Nicotinamide Adenine Dinucleotide (NAD+) and Flavin Adenine Dinucleotide (FAD)
Nicotinamide Adenine Dinucleotide is a small nucleic acid of only two nucleotides, Nicotinamide and Adenine
It is actually NAD+ that accepts a Hydrogen atom and thereby undergoes reduction. This dinucleotide acts as a hydrogen acceptor (oxidizing agent) which then becomes NADH in the reduced form. It is the pyridine like ring containing the Nitrogen atom that is quaternary having four bonds and electron deficient (hence the reason for the positive charge on the NAD+). A hydrogen atom with two electrons (the second electron was taken from a second Hydrogen atom making it a Hydrogen ion) attaches itself to the carbon "para" to the Nitrogen position. Since the Hydrogen atom contains its own bonding electrons with which to bond to the ring carbon that displaces the conjugated ring system giving a pair of the ring electrons to the nitrogen and therefore removing the positive charge (NADH)
The Two Hydrogen atoms came from the oxidation of another organic Biochemical species that was involved in an oxidative process. In Biochemical terms oxidation is thought to occur with the loss of Hydrogen atoms from a molecular system. Reduction would be the adding of hydrogen atoms to a molecular system.
The Nicotinamide Adenine Dinucleotide molecule (NAD+) will accept hydrogen atoms during the oxidation of another molecule, and it will give up the Hydrogen atom in a reduction process of another molecule.
NAD+ + 2H = NADH + H+
Forward reaction predominates during an oxidation. Reverse reaction predominates during the reduction of another molecule.
A similar Dinucleotide which functions in the same way is Nicotinamide Adenine Dinucleotide Phosphate (NADP+).
Both of these function as co-enzymes in complex B vitamins are are referred to as dehydrogenases since they cause other molecules to lose Hydrogen atoms and become oxidized.
Flavin Adenine Dinucleotide (FAD)
Another Dinucleotide which serves as an electron carrier in Oxidation Reduction processes is Flavin Adenine Dinucleotide (FAD)
It is called a dehydrogenase. It picks up two Hydride ions along with their bonding electron pairs to produce FADH2
Chapter 6 – Cellular Respiration
After studying this chapter, you should be able to:
1. describe how living things get the energy they need to carry out life functions.
2. compare aerobic respiration and anaerobic respiration.
3. explain why energy is important for living things.
4. describe the role of ATP in energy transfer.
5. compare oxidation and reduction and explain why one cannot take place without the other.
6. discuss the function of electron carriers in cellular respiration.
7. describe the overall scheme of glycolysis.
8. explain the process of fermentation.
9. describe the function of the Krebs cycle.
10. explain where and how the electron transport chain operates in an organism’s body.
11. relate muscle fatigue to oxygen debt.
6.1 – Energy for Life
All cells require energy to carry on life processes. The energy comes from chemical bonds stored in the organism’s food. The cell must release the energy by breaking these chemical bonds. When the bonds are broken, heat is released as a by-product, which is how we maintain our body temperature. The process of breaking chemical bonds in food to release heat and energy is called cellular respiration.
The energy released during respiration is transferred to a new molecule called ATP (adenosine triphosphate). The molecule is composed of 1 adenine, 1 ribose, and 3 phosphate molecules, refer to Figure 6.2, page 109. ATP can be sent through out the cell to the areas that require energy. The bonds between the phosphate groups contain the energy, thus they are called high-energy bonds. The last bond has the most energy in it. When the third phosphate in ATP is removed and bonded elsewhere, the chemical energy is released and used for the cell’s life processes. The remaining molecule composed of 2 phosphates, 1 ribose, and 1 adenine is called ADP, adenosine diphosphate.
To change ADP to ATP, the cell adds another phosphate group to the ADP. This requires energy. The most common source of energy is glucose. From one glucose molecule, a cell can make several ATP molecules from ADP molecules.
There are several steps by which the energy in glucose is used to make ATP. First, the terms oxidation and reduction need to be defined. Oxidation is the loss of electrons by an atom and reduction is the gain of electrons by an atom. Oxidation and reduction must occur at the same time in the chemical reaction and the loss or gain of electrons forms ions out of the atoms. In addition to the transfer of electrons, oxidation-reduction reactions involve the transfer of energy. Sometimes, a hydrogen ion takes the place of the electron. The substance that loses the electron or hydrogen ion (is oxidized), loses energy because the energy is stored in the electrons or the electron or hydrogen ion.
Electron acceptors or carriers are an important part of cellular respiration. They are the molecules that accept the high-energy electrons or hydrogen ions and transfer them along the biochemical pathway. Examples of this type of molecule are NAD+, nicotinamide adenine dinucleotide and FAD, flavine adenine dinucleotide. Each of these can two high-energy electrons and two hydrogen ions, thus they are reduced, refer to the middle of page 111. When NAD+ and FAD accept the electrons and protons, they are converted to NADH and FADH2, while accepting the energy stored in the electrons and protons. They carry this energy to other parts of the biochemical pathway and eventually it is used to make ATP from ADP. At the end of the pathway, oxygen or another substance will act as the final electron acceptor.
6.2 – Anaerobic Respiration
There are two types of cellular respiration, aerobic and anaerobic. Aerobic respiration requires oxygen and anaerobic respiration, also know as fermentation, does not. Anaerobic respiration does not yield as much ATP as aerobic respiration does, but it is enough to permit the survival of the cell.
The first steps of aerobic and anaerobic respiration are the same, its called glycolysis (the splitting of glucose). Refer to Figure 6.4 on page 113.
A six-carbon glucose molecule is split into 2, three carbon molecules called PGAL (phosphoglyceraldehyde). This requires that the cell expend 2 ATP molecules. PGAL is then oxidized (losing 2 pairs of electrons) and changed into another three-carbon molecule called pyruvic acid. The process of changing PGAL to pyruvic acid gives off energy and some is used to form 2 ATP molecules from 2 ADP molecules. Meanwhile, the two pairs of electrons lost by PGAL, are accepted by NAD+, changing it to NADH. This is glycolysis.
Glycolysis uses 2 ATPs but makes 4 ATPs (two ATPs from the making of each pyruvic acid molecule). Two NADHs are also made. Remember that NADHs store energy that can be used to make ATP later on as long as oxygen is present.
After glycolysis, anaerobic respiration or fermentation takes the following turn. The pyruvic acid is changed into ethyl alcohol and carbon dioxide or lactic acid or some other compound depending on the type of bacteria. Occasionally, a cell that normally respires aerobically can be forced to respire anaerobically by depleting it of oxygen. Over tasking your muscle cells, for example will result in fermentation and the lactic acid “burn” felt by body builders.
6.3 – Aerobic Respiration
After glycolysis, the making of 2 ATPs, and 2 NADHs, the reaction moves to inside the mitochondria of the cell. Each pyruvic acid enters and moves to the cristae (inner membrane). There they each turn into an acetyl CoA by breaking down into carbon dioxide, NADH, and a two carbon compound, which combines with a coenzyme, called CoA (coenzyme A), to form acetyl CoA. Each pyruvic acid becomes an acetyl Co A, thus there is a total of two Acetyl-CoAs.
Once the acetyl CoAs are formed, they are used in the Krebs cycle, which produces 2 molecules of carbon dioxide, 3 molecules of NADH and 1 molecule of FADH2, providing 4 pairs of electrons, and 1 ATP molecule, for each acetyl CoA, thus a total of 2 ATPs, 6 NADHs and 2 FADH2s are made.
Then, the NADHs and FADH2s carry the energy stored in their hydrogen atoms and use it to make ATPs in the inner membrane of the mitochondria. The process is called electron transport chain(ETC) and a series of oxidation and reduction reactions take place where electrons are passed from one compound to another until eventually, free oxygen accepts 2 H+s and a pair of electrons to form water. The water may be used by the cell or excreted as waste. In most cells 32 ATPs are produced by the ETC for each molecule of glucose. Add this to the 2 ATPs from glycolysis and the 2 ATPs from the Krebs cycle and you have 36 ATP total. Compare this to the number of ATP obtained by anaerobic respiration.