2. Carbon was selected as the key element for life due to its versatile bonding capacity 2.1 Carbon accounts for more than one-half the cell dry weight 2.2 Each carbon atom can form very stable single bonds with one, two, three, or four other carbon atoms, and double or triple bonds can also be formed between two carbon atoms 2.3 Covalently linked carbon atoms can form linear chains, branched chains, and cyclic and cagelike (E 的) structures 2.4 To these carbon skeletons are added functional groups conferring specific activities to the molecules.2. Carbon was selected as the key element for life due to its versatile bonding capacity 2.1 Carbon accounts for more than one-half the cell dry weight. 2.2 Each carbon atom can form very stable single bonds with one, two, three, or four other carbon atoms, and double or triple bonds can also be formed between two carbon atoms. 2.3 Covalently linked carbon atoms can form linear chains, branched chains, and cyclic and cagelike（笼形 的）structures. 2.4 To these carbon skeletons are added functional groups conferring specific activities to the molecules