BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. UTILIZATION OF ENERGY

9.3. Cellular Respiration

9.3.5. Aerobic Respiration

During aerobic Respiration, the pyruvic acid produced during Glycolysis is ultimately completely oxidized by oxygen to CO₂ and Water. In The first phase, pyruvic acid is broken down to release CO₂ and hydrogen. This process takes place in the mitochondrial matrix and involves a sequence of reactions known as the Krebs cycle. In the second phase, the released hydrogen undergoes a series of oxidation-reduction reactions—in the so-called Respiratory Chain—and is ultimately oxidized by molecular oxygen to water. This occurs on the cristae (shelf-like folds of The inner mitochondrial membrane).

The Initial Stages of aerobic respiration are shown in Fig. 9.6.

Transition Stage Between Glycolysis and the Krebs Cycle

Each pyruvic acid molecule enters the mitochondrial matrix, where—as an acetyl group (CH₃COO—)—it combines with a substance called coenzyme A (abbreviated as CoA), forming acetyl-coenzyme A (acetyl-CoA). The acetyl group contains two carbon atoms (2C); therefore, for it to form, the pyruvic acid molecule (3C) must lose a carbon atom.

The removal of a carbon atom in the form of CO₂ is called a decarboxylation reaction. This is an oxidative decarboxylation, as it is accompanied by oxidation via dehydrogenation, resulting in The formation of reduced NAD.

The Krebs Cycle

This cycle is named after its discoverer in the 1930s, Sir Hans Krebs. It is also referred to as the tricarboxylic acid (TCA) cycle or The Citric Acid Cycle, because these acids participate in it.

The Krebs cycle (Fig. 9.6) takes place in the mitochondrial matrix. Acetyl groups (2C) enter the cycle by combining with a 4C compound, oxaloacetic acid, to form citric acid (6C). This is followed by a series of reactions in which the incoming acetyl groups are decarboxylated to yield two molecules of CO₂ and dehydrogenated to release four pairs of hydrogen atoms, which bind to carriers to form three molecules of reduced NAD and one molecule of reduced FAD. Each turn of the cycle also yields one molecule of ATP. (Recall that a single glucose molecule yields two acetyl groups, meaning that The oxidation of each glucose molecule requires two turns of the cycle.) At the end of the cycle, oxaloacetic acid is regenerated and is ready to combine with a new acetyl group.

The overall balance sheet of aerobic respiration at this stage is summarized in Table 9.2.

Class="center">

Fig. 9.6. Simplified diagram of the Krebs cycle. Also shown is the reaction linking glycolysis to the Krebs cycle (the reaction between pyruvic acid and coenzyme A, which produces acetyl-coenzyme A). This diagram is a continuation of Fig. 9.5.

Table 9.2. Overall balance of the oxidation of a single glucose molecule during aerobic respiration. Note that the oxidation of one glucose molecule requires two turns of the Krebs cycle.

The overall equation can be written as follows:

All the hydrogen from the glucose molecule ultimately ends up on the carriers (NAD and FAD). All the carbon is lost as CO₂. (The presence of six water molecules in this equation might seem surprising. Water is required as a source of oxygen in decarboxylation reactions—this is THE ORIGIN OF some of the oxygen in CO₂. However, this is a minor detail that can be disregarded.)

The Cell/36.html">Respiratory Chain and Oxidative Phosphorylation

The hydrogen held by the two carriers (10 molecules of reduced NAD and 2 molecules of reduced FAD) is now transported to the inner mitochondrial membrane. This membrane forms folds called cristae, which increase its surface area (Fig. 9.12). Hydrogen acts as the fuel. As noted previously, its oxidation by molecular oxygen produces water and releases energy:

Part of this energy is used to synthesize ATP from ADP and inorganic phosphate via oxidative phosphorylation (Section 9.2.2). The energy is not released all at once in a single reaction; rather, the process is broken down into a series of small steps, some of which release enough energy to drive ATP synthesis. This sequence of reactions is known as the respiratory chain. The respiratory chain involves a series of hydrogen and electron carriers, culminating with oxygen. Hydrogen or electrons are passed from one carrier to the next, moving "down" in an energetic sense until, at the final step, they reduce molecular oxygen to water. A certain amount of energy is released at each step, and at several points this transfer is coupled with ATP synthesis (these points are indicated by arrows in Fig. 9.7). The respiratory chain is described in slightly more detail in the legend to Fig. 9.7. The final step is mediated by a copper-containing carrier called cytochrome c oxidase. Cyanide (or carbon monoxide) blocks cellular respiration at this stage by binding to the copper, which prevents oxygen from attaching to it.

Fig. 9.7. Simplified scheme of the respiratory chain. Hydrogen is transferred from reduced NAD to FAD. Subsequently, hydrogen atoms are split into hydrogen ions (H⁺) and electrons. Electrons are passed from reduced FAD to iron (Fe), copper (Cu), and finally to oxygen, where they combine with hydrogen ions to form water. (The addition of an electron is reduction, and its loss is oxidation; see section 9.3.2.) Iron is part of the heme group of a protein called cytochrome. Like Hemoglobin, another iron-containing protein, cytochrome is colored (pink). Copper is part of a group of Proteins collectively known as cytochrome oxidases. Cytochromes transfer electrons, not hydrogen.

Figure 9.7 shows that for every molecule of reduced NAD entering the respiratory chain, 3 molecules of ATP are formed upon The transfer of hydrogen or electrons to oxygen. However, only two molecules of ATP are formed for each molecule of reduced FAD, because reduced FAD enters the respiratory chain at a lower energy level.

The overall balance for the respiratory chain is given in Table 9.3.

Table 9.3. Respiratory chain balance. Each molecule of reduced NAD yields 3 molecules of ATP and releases hydrogen, which combines with oxygen to form water (H₂O). 10 molecules of reduced NAD therefore yield 30 molecules of ATP and 10 molecules of water. This process consumes 10 oxygen atoms, i.e., 5 oxygen molecules. Each molecule of reduced FAD yields 2 molecules of ATP

The overall equation for the respiratory chain is:

Let us combine the two equations below, 1 and 2:

As a result, we obtain:

Thus, for every glucose molecule oxidized during aerobic respiration, 38 ATP molecules are produced.

9.3. What is The Role of oxygen in respiration?

The General scheme of aerobic respiration is shown in Fig. 9.8.

Fig. 9.8. Scheme of aerobic respiration.

Fatty acid oxidation

When Lipids are used as respiratory substrates, they are first hydrolyzed into glycerol and Fatty acids, after which two-carbon fragments are successively cleaved from the fatty acid molecule, shortening this long molecule by two carbon atoms at each step. The two-carbon acetyl group binds to coenzyme A, and the resulting acetyl-CoA enters the Krebs cycle as usual. A large amount of energy is extracted from each fatty acid molecule: during the oxidation of stearic acid, for example, the ATP yield is 147 molecules. It is hardly surprising, therefore, that Fatty acids are an important source of energy. About half of the normal Energy Expenditure of The Heart Muscle, resting skeletal Muscles, Kidneys, and Liver is met precisely through the Oxidation of Fatty acids.