more often in the combinations of atoms that are molecules. We know of 93 naturally occurring elements, the most common in living matter being carbon, oxygen and hydrogen.
Molecules: their electronic make-up, or chemical properties, allow two or more atoms to join together into combinations called molecules. These are the building blocks of all structures, living and inorganic. The simplest are made up of two atoms of the same element: for example the element oxygen is most often found as a molecule of two oxygen atoms bound by their electrons. Water molecules are a combination of one oxygen and two hydrogen atoms.
Just as with atoms, molecules have chemical properties resulting from their electronic make-up that may cause them to interact with other atoms or molecules. For example, individual water molecules attract each other slightly, giving water the physical property of surface tension that holds a raindrop intact upon a leaf or allows a light insect to walk upon its surface. More complex interactions bring about change in the participating molecules and atoms; for example, water 'dissolves' salt by separating out the sodium and chlorine atoms that make up its molecules.
Living processes such as photosynthesis involve very specific changes in molecules and atoms; they are carried out by large, complex molecules that have very particular chemical properties allowing them to 'direct' change in their chemical environment that would not normally happen.
In healthy, viable ecosystems, resources are constantly being recycled in a complexity of inter-related cycles. For instance nutrients in soil are taken up by plants, and the plants grow, die and decay to become part of the soil to nourish new plants. Within this process, the element nitrogen is taken up by plant roots as a vital nutrient and released again into the soil by the micro-organisms which are the agents of decay. These are two simple inter-related cycles in an ecosystem, but they are part of increasingly large resource-cycles, the largest of all being the Biosphere in which we live.
The recycling of resources is facilitated by both living and non-living processes. For instance, water is re-cycled through plants by transpiration which is a living process, but taken up into the air from the surfaces of leaves to become clouds by evaporation which is a non-living process. Similarly, elephants drink water as a living process, while the percolation of water through soil and into rivers is a non-living process.
