Aerobic respiration, a term often used interchangeably with "cellular respiration," is a marvelously high-yield way for living things to extract energy stored in the chemical bonds of carbon compounds in the presence of oxygen, and put this extracted energy to use in metabolic processes. Eukaryotic organisms (i.e., animals, plants and fungi) all make use of aerobic respiration, thanks chiefly to the presence of cellular organelles called mitochondria. A few prokaryotic organisms (i.e., bacteria) make use of more rudimentary aerobic-respiration pathways, but in general, when you see "aerobic respiration," you should think "multicellular eukaryotic organism."
But that's not all that should jump into your mind. What follows tells you all you need to know about the basic chemical pathways of aerobic respiration, why it is such an essential set of reactions, and how it all got started over the course of biological and geological history.
The Chemical Summary of Aerobic Respiration
All cellular nutrient metabolism begins with molecules of glucose. This six-carbon sugar can be derived from foods in all three macronutrient classes (carbohydrates, proteins and fats), although glucose itself is a simple carbohydrate. In the presence of oxygen, glucose is transformed and broken down in a chain of about 20 reactions to produce carbon dioxide, water, heat, and 36 or 38 molecules of adenosine triphosphate (ATP), the molecule most often used by cells in all living things as a direct source of fuel. The variation in the amount of ATP produced by aerobic respiration reflects the fact that plants cell sometimes squeeze 38 ATP from one glucose molecule, while animal cells generate 36 ATP per glucose molecule. This ATP comes from combining free phosphate molecules (P) and adenosine diphosphate (ADP), with almost all of this occurring in the very latter stages of aerobic respiration in the reactions of the electron transport chain.
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The complete chemical reaction describing aerobic respiration is:
C6H12O6 + 36 (or 38) ADP + 36 (or 38) P + 6O2 → 6CO2 + 6H2O + 420 kcal + 36 (or 38) ATP.
While the reaction itself appears straightforward enough in this form, it belies the multitude of steps it takes to get from the left-hand side of the equation (the reactants) to the right-hand side (the products, including 420 kilocalories of liberated heat). By convention, the entire collection of reactions is divided into three parts based on where each one occurs: glycolysis (cytoplasm), the Krebs cycle (mitochondrial matrix) and the electron transport chain (inner mitochondrial membrane). Before exploring these processes in detail, however, a look at how aerobic respiration got its start on Earth is in order.
The Origins or Aerobic Respiration of Earth
The function of aerobic respiration is to supply fuel for the repair, growth, and maintenance of cells and tissues. This is a somewhat formal way of noting that aerobic respiration keeps eukaryotic organisms alive. You could go many days without food and at least a few without water in most cases, but only a few minutes without oxygen.
Oxygen (O) is found in normal air in its diatomic form, O2. This element was discovered, in some sense, in the 1600s, when it became apparent to scientists that air contained an element vital to the survival of animals, one that could be depleted in a closed environment by flame or, over the longer term, by breathing.
Oxygen constitutes about one-fifth of the mixture of gases you breathe in. But it was not always this way in the 4.5-billion-year history of the planet, and the change in the amount of oxygen in Earth's atmosphere over time has had predictably profound effects on biological evolution. For the first half of the planet's current lifetime, there was no oxygen in the air. By 1.7 billion years ago, the atmosphere consisted of 4 percent oxygen, and unicellular organisms had appeared. By 0.7 billion years ago, O2 made up between 10 and 20 percent of air, and larger, multicellular organisms had emerged. As of 300 million years ago, the oxygen content had risen to 35 percent of air, and correspondingly, dinosaurs and other very large animals were the norm. Later, the share of air held by O2 dropped to 15 percent until again rising to where it is today.
It is clear by tracking this pattern alone that is seems extremely scientifically likely that oxygen's ultimate function is to make animals grow large.
Glycolysis: A Universal Starting Point
The 10 reactions of glycolysis do not themselves require oxygen to proceed, and glycolysis occurs to some extent in all living things, both prokaryotic and eukaryotic. But glycolysis is a necessary precursor for the specific aerobic reactions of cellular respiration, and it normally described along with these.
Once glucose, a six-carbon molecule with a hexagonal ring structure, enters a cell's cytoplasm, it is immediately phosphorylated, meaning that it has a phosphate group attached to one of its carbon. This effectively traps the glucose molecule inside the cell by giving it a net negative charge. The molecule is then rearranged into phosphorylated fructose, with no loss or gain of atoms, before yet another phosphate is added to the molecule. This destabilizes the molecule, which then fragments into a pair of three-carbon compounds, each of them with its own phosphate attached. One of these is transformed into the other, and then, in a series of steps, the two three-carbon molecules give up their phosphates to molecules of ADP (adenosine diphosphate) to yield 2 ATP. The original six-carbon glucose molecule winds up as two molecules of a three-carbon molecule called pyruvate, and in addition, two molecules of NADH (discussed in detail later) are generated.
The Krebs Cycle
Pyruvate, in the presence of oxygen, moves into the matrix (think "middle") of cellular organelles called mitochondria and is converted into a two-carbon compound, called acetyl coenzyme A (acetyl CoA). In the process, a molecule of carbon dioxide (CO2). In the process, a molecule of NAD+ (a so-called high-energy electron carrier) is converted to NADH.
The Krebs cycle, also called the citric acid cycle or the tricarboxylic acid cycle, is referred to as a cycle rather than a reaction because one of its products, the four-carbon molecule oxaloacetate, re-enters the start of the cycle by combining with a molecule of acetyl CoA. This results in a six-carbon molecule called citrate. This molecule is manipulated by a series of enzymes into a five-carbon compound called alpha-ketoglutarate, which then loses another carbon to yield succinate. Each time a carbon is lost, it is in the form of CO2, and because these reactions are energetically favorable, each carbon dioxide loss is accompanied by the conversion of another NAD+ to NAD. The formation of succinate also creates a molecule of ATP.
Succinate is converted to fumarate, generating one molecule of FADH2 from FAD2+ (an electron carrier similar to NAD+ in function). This is converted to malate, yielding another NADH, which is then transformed to oxaloacetate.
If you are keeping score, you can count 3 NADH, 1 FADH2 and 1 ATP per turn of the Krebs cycle. But bear in mind that each glucose molecule supplies two molecules of acetyl CoA for entry into the cycle, so the total number of these molecules synthesized is 6 NADH, 2 FADH2 and 2 ATP. The Krebs cycle thus does not generate much energy directly – only 2 ATP per molecule of glucose supplied upstream – and no oxygen is needed, either. But the NADH and FADH2 are critical to the oxidative phosphorylation steps in the next series of reactions, collectively called the electron transport chain.
The Electron Transport Chain
The various molecules of NADH and FADH2 created in the preceding steps of cellular respiration are ready to be put to use in the electron transport chain, which occurs in folds of the inner mitochondrial membrane called cristae. In brief, the high-energy electrons attached to NAD+ and FAD2+ are used to create a proton gradient across the membrane. This just means that there is a higher concentration of protons (H+ ions) on one side of the membrane than on the other side, creating an impetus for these ions to flow from areas of higher proton concentration to areas of lower proton concentration. In this way, protons behave little differently than, say, water that "wants" to move from an area of higher elevation to an area of lower concentration – here, under the influence of gravity instead of the so-called chemiosmotic gradient observed in the electron transport chain.
Like a turbine at a hydroelectric plant harnessing the energy of flowing water to do work elsewhere (in that case, generate electricity), some of the energy established by the proton gradient across the membrane is captured to attach free phosphate groups (P) to ADP molecules to generate ATP, a process called phosphorylation (and in this instance, oxidative phosphorylation). In fact, this happens over and over in the electron transport chain, until all of the NADH and FADH2 from glycolysis and the Krebs cycle – about 10 of the former and two of the latter – is utilized. This results in the creation of about 34 molecules of ATP per glucose molecule. Since glycolysis and the Krebs cycle each yield 2 ATP per glucose molecule, the total amount if energy released, at least under ideal conditions, is 34 + 2 + 2 = 38 ATP in all.
There are three different points in the electron transport chain at which protons can cross the inner mitochondrial membrane to enter the space between this later and the outer mitochondrial membrane, and four distinct molecular complexes (numbered I, II, III and IV) that form the physical anchor points of the chain.
The electron transport chain requires oxygen because O2 serves as the final electron-pair acceptor in the chain. If no oxygen is present, the reactions in the chain quickly cease because the "downstream" flow of electrons ceases; they have nowhere to go. Among the substances that can paralyze the electron transport chain is cyanide (CN-). This is why you may have seen cyanide used as a deadly poison in homicide shows or spy movies; when it is administered in sufficient doses, aerobic respiration within the recipient stops, and with it, life itself.
Photosynthesis and Aerobic Respiration in Plants
It is often assumed that plants undergo photosynthesis to create oxygen from carbon dioxide, while animals use respiration to generate carbon dioxide from oxygen, thereby helping preserve a neat ecosystem-wide, complementary balance. While this is true on the surface, it is misleading, because plants make use of both photosynthesis and aerobic respiration.
Because plants cannot eat, they must make, rather than ingest, their food. This is what photosynthesis, a series of reactions that takes place in organelles animals lack called chloroplasts, is for. Powered by sunlight, CO2 inside the plant cell is assembled into glucose inside chloroplasts in a series of steps that resemble the electron transport chain in mitochondria. The glucose is then released from the chloroplast; most if it becomes a structural portion of the plant, but some undergoes glycolysis and then proceeds through the rest of aerobic respiration after entering the plant cell mitochondria.