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Figure 2.14. Phosphate groups can attach to COH groups or to other phosphate groups.

Figure 2.15. Nicotine adenine dinucleotide is formed of two nucleotides joined via their phosphate groups.

a pyrimidine, or nicotinamide. In general, a nucleoside is formed by attaching one of these bases to the 1'-carbon atom of ribose.

Phosphorous, although by weight a relatively minor fraction of the whole cell, plays a number of critical roles. In solution phosphorous is mainly found as a phosphate ion with a single hydrogen atom still attached, HPO42- (Fig. 2.14a). We often indicate phosphate ions with the symbol Pi, meaning inorganic phosphate. Where phosphate becomes important, however, is when it is attached to an organic, that is, carbon-containing, molecule. Phosphate can substitute into any C—OH group with the loss of a water molecule (Fig. 2.14ft). The equilibrium in the reaction shown lies far to the left, but cells have other strategies for attaching phosphate groups to organic molecules. Once one phosphate group has been added, more can be added to form a chain (Fig. 2.14c). Once again, the equilibrium in the reaction shown lies far to the left, but cells can achieve this result using other strategies.

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