The Basics of Cellular Respiration

What is Cellular Respiration?

Cellular respiration is the set of reactions that produces ATP. ATP, quite possibly the most talked about molecule in biology, is our energy currency.

Cellular respiration uses glucose to produce the ATP our body needs to perform essential functions. I am going to treat this as an overview article and I will do separate, more detailed articles on each part.

Cellular respiration can be divided into three parts: glycolysis, the citric acid cycle and electron transport chain. This article will give you the basics in order to better understand the more detailed stuff. And believe me, cellular respiration can get pretty complicated.

Important Molecules Involved

Before delving into the parts of cellular respiration, it is important to know a little about the key molecules. The first one is glucose, which is a sugar made of a 6 carbon ring. This is the starting molecule for cellular respiration and an important source of energy in cells.

ADP, or adenosine diphosphate is also one of the starting molecules of cellular respiration. It is formed when one of the phosphate groups on ATP (adenosine triphosphate) is lost. ADP is easily converted back into ATP, which happens in cellular respiration.
ATP is probably one of the most discussed molecule in biology. It is responsible for energy transfer in our cells. It is made of ribose (a type of sugar), three phosphate groups and adenine, which is a nucleotide.

NAD is a coenzyme used in oxidation-reduction reations, or redox reactions for short. Redox reactions remove or add electrons. Oxidation is the loss of elections while reduction is the gain of electrons. The function of NAD is to transport these electrons. NAD is an oxidizing agent, which means it is reduced. NADH is the oxidized form of NAD and is a reducing agent. A reducing agent will lose its electron to another molecule. It is a bit confusing, but just remembering that oxidizing agents are reduced and reducing agents are oxidized. OILRIG (oxidation is loss reduction is gain) will also help.

Parts of Cellular Respiration:

Cellular respiration occurs in the cytosol of the cell and it begins with glycolysis.

Glycolysis is the first cycle of reactions in cellular respiration. This is the process of breaking glucose into two molecules. It starts with glucose, ADP (adenosine diphosphate) and NAD.

Glycolysis is special because it can proceed with or without oxygen (anaerobic). Glucose is a 6 carbon ring and it is broken into two molecules with 3 carbons each. These pieces are called pyruvate, or pyruvic acid.

Glycolysis consists of 10 reactions, each catalyzed by enzymes. In addition to pyruvate, NADH and two net molecules of ATP are produced. When glycolysis proceeds without oxygen, it is called fermentation.

Following glycolysis is the citric acid cycle, or Kreb’s Cycle. The citric acid cycle uses slightly altered forms of pyruvate to produce two net molecules of ATP and even more NADH to be used in the next part of cellular respiration, which is the electron transport chain. The electron transport chain involves the transfer of electrons between a donor and acceptor. This is actually the step that produces the most ATP, which are about 34.

The end products of cellular respiration are roughly 36-38 ATP, water and carbon dioxide. The individual pieces of cellular respiration are more involved, but this a nice overview of the steps and important terms. Stay tuned for a more in depth look at glycolysis.

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