• Introduction
  • The basics of anaerobic respiration
  • Anaerobic glycolysis
  • Lactate removal
  • Final thoughts
  • Sources
  • Aerobic respiration: producing energy (ATP) with the presence of oxygen.
  • Adenosine triphosphate: ATP is a molecule that carries energy within cells.
  • Anaerobic respiration: producing energy (ATP) without the presence of oxygen.
  • Citric acid cycle: a series of chemical reactions used by all aerobic organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats, and proteins into carbon dioxide.
  • Electron carrier: Small organic molecules that switch between oxidized and reduced forms and transport electrons during metabolic reactions
  • Glycolysis: the breakdown of glucose, which releases energy and produces two molecules of pyruvate, ATP, NADH, and water.
  • Lactate: a byproduct of anaerobic respiration. Known to cause fatigue and nausea.
  • Maximum oxygen uptake: The maximum amount of oxygen a person can use during intense exercise.
  • NAD+: An oxidized form of Nicotinamide adenine dinucleotide that accepts electrons from other reactions and becomes reduced.
  • NADH: A reduced form of Nicotinamide adenine dinucleotide that carries electrons from one reaction to another. An important cofactor in metabolism consisting of two nucleotides and their phosphate groups.
  • Pyruvate: The end product of glycolysis, and an important molecule in multiple biological pathways, such as the citric acid cycle.

Introduction

Energy metabolism consists of complex interconnected pathways that break down nutrients to form adenosine triphosphate (ATP). These processes can be divided into three separate systems; aerobic respiration, anaerobic respiration, and the phosphagen system. 

While each of these systems plays an important part in energy production, they rely on different mechanisms to do so. The energy gained through these systems is used during exercise as well as to maintain bodily functions such as breathing, heartbeat, hormonal activity, cell repair, etc.

Anaerobic respiration is the second fastest way to produce ATP after the phosphagen system. It is especially useful during high-intensity exercises that last from 10s to approximately four minutes. However, because anaerobic respiration produces lactate, it can only be maintained for a relatively short amount of time before a significant drop in performance.

This article describes the basic mechanisms of anaerobic respiration and what makes it so important in everyday life and physical performance. 

The basics of anaerobic respiration

Anaerobic respiration, also known as the lactic acid system, describes breaking down blood glucose and muscle glycogen to form ATP without the presence of oxygen. This can occur during high-intensity exercise when the muscles’ oxygen need surpasses the oxygen supply.

In the absence of oxygen, energy must be generated via fermentation. This process also produces lactate as a side product, which is considered to cause fatigue and muscle cramps. The lack of oxygen also means that glucose molecules cannot be oxidized in the mitochondria. Therefore, anaerobic respiration takes place in the cytoplasm of the cell. 

Although this produces ATP at a rapid rate, it is significantly less efficient than aerobic respiration, producing only two molecules of ATP for every molecule of glucose. For comparison, a fully oxidized glucose molecule produces 30-32 ATP molecules. 

Even though the three energy systems are conceptually different, it is important to remember that they all work simultaneously – be it during a workout or at rest. However, the amount of energy generated by each system depends on the intensity and duration of the exercise.

Duration

Classification

Energy Source

1-3s

Anaerobic

Stored ATP

3-10s

Anaerobic

ATP + CP

10-45s

Anaerobic

ATP + CP + Muscle Glycogen

45s-2mins

Anaerobic, Lactic

Muscle Glycogen

2-4mins

Aerobic + Anaerobic

Muscle Glycogen + Lactic Acid

>4mins

Aerobic

Muscle Glycogen + Fatty Acids

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Anaerobic Energy System

Without oxygenOccurs in the cytoplasmFast energy productionSmall energy storages (glucose)Produces lactic acid

Anaerobic glycolysis

The name glycolysis comes from the Greek words of glyk (sweet) and lysis (dissolution). Thus, it is aptly named to describe the process of breaking down simple sugars (glucose, fructose, galactose) to form ATP. Glycolysis is also an anaerobic process that takes place in the cytoplasm of the cell. This also makes it the main energy production method in fast-twitch muscle fibers that do not contain mitochondria.

Glycolysis starts by breaking down a six-carbon glucose molecule (C₆H₁₂O₆) into a three-carbon molecule called pyruvate (CH3COCOO−). In aerobic conditions, pyruvate can be fully oxidized in the mitochondria.

However, in anaerobic conditions, pyruvate molecules are reduced into lactate by NADH, leaving NAD+ (an oxidizing cofactor) after the reduction. This reaction is catalyzed by an enzyme called lactate dehydrogenase, which basically recycles NAD+ and allows glycolysis to continue.

Pyruvate + NADH

Lactate + NAD+

Lactate removal

Because skeletal muscle cells and red blood cells cannot utilize lactate as a direct source of fuel, they must dispose of it by releasing it to the bloodstream. Through a process known as the Cori cycle, the lactate produced in the muscles is transported to the liver. There, these molecules are converted back into glucose in a process called gluconeogenesis and sent back to the muscles.

Cardiac muscle cells are also able to transform lactate into pyruvate via lactate dehydrogenase. As you may remember, pyruvate is the end product of glycolysis, which allows more ATP to be produced via the citric acid cycle in the mitochondria (in aerobic conditions), or anaerobically in the cytoplasm. Thus, cardiac muscle cells are able to recycle lactate for energy while conserving blood glucose levels.

Anaerobic respiration is the main energy production system in exercises lasting from 10s to 4mins.

Final thoughts

Although anaerobic respiration provides energy for a limited amount of time, it does so incredibly quickly. This makes it especially important in high-intensity exercises that last anywhere from 10 seconds to several minutes. This includes a variety of sprint events as well as some court-based sports like basketball and futsal.

Interestingly, anaerobic respiration is also strongly connected to the muscle fiber type. Fast-twitch fibers (type IIa & type IIb/IIx) do not contain mitochondria like slow muscle fibers (type I), meaning that they must generate ATP anaerobically. However, being larger in size, they also contract with more force. 

The good news is that anaerobic exercise can significantly improve your lactate threshold and lactate buffering. As a result, you can maintain a high level of performance for a longer amount of time.

Did you learn anything new about anaerobic respiration? Let us know in the comments.

Sources

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