'Constructive' metabolism refers to synthesising larger molecules from smaller ones. Or even transforming precursors to active chemicals. Making retinal from beta carotene, for example, is constructive in that you end up with a visual pigment - even though the carotene is 'broken down' by chopping it in half. Many of the starting points of anabolic pathways lie in the TCA cycle, such as provision of carbon skeletons for amino acids. Since some chemicals can be absorbed while already in a useful state, not all possible anabolic pathways are necessary for comlex organisms. Those creatures that live only on rocks and air, on the other hand, need to make the majority of thier essential components.