Cellular Respiration

I. Many cells (including our own) are capable of generating adenosine triphosphate (ATP) using pathways that either rely upon oxygen (aerobic) or do not rely upon oxygen (anaerobic). Though both types of respiration convert the energy found in sugars, fats, amino acids or nucleotides into ATP, aerobic pathways in general produce far more ATP per sugar molecule than do the anaerobic pathways. For this reason, aerobic organisms usually grow much faster than anaerobic organisms. ATP is the form of energy which cells will use to synthesize those molecules needed to keep the cell alive. We will concentrate on the metabolic pathways involved in the breakdown of carbohydrates and especially glucose.

A. The breakdown of glucose begins with an anaerobic pathway known as glycolysis. In both eukaryotic and prokaryotic cells this pathway occurs. The products of this pathway can be introduced into anaerobic pathways, collectively referred to as fermentation, or into aerobic respiration which involves the pathways known as the tricarboxylic acid cycle (also known as the citric acid cycle or Kreb's cycle), the electron transport chain, and chemiosmosis.

B. During glycolysis and the Kreb's cycle high energy electrons are released. These electrons reduce nicotinamide adenine dinucleotide (NAD+) to NAD- which is then converted to NADH. The high energy electrons are carried by NADH to the electron transport chain (ETC). The ETC requires oxygen at the final step to accept the electrons from the last cytochrome in hte ETC.. Without oxygen the ETC and the Kreb's cycle stop functioning.

1. Oxidation - reduction reactions Oxidation and reduction are coupled reactions involving the transfer of electrons. When one molecule gives up an electron it is said to be oxidized. That electron is given to another molecule. The molecule that receives the electron is reduced. Because the electron can not vanish into thin air, oxidation and reduction reactions occur in pairs. When one molecule is oxidized another is reduced.
2. In the case of the Kreb's cycle oxidation of the substrates results in reduction of NAD-.
 

NAD+  +  2e- becomes  NAD-

In most cases a H+ ion  will be attracted coincident with the addition of the electrons. So the reduction of NAD+ by two electrons ultimately results in the formation of NADH.

II. Glycolysis (also known as the Embden-Meyerhof-Parnas pathway) is the anaerobic pathway that results in the breakdown of glucose into two three-carbon molecules known as pyruvic acid. This results in the net production of only two ATP molecules per glucose molecule broken down. Glycolysis also results in the reduction of two molecules of NAD+ to NADH.

A. Organisms that are incapable of further aerobic breakdown of pyruvic acid carry out fermentation to convert the pyruvic acid down into either vinegar (ascetic acid), alcohol (ethanol) or one of many other organic compounds. Often this is accompanied by the liberation of carbon dioxide. One of the first steps in the transformation of pyruvic acid into some other organic compound often involves reduction of the pyruvic acid by the NADH. This results in the conversion of NADH back to NAD+ thus allowing the NAD+ to be used again in the glycolytic pathway.

B. For those organisms that have the capacity for aerobic respiration the products of glycolysis must be introduced into the Kreb's cycle. To accomplish this, pyruvate is linked to a molecule of Coenzyme A (CoA). Linkage of pyruvate to CoA results in the loss of one carbon molecule as carbon dioxide (CO2) and two carbons covalently attached to CoA. This two carbon group is referred to as an acetyl group and the entire molecule is now referred to as acetyl-CoA. This coupling of pyruvic acid to coenzyme A is referred to as the transition reaction.

III. The Kreb's cycle occurs within the cytoplasm of prokaryotic cells. The acetyl group of acetyl-CoA is introduced into the cycle and CoA is liberated and allowed to pick up another acetyl group. For every acetyl group introduced into the Kreb's cycle two CO2 molecules are produced and four pairs of high energy electrons reduce either nicotinamide adenine dinucleotide (NAD+) or flavin adenine dinucleotide (FAD+; FAD is a molecule very similar to NAD). One ATP molecule is also produced.

A. Acetyl-CoA contributes its acetyl group (2 carbons) to a four carbon oxaloacetic acid molecule. A covalent bond forms resulting in a six carbon molecule known as citric acid. The CoA is released to bind with another acetyl group.

B. In a stepwise manner the six carbon molecule is converted to a four carbon molecule one carbon at a time. The carbon atoms are released in the form of carbon dioxide (CO2). At specific steps in this breakdown, energy is trapped and transferred as pairs of high energy electrons. Three molecules of NAD+ and one molecule of FAD are reduced leading to the formation of three NADH and one FADH2 molecules. Eventually the original oxaloacetic acid is regenerated.

IV. Electron transport chain The electrons which reduced NADH and FADH2 are contributed to coenzymes and cytochromes embedded in the cytoplasmic membrane of the prokaryotic cell (they are located in the inner membrane of the eukaryotic mitochondria).

A. Either Flavin mononucleotide (FMN) or Coenzyme Q is the initial electron acceptor. These electrons are stepwise passed from cytochrome to cytochrome in a pathway known as the electron transport chain. The final acceptor of the electrons is a oxygen atom. The reduced (O2-) is known as a superoxide. The enzyme superoxide dismutase converts the superoxide to hydrogen peroxide (H2O2) which is converted to water by the enzyme known as catalase. These steps are very important because superoxides and hydrogen peroxide will combine with proteins or with DNA to disrupt the normal functioning of these molecules.    Overabundance of superoxides or hydrogen peroxide can lead to death of the cell.

B. As the electrons are passed from cytochrome to cytochrome, some of the energy they are carrying is used to pump  hydrogen ions (H+) from the cytoplasm into the extracellular environment. (In the eukaryotic cell this is occurring across the inner membrane of the mitochondria.) This results in the establishment of a gradient of H+ with high levels of H+ outside the cell and relatively low levels of H+ inside the cell. Considerable energy is stored in this gradient.

V. A complex protein known as ATP synthetase (this is also referred to as the F0F1 particle) is embedded in the cytoplasmic membrane and protrudes into the cytoplasm (in the eukaryotic cell it is found in the inner membrane of the mitochondria, it extends into the matrix of the mitochondria). This structure allows the hydrogen ions to move across the membrane back into the cell down their concentration gradient. The energy of this movement is used by ATP synthase to make ATP. This is referred to as chemiosmosis.

Here are some good links!!!

Grapes of Staph  Tutorials for Cellular Respiration http://www.cat.cc.md.us/courses/bio141/lecguide/unit4/index.html

Good discussion of basic principles discussed in class.  Go to the section marked "Bacterial Growth and Microbial Metabolism"

 

MIT Biology    http://web.mit.edu/esgbio/www/glycolysis/dir.html

A brief chemistry review with helpful diagrams.