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Chapter 2 The Chemical Basis of Life Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 1 Basic Chemistry • Elements and compounds (Figure 2-1) Matter—anything that has mass and occupies space Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 2 Basic Chemistry • Element—simple form of matter, a substance that cannot be broken down into two or more different substances There are 26 elements in the human body There are 11 “major elements,” four of which (carbon, oxygen, hydrogen, and nitrogen) make up 96% of the human body There are 15 “trace elements” that make up less than 2% of body weight • Compound—atoms of two or more elements joined to form chemical combinations Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 3 Basic Chemistry • Atoms (Figure 2-2) The concept of an atom was proposed by the English chemist John Dalton Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 4 Basic Chemistry • Atomic structure—atoms contain several different kinds of subatomic particles; the most important are the following: Protons (+ or p)—positively charged subatomic particles found in the nucleus Neutrons (n)—neutral subatomic particles found in the nucleus Electrons (– or e)—negatively charged subatomic particles found in the electron cloud Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 5 Basic Chemistry • Atomic number and atomic weight Atomic number (Table 2-1) • The number of protons in an atom’s nucleus • The atomic number is critically important; it identifies the kind of element Atomic weight • The mass of a single atom • It is equal to the number of protons plus the number of neutrons in the nucleus (p + n) Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 6 Basic Chemistry • Energy levels (Figures 2-3 and 2-4) The total number of electrons in an atom equals the number of protons in the nucleus (in a stable atom) The electrons form a “cloud” around the nucleus Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 7 Basic Chemistry • Energy levels (cont.) “Bohr model”—a model resembling planets revolving around the sun, useful in visualizing the structure of atoms • Exhibits electrons in concentric circles showing relative distances of the electrons from the nucleus • Each ring or shell represents a specific energy level and can hold only a certain number of electrons • The number and arrangement of electrons determine if an atom is chemically stable • An atom with eight, or four pairs of, electrons in the outermost energy level is chemically inert • An atom without a full outermost energy level is chemically active Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 8 Basic Chemistry • Energy levels (cont.) Octet rule—atoms with fewer or more than eight electrons in the outer energy level will attempt to lose, gain, or share electrons with other atoms to achieve stability Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 9 Basic Chemistry • Isotopes (Figure 2-5) Isotopes of an element contain the same number of protons but contain different numbers of neutrons Isotopes have the same atomic number, and therefore the same basic chemical properties, as any other atom of the same element, but they have a different atomic weight Radioactive isotope—an unstable isotope that undergoes nuclear breakdown and emits nuclear particles and radiation Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 10 Basic Chemistry • Attractions between atoms—chemical bonds Chemical reaction—interaction between two or more atoms that occurs as a result of activity between electrons in their outermost energy levels Molecule—two or more atoms joined together Compound—consists of molecules formed by atoms of two or more elements Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 11 Basic Chemistry • Chemical bonds—two types unite atoms into molecules: Ionic, or electrovalent, bond (Figure 2-6)—formed by transfer of electrons; strong electrostatic force that binds positively and negatively charged ions together Covalent bond (Figure 2-7)—formed by sharing of electron pairs between atoms Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 12 Basic Chemistry • Hydrogen bond (Figures 2-8 and 2-9) Much weaker than ionic or covalent bonds Results from unequal charge distribution on molecules Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 13 Basic Chemistry • Attractions between molecules Hydrogen bonds • Form when electrons are unequally shared Example: water molecule Polar molecules have regions with partial electrical charges resulting from unequal sharing of electrons among atoms • Areas of different partial charges attract one another, forming hydrogen bonds Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 14 Basic Chemistry • Attractions between molecules Other weak forces attract molecules to each other through differences in electrical charge Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 15 Basic Chemistry • Chemical reactions Involve the formation or breaking of chemical bonds There are three basic types of chemical reactions involved in physiology: • Synthesis reaction—combining of two or more substances to form a more complex substance; formation of new chemical bonds: A + B → AB • Decomposition reaction—breaking down of a substance into two or more simpler substances; breaking of chemical bonds: AB → A + B • Exchange reaction—decomposition of two substances and, in exchange, synthesis of two new compounds from them: AB + CD → AD + CB • Reversible reactions—occur in both directions Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 16 Metabolism • Metabolism—all of the chemical reactions that occur in body cells (Figure 2-10) • Catabolism Chemical reactions that break down complex compounds into simpler ones and release energy; hydrolysis is a common catabolic reaction Ultimately, the end products of catabolism are carbon dioxide, water, and other waste products More than half the energy released is transferred to ATP, which is then used to do cellular work (Figure 2-28) Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 17 Metabolism • Anabolism Chemical reactions that join simple molecules together to form more complex molecules Chemical reaction responsible for anabolism is dehydration synthesis Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 18 Organic and Inorganic Compounds • Inorganic compounds—few have carbon atoms and none have C–C or C–H bonds • Organic molecules Have at least one carbon atom and at least one C–C or C–H bond in each molecule Often have functional groups attached to the carbon-containing core of the molecule (Figure 2-11) Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 19 Inorganic Compounds • Water The body’s most abundant and important compound Properties of water (Table 2-2) • Polarity—allows water to act as an effective solvent; ionizes substances in solution (Figure 2-8) • The solvent allows transportation of essential materials throughout the body (Figure 2-12) Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 20 Inorganic Compounds • Properties of water (cont.) High specific heat—water can lose and gain large amounts of heat with little change in its own temperature; enables the body to maintain a relatively constant temperature High heat of vaporization—water requires absorption of significant amounts of heat to change water from a liquid to a gas, allowing the body to dissipate excess heat Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 21 Inorganic Compounds • Oxygen and carbon dioxide—closely related to cellular respiration Oxygen—required to complete decomposition reactions necessary for the release of energy in the body Carbon dioxide—produced as a waste product, also helps maintain the appropriate acid-base balance in the body Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 22 Inorganic Compounds • Electrolytes Large group of inorganic compounds, which includes acids, bases, and salts Substances that dissociate in solution to form ions Positively charged ions are cations; negatively charged ions are anions Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 23 Inorganic Compounds • Electrolytes (cont.) Acids and bases—common and important chemical substances that are chemical opposites • Acids Any substance that releases a hydrogen ion (H+) when in solution; “proton donor” Level of “acidity” depends on the number of hydrogen ions a particular acid will release • Bases Electrolytes that dissociate to yield hydroxide ions (OH–) or other electrolytes that combine with hydrogen ions (H+) Described as “proton acceptors” Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 24 Inorganic Compounds • pH scale—measuring acidity and alkalinity (Figure 2-13) pH indicates the degree of acidity or alkalinity of a solution pH of 7 indicates neutrality (equal amounts of H+ and OH–); a pH of less than 7 indicates acidity; a pH of more than 7 indicates alkalinity Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 25 Inorganic Compounds • Buffers Maintain the constancy of the pH Minimize changes in the concentrations of H+ and OH– ions Act as a “reservoir” for hydrogen ions Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 26 Inorganic Compounds • Salts (Table 2-3) Compounds that result from chemical interaction of an acid and a base Reaction between an acid and a base to form a salt and water is called a neutralization reaction Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 27 Organic Molecules (Figure 2-14; Table 2-4) • “Organic” describes compounds that contain C–C or C–H bonds • Carbohydrates—organic compounds containing carbon, hydrogen, and oxygen; commonly called sugars and starches Monosaccharides—simple sugars with short carbon chains; those with six carbons are hexoses (e.g., glucose), whereas those with five are pentoses (e.g., ribose, deoxyribose) (Figure 2-15) Disaccharides and polysaccharides—two (di-) or more (poly-) simple sugars that are bonded together through a synthesis reaction (Figure 2-16) Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 28 Organic Molecules • Proteins (Table 2-5) Most abundant organic compounds Chainlike polymers Amino acids—building blocks of proteins (Figures 2-17 to 2-19) • Essential amino acids—eight amino acids that cannot be produced by the human body • Nonessential amino acids—12 amino acids can be produced from molecules available in the human body • Amino acids consist of a carbon atom, an amino group, a carboxyl group, a hydrogen atom, and a side chain Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 29 Organic Molecules • Levels of protein structure (Figure 2-20) Protein molecules are highly organized and show a definite relationship between structure and function There are four levels of protein organization: • Primary structure—refers to the number, kind, and sequence of amino acids that make up the polypeptide chain • Secondary structure—polypeptide is coiled or bent into pleated sheets stabilized by hydrogen bonds • Tertiary structure—a secondary structure can be further twisted, resulting in a globular shape; the coils touch in many places and are “welded” by covalent and hydrogen bonds • Quaternary structure—highest level of organization occurring when protein contains more than one polypeptide chain Two broad categories Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 30 Organic Molecules • Structural proteins form the structures of the body • Functional proteins cause chemical changes in the molecules Shape of a protein’s molecules determines its function (Figure 221) • Denatured proteins have lost their shape and therefore their function (Figure 2-22) • Proteins can be denatured by changes in pH, temperature, radiation, and other chemicals • If the chemical environment is restored, proteins may be renatured and function normally Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 31 Organic Molecules • Lipids (Table 2-6) Water-insoluble organic molecules that are critically important biological compounds Major roles: • Energy source • Structural role • Integral parts of cell membranes Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 32 Organic Molecules • Lipids (cont.) Triglycerides, or fats (Figures 2-23 and 2-24) • Most abundant lipids and most concentrated source of energy • The building blocks of triglycerides are glycerol (the same for each fat molecule) and fatty acids (different for each fat, they determine its chemical nature) Types of fatty acids—saturated fatty acid (all available bonds are filled) and unsaturated fatty acid (has one or more double bonds) Triglycerides are formed by a dehydration synthesis Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 33 Organic Molecules • Lipids (cont.) Phospholipids (Figure 2-25) • Fat compounds similar to triglyceride • One end of the phospholipid is water-soluble (hydrophilic); the other end is fat-soluble (hydrophobic) • Phospholipids can join two different chemical environments • Phospholipids may form double layers called bilayers that make up cell membranes Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 34 Organic Molecules • Lipids (cont.) Steroids (Figure 2-26) • Main component is steroid nucleus • Involved in many structural and functional roles Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 35 Organic Molecules • Lipids (cont.) Prostaglandins (Figure 2-27) • Commonly called “tissue hormones”; produced by cell membranes throughout the body • Effects are many and varied; however, they are released in response to a specific stimulus and are then inactivated Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 36 Organic Molecules • Nucleic acids and related molecules DNA (deoxyribonucleic acid) • Composed of deoxyribonucleotides; that is, structural units composed of the pentose sugar (deoxyribose), phosphate group, and nitrogenous base (cytosine, thymine, guanine, or adenine) • DNA molecule consists of two long chains of deoxyribonucleotides coiled into double-helix shape (Figure 2-28) • Alternating deoxyribose and phosphate units form backbone of the chains Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 37 Organic Molecules • DNA (cont.) Base pairs hold the two chains of DNA molecule together Specific sequence of more than 100 million base pairs constitute one human DNA molecule; all DNA molecules in one individual are identical and different from those in all other individuals DNA functions as the molecule of heredity Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 38 Organic Molecules • Nucleic acids and related molecules (cont.) RNA (ribonucleic acid) (Figure 2-29, Table 2-7) • Composed of the pentose sugar (ribose), phosphate group, and a nitrogenous base • Nitrogenous bases for RNA are adenine, uracil, guanine, or cytosine (uracil replaces thymine) • Some RNA molecules are temporary copies of segments (genes) of the DNA code and are involved in synthesizing proteins • Some RNA molecules are regulatory, acting as enzymes (ribozymes) or silencing gene expression (RNA interference) Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 39 Organic Molecules • Nucleic acids and related molecules (cont.) Nucleotides • Nucleotides have other important roles in the body • ATP (Figure 2-30) Composed of – Adenosine – Ribose—a pentose sugar – Adenine—a nitrogen-containing molecule – Three phosphate subunits Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 40 Organic Molecules • Nucleotides (cont.) • High-energy bonds present between phosphate groups • Cleavage of high-energy bonds releases energy during catabolic reactions • Energy stored in ATP is used to do the body’s work • ATP often called the energy currency of cells • ATP is split into adenosine diphosphate (ADP) and an inorganic phosphate group by a special enzyme • If ATP is depleted during prolonged exercise, creatine phosphate (CP) or ADP can be used for energy Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 41 Organic Molecules • Nucleic acids and related molecules (cont.) NAD and FAD (Figure 2-31) • Used as coenzymes to transfer energy-carrying molecules from one chemical pathway to another cAMP (cyclic AMP) • Made from ATP by removing two phosphate groups to form a monophosphate • Used as an intracellular signal Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 42 Organic Molecules • Combined forms—large molecules can be joined together to form even larger molecules Gives the molecules a completely different function Names of combined molecules reveals what is in them • Base word tells which component is dominant • Prefix shows the component present in a lesser amount Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 43 Organic Molecules • Combined forms (cont.) Examples: • Adenosine triphosphate (ATP)—two extra phosphate groups to a nucleotide • Lipoproteins—lipid and protein groups combined into a single molecule • Glycoproteins—carbohydrate (glyco, “sweet”) and protein • Examples of combined forms and their functions in the body shown in Table 2-4 Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 44