Many of the structures present in multicellular organisms can be seen as solutions to problems of pick-up and transport for various substances (oxygen, nutrients, minerals, or wastes) around the body. "Transport tubes" typically form circulatory or vascular systems in plants and animals. These are often accompanied by specialized organs that make hormones and serve other functions. All of these structures add to body size, but we know that bodies are not just mounds of cells. Support systems or structural elements, like bones, are usually present. Bone is one example of a structural element, but there are others, including wood, shell, and exoskeleton, to name just a few. These elements are made of proteins and minerals, among other things. In the section below, refresh your memory on the chemical constituents of a few structural elements and then find out where these constituents occur in the non-living world. Q12. Proteins are made of amino acids. What elements make up amino acids?