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Metabolic pathways are defined for groups of substances (metabolites) that are interconverted by a corresponding group of enzymes. Conceptually, they start with one substance (glucose, say) and end with another (carbon dioxide). However, there is no reason why they shouldn't be connected in a circle rather than an arrow, as is the case for the TCA cycle.

A  -> B ->  C -> D -> E pathway

            F   <-   E <
          /              \
           A            D
         \               / 
            B  -> C     
 cycle

Naturally, the whole of the cell is part of one large metabolic net consisting of many pathways that intertwine and mix. This is the basis behind the concept of autopoesis - that the cell is catalytically closed. One large cycle.

Metabolic pathways are the basic subcomponents of metabolism. They refer to specific sequences of reactions aimed towards a single goal. Metabolic pathways are rather similar throughout organisms, significant evidence for both the conservation of energy by Nature and evolution. They are specified by an organism's DNA, since DNA is the blueprint for proteins and enzymes, which catalyze the reactions that form metabolic pathways.

There are several classifications of metabolic pathways. The broadest are anabolic and catabolic:

  • Anabolic pathways initiate synthesis reactions. They serve in the creation of large molecules. The reactions are endrogonic, they require energy which is gathered from...
  • Catabolic pathways, which break down larger molecules into small molecules and energy. This decomposition fuels the processes of anabolic pathways.
It is sometimes very difficult to determine the starting and ending points of a pathway. Pathways are extremely interwoven in any organism, they feed and initiate each other in complex interactions that are hard to track. Many times the start and the end of a pathway are really just up to the arbitrary definition of whoever is currently studying the pathway.

Based off of this, there are more specific subcategories that classify the shape of a metabolic pathway:

  • Linear pathways use each product as the substrate for the next reaction. They go from point A to B to C in a straight line.
  • Cyclic pathways follow reactions in a closed loop; the intermidiates (metabolites, middle-step products) are constantly regenerated. The_Alchemist's metabolic pathway above is a cyclic pathway.
  • Spiral pathways use the same enzymes to increase or decrease the size of a molecule. Sort of like cyclic pathways, but they have an end. Basically, the pathway loops back on itself over and over again until the very large or very small target molecule is finally reached, whereupon the pathway ceases. This is what differentiates spiral pathways from cyclic pathways.

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