The top-down approach aims at simplifying existing small organisms, possibly arriving at a minimal genome.
The bottom-up approach aims at constructing artificial chemical supersystems that could be considered alive. No such experimental system exists yet; at least one component is always missing.
Metabolism seems to be the stepchild in the family. Basically, what most researchers in the field used to call 'metabolism' is typically the trivial outcome of need for material input of both template replication and membrane growth. This input is usually simplified to a conversion reaction from precursors to products."