# Why do some chemical reactions require many steps? [duplicate]

I was a college student of physics ten years ago, and recently started to learn biochemistry. I enjoy finding out that some familiar concepts in physics play important roles in biochemistry such as Entropy and Gibbs free energy.

For example, as a (ex-)student of physics, I am happy to know that Gibbs free energy helps to decide the directions of chemnical reactions. I feel this is a good example where a sort of fundamental law of physics determines how phenomenon looks like.

However, I still can not understand why the chemical reactions in a body need to be so complex. Many chemnical systems consists of more than several steps to acheive their purposes. According to wikipedia, glycolysis takes ten steps through its process. Why so many steps are necessary?

I tried to find out a physical law which prohibit that glycolysis process is achieved by one or two steps. But I could not find an answer.

I would like to know (or discuss) whether there is a physical law which makes the chemical system such a complex one (many steps required).

My assumption is that some physical law prohibit the existence of a enzyme which realize a one-step process of glycolysis.

## migrated from physics.stackexchange.comJun 22 '18 at 17:47

This question came from our site for active researchers, academics and students of physics.

• ...why the chemical reactions in a body need to be so complex... more than several steps to acheive their purposes....Why so many steps are necessary? What need? What purpose? Biochemical reactions do not have a mind of their own. They were not planned or designed. – sammy gerbil Jun 19 '18 at 3:46
• Because you can find many models of biological systems in Physical Review L/E/X to explain the fundamental property of them, I think physicians are suitable for answer/discuss about this topic. But I might be wrong. I am going to transfer this question to SE.Chemistry. Due to the reputation limnitation, I can not close this. Sorry... – rkjt50r983 Jun 19 '18 at 9:47

The number of steps required to model a reaction really depends on what time scales you are interested in looking at. Let's take the process of burning natural gas with air. If you only care about the final products of the combustion after a really long time, like let's say hours, then a single step reaction would be a perfectly good model. In fact, in situations where you only care about the global heat release and maybe the speed of the reaction at one particular condition, single-step or few-step reactions are just fine.

But let's say you are really interested in what's happening. When we look at a global reaction, let's say:

$$O_2 + 2H_2 \rightarrow 2H_2 O$$

We'll assume this is in air, but the nitrogen doesn't really react. Nice and simple, right? Well, for the reactions to occur, you need some $O_2$ molecules to collide with $H_2$ molecules. But it's not enough that they collide. They need to collide with enough energy to break the bonds between them. And it's possible that one of the molecules breaks at lower energy than the the other. When they break, they break into 2 O's or 2 H'2. So our simple reaction really starts out as:

$$O_2 + M \rightarrow 2 O + M$$ and $$H_2 + M \rightarrow 2 H + M$$

where $M$ is any other molecule. Okay, so now we've got a mixture of $H$, $O$, $H_2$, $O_2$, and $N_2$ floating around and bumping into each other. If they bump together with enough energy, we can start forming other things. Things like $HO_2$, $OH$, $H_2O_2$ and of course, $H_2O$.

Because these reactions rely on collisions, you need to form some things before you can form other things. So before an $O$ and an $H$ can collide to form $OH$, the $O_2$ and $H_2$ have to collide with other things and break apart. So there's finite time scales with all of the reactions, and they tend to cascade through a series of reactions before reaching the final product. If you are looking at time scales similar to those of the reactions, then these details really matter.

Hydrogen reactions are super simple. Imagine how complex organic molecules are. Even things like methane or propane have hundreds or thousands of steps to go from the complex molecule through all of the radicals to the final species. And the time scales the reactions occur over are all based on how frequently the collisions between them occur, and how much energy those collisions have.

Better question on Chemistry.

There is no physical law that prohibits multiple steps. Biochemical systems in particular tend to have complex mechanisms since very high specificity is required for the products.

Even for many nonbiological systems all the intermediates are often much more complex than the overall chemical reaction would indicate.