RESPIRATION

CONCEPT

Respiration is much more than just breathing; in fact, the term refers to two separate processes, only one of which is the intake and outflow of breath. At least cellular respiration, the process by which organisms convert food into chemical energy, requires oxygen; on the other hand, some forms of respiration are anaerobic, meaning that they require no oxygen. Such is the case, for instance, with some bacteria, such as those that convert ethyl alcohol to vinegar. Likewise, an anaerobic process can take place in human muscle tissue, producing lactic acid—something so painful that it feels as though vinegar itself were being poured on an open sore.

HOW IT WORKS

Forms of Respiration

Respiration can be defined as the process by which an organism takes in oxygen and releases carbon dioxide, one in which the circulating medium of the organism (e.g., the blood) comes into contact with air or dissolved gases. Either way, this means more or less the same thing as breathing. In some cases, this meaning of the term is extended to the transfer of oxygen from the lungs to the bloodstream and, eventually, into cells or the release of carbon dioxide from cells into the bloodstream and thence to the lungs, from whence it is expelled to the environment. Sometimes a distinction is made between external respiration, or an exchange of gases with the external environment, and internal respiration, an exchange of gases between the body's cells and the blood, in which the blood itself "bathes" the cells with oxygen and receives carbon dioxide to transfer to the environment.

This is just one meaning—albeit a more familiar one—of the word respiration. Respiration also can mean cellular respiration, a series of chemical reactions within cells whereby food is "burned" in the presence of oxygen and converted into carbon dioxide and water. This type of respiration is the reverse of photosynthesis, the process by which plants convert dioxide and water, with the aid of solar energy, into complex organic compounds known as carbohydrates. (For more about carbohydrates and photosynthesis, see Carbohydrates.)

How Gases Move Through the Body

Later in this essay, we discuss some of the ways in which various life-forms breathe, but suffice it to say for the moment—hardly a surprising revelation!—that the human lungs and respiratory system are among the more complex mechanisms for breathing in the animal world. In humans and other animals with relatively complex breathing mechanisms (i.e., lungs or gills), oxygen passes through the breathing apparatus, is absorbed by the bloodstream, and then is converted into an unstable chemical compound (i.e., one that is broken down easily) and carried to cells. When the compound reaches a cell, it is broken down and releases its oxygen, which passes into the cell.

On the "return trip"—that is, the reverse process, which we experience as exhalation—cells release carbon dioxide into the bloodstream, where it is used to form another unstable chemical compound. This compound is carried by the bloodstream back to the gills or lungs, and, at the end of the journey, it breaks down and releases the carbon dioxide to the surrounding environment. Clearly, the one process is a mirror image of the other, with the principal difference being the fact that oxygen is the key chemical component in the intake process, while carbon dioxide plays the same role in the process of outflow.

HEMOGLOBIN AND OTHER COMPOUNDS.

In humans the compound used to transport oxygen is known by the name hemoglobin. Hemoglobin is an iron-containing protein in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them. In the lungs, hemoglobin, known for its deep red color, reacts with oxygen to form oxyhemoglobin. Oxyhemoglobin travels through the bloodstream to cells, where it breaks down to form hemoglobin and oxygen, and the oxygen then passes into cells. On the return trip, hemoglobin combines with carbon dioxide to form carbaminohemoglobin, an unstable compound that, once again, breaks down—only this time it is carbon dioxide that it releases, in this case to the surrounding environment rather than to the cells.

In other species, compounds other than hemoglobin perform a similar function. For example, some types of annelids, or segmented worms, carry a green blood protein called chlorocruorin that functions in the same way as hemoglobin does in humans. And whereas hemoglobin is a molecule with an iron atom at the center, the blood of lobsters and other large crustaceans contains hemocyanin, in which copper occupies the central position. Whatever the substance, the compound it forms with oxygen and carbon dioxide must be unstable, so that it can break down easily to release oxygen to the cells or carbon dioxide to the environment.

Cellular Respiration

Both forms of respiration involve oxygen, but cellular respiration also involves a type of nutrient—materials that supply energy, or the materials for forming new tissue. Among the key nutrients are carbohydrates, naturally occurring compounds that consist of carbon, hydrogen, and oxygen. Included in the carbohydrate group are sugars, starches, cellulose, and various other substances.

Glucose is a simple sugar produced in cells by the breakdown of more complex carbohydrates, including starch, cellulose, and such complex sugars as sucrose (cane or beet sugar) and fructose (fruit sugar). In cellular respiration, an organism oxidizes glucose (i.e., combines it with oxygen) so as to form the energy-rich compound known as adenosine triphosphate (ATP). ATP, critical to metabolism (the breakdown of nutrients to provide energy or form new material), is the compound used by cells to carry out most of their ordinary functions. Among those functions are the production of new cell parts and chemicals, the movement of compounds through cells and the body as a whole, and growth.

In cellular respiration, six molecules of glucose (C₆H₁₂O₆) react with six molecules of oxygen (O₂) to form six molecules of carbon dioxide (CO₂), six molecules of water (H₂O), and 36 molecules of ATP. This can be represented by the following chemical equation:

6C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + 36 ATP

The process is much more complicated than this equation makes it appear: some two dozen separate chemical reactions are involved in the overall conversion of glucose to carbon dioxide, water, and ATP.

The Mechanics of Breathing

All animals have some mechanism for removing oxygen from the air and transmitting it into the bloodstream, and this same mechanism typically is used to expel carbon dioxide from the bloodstream into the surrounding environment. Types of animal respiration, in order of complexity, include direct diffusion, diffusion into blood, tracheal respiration, respiration with gills, and finally, respiration through lungs. Microbes, fungi, and plants all obtain the oxygen they use for cellular respiration directly from the environment, meaning that there are no intermediate organs or bodily chemicals, such as lungs or blood. More complex organisms, such as sponges, jellyfish, and terrestrial (land) flatworms, all of which have blood, also breathe through direct diffusion. The latter term describes an exchange of oxygen and carbon dioxide directly between an organism, or its bloodstream, and the surrounding environment.

More complex is the method of diffusion into blood whereby oxygen passes through a moist layer of cells on the body surface and then through capillary walls (capillaries are small blood vessels that form a network throughout the body) and into the bloodstream. Once oxygen is in the blood, it moves throughout the body to different tissues and cells. Among the organisms that rely on diffusion into blood are annelids, a group that includes earthworms, various marine worms, and leeches.

In tracheal respiration air moves through openings in the body surface called spiracles. It then passes into special breathing tubes called tracheae that extend into the body. The tracheae divide into many small branches that are in contact with muscles and organs. In small insects, air simply moves into the tracheae, while in large insects, body movements assist tracheal air movement. Insects and terrestrial arthropods (land-based organisms with external skeletons) use this method of respiration.

Much more complicated than tracheae, gills are specialized tissues with many infoldings. Each gill is covered by a thin layer of cells and filled with blood capillaries. These capillaries take up oxygen dissolved in water and expel carbon dioxide dissolved in blood. Fish and other aquatic animals use gills, as did the early ancestors of humans and other higher animals. A remnant of this chapter from humans' evolutionary history can be seen in the way that an embryo breathes in its mother's womb, not by drawing in oxygen through its lungs but through gill-like mechanisms that disappear as the embryo develops.

LUNGS.

Lungs are composed of many small chambers or air sacs surrounded by blood capillaries. Thus, they work with the circulatory system, which transports oxygen from inhaled air to all tissues of the body and also transports carbon dioxide from body cells to the lungs to be exhaled. After air enters the lungs, oxygen moves into the bloodstream through the walls of these capillaries. It then passes from the lung capillaries to the different muscles and organs of the body.

Although they are common to amphibians, reptiles, birds, and mammals, lungs differ enormously throughout the animal kingdom. Frogs, for instance, have balloon-like lungs that do not have a very large surface area. By contrast, if the entire surface of an adult male human's lungs were spread flat, it would cover about 750 sq. ft. (70 m²), approximately the size of a handball court. The reason is that humans have about 300 million gas-filled alveoli, tiny protrusions inside the lungs that greatly expand the surface area for gas exchange.

Birds have specialized lungs that use a mechanism called crosscurrent exchange, which allows air to flow in one direction only, making for more efficient oxygen exchange. They have some eight thin-walled air sacs attached to their lungs, and when they inhale, air passes through a tube called the bronchus and enters posterior air sacs—that is, sacs located toward the rear. At the same time, air in the lungs moves forward to anterior air sacs, or ones located near the bird's front. When the bird exhales, air from the rear air sacs moves to the outside environment, while air from the front moves into the lungs. This efficient system moves air forward through the lungs when the bird inhales and exhales and makes it possible for birds to fly at high altitudes, where the air has a low oxygen content.

Humans and other mammals have lungs in which air moves in and out through the same pathway. This is true even of dolphins and whales, though they differ from humans in that they do not take in nutrition through the same opening. In fact, terrestrial mammals, such as the human, horse, or dog, are some of the only creatures that possess two large respiratory openings: one purely for breathing and smelling and the other for the intake of nutrients as well as air (i.e., oxygen in and carbon dioxide out).

REAL-LIFE APPLICATIONS

Anaerobic Respiration

Activity that involves oxygen is called aerobic; hence the term aerobic exercise, which refers to running, calisthenics, biking, or any other form of activity that increases the heart rate and breathing. Activity that does not involve oxygen intake is called anaerobic. Weightlifting, for instance, will increase the heart rate and rate of breathing if it is done intensely, but that is not its purpose and it does not depend on the intake and outflow of breath. For that reason, it is called an anaerobic exercise—though, obviously, a person has to keep breathing while doing it.

In fact, a person cannot consciously stop breathing for a prolonged period, and for this reason, people cannot kill themselves simply by holding their breath. A buildup of carbon dioxide and hydrogen ions (electrically charged atoms) in the bloodstream stimulates the breathing centers to become active, no matter what we try to do. On the other hand, if a person were underwater, the lungs would draw in water instead of air, and though water contains air, the drowning person would suffocate.

ANAEROBIC BACTERIA.

Some creatures, however, do not need to breathe air but instead survive by anaerobic respiration. This is true primarily of some forms of bacteria, and indeed scientists believe that the first organisms to appear on Earth's surface were anaerobic. Those organisms arose when Earth's atmosphere contained very little oxygen, and as the composition of the atmosphere began to incorporate more oxygen over the course of many millions of years, new organisms evolved that were adapted to that condition.

The essay on paleontology discusses Earth's early history, including the existence of anaerobic life before the formation of oxygen in the atmosphere. The appearance of oxygen is a result of plant life, which produces it as a byproduct of the conversion of carbon dioxide that takes place in photosynthesis. Plants, therefore, are technically anaerobic life-forms, though that term usually refers to types of bacteria that neither inhale nor exhale oxygen. Anaerobic bacteria still exist on Earth and serve humans in many ways. Some play a part in the production of foods, as in the process of fermentation. Other anaerobic bacteria have a role in the treatment of sewage. Living in an environment that would kill most creatures—and not just because of the lack of oxygen—they consume waste materials, breaking them down chemically into simpler compounds.

HUMANS AND ANAEROBIC RESPIRATION.

Even in creatures, such as humans, that depend on aerobic respiration, anaerobic respiration can take place. Most cells are able to switch from aerobic to anaerobic respiration when necessary, but they generally are not able to continue producing energy by this process for very long. For example, a person who exercises vigorously may be burning up glucose faster than oxygen is being pumped to the cells, meaning that cellular respiration cannot take place quickly enough to supply all the energy the body needs. In that case, cells switch over to anaerobic respiration, which results in the production of lactic acid, or C₃H₆O₃. One advantage of anaerobic respiration is that it can take place very quickly and in short bursts, as opposed to aerobic respiration, which is designed for slower and steadier use of muscles. The disadvantage is that anaerobic respiration produces lactic acid, which, when it builds up in muscles that are overworked, causes soreness and may even lead to cramps.

LACTIC ACID IN THE BODY.

Eventually, the buildup of lactic acid is carried away in the bloodstream, and the lactic acid is converted to carbon dioxide and water vapor, both of which are exhaled. But if lactic acid levels in the bloodstream rise faster than the body can neutralize them, a state known as lactic acidosis may ensue. Lactic acidosis rarely happens in healthy people and, more often than not, is a result of the body's inability to obtain sufficient oxygen, as occurs in heart attacks or carbon monoxide or cyanide poisoning or in the context of diseases such as diabetes.

The ability of the body to metabolize lactic acid is diminished significantly by alcohol, which impairs the liver's ability to carry out normal metabolic reactions. For this reason, alcoholics often have sore muscles from lactic acid buildup, even though they may not exercise. Lactic acid also can lead to a buildup of uric acid crystals in the joints, in turn causing gout, a very painful disease.

LACTIC ACID IN FOOD AND INDUSTRY.

Lactic acid is certainly not without its uses, and it is found throughout nature. When lactose, or milk sugar, is fermented by the action of certain bacteria, it causes milk to sour. The same process is used in the manufacture of yogurt, but the reaction is controlled carefully to ensure the production of a consumable product. Lactic acid also is applied by the dairy industry in making cheese. Molasses contains lactic acid, a product of the digestion of sugars by various species of bacteria, and lactic acid also is used in making pickles and sauerkraut, foods for which a sour taste is desired.

A compound made from lactic acid is used as a food preservative, but the applications of lactic acid extend far beyond food production. Lactic acid is important as a starting material for making drugs in the pharmaceutical industry. Additionally, it is involved in the manufacturing of lacquers and inks; is used as a humectant, or moisturizer, in some cosmetics; is applied as a mordant, or a chemical that helps fabrics accept dyes, to textiles; and is employed in tanning leather.

Respiratory Disorders

In almost any bodily system, there are bound to be disorders, or at least the chance that disorders may occur. This is particularly the case with something as complex as the respiratory system, because the more complex the system, the more things that can go wrong. Among the respiratory disorders that affect humans is a whole range of ailments from the common cold to emphysema, and from the flu to cystic fibrosis.

THE COMMON COLD.

Colds are among the most common conditions that affect the respiratory system, though what we call the common cold is actually an invasion by one of some 200 different types of virus. Thus, it is really not one ailment but 200, though these are virtually identical, but the large number of viral causative agents has made curing the cold an insurmountable task.

When you get a cold, viruses establish themselves on the mucus membrane that coats the respiratory passages that bring air to your lungs. If your immune system is unsuccessful in warding off this viral infection, the nasal passages become inflamed, swollen, and congested, making it difficult to breathe.

Coughing is a reflex action whereby the body attempts to expel infected mucus or phlegm. It is essential to removing infected secretions from the body, but of course it plays no role in actually bringing a cold to an end. Nor do antibiotics, which are effective against bacteria but not viruses (see Infection). Only when the body builds up its own defense to the cold—assuming the sufferer has a normally functioning immune system—is the infection driven away.

INFLUENZA AND ALLERGIES.

Influenza, a group of viral infections that can include swine flu, Asian flu, Hong Kong flu, and Victoria flu, is often far more serious than the common cold. A disease of the lungs, it is highly contagious, and can bring about fever, chills, weakness, and aches. In addition, influenza can be fatal: a flu epidemic in the aftermath of World War I, spread to far corners of the globe by returning soldiers, killed an estimated 20 million people.

Respiratory ailments often take the form of allergies such as hay fever, symptoms of which include sneezing, runny nose, swollen nasal tissue, headaches, blocked sinuses, fever, and watery, irritated eyes. Hay fever is usually aggravated by the presence of pollen or ragweed in the air, as is common in the springtime. Other allergy-related respiratory conditions may be aggravated by dust in the air, and particularly by the feces of dust mites that live on dust particles.

BRONCHIAL AILMENTS.

Allergic reactions can be treated by antihistamines (see The Immune System for more about allergies), but simple treatments are not available for such complex respiratory disorders as asthma, chronic bronchitis, and emphysema. All three are characterized by an involuntary constriction in the walls of the bronchial tubes (the two divisions of the trachea or windpipe that lead to the right and left lungs), which causes the tubes to close in such a way that it becomes difficult to breathe.

Emphysema can be brought on by cigarette smoking, and indeed some heavy smokers die from that ailment rather than from lung cancer. On the other hand, a person can contract a bronchial illness without engaging in smoking or any other activity for which the sufferer could ultimately be blamed. Indeed, small children may have asthma. One treatment for such disorders is the use of a bronchodilator, a medicine used to relax the muscles of the bronchial tubes. This may be administered as a mist through an inhaler, or given orally like other medicine.

TUBERCULOSIS AND PNEUMONIA.

More severe is tuberculosis, an infectious disease of the lungs caused by bacteria. Tuberculosis attacks the lungs, leading to a chronic infection with such symptoms as fatigue, loss of weight, night fevers and chills, and persistent coughing that brings up blood. Without treatment, it is likely to be fatal. Indeed, it was a significant cause of death until the introduction of antibiotics in the 1940s, and it has remained a problem in underdeveloped nations. Additionally, thanks to mutation in the bacteria themselves, strains of the disease are emerging that are highly resistant to antibiotics.

Another life-threatening respiratory disease is pneumonia, an infection or inflammation of the lungs caused by bacteria, viruses, mycoplasma (microorganisms that show similarities to both viruses and bacteria), and fungi, as well as such inorganic agents as inhaled dust or gases. Symptoms include pleurisy (chest pain), high fever, chills, severe coughing that brings up small amounts of mucus, sweating, blood in the sputum (saliva and mucus expelled from the lungs), and labored breathing.

In 1936, pneumonia was the principal cause of death in the United States. Since then, it has been controlled by antibiotics, but as with tuberculosis, resistant strains of bacteria have developed, and therefore the number of cases has increased. Today, pneumonia and influenza combined are among the most significant causes of death in the United States (see Diseases).

LUNG CANCER AND CYSTIC FIBROSIS.

Respiratory ailments may also take the form of lung cancer, which may or may not be a result of smoking. Cigarette smoking and air pollution are considered to among the most significant causes of lung cancer, yet people have been known to die of the disease without being smokers or having been exposed to significant pollution.

One particularly serious variety of respiratory illness is cystic fibrosis, a genetic disorder that causes a thick mucus to build up in the respiratory system and in the pancreas, a digestive organ. (For more about genetic disorders, see Heredity; for more on role of the pancreas, see Digestion.) In the United States, the disease affects about one in every 3,900 babies born annually. No cure for cystic fibrosis exists, and the disease is invariably fatal, with only about 50% of sufferers surviving into their thirties.

Lung complications are the leading cause of death from cystic fibrosis, and most symptoms of the disease are related to the sticky mucus that clogs the lungs and pancreas. People with cystic fibrosis have trouble breathing, and are highly susceptible to bacterial infections of the lungs. Coughing, while it may be irritating and painful if you have a cold, is necessary for the expulsion of infected mucus, but mucus in the lungs of a cystic fibrosis is too thick to be moved. This makes it easy for bacteria to inhabit the lungs and cause infection.

WHERE TO LEARN MORE

Bryan, Jenny. Breathing: The Respiratory System. New York: Dillon Press, 1993.

Cellular Metabolism and Fermentation. Estrella Mountain Community College (Web site). <http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookGlyc.html>.

Kimball, Jim. "The Human Respiratory System." Kim ball's Biology Pages (Web site). <http://www.ultranet.com/~jkimball/BiologyPages/P/Pulmonary.html>.

Levesque, Mireille, Letitia Fralick, and Joni McDowell. "Respiration in Water: An Overview of Gills." University of New Brunswick (Web site). <http://www.unb.ca/courses/biol4775/SPAGES/SPAGE13.HTM>.

Llamas, Andreu. Respiration and Circulation. Milwaukee: Gareth Stevens, 1998.

Paustian, Timothy. Anaerobic Respiration. Department of Bacteriology, University of Wisconsin-Madison (Web site). <http://www.bact.wisc.edu/microtextbook/Metabolism/RespAnaer.html>.

Roca, Núria, and Marta Serrano. The Respiratory System, the Breath of Life. Illus. Antonio Tenllado. New York: Chelsea House Publishers, 1995.

Silverstein, Alvin, and Virginia B. Silverstein. The Respiratory System. New York: Twenty-First Century Books, 1994.

KEY TERMS

AEROBIC:

Oxygen-breathing.

ANAEROBIC:

Non-oxygen-breathing.

ATP:

Adenosine triphosphate, an energy carrier formed when a simpler compound, adenosine diphosphate (ADP), combines with a phosphate group.

CAPILLARY:

A very small blood vessel. Capillaries form networks throughout the body.

CARBOHYDRATES:

Naturally occurring compounds, consisting of carbon, hydrogen, and oxygen, whose primary function in the body is to supply energy. Included in the carbohydrate group aresugars, starches, cellulose, and various other substances. Most carbohydrates are produced by green plants in the process of undergoing photosynthesis.

CELLULAR RESPIRATION:

A process that, when it takes place in the presence of oxygen, involves the intake of organic substances, which are broken down into carbon dioxide and water, with the release of considerable energy.

CIRCULATORY SYSTEM:

The parts of the body that work together to move blood and lymph. They include the heart, blood vessels, blood, and the lymphatic glands, such as the lymph nodes.

COMPOUND:

A substance in which atoms of more than one element are bonded chemically to one another.

FERMENTATION:

A process, involvingenzymes, in which a compound rich in energy is broken down into simpler substances.

GLUCOSE:

A monosaccharide (sugar) that occurs widely in nature and which is the form in which animals usually receive carbohydrates. Also known as dextrose, grape sugar, and corn sugar.

HEMOGLOBIN:

An iron-containing protein in human red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide fromthem. Hemoglobin is known for its deep red color.

LYMPH:

The portion of the blood that includes white blood cells and plasma but not red blood cells.

LYMPH NODES:

Masses of tissue, at certain places in the body, that act as filters for blood.

METABOLISM:

The chemical process by which nutrients are broken down and converted into energy or are used in the construction of new tissue or other material in the body. All metabolic reactions are either catabolic or anabolic.

MONOSACCHARIDE:

The simplest type of carbohydrate. Monosaccharides, which cannot be broken down chemically into simpler carbohydrates, also are known as simple sugars.

NUTRIENT:

Materials that supply energy or the materials to form new tissue for organisms. They include proteins, carbohydrates, lipids (fats), vitamins, and minerals.

PHOSPHATE GROUP:

A group (that is, a combination of atoms from two or more elements that tend to bond with other elements or compounds in certain characteristic ways) involving a phosphate, or a chemical compound that contains oxygen bonded to phosphorus.

PHOTOSYNTHESIS:

The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. In this process carbondioxide and water are converted to carbohydrates and oxygen.

RESPIRATION:

A term that can refer either to cellular respiration (see definition) or, more commonly, to the process by which an organism takes in oxygen and releases carbon dioxide. Sometimes a distinction is made between external respiration, or an exchange of gases with the external environment, and internal respiration, an exchange of gases between the body's cells and the blood.

SIMPLE SUGAR:

A monosaccharide, or simple carbohydrate.

TISSUE:

A group of cells, along with the substances that join them, that forms part of the structural materials in plants oranimals.

Respiration

Respiration is the physiological process by which organisms supply oxygen to their cells and the cells use that oxygen to produce high energy molecules. Respiration occurs in all types of organisms, including bacteria , protists, fungi , plants, and animals. In higher animals, respiration is often separated into three separate components: (a) external respiration, the exchange of oxygen and carbon dioxide between the environment and the organism ; (b) internal respiration, the exchange of oxygen and carbon dioxide between the internal body fluids, such as blood , and individual cells; and (c) cellular respiration, the biochemical oxidation of glucose and consequent synthesis of ATP (adenosine triphosphate ).

External respiration

External respiration, commonly known as breathing, is the exchange of oxygen and carbon dioxide between an animal and its environment. Most animals use specialized organs or organ systems, such as lungs, trachea, or gills, for external respiration.

In all cases, exchange of gases between the environment and an animal occurs by diffusion through a wet surface on the animal which is permeable to oxygen and carbon dioxide. Diffusion is the random movement of molecules and causes a net movement of molecules from a region of high concentration to a region of low concentration. Thus, oxygen moves into an organism because its concentration is lower inside than in the environment (air or water ); carbon dioxide moves out of an organism because its concentration is higher inside than in the environment.

Different organisms have different mechanisms for extracting oxygen from their environments. Below, animal-gas exchange mechanisms have been classified into five categories.

1. Direct diffusion. Sponges , jellyfish , and terrestrial flatworms use this primitive method. In direct diffusion, oxygen diffuses from the environment through cells on the animal's surface and then diffuses to individual cells inside. The primitive animals that use this method do not have respiratory organs. Obviously, an animal with small surface areas and large volume cannot rely on direct diffusion, since little oxygen would reach the interior of the body. Microbes, fungi, and plants all obtain the oxygen they use for cellular respiration by direct diffusion through their surfaces.
2. Diffusion into blood. Annelids (segmented worms ) and amphibians use this method. In this method, oxygen diffuses through a moist layer of epidermal cells on the body surface and from there through capillary walls and into the blood stream. Once oxygen is in the blood, it moves throughout the body to different tissues and cells. While this method does not rely upon respiratory organs and is thus quite primitive, it is somewhat more advanced than direct diffusion.
3. Tracheae. Insects and terrestrial arthropods use this method. In tracheal respiration, air moves through openings in the body surface called spiracles and then into special tubes called tracheae (singular, trachea) which extend into the body. The tracheae divide into many small branches which contact the muscles and organs. In small insects, air moves into the tracheae passively, whereas in large insects, body movements facilitate tracheal air movement. An advantage of tracheal respiration is that it provides oxygen directly to the muscles. Muscle cells use this oxygen, together with the carbohydrates and other energetic molecules in the hemolymph (insect blood), to generate the energy needed for flight.
4. Gills. Fish and other aquatic animals use this method. Gills are specialized tissues with many infoldings, each covered by a thin layer of cells and impregnated with blood capillaries . They take up oxygen dissolved in water and expel carbon dioxide dissolved in blood. Gills work by a mechanism called countercurrent exchange, in which blood and water flow in discrete pathways and opposite directions. This allows gills to more efficiently extract oxygen from water and expel carbondioxide into the water. Certain details of gill anatomy differ among different species .
5. Lungs. Terrestrial vertebrates use this method. Lungs are special organs in the body cavity that are composed of many small chambers impregnated with blood capillaries. After air enters the lungs, oxygen diffuses into the blood stream through the walls of these capillaries. It then moves from the lung capillaries to the different muscles and organs of the body. Humans and other mammals have lungs in which air moves in and out through the same pathway. In contrast, birds have more specialized lungs which use a mechanism called cross-current exchange. Like the countercurrent exchange mechanism of gills, air flows through the crosscurrent exchange system of bird lungs in one direction only, making for more efficient oxygen exchange.

Internal respiration

Internal respiration is the exchange of oxygen and carbon dioxide between blood and cells in different tissues of an animal's body. Internal respiration occurs in animals with a circulation system (categories 2, 4, and 5 above). Animals with gills or lungs take up oxygen and transport oxygen-rich blood throughout the body; they transport carbon dioxide-rich blood from the body back into the respiratory organs where it is expelled. The oxygen-rich blood and carbon dioxide-rich blood do not mix, making for an efficient internal respiration system. Mammals and birds have a double circulation system for blood, in which separate pumps in the left and right chambers of the heart move the oxygen-rich blood in the arteries and carbon dioxide-rich blood in the veins .

The blood of vertebrates and some invertebrates contains a protein (such as hemoglobin, hemocyanin, or chlorocruorin), which binds oxygen and transports it from the respiratory organs throughout the body. These oxygen-binding proteins greatly improve the oxygen carrying ability of blood. For example, human hemoglobin contains about 98% of the oxygen in a human's blood.

Hemoglobin is a red protein which binds oxygen and occurs in the red blood cells of vertebrates. Each molecule of hemoglobin contains an iron atom and can bind up to four molecules of oxygen. In muscles, hemoglobin passes its oxygen to myoglobin. Myoglobin is an oxygen-binding protein that makes muscles red and transports oxygen to the cells of the muscle. In turn, muscle cells use the oxygen from myoglobin to power muscle movement by cellular respiration.

Some segmented worms (annelids) have a green blood protein, called chlorocruorin, which binds iron and serves as an oxygen carrier. Some invertebrates have a blue blood protein, called hemocyanin, which binds copper and serves as an oxygen carrier.

Cellular respiration

Cellular respiration is an intracellular process in which glucose (C6H12O6) is oxidized and the energy is used to make ATP (adenosine triphosphate). ATP is a high energy molecule which organisms use to drive energy-requiring processes such as biosynthesis, transport, growth, and movement. The general features of cellular respiration are the same in most organisms.

Cellular respiration consists of many separate enzymatic reactions. The entire process can be summarized in the chemical equation:

Cellular respiration is divided into three sequential series of reactions: glycolysis , the citric acid cycle, and the electron transport chain. In higher organisms (eukaryotes), glycolysis occurs in the cytosol of the cell , the aqueous region outside the nucleus; the citric acid cycle and electron transport chain occur in the mitochondria, cellular organelles (intracellular organ-like structures) which have characteristic double membranes and are specialized for ATP production.

Glycolysis

Glycolysis can be defined simply as the lysis, or splitting, of sugar. More particularly, it is the controlled breakdown of glucose, a 6-carbon carbohydrate , into pyruvate, a 3-carbon carbohydrate. Organisms frequently store complex carbohydrates, such as glycogen or starch, and break these down into glucose units which can then enter into glycolysis.

Two features of glycolysis suggest that it has an ancient evolutionary origin. First, the same series of reactions occur in virtually all cells, including bacteria, plants, fungi, and animals. Second, glycolysis does not require oxygen, making it appropriate for primeval cells which had to live in a world with very little atmospheric oxygen.

Glycolysis has several important features:

1. It breaks down one molecule of glucose, a 6-carbon molecule, into two molecules of pyruvate, a 3-carbon molecule, in a controlled manner by ten or more enzymatic reactions. The oxidation of glucose is controlled so that the energy in this molecule can be used to manufacture other high energy compounds (see 2 and 3 below).
2. It makes a small amount of ATP, a process known as substrate-level phosphorylation. For each glucose molecule that is broken down by glycolysis, there is a net gain of two molecules of ATP.
3. It makes NADH (reduced nicotinamide adenine dinucleotide), a high energy molecule which can be used to make ATP in the electron transfer chain (see below). For each glucose molecule that is broken down by glycolysis, there is a net gain of two molecules of NADH.
4. It makes compounds which can be used to synthesize fatty acids . In particular, some of the carbohydrate intermediates of glycolysis are used by other enzymatic reactions to synthesize fatty acids, the major constituents of lipids, important energy storage molecules.

Cirtric acid cycle

After pyruvate (a 3-carbon molecule) is synthesized by glycolysis, it moves into the mitochondria and is oxidized to form carbon dioxide (a 1-carbon molecule) and acetyl CoA (a two carbon molecule). Cells can also make acetyl CoA from fats and amino acids and this is how cells often derive energy, in the form of ATP, from molecules other than glucose or complex carbohydrates.

After acetyl CoA forms, it enters into a series of nine sequential enzymatic reactions, known as the citric acid cycle. These reactions are so named because the first reaction makes one molecule of citric acid (a 6-carbon molecule) from one molecule of acetyl CoA (a 2-carbon molecule) and one molecule of oxaloacetic acid (a 4-carbon molecule). A complete round of the citric acid cycle expels two molecules of carbon dioxide and regenerates one molecule of oxaloacetic acid, hence the cyclic nature of these reactions. The citric acid cycle is sometimes called the Krebs cycle , in honor of Hans Krebs, the English biochemist who first proposed that pyruvate is broken down by a cycle of biochemical reactions.

The citric acid cycle has several important features:

1. It makes NADH (reduced nicotinamide adenine dinucleotide) and FADH2 (reduced flavin adenine dinucleotide), high energy molecules which are used to make ATP in the electron transfer chain (see below). For each glucose molecule which initially enters glycolysis, the citric acid cycle makes 6 molecules of NADH and 2 molecules of FADH2.
2. It makes GTP (guanosine triphosphate) by a process known as substrate-level phosphorylation. GTP is a high energy molecule which cells can easily use to make ATP by a separate mitochondrial reaction. For each molecule of glucose which initially enters glycolysis, the citric acid cycle makes two molecules of ATP.
3. Some of the intermediates of the citric acid cycle reactions are used to make other important compounds. In particular, certain intermediates are used to synthesize amino acids, the building blocks of proteins, nucleotides, the building blocks of DNA, and other important molecules.

Electron transfer chain

The electron transfer chain is the final series of biochemical reactions in cellular respiration. It consists of a series of organic electron carriers associated with the inner membrane of the mitochondria. Cytochromes are among the most important of these electron carriers. Like hemoglobin, cytochromes are colored proteins which contain iron in a nitrogen-containing heme group. The final electron acceptor of the electron transfer chain is oxygen, which produces water as a final product of cellular respiration (see equation 1).

The main function of the electron transfer chain is the synthesis of 32 molecules of ATP from the controlled oxidation of the eight molecules of NADH and two molecules of FADH2, made by the oxidation of one molecule of glucose in glycolysis and the citric acid cycle. This oxygen-requiring process is known as oxidative phosphorylation.

The electron transfer chain slowly extracts the energy from NADH and FADH2 by passing electrons from these high energy molecules from one electron carrier to another, as if along a chain. As this occurs, protons (H+) are pumped across the inner membrane of mitochondria, creating a proton gradient which is subsequently used to make ATP by a process known as chemiosmosis.

Anaerobic respiration

The above reactions of cellular respiration are often referred to as aerobic respiration because the final series of reactions, the electron transfer chain, require oxygen as an electron acceptor. When oxygen is absent or in short supply, cells may rely upon glycolysis alone for their supply of ATP. Glycolysis presumably originated in primitive cells early in Earth's history when very little oxygen was present in the atmosphere.

In an anaerobic environment, pyruvate is typically broken down into lactate or into acetaldehyde and then ethanol , instead of being degraded to acetyl CoA and then introduced to the citric acid cycle. The NADH made during glycolysis (see above) is required for synthesis of ethanol or lactate. Obviously, exclusive reliance upon glycolysis for the manufacture of ATP is very inefficient, since only two molecules of ATP are made from each glucose molecule, whereas aerobic respiration makes 36 molecules of ATP from each glucose molecule.

Needless to say, synthesis of ethanol is essential in the making of wine and beer. In this case, the sugars present in the must (sweet juice of the crushed grapes ) or wort (sweet liquid from the malted barley ) are broken down to pyruvate and from there into ethanol. Interestingly, when humans drink ethanol, our livers metabolize it in the reverse direction, into acetaldehyde and other carbohydrates. Accumulation of acetaldehyde has been implicated in causing hangovers as well as in fetal alcohol syndrome , a suite of developmental abnormalities in an infant caused by exposure to alcohol as a fetus.

Efficiency of cellular respiration

One can easily determine the energy efficiency of cellular respiration by calculating the standard free energy change, a thermodynamic quantity, between the reactants and products. On this basis, biochemists often quote the overall efficiency of cellular respiration as about 40%, with the additional 60% of the energy given off as heat .

However, many cells regulate the different enzymes of respiration so that they are in nonequilibrium states, leading to a higher overall efficiency. Calculations of the free energy change, a different thermodynamic quantity, account for these regulatory effects and show that cellular respiration often has an efficiency of 60% or more.

Interestingly, some plants have two separate electron transfer chains in their mitochondria. The alternate electron transfer chain only operates occasionally, but when it does, it gives off most of its energy as heat, rather than ATP. This seemingly wasteful generation of heat is so great in some species that it volatilizes chemicals in their flowers which attract insect pollinators.

See also Respiratory system.

Resources

books

Galston, A. W. Life Processes of Plants: Mechanisms for Survival. New York: W. H. Freeman, 1993.

Hall, D. L. Why Do Animals Breathe? Manchester, NH: Ayer Press, Inc., 1981.

Nicholls, P. The Biology of Oxygen. Burlington, NC: Carolina Biological, Inc., 1982.

Randall, D. J., et al. The Evolution of Air Breathing in Vertebrates. Cambridge: Cambridge University Press, 1981.

Salisbury, F.B., and C.W. Ross. Plant Physiology. 4th ed. Belmont, CA: Wadsworth Inc., 1991.

Storer, T.I., R.L. Usinger, R.C. Stebbins, and J.W. Nybakken. General Zoology. 6th ed. New York: McGraw-Hill, Inc., 1979.

Stryer, L. Biochemistry. 4th ed. New York: W.H. Freeman and Company, 1999.

Peter A. Ensminger

KEY TERMS

ATP

—Adenosine triphosphate; a high energy molecule that cells use to drive energy-requiring processes such as biosynthesis, transport, growth, and movement.

Chemiosmosis

—Process in which a difference in H+ concentration on different sides of the inner mitochondrial membrane drives ATP synthesis.

Diffusion

—Random movement of molecules which leads to a net movement of molecules from a region of high concentration to a region of low concentration.

Eukaryote

—A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.

Fetal alcohol syndrome

—Suite of developmental abnormalities of an infant, caused by exposure to alcohol as a fetus.

Hemoglobin

—An iron-containing, protein complex carried in red blood cells that binds oxygen for transport to other areas of the body.

Mitochondrion (plural, mitochondria)

—Cellular organelle of eukaryotes which produces ATP.

Respiration

External respiration

Internal respiration

Cellular respiration

Anaerobic respiration

Resources

Respiration is the physiological process that organisms use to supply oxygen to their cells, and that the cells use that oxygen to produce high energy molecules. Respiration occurs in all types of organisms, including bacteria, protists, fungi, plants, and animals.

In higher animals, respiration is often separated into three separate components. The first component is external respiration (breathing), which is the exchange of oxygen and carbon dioxide between the environment and the organism. The second component is internal respiration, which is the exchange of oxygen and carbon dioxide between the internal body fluids, such as blood, and individual cells. The final component is cellular respiration, which is the biochemical oxidation (the loss of an electron) of glucose; the energy is used in the synthesis of adenosine tri-phosphate (ATP).

External respiration

External respiration is the exchange of oxygen and carbon dioxide between an animal and its environment. For this, most animals use specialized organs or organ systems such as lungs, trachea, or gills.

Exchange of gases between the environment and an animal occurs by diffusion through a surface on the animal which is permeable to oxygen and carbon dioxide. Diffusion is the random movement of molecules and causes a net movement of molecules from a region of high concentration to a region of low concentration. Thus, oxygen moves into an organism because its concentration is lower inside than in the environment (air or water); carbon dioxide moves out of an organism because its concentration is higher inside than in the environment.

Different organisms have different mechanisms for extracting oxygen from their environments. Direct diffusion occurs on organisms including sponges, jellyfish, and terrestrial flatworms. It is a relatively primitive method, which does not require organs dedicated to respiration, in which oxygen diffuses from the environment through cells on the animal’s surface and then diffuses to individual cells inside. This mechanism is not suitable for many small creatures, since their internal volume is much greater than their surface area, and so the oxygen that can diffuse inward is insufficient to meets the organisms’ need. Microorganisms are able to use direct diffusion.

Another rout of respiration involves diffusion of oxygen into blood. Annelids (segmented worms) and amphibians use this method, in which oxygen diffuses through a moist layer of epidermal cells on the body surface and from there through capillary walls and into the blood stream. Once oxygen is in the blood, it moves throughout the body to different tissues and cells. While this method does not rely upon respiratory organs and is thus quite primitive, it is somewhat more advanced than direct diffusion.

A third repiratory route involves structures called tracheae. Insects and terrestrial arthropods use this method. In tracheal respiration, air moves through openings in the body surface called spiracles and then into special tubes called tracheae (singular, trachea) which extend into the body. The tracheae divide into many small branches which contact the muscles and organs. In small insects, air moves into the tracheae passively, whereas in large insects, body movements facilitate tracheal air movement. An advantage of tracheal respiration is that it provides oxygen directly to the muscles. Muscle cells use this oxygen, together with the carbohydrates and other energetic molecules in the hemolymph (insect blood), to generate the energy needed for flight.

Fish and other aquatic animals acqurie oxygen through gills. Gills are specialized tissues with many infoldings, each covered by a thin layer of cells and impregnated with blood capillaries. They take up oxygen dissolved in water and expel carbon dioxide dissolved in blood. Gills work by a mechanism called countercurrent exchange, in which blood and water flow in discrete pathways and opposite directions. This allows gills to more efficiently extract oxygen from water and expel carbondioxide into the water. Certain details of gill anatomy differ among different species.

Another route of respiration involves lungs. Terrestrial vertebrates use this method. Lungs are special organs in the body cavity that are composed of many small chambers impregnated with blood capillaries. After air enters the lungs, oxygen diffuses into the blood stream through the walls of these capillaries. It then moves from the lung capillaries to the different muscles and organs of the body. Humans and other mammals have lungs in which air moves in and out through the same pathway. In contrast, birds have more specialized lungs which use a mechanism called crosscurrent exchange. Like the countercurrent exchange mechanism of gills, air flows through the crosscurrent exchange system of bird lungs in one direction only, making for more efficient oxygen exchange.

Internal respiration

Internal respiration is the exchange of oxygen and carbon dioxide between blood and cells in different tissues of an animal’s body. Internal respiration occurs in animals with a circulation system. Animals with gills or lungs take up oxygen and transport oxygen-rich blood throughout the body; they transport carbon dioxide-rich blood from the body back into the respiratory organs where it is expelled. The oxygen-rich blood and carbon dioxide-rich blood do not mix, making for an efficient internal respiration system.

Mammals and birds have a double circulation system for blood, in which separate pumps in the left and right chambers of the heart move the oxygen-rich blood in the arteries and carbon dioxide-rich blood in the veins.

The blood of vertebrates and some invertebrates contains a protein (such as hemoglobin, hemocyanin, or chlorocruorin), which binds oxygen and transports it from the respiratory organs throughout the body. These oxygen-binding proteins greatly improve the oxygen carrying ability of blood. For example, human hemoglobin contains about 98% of the oxygen in a human’s blood.

Hemoglobin is a red protein which binds oxygen and occurs in the red blood cells of vertebrates. Each molecule of hemoglobin contains an iron atom and can bind up to four molecules of oxygen. In muscles, hemoglobin passes its oxygen to myoglobin. Myoglobin is an oxygen-binding protein that makes muscles red and transports oxygen to the cells of the muscle. In turn, muscle cells use the oxygen from myoglobin to power muscle movement by cellular respiration.

Some segmented worms (annelids) have a green blood protein, called chlorocruorin, which binds iron and serves as an oxygen carrier. Some invertebrates have a blue blood protein, called hemocyanin, which binds copper and serves as an oxygen carrier.

Cellular respiration

Cellular respiration is an intracellular process in which glucose (C₆H₁₂O₆) is oxidized and the energy is used to make ATP (adenosine triphosphate). ATP is a high energy molecule which organisms use to drive energy-requiring processes such as biosynthesis, transport, growth, and movement. The general features of cellular respiration are the same in most organisms.

Cellular respiration is divided into three sequential series of reactions: glycolysis, the citric acid cycle, and the electron transport chain. In higher organisms (eukaryotes), glycolysis occurs in the cytosol of the cell, the aqueous region outside the nucleus; the citric acid cycle and electron transport chain occur in the mitochondria, cellular organelles (intracellular organ-like structures) which have characteristic double membranes and are specialized for ATP production.

Glycolysis can be defined simply as the lysis, or splitting, of sugar. More particularly, it is the controlled breakdown of glucose, a six-carbon carbohydrate, into pyruvate, a three-carbon carbohydrate. Organisms frequently store complex carbohydrates, such as glycogen or starch, and break these down into glucose units which can then enter into glycolysis.

Two features of glycolysis suggest that it has an ancient evolutionary origin. First, the same series of reactions occur in virtually all cells, including bacteria, plants, fungi, and animals. Second, glycolysis does not require oxygen, making it appropriate for primeval cells which had to live in a world with very little atmospheric oxygen.

After pyruvate is synthesized by glycolysis, it moves into the mitochondria and is oxidized to form carbon dioxide (a 1-carbon molecule) and acetyl CoA (a two carbon molecule). Cells can also make acetyl CoA from fats and amino acids and this is how cells often derive energy, in the form of ATP, from molecules other than glucose or complex carbohydrates.

After acetyl CoA forms, it enters into a series of nine sequential enzymatic reactions, known as the citric acid cycle. These reactions are so named because the first reaction makes one molecule of citric acid (a six-carbon molecule) from one molecule of acetyl CoA (a two-carbon molecule) and one molecule of oxaloacetic acid (a 4-carbon molecule). A complete round of the citric acid cycle expels two molecules of carbon dioxide and regenerates one molecule of oxaloacetic acid, hence the cyclic nature of these reactions. The citric acid cycle is sometimes called the Krebs cycle, in honor of Hans Krebs, the English biochemist who first proposed that pyruvate is broken down by a cycle of biochemical reactions.

The electron transfer chain is the final series of biochemical reactions in cellular respiration. It consists of a series of organic electron carriers associated with the inner membrane of the mitochondria. Cytochromes are among the most important of these electron carriers. Like hemoglobin, cytochromes are colored proteins which contain iron in a nitrogen-containing heme group. The final electron acceptor of the electron transfer chain is oxygen, which produces water as a final product of cellular respiration.

The main function of the electron transfer chain is the synthesis of 32 molecules of ATP from the controlled oxidation of the eight molecules of NADH and two molecules of FADH₂, made by the oxidation of one molecule of glucose in glycolysis and the citric acid cycle. This oxygen-requiring process is known as oxidative phosphorylation.

The electron transfer chain slowly extracts the energy from NADH and FADH₂ by passing electrons from these high energy molecules from one electron carrier to another, as if along a chain. As this occurs, protons (H⁺) are pumped across the inner membrane of mitochondria, creating a proton gradient which is subsequently used to make ATP by a process known as chemiosmosis.

KEY TERMS

ATP— Adenosine triphosphate; a high energy molecule that cells use to drive energy-requiring processes such as biosynthesis, transport, growth, and movement.

Chemiosmosis— Process in which a difference in H⁺ concentration on different sides of the inner mitochondrial membrane drives ATP synthesis.

Diffusion— Random movement of molecules which leads to a net movement of molecules from a region of high concentration to a region of low concentration.

Eukaryote— A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.

Fetal alcohol syndrome— Suite of developmental abnormalities of an infant, caused by exposure to alcohol as a fetus.

Hemoglobin— An iron-containing, protein complex carried in red blood cells that binds oxygen for transport to other areas of the body.

Mitochondrion (plural, mitochondria)— Cellular organelle of eukaryotes which produces ATP.

Anaerobic respiration

The above reactions of cellular respiration are often referred to as aerobic respiration because the final series of reactions, the electron transfer chain, require oxygen as an electron acceptor. When oxygen is absent or in short supply, cells may rely upon glycolysis alone for their supply of ATP. Glycolysis presumably originated in primitive cells early in earth’s history when very little oxygen was present in the atmosphere.

In an anaerobic environment, pyruvate is typically broken down into lactate or into acetaldehyde and then ethanol, instead of being degraded to acetyl CoA and then introduced to the citric acid cycle. The NADH made during glycolysis is required for synthesis of ethanol or lactate. Obviously, exclusive reliance upon glycolysis for the manufacture of ATP is very inefficient, since only two molecules of ATP are made from each glucose molecule, whereas aerobic respiration makes 36 molecules of ATP from each glucose molecule.

Resources

BOOKS

Alberts, Bruce, Alexander Johnson, Julian Lewis, Martin Raff, Dennis Bray, Karen Hopkin, Keith Roberts, and Peter Walter. Essential Cell Biology. 2nd ed. New York: Garland Science/Taylor & Francis Group,.

Dyer, Betsey D. A Field Guide to Bacteria. Cornell: Cornell University Press, 2003.

Gilbert, Scott F. Developmental Biology. Sunderland, MA: Sinauer Associates, 2006.

Lodish, Harvey F. Molecular Cell Biology. New York: W.H. Freeman & Company, 2003.

Peter A. Ensminger

Respiration

The term respiration has two relatively distinct meanings in biology. First, respiration is the process by which an organism takes oxygen into its body and then releases carbon dioxide from its body. In this respect, respiration can be regarded as roughly equivalent to "breathing." In some cases, this meaning of the term is extended to mean the transfer of the oxygen from the lungs to the bloodstream and, eventually, into cells. On the other hand, it may refer to the release of carbon dioxide from cells into the bloodstream and, thence, to the lungs.

Words to Know

Aerobic respiration: Respiration that requires the presence of oxygen.

Anaerobic respiration: Respiration that does not require the presence of oxygen.

ATP (adenosine triphosphate): High-energy molecule that cells use to drive energy-requiring processes such as biosynthesis (the production of chemical compounds), growth, and movement.

Capillaries: Very thin blood vessels that join veins to arteries.

Diffusion: Random movement of molecules that leads to a net movement of molecules from a region of high concentration to a region of low concentration.

Fermentation: A chemical reaction by which carbohydrates, such as sugar, are converted into ethyl alcohol.

Gill: An organ used by some animals for breathing consisting of many specialized tissues with infoldings. It allows the animal to absorb oxygen dissolved in water and expel carbon dioxide to the water.

Glucose: also known as blood sugar, a simple sugar broken down in cells to produce energy.

Glycolysis: A series of chemical reactions that takes place in cells by which glucose is converted into pyruvate.

Hemoglobin: Blood protein that can bind with oxygen.

Lactic acid: Similar to lactate, a chemical compound formed in cells from pyruvate in the absence of oxygen.

Pyruvate: The simpler compound glucose is broken down into during the process of glycolysis.

Trachea: A tube used for breathing.

Second, respiration also refers to the chemical reactions that take place within cells by which food is "burned" and converted into carbon dioxide and water. In this respect, respiration is the reverse of photosynthesis, the chemical change that takes place in plants by which carbon dioxide and water are converted into complex organic compounds. To distinguish from the first meaning of respiration, this "burning" of foods is also referred to as aerobic respiration.

Respiration mechanisms

All animals have some mechanism for removing oxygen from the air and transmitting it into their bloodstreams. The same mechanism is used to expel carbon dioxide from the bloodstream into the surrounding environment. In many cases, a special organ is used, such as lungs, trachea, or gills. In the simplest of animals, oxygen and carbon dioxide are exchanged directly between the organism's bloodstream and the surrounding environment. Following are some of the mechanisms that animals have evolved to solve this problem.

Direct diffusion. In direct diffusion, oxygen passes from the environment through cells on the animal's surface and then into individual cells inside. Sponges, jellyfish, and terrestrial flatworms use this primitive method of respiration. These animals do not have special respiratory organs. Microbes, fungi, and plants all obtain the oxygen they use for cellular respiration by direct diffusion through their surfaces.

Diffusion into blood. In diffusion into the blood, oxygen passes through a moist layer of cells on the body surface. From there, it passes through capillary walls and into the blood stream. Once oxygen is in the blood, it moves throughout the body to different tissues and cells. This method also does not rely upon special respiratory organs and is thus quite primitive. However, it is somewhat more advanced than direct diffusion. Annelids (segmented worms) and amphibians use this method of respiration.

Tracheae. In tracheal respiration, air moves through openings in the body surface called spiracles. It then passes into special breathing tubes called tracheae (singular, trachea) that extend into the body. The tracheae divide into many small branches that are in contact with muscles and organs. In small insects, air moves into the tracheae simply by molecular motion. In large insects, body movements assist tracheal air movement. Insects and terrestrial arthropods (organisms with external skeletons) use this method of respiration.

Gills. Fish and other aquatic animals use gills for respiration. Gills are specialized tissues with many infoldings. Each gill is covered by a thin layer of cells and filled with blood capillaries. These capillaries take up oxygen dissolved in water and expel carbon dioxide dissolved in blood.

Lungs. Lungs are special organs in the body cavity composed of many small chambers filled with blood capillaries. After air enters the lungs, oxygen diffuses into the blood stream through the walls of these capillaries. It then moves from the lung capillaries to the different muscles and organs of the body. Humans and other mammals have lungs in which air moves in and out through the same pathway. In contrast, birds have more specialized lungs that use a mechanism called crosscurrent exchange. Crosscurrent exchange allows air to flow in one direction only, making for more efficient oxygen exchange.

Movement of gases through the body

In direct diffusion and tracheal systems, oxygen and carbon dioxide move back and forth directly between cells and the surrounding environment. In other systems, some mechanism is needed to carry these gases between cells and the outside environment. In animals with lungs or gills, oxygen is absorbed by the bloodstream, converted into an unstable (easily broken down) chemical compound, and then carried to cells. When the compound reaches a cell, it breaks down and releases the oxygen. The oxygen then passes into the cell.

In the reverse process, carbon dioxide is released from a cell into the bloodstream. There the carbon dioxide is used to form another unstable chemical compound, which is carried by the bloodstream back to the gills or lungs. At the end of this journey, the compound breaks down and releases the carbon dioxide to the surrounding environment.

Various animals use different substances to form these unstable compounds. In humans, for example, the substance is a compound known as hemoglobin. In the lungs, hemoglobin reacts with oxygen to form oxyhemoglobin. Oxyhemoglobin travels through the bloodstream to cells, where it breaks down to form hemoglobin and oxygen. The oxygen then passes into cells.

On the return trip, hemoglobin combines with carbon dioxide to form carbaminohemoglobin. In this (and other) forms, carbon dioxide is returned to the surrounding environment.

Animals other than humans use compounds other than hemoglobin for the transport of oxygen and carbon dioxide. Certain kinds of annelids (earthworms, various marine worms, and leeches), for example, contain a green blood protein called chlorocruorin that functions in the same way that hemoglobin does in humans.

Whatever substance is used, the compound it forms with oxygen and carbon dioxide must be unstable, it must break down easily. This property is essential if the oxygen and carbon dioxide are to be released easily at the end of their journeys into and out of cells, lungs, and gills.

Cellular respiration. Cellular respiration is a process by which the simple sugar glucose is oxidized (combined with oxygen) to form the energy-rich compound adenosine triphosphate (ATP). Glucose is produced in cells by the breakdown of more complex carbohydrates, including starch, cellulose, and complex sugars such as sucrose (cane or beet sugar) and fructose (fruit sugar). ATP is the compound used by cells to carry out most of their ordinary functions, such as production of new cell parts and chemicals, movement of compounds through cells and the body as a whole, and growth.

The overall chemical change that occurs in cellular respiration can be represented by a fairly simple chemical equation:

6C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + 36 ATP

That equation says that six molecules of glucose (C₆H₁₂O₆) react with six molecules of oxygen (O₂) to form six molecules of carbon dioxide (CO₂), six molecules of water (H₂O) and 36 molecules of ATP.

Cellular respiration is, however, a great deal more complicated that this equation would suggest. In fact, nearly two dozen separate chemical reactions are involved in the overall conversion of glucose to carbon dioxide, water, and ATP. Those two dozens reactions can be grouped together into three major cycles: glycolysis, the citric acid (or Krebs) cycle, and the electron transport chain.

In glycolysis, glucose is broken down into a simpler compound known as pyruvate. Pyruvate, in turn, is converted in the citric acid cycle to a variety of energy-rich compounds, such as ATP and NADH (nicotinamide adenine dinucleotide). Finally, all of these energy-rich compounds are converted in the electron transport chain to ATP.

Anaerobic respiration. As the equation above indicates, cellular respiration usually requires the presence of oxygen and is, therefore, often known as aerobic (or "using oxygen") respiration. Another form of respiration is possible, one that does not make use of oxygen. That form of respiration is known as anaerobic (or "without oxygen") respiration.

Anaerobic respiration begins, as does aerobic respiration, with glycolysis. In the next step, however, pyruvate is not passed onto the citric acid cycle. Instead, it undergoes one of two other chemical reactions. In the first of these reactions, the pyruvate is converted to ethyl alcohol in a process known as fermentation. Fermentation is a well-known chemical reaction by which grapes, barley, rice, and other grains are used to make wine, beer, and other alcoholic beverages.

The second anaerobic reaction occurs when cells are unable to obtain oxygen by methods they normally use. For example, a person who exercises vigorously may not be able to inhale oxygen fast enough to meet the needs of his or her cells. (Glucose is used up faster than oxygen is supplied to the cells.) In that case, cells switch over to anaerobic respiration. They convert glucose to pyruvate and then to another chemical known as lactate or lactic acid (two forms of the same compound). As lactic acid begins to build up in cells, it causes an irritation similar to placing vinegar (acetic acid) in an open wound.

Most cells are able to switch from aerobic to anaerobic respiration when necessary. But they are generally not able to continue producing energy by this process for very long.

Scientists believe that the first organisms to appear on Earth's surface were anaerobic organisms. Those organisms arose when Earth's atmosphere contained very little oxygen. They had to produce the energy they needed, therefore, by mechanisms that did not require oxygen. As the composition of Earth's atmosphere changed to include more oxygen, organisms evolved to adapt to that condition.

[See also Bacteria; Blood; Diffusion; Fermentation; Metabolism; Oxygen family; Respiratory system; Yeast ]

respiration is the absorption of oxygen and the output of carbon dioxide that take part in the metabolic processes in the body; it includes the burning of foodstuffs in the tissues, the transport of the gases in the blood and their exchange in the lungs. All but the smallest animals, for example single cells like amoeba, require specialized transport and exchange organs to provide sufficient supply and removal of the large quantities of gases involved. A vigorously exercising athletic human might take up as much as six litres of oxygen per minute, and excrete a similar amount or more of carbon dioxide.

It is usual to divide respiration into external and internal components, linked by the bloodstream that transports the gases but has little metabolism of its own. In mammals, external respiration — the ventilation of the lungs — is achieved by breathing, the mechanical basis of respiration: the terms are sometimes used synonymously. In fishes there is equivalent ‘ventilation’ of the gills with water. At rest an adult human inhales about 6–8 litres of air per minute, of which about two thirds gets to the alveoli and takes part in gas exchange, the rest remaining in the air passages. Fresh air contains 21% oxygen and almost no carbon dioxide. In the lungs, the entry of oxygen into the blood and the release of carbon dioxide results in exhaled gas having about 5% less oxygen and 5% more carbon dioxide. The gases pass in and out of the blood by passive diffusion, due to their relative pressures in the blood and in the alveolar gas, just as a fizzy drink will lose its gas and go flat when exposed to air. At rest an average-sized adult will take up about 250 ml of oxygen each minute, and exhale about 200 ml of carbon dioxide. In vigorous exercise these values can increase over 20-fold in a trained athlete as a result of the increased metabolism in the muscles. The blood in the capillaries leaving the lungs, and therefore in the arteries carrying it round the body, have the same pressures of oxygen and carbon dioxide as those in the alveoli, at least in health. In some lung and heart disease, involving a problem of gas transfer, the arterial blood may have a lower oxygen and a higher carbon dioxide pressure than that in the alveoli, causing hypoxia and hypercapnia. In other types of lung disease these abnormalities in the blood result from inadequate breathing — failure to ventilate the alveoli with sufficient fresh air.

In the blood almost all the oxygen is carried combined with haemoglobin in the red cells, while carbon dioxide is taken up in both plasma and red cells. Once the blood has reached the tissues, internal respiration takes over. Oxygen will diffuse from the capillaries into metabolizing cells, again passively following its pressure gradient. Vigorously contracting muscle cells use up almost all the available oxygen inside them, and strongly pull a fresh supply from the blood by diffusion. The oxygen combines with food materials, mainly sugars and fats, to release heat and energy for contraction of muscle, with water and carbon dioxide as the main waste products. The water enters the general water pool of the body, while the carbon dioxide enters the blood and is carried to the lungs mainly as bicarbonate but also in combination with hemoglobin and plasma proteins. An appreciable amount of carbon dioxide, unlike oxygen, is also free in solution in the plasma. Such aerobic metabolism occurs similarly in the great majority of body cells.

This understanding of the chemical basis of respiration was only developed in the eighteenth century, with the chemical identification of oxygen and carbon dioxide, mainly by Joseph Priestley (1733–1804) and Antoine-Laurent Lavoisier (1743–94). For two thousand years before then there were many speculations about the meaning of breathing, the main idea being that its function was to cool the blood by physically mixing air with it in the arteries and veins. The cooling function seemed proved by the fact that exhaled gas was usually warmer than inhaled air, and the mixing with blood was deduced from the fact that when arteries and veins were cut the blood flowing out was often frothy. (We now know that if large veins are cut they will suck air into the circulation, and it will pass through the heart and lungs and make the haemorrhaging blood bubbly.) In the second century ad Galen wrote ‘respiration is useful to animals for the sake of the heart, which to some extent requires the substance of the air and besides needs very greatly to be cooled because of its burning heat.’ Even primitive men must have known that breathing was necessary for life; according to the Judaeo–Christian tradition, breath created it. ‘And the Lord God formed man of the dust of the ground and breathed into his nostrils the breath of life; and man became a living soul’ (Genesis 2:7). In the seventeenth century much experimental work established that one component of air was essential for life, and that another different fraction was exhaled. These were identified as oxygen and carbon dioxide a century later. There was much speculation about the ‘respiration’ of fetuses. Clearly they could not breathe air, since their lungs were full of liquid. Since the role of the blood circulation in gas exchange was not understood, the respiratory function of the placenta, the ‘external’ respiratory organ of the fetus, was not apparent.

Sophisticated and accurate methods of analysing respiratory gases were developed in the twentieth century, and the mechanisms of external respiration are now well defined. At about the same time development of biochemistry and, later, of molecular biology, led to an understanding of internal respiration. When oxygen combines with carbohydrates every molecule of oxygen creates one molecule of water and one in carbon dioxide. Thus the ratio of the exchange volumes of the two gases, the respiratory quotient, is 1; in practice this would only occur in someone living on a diet of, and metabolizing only carbohydrate — polished rice, for example. Fat and protein contain less oxygen than does carbohydrate, so more oxygen is needed for their consumption than the carbon dioxide that is produced. On an average diet in a developed country the ratio of carbon dioxide excreted to oxygen absorbed is about 0.8. However these values assume that there is enough oxygen for the metabolic needs of all the body — that we are at rest or in a state of entirely aerobic metabolism. ‘Aerobic exercise’ is exercise within this limit. If exercise is severe, not enough oxygen is available for the muscles, which pass an ‘anaerobic threshold’, and energy is provided additionally by the breakdown of carbohydrates without using oxygen and with the formation of lactic acid. This diffuses into the blood, causing a mild acidosis and acting on the bicarbonate to release carbon dioxide which is excreted in the lungs. (Lemon juice, put on self-raising flour, or bicarbonate of soda, will make it fizz as it releases carbon dioxide.) Thus in severe exercise we exhale more carbon dioxide than the oxygen we absorb. After the exercise we retain carbon dioxide in the body and most of the lactic acid is converted back into carbohydrate in the liver. Extra oxygen is needed for this, and the process is referred to as ‘repaying the oxygen debt’.

John Widdicombe