What is the difference betwween a linear and a cyclic metabolic pathway?

Question

Illustrate your answer with an example of each type of pathway.

Answer 1

A linear pathway is a set of reactions where the starting material and the product are not the same thing.  In this case, the reactions lead to a dead end, as far as being able to continue the process.  For example, enzymes in your cells convert vitamin A into retinal, which is the major light-absorbing pigment in your eyes.  This is a linear pathway because almost all of the vitamin A is used to generate retinal, and none can be re-used to help out with another reaction.  

Cyclic pathways are ones that have complex starting materials with are not completely consumed in each reaction.  In order to keep the reaction going, the process has a way to regenerate the complex starting materials.  For example, the starting materials for the Calvin cycle are carbon dioxide and a five-carbon molecule called RuBP.  The product is calle G3P, which is used to make sugar, but it takes three cycles of the reactions just to net one G3P.  So it is very important that the cell be able to keep the process going.  For that reason, the Calvin cycle regenerates RuBP so that it can continue as long as there are carbon dioxide molecules around.

Answer 2

A linear pathway begins with one substance and converts that into something different. For example, glycolysis begins with glucose and ends with the formation of 2 pyruvate molecules (plus some other stuff).

A cyclic pathway begins and ends with the same thing. So, the citric acid cycle (Krebs' cycle or TCA cycle) begins with oxaloacetate (plus acetyl-CoA) and ends back with oxaloacetate.