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. Every time energy is converted from one form to another, some of the energy is lost as heat. That means that during each of these enzyme-catalyzed reactions, some of the original energy from the sugar molecule is lost as heat.

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, but here is a basic summary:

• During glycolysis, 6-carbon glucose is broken in half and a small amount of energy is transferred to ATP and other energy carrier molecules.
• One carbon atom is broken off of each of the two halves of the glucose molecule (3-carbon molecules known as pyruvate) and released as carbon dioxide. This leaves two 2-carbon molecules called acetyls, which are attached to Coenzyme A to make acetyl-CoA.
• Acetyl-CoA enters the citric acid cycle, where it is completely broken down into carbon dioxide and all the energy from the molecule is transferred to ATP and other energy carrier molecules. The carbon atoms are released as carbon dioxide.
• The energy carrier molecules produced during glycolysis and the citric acid cycle are used to power the electron transport chain and chemiosmosis (together known as oxidative phosphorylation). The end result of this is the majority of ATP produced during aerobic respiration.

Anaerobic Processes

Anaerobic Cellular Respiration

Some organisms (mostly bacteria) perform anaerobic cellular respiration. During anaerobic cellular respiration, cells use the same three basic stages: glycolysis, the citric acid cycle, and the electron transport chain / chemiosmosis, but another molecule is used in place of oxygen gas. This is not a common form of cellular respiration and isn’t used by humans, so we will not be focusing on it.

Just as a side note here, typically when “anaerobic respiration” is mentioned in videos and animations, they are not talking about this process. They are talking about fermentation (below).

Fermentation

Fermentation is also a form of cellular respiration that occurs in the absence of oxygen. There are several different types of fermentation, which will be discussed in more detail later. Fermentation releases a much smaller amount of energy than aerobic respiration. In fact, it 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. Fermentation 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.

All types of fermentation 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 Fermentation

Table 8.31 Comparison of aerobic respiration and fermentation.

	Aerobic Respiration	Fermentation
Requires oxygen?	Yes	No
Glucose breakdown	Complete	Incomplete
End products	CO2 and H2O	Animal cells: lactic acid Plant cells and yeast: carbon dioxide and ethanol
ATP produced	About 38	2

Aerobic respiration is much more efficient than fermentation. 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 fermentation.

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

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