The backbone of the double helix has an alternating molecules. what do you call these alternating molecule?

This backbone is composed of alternatingsugar and phosphate groups, and defines directionality of the molecule. Thesebonds are called phosphodiester bonds, and the sugar-phosphate backbone is described as extending, or growing, in the 5' to 3' direction when the molecule is synthesized.

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.