5. What do the terms 'reactant' and 'product' mean?
The terms reactant and product can be used to describe constituents of individual enzymatic reactions or entire metabolic pathways. Reactants (or 'substrates') are the starting materials for a reaction, In the generic reaction shown below, A and B are the reactants. C and D are the products.
The 'first' step of the Calvin-Benson cycle represents a specific example:
CO2 and RUBP are the reactants in this reaction, Rubisco is the enzyme, and the product is two molecules of 3PGA.
We can also refer to the reactants and products of entire metabolic pathways, such as of the light-independent reactions.
Often when we discuss a multi-step pathway, the molecules produced by the intermediate enzymatic steps are referred to as "intermediate metabolites." For example, if we diagram the Calvin-Benson cycle in more detail, we see some of the intermediate metabolites that are formed between the CO2 fixation step, synthesis of carbohydrate, and regeneration of RUBP.
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