Metabolism 101: Energy & Reactions

Metabolism! Fascinating topic! Do you know what the word actually means? Technically metabolism is a series of interconnected pathways that are responsible for energy production or synthesis of molecules using energy. Woaa! That’s a mouthful! Metabolism is a very vast and complicated field. Most biochemistry students have to take a course specifically on metabolism which only covers the basics. Hence, in the next series of posts I have decided to go over the most important aspects of metabolism which will help you understand how it works and its relation to nutrition.

I’ve decided to begin by addressing an important question. What is energy within a biological system and why do we need to consume calories?

Energy is an interesting concept because it can neither be created nor destroyed.  However, it can be transformed into a different form of energy or be transferred between a system and its surroundings. In a living organism energy is obtained by breaking down macronutrients and trapping their energy as adenosine triphosphate (ATP). The ATP can then be used to make vital bodily reactions occur.

The thing is that there’re two types of reactions those that require energy (endothermic) and those that give off energy (exothermic). These reactions can be coupled in that the energy from an exothermic reaction will power the endothermic reaction. 

An exothermic reaction is when  the reaction’s reactants are more energetic than the products. Thus, there is energy given off during the reaction which can then be stored or used to make another reaction happen. A typical exothermic reaction would be the metabolism of glucose. The glucose molecule is highly energetic and our body has learned to break it down through a series of reactions that turn glucose into carbon dioxide and water. Do note that there is an initial energy investment required but using enzymes this step can be skipped.

Glucose-Exothermic Rx

An endothermic reaction is the complete opposite of an exothermic reaction. The initial reactants have little energy but after the reaction the products have a high level of energy. Therefore, energy must be absorbed or used up in order for the reaction to occur. In the case of ATP, energy from glucose breakdown is used to attach a phosphate group to ADP to form the highly energetic ATP molecule.

ATP-Endothermic Rx

The energy trapped in the form of ATP can then be used to make other reactions occur. In order to move our muscles, ATP molecules are broken down into ADP and phosphate. This reaction involves an exothermic reaction of ATP being broken down into ADP and phosphate using an enzyme called actomyosin ATPase. The potential energy from ATP is then converted from into kinetic energy and thus displacement.

So back to the question. If energy cannot be made or destroyed, why do we need to consume calories? Well when we use up ATP, the energy gets transformed into something we can no longer use or is lost to the surroundings in the form of heat. Technically, this energy is not “destroyed” but is no longer available to us and must be replenished.

Now that I’ve covered the role of energy in our bodies, I will follow up on posts on oxidation & reduction and enzymes. Although these ideas might sound dry they’re fundamental to understanding metabolism 😉

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