DESIGN OF THE OXYGEN AND SUBSTRATE PATHWAYS .1. MODEL AND STRATEGY TOTEST SYMMORPHOSIS IN A NETWORK STRUCTURE

This first paper in a series develops a model of structure-function re lationships for the oxygen and substrate pathways of oxidative metabol ism in working muscle, This will be used in the subsequent experimenta l papers in asking how biological structures are designed if they serv e more than one function and whether one function can be served by mor e than one structural pathway, We have used the concept of symmorphosi s to address this question; in its original form, it postulates that n o more structure is built and maintained at each step in a pathway tha n is required to meet functional demands, The concept of symmorphosis was developed to deal with the problem of modelling the pathway for ox ygen from the environment to mitochondria, essentially a single series of interconnected transfer steps, In the present context, the applica tion of this concept is more complex, Both oxygen and substrates are t ransported directly from the blood to the mitochondria in what appear to be shared steps, The flows along this direct pathway are adjusted d uring muscular work, However, substrates have an additional option, Th ey can be stored intracellularly as lipid droplets or glycogen, and th us their supply to mitochondria can occur in two steps separated in ti me: from capillaries to stores during rest, and from stores to mitocho ndria during work, The integrated pathways have a network structure an d the functional flows are partitioned to different branches of the ne twork, and we must ask whether the partitioning of fluxes is related t o design constraints, The principle of symmorphosis predicts that the best use is made of the available options and that the design of each step is matched to the specific functional demand in view of a balance to be achieved over the entire network, This will be tested in subseq uent papers by determining maximal flows for oxygen, carbohydrates and lipids through each of the transport steps and their respective struc tural capacities, comparing dogs and goats, animals of the same size w hose maximal oxidative capacities differ by more than twofold, Finally , we will ask whether the principle of symmorphosis can be extended to apply to network systems.