Glucose and other molecules from food are broken down to release energy in a complex series of chemical reactions that together are called cellular respiration.

Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into ATP, and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process. These processes require a large number of enzymes which each perform one specific chemical reaction.

Aerobic Respiration

Aerobic respiration requires oxygen. This is the reason why we breathe oxygen in from the air. This type of respiration releases a large amount of energy from glucose that can be stored as ATP. Aerobic respiration happens all the time in animals and plants, where most of the reactions occur in the mitochondria. Even some prokaryotes can perform aerobic respiration (although since prokaryotes don’t contain mitochondria, the reactions are slightly different). The overall chemical formula for aerobic respiration can be written as:

C₆H₁₂O₂ + 6 O₂ → 6 CO₂ + 6 H₂O + (approximately) 38 ATP

Translating that formula into English: One molecule of glucose can be broken down in the presence of oxygen gas to produce waste products of carbon dioxide (which we breathe out) and water. This process has an overall release of energy which is captured and stored in 38 molecules of ATP.

Aerobic respiration is a complex process that can be divided into three basic stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. The next several sections in the textbook address the details of these stages.

Anaerobic Respiration

Anaerobic respiration occurs in the absence of oxygen. It releases a much smaller amount of energy than aerobic respiration. Anaerobic respiration does not release enough energy to power human cells for long – think about how long a person can live if they are not able to breathe. Anaerobic respiration occurs in muscle cells during hard exercise (after the oxygen has been used up). It also occurs in yeast when brewing beer. Many prokaryotes perform anaerobic respiration.

There are several different types of anaerobic respiration, which will be discussed in more detail later. All the types of anaerobic respiration involve glycolysis, and none of them go through the citric acid cycle or oxidative phosphorylation. Instead, various other methods are used to regenerate the molecules needed for glycolysis, For now, we will summarize them all using this chemical formula:

C₆H₁₂O₂  NAD+ → various waste products + NADH + 2 ATP

NAD+ and NADH are two states of a molecule that will carry energy during this process. It will be addressed further in a later section. For right now, just know that NADH carries energy (similar to ATP) and NAD+ is the form that carries less energy (similar to ADP)

Aerobic vs anaerobic respiration

Aerobic respiration is much more efficient than anaerobic respiration. One molecule of glucose can generate up to 38 molecules of ATP if aerobic respiration is used. In contrast, only 2 molecules of ATP are generated in anaerobic respiration.

To put it another way, a cellular process which requires 100 molecules of ATP:

• Will require about 2.5 molecules of glucose to be broken down using aerobic respiration (100 / 38 = 2.63)
• Will require 50 molecules of glucose to be broken down using anaerobic respiration (100 / 2 = 50)

References

Wikipedia.