Figure 5.1
The human respiratory system is only part of the story of respiration. Diaphragm, lungs, and trachea take air deep into the body and provide oxygen gas to the bloodstream. The fate of that oxygen is the story of cellular respiration.
You know that humans deprived of oxygen for more than a few minutes will quickly become unconscious and die. Breathing, also known as respiration, is essential for human life, because the body cannot store oxygen for later use as it does food. The mammalian respiratory system, shown in Figure below features a diaphragm, trachea, and a thin membrane whose surface area is equivalent to the size of a handball court - all for efficient oxygen intake. Other forms of life employ different types of respiratory organs: fish and aquatic amphibians and insects flaunt gills, spiders and scorpions develop "book lungs," and terrestrial insects use an elaborate network of tubes called tracheae, which open via spiracles, as shown in Figure below and Figure below. A constant supply of oxygen gas is clearly important to life. However, do you know why you need oxygen?
Figure 5.1
The human respiratory system is only part of the story of respiration. Diaphragm, lungs, and trachea take air deep into the body and provide oxygen gas to the bloodstream. The fate of that oxygen is the story of cellular respiration.
Figure 5.2
Spiracles in this Indian Luna Moth () caterpillar connect to a system of internal tubes (tracheae) which carry oxygen throughout the animal's body.
Figure 5.3
Gills in this alpine newt larva, , bring blood close to an extensive surface area so that the newt can absorb dissolved oxygen gas from its watery habitat.
Many people would answer that oxygen is needed to make carbon dioxide, the gas exhaled or released by each of the respiratory systems listed above. However, CO2 is waste product. Surely, there is more to the story than just gas exchange with the environment! To begin to appreciate the role of oxygen inside your body, think about when your breathing rate increases: climbing a steep slope, running a race, or skating a shift in a hockey game. Respiration rate correlates with energy use, and that correlation reflects the link between oxygen and energy metabolism. For this reason, the chemical reactions inside your cells that consume oxygen to produce usable energy are known as cellular respiration. This chapter will introduce you to the overall process of cellular respiration, and then focus on the first stage, which by itself does not require oxygen.
Another way to think about the role of oxygen in your body - and a good starting point for understanding the whole process of cellular respiration - is to recall the last time you sat by a campfire (see below figure) and noticed that it was "dying." Often people will blow on a campfire to keep it from "dying out." How does blowing help? What happens in a campfire?
Figure 5.4
Analyzing what happens when wood burns in a campfire is a good way to begin to understand cellular respiration.
You know that a fire produces light and heat energy. However, it cannot "create" energy (remember that energy cannot be created or destroyed). Fire merely transforms the energy stored in its fuel – chemical energy – into light and heat. Another way to describe this energy transformation is to say that burning releases the energy stored in fuel. As energy is transformed, so are the compounds that make up the fuel. In other words, burning is a chemical reaction. We could write our understanding of this energy-releasing chemical reaction up to this point as:
Now return to what happens when you blow on a fire. The fire was "dying out," so you blew on it to get it going again. Was it movement or something in the air that promoted the chemical reaction? If you have ever "smothered" a fire, you know that a fire needs something in the air to keep burning. That something turns out to be oxygen. Oxygen gas is a reactant in the burning process. At this point, our equation is:
To complete this equation, we need to know what happens to matter, to the atoms of oxygen, and to the atoms of the fuel during the burning. If you collect the gas rising above a piece of burning wood in an inverted test tube, you will notice condensation - droplets appearing on the sides of the tube. Cobalt chloride paper will change from blue to pink, confirming that these droplets are water. If you add bromothymol blue (BTB) to a second tube of collected gases, the blue solution will change to green or yellow (Figure below), indicating the presence of carbon dioxide. Thus, carbon dioxide and water are products of burning wood.
Figure 5.5
Bromothymol blue (BTB) changes from blue to green to yellow as carbon dioxide is added. Thus, it is a good indicator for this product of burning or cellular respiration.
Now we know what happened to those oxygen atoms during the chemical reaction, but we need to be sure to identify the sources of the carbon atoms in the CO2 and of the hydrogen atoms in the water. If you guessed that these atoms make up the wood fuel – and nearly all fuels we burn, from coal to propane to candle wax to gasoline (hydrocarbons!), you have solved the equation completely. Overall, burning is the combining of oxygen with hydrogen and carbon atoms in a fuel (combustion or oxidation) to release the stored chemical energy as heat and light. Products of combustion are CO2 (oxidized carbon) and H2O (oxidized hydrogen). Or in symbols,
Return to the fate of the oxygen gas you breathe in and absorb. Recall that we related breathing rate and oxygen intake to energy use. Burning consumes oxygen as it releases stored chemical energy, transforming it into light and heat. Cellular respiration is actually a slow burn. Your cells absorb the oxygen carried by your blood from your lungs, and use the O2 to release stored chemical energy so that you can use it.
However, releasing energy within cells does not produce light or intense heat. Cells run on chemical energy – specifically, the small amount temporarily stored in adenine triphosphate (ATP) molecules. Cellular respiration transfers chemical energy from a "deliverable" fuel molecule – glucose – to many "usable" molecules of ATP. Like oxygen, glucose is delivered by your blood to your cells. If ATP were delivered to cells, more than 60,221,417,930,000,000,000,000,000 of these large molecules (which contain relatively small amounts of energy) would clog your capillaries each day. Pumping them across cell membranes would "cost" a great deal of energy. A molecule of glucose contains a larger amount of chemical energy in a smaller package. Therefore, glucose is much more convenient for bloodstream delivery, but too "powerful" to work within the cell. The process of cellular respiration uses oxygen to help transfer the chemical energy from glucose to ATP, which can be used to do work in the cell. This chemical equation expresses what we have worked out:
As with burning, we must trace what happens to atoms during cellular respiration. You can readily see that when the carbon atoms in glucose are combined with oxygen, they again form carbon dioxide. And when the hydrogen atoms in glucose are oxidized, they form water, as in burning. You can detect these products of cellular respiration in your breath on a cold day (as water condensation) and in the lab (BTB turns yellow when you blow into it through a straw). The equation:
This accounts for the energy transfer and the carbon, hydrogen, and oxygen atoms, but it does not show the "raw materials" or reactants which build ATP. Recall that the energy temporarily stored in ATP is released for use when the bond between the second and third phosphates is broken. The resulting ADP can be recycled within the cell by recombining it with inorganic phosphate (Pi).
Now you should be able to see that the source of energy for re-attaching the phosphate is the chemical energy in glucose! Materials cycle and recycle, but energy gets used up and must be replaced. That is the key to understanding cellular respiration: it is a "recharging of the batteries" - ATP molecules – which power cellular work. How many ATP can be made by harnessing the energy in a single glucose molecule? Although this number varies under certain conditions, most cells can capture enough energy from one molecule of glucose to build 38 molecules of ATP. Our equation becomes:
This equation for cellular respiration is not quite complete, however, because we can easily mix air and glucose sugar (even adding ADP and Pi) and nothing will happen. For the campfire, we indicated above the arrow that a necessary condition was a spark or match to start the reaction. A spark or match would damage or destroy living tissue. What necessary condition initiates the slow burn that is cellular respiration? Recall that enzymes are highly specific proteins which "speed up" chemical reactions in living cells. More than 20 kinds of enzymes carry out cellular respiration! If you also recall that membranes within organelles often sequence enzymes for efficiency, as in chloroplasts for photosynthesis, you will not be surprised that a specific organelle, the mitochondrion (Figure below), is also a necessary condition of cellular respiration - at least in eukaryotes.
Figure 5.6
Mitochondria are membranous organelles which sequence enzyme and electron carrier molecules to make cellular respiration highly efficient.
Within each eukaryotic cell, the membranes of 1000-2000 mitochondria sequence enzymes and electron carriers and compartmentalize ions so that cellular respiration proceeds efficiently. Mitochondria, like chloroplasts, contain their own DNA and ribosomes and resemble certain bacteria. The endosymbiotic theory holds that mitochondria, too, were once independently living prokaryotes. Larger prokaryotes engulfed (or enslaved) these smaller aerobic cells, forming eukaryotic cells. Many prokaryotes today can perform cellular respiration; perhaps they and mitochondria have common ancestors. Their expertise in generating ATP made mitochondria highly valued symbionts.
Including these necessary conditions and balancing numbers of atoms on both sides of the arrow, our final equation for the overall process of cellular respiration is:
In words, cellular respiration uses oxygen gas to break apart the carbon-hydrogen bonds in glucose and release their energy to build 38 molecules of ATP. Most of this process occurs within the mitochondria of the cell. Carbon dioxide and water are waste products. This is similar to burning, in which oxygen breaks the carbon-hydrogen bonds in a fuel and releases their chemical energy as heat and light. Again, carbon dioxide and water are waste.
If you have studied the process of photosynthesis, you’ve probably already noticed its similarity to the process of cellular respiration. Both are processes within the cell which make chemical energy available for life. Photosynthesis transforms light energy into chemical energy stored in glucose, and cellular respiration releases the energy from glucose to build ATP, which does the work of life. Moreover, photosynthesis reactants CO2 and H2O are products of cellular respiration. And the reactants of respiration, C6H12O6 and O2, are the products of photosynthesis. This interdependence is the basis of the carbon-oxygen cycle (Figure below), which connects producers to consumers and their environment. At first glance, the cycle merely seems to show mitochondria undoing what chloroplasts do; but the cycle’s energy transformations power all the diversity, beauty, and mystery of life.
Figure 5.7
Photosynthesis in the chloroplast and cellular respiration in the mitochondrion show the interdependence of producers and consumers, the flow of energy from sunlight to heat, and the cycling of carbon and oxygen between living world and environment.
An excellent animation demonstrating cellular respiration can be found at the following web site:
When was the last time you enjoyed yogurt on your breakfast cereal, or had a tetanus shot? These experiences may appear unconnected, but both relate to bacteria which do not use oxygen to make ATP. In fact, tetanus bacteria cannot survive if oxygen is present. However, Lactobacillus acidophilus (bacteria which make yogurt) and Clostridium tetani (bacteria which cause tetanus or lockjaw) share with nearly all organisms the first stage of cellular respiration, glycolysis (Figure below). Because glycolysis is universal, whereas aerobic (oxygen-requiring) cellular respiration is not, most biologists consider it to be the most fundamental and primitive pathway for making ATP.
Figure 5.8
bacteria are obligate anaerobes, which cannot grow in the presence of oxygen and use a variation of glycolysis to make ATP. Because they can grow in deep puncture wounds and secrete a toxin, which can cause muscle spasms, seizures, and death, most people receive tetanus vaccinations at least every ten years throughout life.
Return to the overall equation for cellular respiration:
Like photosynthesis, the process represented by this equation is actually many small, individual chemical reactions. We grouped the reactions of photosynthesis into two stages, the light reactions and the Calvin Cycle. We will divide the reactions of cellular respiration into three stages: glycolysis, the Krebs Cycle, and the electron transport chain (Figure below). In this section, we will explore Stage 1, glycolysis - the oldest and most widespread pathway for making ATP. Before diving into the details, we must note that this first stage of cellular respiration is unique among the three stages: it does not require oxygen, and it does not take place in the mitochondrion. The chemical reactions of glycolysis occur without oxygen in the cytosol of the cell (Figure below).
Figure 5.9
The many steps in the process of aerobic cellular respiration can be divided into three stages. The first stage, glycolysis, produces ATP without oxygen. Because this part of the cellular respiration pathway is universal, biologists consider it the oldest segment. Note that and fats can also enter the glycolysis pathway.
Figure 5.10
Glycolysis, unlike the latter two stages of cellular respiration, takes place without oxygen in the cytosol of the cell. For many organisms, aerobic respiration continues with the Krebs cycle and the electron transport chain in the mitochondria.
The name for Stage 1 clearly indicates what happens during that stage: glyco- refers to glucose, and -lysis means "splitting." In glycolysis, within the cytosol of the cell, a minimum of eight different enzymes break apart glucose into two 3-carbon molecules. The energy released in breaking those bonds is transferred to carrier molecules, ATP and NADH. NADH temporarily holds small amounts of energy which can be used later to build ATP. The 3-carbon product of glycolysis is pyruvate, or pyruvic acid (Figure below). Overall, glycolysis can be represented as shown below:
Figure 5.11
Glycolysis breaks the 6-carbon molecule glucose into two 3-carbon pyruvate molecules, releasing some of the chemical energy which had been stored in glucose.
However, even this equation is deceiving. Just the splitting of glucose requires many steps, each transferring or capturing small amounts of energy. Individual steps appear in Figure below. Studying the pathway in detail reveals that cells must "spend" or "invest" two ATP in order to begin the process of breaking glucose apart. Note that the phosphates produced by breaking apart ATP join with glucose, making it unstable and more likely to break apart. Later steps harness the energy released when glucose splits, and use it to build "hot hydrogens" (NAD+ is reduced to NADH) and ATP (ADP + Pi ATP). If you count the ATP produced, you will find a net yield of two ATP per glucose (4 produced – 2 spent). Remember to double the second set of reactions to account for the two 3-carbon molecules which follow that pathway! The "hot hydrogens" can power other metabolic pathways, or in many organisms, provide energy for further ATP synthesis.
Figure 5.12
This detailed diagram demonstrates that glycolysis "costs" 2 ATP, but harnesses enough energy from breaking bonds in glucose to produce 4 ATP and 2 pairs of "hot hydrogens" (NADH + H). Note the multiplier (2X) required for the 3-carbon steps.
To summarize: In the cytosol of the cell, glycolysis transfers some of the chemical energy stored in one molecule of glucose to two molecules of ATP and two NADH. This makes (some of) the energy in glucose, a universal fuel molecule for cells, available to use in cellular work - moving organelles, transporting molecules across membranes, or building large organic molecules.
Although glycolysis is universal, pathways leading away from glycolysis vary among species depending on the availability of oxygen. If oxygen is unavailable, pyruvate may be converted to lactic acid or ethanol and carbon dioxide in order to regenerate NAD+, ending anaerobic respiration. Anaerobic respiration is also called fermentation, which we will discuss in a later section.
If oxygen is present, pyruvate enters the mitochondria for further breakdown, releasing far more energy and producing many more molecules of ATP in the latter two stages of aerobic respiration - the Krebs cycle and electron transport chain. We will explore these, too, in a later section.
Enticing clues - volcanic gases, vast iron ore sediments, and bubbles of ancient air trapped in amber – suggest dramatic changes during the history of earth’s atmosphere. Correlating these clues with the fossil record leads to two major conclusions: that early life evolved in the absence of oxygen, and that oxygen first appeared between 2 and 3 billion years ago (Figure below) because of photosynthesis by bluegreen bacteria (Figure below). The chemistry of cellular respiration reflects this history. Its first stage, glycolysis, is universal and does not use oxygen.
Figure 5.13
Oxygen has increased in the atmosphere throughout the history of the earth. Note the logarithmic scale, which indicates great increases after first photosynthesis and then land plants evolved. Related geological events: A = no oxidized iron; B = oxidized iron bands in seabed rock - evidence for O in the oceans; C= oxidized iron bands on land and ozone layer formation- evidence for O in the atmosphere.
Figure 5.14
Bubbles of oxygen appear at the surface above a mat of bluegreen bacteria in a freshwater pond. Studies of the fossil record and earths atmosphere suggest that life evolved before bacteria similar to these first added oxygen.
Absolutely dependent on oxygen gas, we find it difficult to imagine that its appearance must have been disastrous for the anaerobic organisms that evolved in its absence. But oxygen is highly reactive, and at first, its effect on evolution was so negative that some have named this period the “oxygen catastrophe.” However, as oxygen gradually formed a protective ozone layer, life rebounded. After the first organisms “discovered” how to use oxygen to their advantage – in ways we will explore in this chapter – the diversity of aerobic organisms exploded. According to the endosymbiotic theory, engulfing of some of these aerobic bacteria led to eukaryotic cells with mitochondria, and multicellularity followed. Today, we live in an atmosphere which is 21% oxygen, and most of life follows glycolysis with the last two, aerobic stages of cellular respiration.
Recall the purpose of cellular respiration: to release energy from glucose to make ATP - the universal “currency” for cellular work. The following equation describes the overall process, although it summarizes many individual chemical reactions.
Once again, the first stage of this process, glycolysis, is ancient, universal, and anaerobic. In the cytoplasm of most cells, glycolysis breaks each 6-carbon molecule of glucose into two 3-carbon molecules of pyruvate. Chemical energy, which had been stored in the now broken bonds, is transferred to 2 ATP and 2 “hot hydrogens,” NADH.
The fate of pyruvate depends on the species and the presence or absence of oxygen. If oxygen is present to drive subsequent reactions, pruvate enters the mitochondrion, where the Krebs Cycle (Stage 2) and electron transport chain (Stage 3) break it down and oxidize it completely to CO2 and H2O. The energy thus released builds many more ATP molecules, though of course some is lost as heat. Let’s explore the details of how mitochondria use oxygen to make more ATP from glucose by aerobic respiration.
Aerobic respiration begins with the entry of pyruvate (product of glycolysis) into the mitochondria. We will follow the six carbons of the original glucose molecule, so we will consider two 3-carbon pyruvates. The fate of pyruvate’s energy and carbon atoms can be followed in the examples below:
Figure 5.15
After glycolysis, two 3-carbon pyruvates enter the mitochondrion, where they are converted to two 2-carbon acetyl CoenzymeA (CoA) molecules. Acetyl CoA then enters the Krebs Cycle. Note that the carbons removed become carbon dioxide, accounting for two of the six such end products of glucose oxidation. The energy released by this breakdown is carried by hot hydrogen."
Figure 5.16
The Krebs or Citric Acid Cycle completes the breakdown of glucose begun in glycolysis. If oxygen is present, pyruvate enters the mitochondria and is converted to Acetyl CoA. Acetyl CoA enters the cycle by combining with 4-carbon oxaloacetate. Study the diagram to confirm that each turn of the cycle (two for each glucose) stores energy in 3 NADH+H, one FADH, and one ATP (from GTP), and releases 2 CO.
In summary, the Krebs Cycle completes the breakdown of glucose which began with glycolysis. Its chemical reactions oxidize all six of the original carbon atoms to CO2, and capture the energy released in 2 ATP, 6 NADH, and 2 FADH2. These energy carriers join the 2 ATP and 2 NADH produced in glycolysis and the 2 NADH produced in the conversion of 2 pyruvates to 2 Acetyl CoA.
At the conclusion of the Krebs Cycle, glucose is completely broken down, yet only four ATP have been produced. Moreover, although oxygen is required to drive the Krebs Cycle, the cycle’s chemical reactions do not themselves consume O2. The conclusion of cellular respiration – its “grand finale!" – produces the majority of the ATP. The next section will explore the electron transport chain, where Stage 3 concludes aerobic cellular respiration.
As noted earlier, the aerobic phases of cellular respiration in eukaryotes occur within organelles called mitochondria. A detailed look at the structure of the mitochondrion (Figure below) helps to explain its role in the last stage of respiration, the electron transport chain.
Figure 5.17
Mitochondria, organelles specialized to carry out aerobic respiration, contain an inner membrane folded into cristae, which form two separate kinds of compartments: inner membrane space and matrix. The Krebs Cycle takes place in the . The electron transport chain is embedded in the inner membrane and uses both compartments to make ATP by .
Two separate membranes form the mitochondrion. The inner membrane folds into cristae which divide the organelle into three compartments – intermembrane space (between outer and inner membranes), cristae space (formed by infoldings of the inner membrane), and matrix (within the inner membrane). The Krebs Cycle takes place within the matrix. The compartments are critical for the electron transport chain, as we’ll see in the final section of this lesson. Glycolysis occurs in the cytoplasm of the cell, with the products of glycolysis entering the mitochondria to continue cellular respiration.
At the end of the Krebs Cycle, energy from the chemical bonds of glucose is stored in diverse energy carrier molecules: four ATP, but also two FADH2 and ten NADH. The primary task of the last stage of cellular respiration, the electron transport chain (ETC), is to transfer energy from these carriers to ATP, the “batteries” which power work within the cell.
Pathways for making ATP in stage 3 of aerobic respiration closely resemble the electron transport chains used in photosynthesis. In both ETCs, energy carrier molecules are arranged in sequence within a membrane so that energy-carrying electrons cascade from one to another, losing a little energy in each step. In both photosynthesis and aerobic respiration, the energy lost is harnessed to pump hydrogen ions into a compartment, creating an electrochemical or chemiosmotic gradient across the enclosing membrane. And in both processes, the energy stored in the chemiosmotic gradient is used to build ATP.
For aerobic respiration, the electron transport chain or “respiratory chain” is embedded in the inner membrane of the mitochondria (Figure below). FADH2 and NADH (produced in glycolysis and the Krebs Cycle) donate high-energy electrons to energy carrier molecules within the membrane. As they pass from one carrier to another, the energy they lose is used to pump hydrogen ions into the intermembrane space, creating an electrochemical gradient. Hydrogen ions flow “down” the gradient – from outer to inner compartment – through an ion channel/enzyme, ATP synthase, which transfer their energy to ATP. Note the paradox that it requires energy to create and maintain a concentration gradient of hydrogen ions that are then used by ATP synthase to create stored energy (ATP). In broad terms, it takes energy to make energy. Coupling the electron transport chain to ATP synthesis with a hydrogen ion gradient is chemiosmosis, first described by Nobel laureate Peter D. Mitchell.
Figure 5.18
The third stage of photosynthesis uses the energy stored earlier in NADH and FADH to make ATP. Electron transport chains embedded in the inner membrane capture high-energy electrons from the carrier molecules and use them to concentrate hydrogen ions in the intermembrane space. Hydrogen ions flow down their electrochemical gradient back into the matrix through channels which capture their energy to convert ADP to ATP.
After passing through the ETC, low-energy electrons and low-energy hydrogen ions combine with oxygen to form water. Thus, oxygen’s role is to drive the entire set of ATP-producing reactions within the mitochondrion by accepting “spent” hydrogens. Oxygen is the final electron acceptor; no part of the process - from the Krebs Cycle through electron transport chain – can happen without oxygen.
The electron transport chain can convert the energy from one glucose molecule’s worth of FADH2 and NADH+H+ into as many as 34 ATP. When the four ATP produced in glycolysis and the Krebs Cycle are added, the total fits the overall equation for aerobic cellular respiration:
Aerobic respiration is complete. If oxygen is available, cellular respiration transfers the energy from one molecule of glucose to 38 molecules of ATP, releasing carbon dioxide and water as waste. “Deliverable” food energy has become energy which can be used for work within the cell – transport within the cell, pumping ions and molecules across membranes, and building large organic molecules. Can you see how this could lead to “life in the fast lane” compared to anaerobic respiration (glycolysis alone)?
Introduction to Aerobic Respiration:
The Krebs Cycle harnesses the energy which remains in pyruvate after glycolysis.
Mitochondria are organelles whose membranes are specialized for aerobic respiration.
The third and final stage of aerobic cellular respiration, the electron transport chain, accounts for most of the ATP.
After the photosynthetic “oxygen catastrophe” challenged life between 2.5 and 3 billion years ago, evolution rebounded with biochemical pathways to harness and protect against oxygen’s power. Today, most organisms use O2 in aerobic respiration to produce ATP. Almost all animals, most fungi, and some bacteria are obligate aerobes, which require oxygen. Some plants and fungi and many bacteria retain the ability to make ATP without oxygen. These facultative anaerobes use ancient anaerobic pathways when oxygen is limited. A few bacteria remain as obligate anaerobes, which die in the presence of oxygen and depend on only the first (anaerobic) stage of cellular respiration.
Aerobic and anaerobic pathways diverge after glycolysis splits glucose into two molecules of pyruvate:
Figure 5.19
Anaerobic and aerobic respiration share the glycolysis pathway. If oxygen is not present, fermentation may take place, producing lactic acid or ethyl alcohol and carbon dioxide. Products of fermentation still contain chemical energy, and are used widely to make foods and fuels.
Pyruvate still contains a great deal of chemical energy. If oxygen is present, pyruvate enters the mitochondria for complete breakdown by the Krebs Cycle and electron transport chain. If oxygen is not present, cells must transform pyruvate to regenerate NAD+ in order to continue making ATP. Two different pathways accomplish this with rather famous products: lactic acid and ethyl alcohol (Figure above). Making ATP in the absence of oxygen by glycolysis alone is known as fermentation. Therefore, these two pathways are called lactic acid fermentation and alcoholic fermentation. If you lack interest in organisms, such as yeast and bacteria, which have “stuck with” the anaerobic tradition, the products of these chemical reactions may still intrigue you. Fermentation makes bread, yogurt, beer, wine, and some new biofuels. In addition, some of your body’s cells are facultative anaerobes, retaining one of these ancient pathways for short-term, emergency use.
For chicken or turkey dinners, do you prefer light meat or dark? Do you consider yourself a sprinter, or a distance runner? (Figure below)
Figure 5.20
Light meat or dark? Sprinting or endurance? Muscle cells know two ways of making ATP aerobic and anaerobic respiration.
Yes! Muscle color reflects its specialization for aerobic or anaerobic metabolism. Although humans are obligate aerobes, our muscle cells have not given up on ancient pathways which allow them to keep producing ATP quickly when oxygen runs low. The difference is more pronounced in chickens and grouse (Figure below), which stand around all day on their legs. For long periods of time, they carry out aerobic respiration in their “specialized-for-endurance” red muscles. If you have ever hunted grouse, you know that these birds “flush” with great speed over short distances. Such “sprinting” flight depends on anaerobic respiration in the white cells of breast and wing muscle. No human muscle is all red or all white, but chances are, if you excel at running short distances or at weight lifting, you have more white glycolytic fibers in your leg muscles. If you run marathons, you probably have more red oxidative fibers.
Figure 5.21
Ruffed grouse use anaerobic respiration (lactic acid fermentation) in wing and breast muscles for quick bursts of speed to escape from predators (and hunters!).
You probably were not aware that muscle cells “ferment.” Lactic acid fermentation is the type of anaerobic respiration carried out by yogurt bacteria (Lactobacillus and others) and by your own muscle cells when you work them hard and fast. Converting pyruvate to 3-carbon lactic acid (see Figure below) regenerates NAD+ so that glycolysis can continue to make ATP in low-oxygen conditions.
For Lactobacillusbacteria, the acid resulting from fermentation kills bacterial competitors in buttermilk, yogurt, and some cottage cheese. The benefits extend to humans who enjoy these foods, as well (Figure below).
Figure 5.22
bacteria use the same type of anaerobic respiration as our muscle cells. Lactic acid reduces competition from other bacteria, and flavors yogurt, as well!
You may have noticed this type of fermentation in your own muscles, because muscle fatigue and pain are associated with lactic acid. Keep this in mind, however, as we discuss a second type of fermentation, which produces alcohol. Imagine what would happen as you ran a race if muscle cells conducted alcoholic rather than lactic acid fermentation!
Have you fueled your car with corn? You have, if you bought gas within the city of Portland, Oregon. Portland was the first city to require that all gasoline sold within the city limits contain at least 10% ethanol. By mid-2006, nearly 6 million “flex-fuel” vehicles – which can use gasoline blends up to 85% ethanol (E85 – Figure below) were traveling US roads. This “new” industry employs an “old” crew of yeast and bacteria to make ethanol by an even older biochemical pathway – alcoholic fermentation. Many people consider “renewable” biofuels such as ethanol a partial solution to the declining availability of “nonrenewable” fossil fuels. Although controversy still surrounds the true efficiency of producing fuel from corn, ethanol is creeping into the world fuel resource picture (Figure below).
Figure 5.23
Ethanol provides up to 85% of the energy needs of new fuel-flex cars. Although its energy efficiency is still controversial, ethanol from corn or cellulose appears to be more renewable than fossil fuels.
Figure 5.24
One of the newest kids on the block, ethanol from corn or cellulose is produced by yeasts through alcoholic fermentation an anaerobic type of respiration.
You are probably most familiar with the term "fermentation" in terms of alcoholic beverages. You may not have considered that the process is actually a chemical reaction certain bacteria and yeasts use to make ATP. Like lactic acid fermentation, alcoholic fermentation processes pyruvate one step further in order to regenerate NAD+ so that glycolysis can continue to make ATP. In this form of anaerobic respiration, pyruvate is broken down into ethyl alcohol and carbon dioxide:
We have domesticated yeast (Figures below and Figure below) to carry out this type of anaerobic respiration for many commercial purposes. When you make bread, you employ the yeast to make the bread “rise” by producing bubbles of carbon dioxide gas. Why do you suppose that eating bread does not intoxicate you?
Figure 5.25
Yeasts are facultative anaerobes, which means that in the absence of oxygen, they use alcoholic fermentation to produce ethyl alcohol and carbon dioxide. Both products are important commercially.
Figure 5.26
We employ yeasts to use their anaerobic talents to help bread rise (via bubbles of CO2) and grapes ferment (adding ethanol).
Brewers of beer and wine use yeast to add alcohol to beverages. Traditional varieties of yeast not only make but also limit the quantity of alcohol in these beverages, because above 18% by volume, alcohol becomes toxic to the yeast itself! We have recently developed new strains of yeast which can tolerate up to 25% alcohol by volume. These are used primarily in the production of ethanol fuel.
Human use of alcoholic fermentation depends on the chemical energy remaining in pyruvate after glycolysis. Transforming pyruvate does not add ATP to that produced in glycolysis, and for anaerobic organisms, this is the end of the ATP-producing line. All types of anaerobic respiration yield only 2 ATP per glucose. In the next section, we will compare the advantages and disadvantages of aerobic and anaerobic respiration.
As aerobes in a world of aerobic organisms, we tend to consider aerobic respiration “better” than fermentation. In some ways, it is. However, anaerobic respiration has persisted far longer on this planet, through major changes in atmosphere and life. There must be value in this alternative way of making ATP. In this last section, we will compare the advantages and disadvantages of these two types of respiration.
A major argument in favor of aerobic over anaerobic respiration is overall energy production. Without oxygen, organisms can only break 6-carbon glucose into two 3-carbon molecules. As we saw earlier, glycolysis releases only enough energy to produce two (net) ATP per molecule of glucose. In contrast, aerobic respiration breaks glucose all the way down to CO2, producing up to 38 ATP. Membrane transport costs can reduce this theoretical yield, but aerobic respiration consistently produces at least 15 times as much ATP as anaerobic respiration. This vast increase in energy production probably explains why aerobic organisms have come to dominate life on earth. It may also explain how organisms were able to increase in size, adding multicellularity and great diversity.
However, anaerobic pathways persist, and a few obligate anaerobes have survived over 2 billion years beyond the evolution of aerobic respiration. What are the advantages of fermentation?
One advantage is available to organisms occupying the few anoxic (lacking oxygen) niches remaining on earth. Oxygen remains the highly reactive, toxic gas which caused the “Oxygen Catastrophe.” Aerobic organisms have merely learned a few tricks – enzymes and antioxidants - to protect themselves. Organisms living in anoxic niches do not run the risk of oxygen exposure, so they do not need to spend energy to build these elaborate chemicals.
Individual cells which experience anoxic conditions face greater challenges. We mentioned earlier that muscle cells “still remember” anaerobic respiration, using lactic acid fermentation to make ATP in low-oxygen conditions. Brain cells do not “remember”, and consequently cannot make any ATP without oxygen. This explains why death follows for most humans who endure more than four minutes without oxygen.
Variation in muscle cells gives further insight into some benefits of anaerobic respiration. In vertebrate muscles, lactic acid fermentation allows muscles to produce ATP quickly during short bursts of strenuous activity. Muscle cells specialized for this type of activity show differences in structure as well as chemistry. Red muscle fibers are “dark” because they have a rich blood supply for a steady supply of oxygen, and a protein, myoglobin, which holds extra oxygen. They also contain more mitochondria, the organelle in which the Krebs cycle and electron transport chain conclude aerobic respiration. White muscle cells are “light” because they lack the rich blood supply, have fewer mitochondria, and store glycogen rather than oxygen. When you eat dark meat, you are eating endurance muscle. When you eat white meat, you are eating muscle built for sprinting.
Each type of muscle fiber has advantages and disadvantages, which reflect their differing biochemical pathways. Aerobic respiration in red muscles produces a great deal of ATP from far less glucose - but slowly, over a long time. Anaerobic respiration in white muscles produces ATP rapidly for quick bursts of speed, but a predator who continues pursuit may eventually catch a white-muscled prey.
In summary, aerobic and anaerobic respiration each have advantages under specific conditions. Aerobic respiration produces far more ATP, but risks exposure to oxygen toxicity. Anaerobic respiration is less energy-efficient, but allows survival in habitats which lack oxygen. Within the human body, both are important to muscle function. Muscle cells specialized for aerobic respiration provide endurance, and those specialized for lactic acid fermentation support short but intense energy expenditures. Both ways of making ATP play critical roles in life on earth.