming a living system: lipids, carbohydrates, proteins, and lastly, nucleic acids.
The lipids are a group of polymers, one end of which is hydrophilic, with the other by hydrophobic (Fig. 9.1). Overall, they are poorly soluble in water, and form aggregates that are weakly bound chemically, which allows them to store energy, while retaining a high degree of flexibility, which is particularly useful in the formation of membranes.
Carbohydrates are polar molecules, possessing hydroxyl groups (OH bonds), and are thus soluble in water. The sugars, in particular (Fig. 9.2), are carbohydrates, which, dissolved in water, adopt a circular form. They are able to link with one another to form polymers, such as the polysaccharides. These can store energy and provide structure to organisms.
Fig. 9.1 Examples of the molecular structure of two lipids, octadecanoic acid (stearic acid) and octade-canoic acid (oleic acid). Both have a hydrophilic head and a hydrophobic tail (After Gilmour and Sephton, 2003 © Cambridge University Press and The Open University)
Octadecanoic acid (stearic acid, C18)
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