Aerobic Respiration

Like fermentation, aerobic respiration begins with the pyruvic acid produced through glycolysis. Although glycolysis takes place in the cytoplasm of the cell, aerobic respiration takes place on the folded membranes inside the mitochondria. Associated with these folded membranes are all the enzymes and coenzymes needed in the reactions that make up the process of aerobic respiration.

The equation for aerobic respiration (2.5) is essentially the reverse of that for photosynthesis:

enzymes

C₆H₁₂O₆ + 6O₂ è 6CO₂ + 6H₂O + energy (2.5)

Glucose

Aerobic respiration results in a maximum energy gain of 38 molecules of ATP from each molecule of glucose—that is, 36 ATPs are produced in the mitochondria in addition to the 2 gained from cytoplasmic glycolysis. Some cells, because of a high metabolic rate, must produce a greater amount of energy. Most species of living things carry on aerobic respiration because of its high energy yields. The highly active and complex organisms that inhabit the earth today could not have evolved without the energy provided by aerobic respiration.

The complete breakdown of one molecule of glucose results in a maximum of 38 molecules of ATP.

The same process of respiration that occurs in humans also occurs in the cells of all living things. Cabbage, mushrooms, and bacteria all carry out respiration.

The metabolism of sugars is important not only in making alcoholic beverages but in providing the energy that organisms store in ATP—the energy you use all the time to fuel both conscious actions such as turning the pages of this book and automatic ones such as the beating of the heart.

Energy and Electrons from Glucose

We are all familiar with fuels and their uses. Petroleum fuels contain stored energy that is harvested to move cars and heat homes. Wood burning in a stove or campfire releases energy as light and heat. Living organisms also need fuels, which must be obtained from foods. This is true whether we are speaking of organisms that make their own foods through photosynthesis or organisms that obtain foods by eating other organisms. The most common fuel for living cells is the sugar glucose (C6H12O6). Many other compounds serve as foods but almost all of them are converted to glucose or to intermediate compounds in the step-by-step metabolism of glucose. As you will see in this section, cells obtain energy from glucose by the chemical process of oxidation which is carried out through a series of metabolic pathways. Before we examine that process, let’s take a brief look at how metabolic pathways operate in the cell. Several principles govern metabolic pathways:

Complex chemical transformations in the cell do not occur in a single reaction, but in a number of separate reactions that form a metabolic pathway.

Each reaction in a pathway is catalyzed by a specific enzyme.

Metabolic pathways are similar in all organisms, from bacteria to humans.

Many metabolic pathways are compartmentalized in eukaryotes, with certain reactions occurring inside an organelle.

The operation of each metabolic pathway can be regulated by the activities of key enzymes.

Cells trap free energy while metabolizing glucose

The familiar process of combustion (burning) is very similar to the chemical processes that release energy in cells. If glucose is burned in a flame, it reacts with O₂, rapidly forming carbon dioxide and water and releasing a lot of energy. The balanced equation for this combustion reaction is C₆H₁₂O₆ + 6 O₂ ®6 CO₂ + 6 H₂O + energy (heat and light).

The same equation applies to the metabolism of glucose in cells. The metabolism of glucose, however, is a multistep, controlled series of reactions. The multiple steps of the process permit about one-third of the energy released to be captured in ATP. That ATP can be used to do cellular work such as movement or active transport across a membrane, just as energy captured from combustion can be used to do work. The change in free energy (∆G) for the complete conversion of glucose and O₂ to CO₂ and water, whether by combustion or by metabolism, is –686 kcal/mol (–2,870 kJ/mol). Thus the overall reaction is highly exergonic and can drive the endergonic formation of a great deal of ATP from ADP and phosphate. It is the capture of this energy in ATP that requires the many steps characteristic of glucose metabolism. Three metabolic processes play roles in the utilization of glucose for energy: glycolysis, cellular respiration, and fermentation. All three involve metabolic pathways made up of many distinct chemical reactions.

Glycolysisbegins glucose metabolism in all cells and produces two molecules of the three-carbon product pyruvate. Asmall amount of the energy stored in glucose is captured in usable forms. Glycolysis does not use O₂.

Cellular respirationuses O₂ from the environment and completely converts each pyruvate molecule to three molecules of CO₂ through a set of metabolic pathways. In the process, a great deal of the energy stored in the covalent bonds of pyruvate is released and transferred to ADP and phosphate to form ATP.

Fermentationdoes not involve O₂. Fermentation converts pyruvate into products such as lactic acid or ethyl alcohol (ethanol), which are still relatively energy-rich molecules. Because the breakdown of glucose is incomplete, much less energy is released by fermentation than by cellular respiration, and no ATP is produced. Glycolysis and fermentation are anaerobicmetabolic processes — that is, they do not involve O₂. Cellular respiration is an aerobicmetabolic process, requiring the direct participation of O₂.