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.