Metabolism 101: Why Anaerobic Respiration?

Oxygen, the essence of life. Why? Well that’s what this post is all about 😉

Our bodies can break down glucose through two ways, with or without oxygen. What determines how it’s broken down? Well remember that reducing agent, NAD+. Yeah… that little guy that accepts electrons from others to become NADH+ H+. The balance between NAD+ and NADH+ H+ is the key, let me explain to you why.

Typically glucose breakdown consists of 3 distinct parts- glycolysis, Kreb’s cycle (aka citric acid cycle aka tricarboxylic acid cycle), and electron transport chain (ETC). I will focus on the big picture as I have memorized these pathways in detail multiple times for school just to forget them.

In glycolysis, the 6 carbon glucose is broken down into two 2 carbon pyruvate molecules. Through the process there’s a net production of 2 ATP molecules and 2 NAD+ molecules get reduced to 2 NADH+ H+. Subsequently, each pyruvate molecule loses a carbon as carbon dioxide, generates another NADH+ H+, and eventually becomes acetyl-CoA.

Then each acetyl-CoA will enter the Kreb’s cycle to be fully broken down into 2 carbon dioxide molecules, 3 NADH+ H+, 1 FADH2, and 1 GTP (like ATP). At this point the glucose has been fully broken down.

Now the big money (ATP) maker is the electron transport chain reaction. Here NADH+ H+ and FADH2 will unload their gained electrons to produce ATP. The process is driven by oxygen which will react with NADH+ H+’s electrons and hydrogens to form water. At the same time NAD+ and FAD will be reconstituted. In total, aerobic respiration will yield 38 ATPs.

Without oxygen, the electron transport chain will not occur and NAD+ with FAD will not be regenerated. With no NAD+, glycolysis or the first step of glucose metabolism cannot occur and absolutely no ATP can be generate. That’s when the body goes to plan b! Anaerobic respiration! By converting glucose into lactate we can regenerate NAD+. Unfortunately, this process is highly inefficient and only produces 2 ATP molecules. That’s why you can only sustain anaerobic activity for such short bursts. Think about it you can only make 2 ATPs instead of 38 ATPs through aerobic respiration. Better then nothing during the fight or flight response though 😉

This is the reason why we undergo anaerobic respiration. We can only sustain anaerobic respiration for a short time and hence will die without oxygen. In the next post I’ll talk about what happens to that lactose and how anaerobic respiration has been taken advantage off in food production.

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