The role of ATP and NAD / FAD in Respiration.

Tim Grayson

The role of ATP and NAD / FAD in Respiration

Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water. The energy released is trapped in the form of ATP for use by all the energy-consuming activities of the cell.

The process occurs in two phases:

Glycolysis, the breakdown of glucose to pyruvic acid
The complete oxidation of pyruvic acid to carbon dioxide and water

These processes take place inside the mitochondria.

Mitochondria are membrane-enclosed organelles distributed through the cytosol of most eukaryotic cells. Their main function is the conversion of the potential energy of food molecules into ATP. Mitochondria have:

An outer membrane that encloses the entire structure
An inner membrane that encloses a fluid-filled matrix
Between the two is the intermembrane space
The inner membrane is elaborately folded with cristae projecting into the matrix.
A small number (some 5-10) circular molecules of DNA

The glycolytic Pathway

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The glycolytic Pathway

Glycolysis is the splitting, or lysis of glucose. It is a multi-step process in which a glucose molecule with six carbon atoms is eventually split into two molecules of pyruvate, each with three carbon atoms. Energy from ATP is needed in the first steps, but energy is released in later steps, when it can be used to make ATP. There is a net gain of two ATP molecules per molecule of glucose broken down. Glycolysis takes place in the cytoplasm of the cell. In the first stage phosphorylation glucose is phosphorylated using ATP. Glucose is energy rich but does not react easily. To tap the bond energy of glucose, energy must first be used to make the reaction easier. Two ATP molecules are used for each molecule of glucose to make hexose biphosphate, which breaks down to produce two molecules of triose phosphate. Hydrogen is then removed from triose phosphate and transferred to the carrier molecule NAD (nictoinamide adenine dinucleotide). Two molecules of reduced NAD are produced for each molecule of glucose entering glycolysis. The hydrogens carried by the reduced NAD can easily be transferred to other molecules and are used in oxidative phosphorylation to generate ATP. The end product of glycolysis, pyruvate, still contains a great deal of chemical potential energy. When free oxygen is available, some of this energy can be released via the krebs cycle and oxidative phosphorylation. However, the pyruvate first enters the link reaction, which takes place in the mitochondria.

The Link Reaction

In aerobic respiration, each pyruvate molecule is decarboxylated (co2 removed), the remaining two-carbon molecule (ethanoyl or acetyl group) react with reduced Coenzyme A, and at the same time one NADH+ and HADH+ is formed. This is known as the link reaction.

Coenzyme A is a complex molecule of a nucleotide (adenine + ribose) with a vitamin (pantothenic acid), and acts as a carrier of acetyl groups to the krebs cycle. The hydrogen removed from pyruvate is transferred to NAD. Fatty acids from fat metabolism may also be used to produce acetyl coenzyme A. Fatty acids are broken down in the mitochondrion in a cycle of reactions in which each turn of the cycle shortens the fatty acid chain by a two carbon acetyl unit. Each of these can react with coenzyme A to produce acetyl coenzyme A, which, like that produced from pyruvate now enters the Krebs cycle.

The Krebs cycle

The Krebs cycle is also known as the tricarboxylic acid (TCA) cycle and as the citric acid cycle. The Krebs cycle takes place in the mitochondria and consists of eight steps.

The first reaction of the cycle occurs when acetyl CoenzymeA transfers its two-carbon acetyl group to the four-carbon compound oxaloacetate, forming citrate, a six-carbon compound. The citrate then goes through a series of chemical changes, losing first one and then a second carboxyl group as carbon dioxide. Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD+, forming NADH. For each acetyl group that enters the Krebs cycle, three molecules of NAD+ are reduced to NADH. In Step 6, electrons are transferred to the electron acceptor FAD rather than to NAD+.

In one turn of the citric acid cycle, two molecules of carbon dioxide and eight hydrogen atoms are removed, forming three NADH and one FADH2. The carbon dioxide produced accounts for the two carbon atoms of the acetyl group that entered the citric acid cycle.

Because two acetyl CoenzymeA molecules are produced from each glucose molecule, the cycle must turn twice to process each glucose. At the end of each turn of the cycle, the four-carbon oxaloacetate is left, and the cycle is ready for another turn. After two turns of the cycle, the original glucose molecule has lost all of its carbons and may be regarded as having been completely consumed. Only one molecule of ATP is produced directly by a substrate-level phosphorylation with each turn of the ciric acid cycle. The rest of the ATP that is formed during aerobic respiration is produced by the electron transport system and chemiosmosis.