1. Chapter 2
Chemistry of Life

2. Objectives Define the terms atom, element, molecule, and compound
Describe the structure of an atom
Compare and contrast ionic and covalent types of chemical bonding
Distinguish between organic and inorganic chemical compounds

3. Objectives Discuss the chemical characteristics of water
Explain the concept of pH
Discuss the structure and function of the following types of organic molecules: carbohydrate, lipid, protein, and nucleic acid

4. Levels of Chemical Organization
Atoms
Nucleus—central core of atom
Proton—positively charged particle in nucleus
Neutron—uncharged particle in nucleus
Atomic number—number of protons in nucleus
Atomic mass—number of protons and neutrons combined
What makes up an atom’s atomic mass? The number of protons and neutrons combined.

5. Levels of Chemical Organization
Atoms
Energy levels—orbital regions surrounding atomic nucleus that contain electrons
Electron—negatively charged particle
May contain up to 8 electrons in each level
Energy level increases the farther away it is from the nucleus
Describe an energy level. Energy levels (shells) are the limiting regions in which orbitals are arranged.

6. Models of the atom. The nucleus—protons (+) and uncharged neutrons—is at the core. Electrons inhabit outer regions called energy levels (A), which look more like a cloud because the electrons do not stay in one place (B). This is a carbon atom, a fact that is determined by the number of its protons. All carbon atoms (and only carbon atoms) have six protons.(One proton in the nucleus is not visible in this illustration.)

7. Levels of Chemical Organization
Elements, molecules, and compounds
Element—a pure substance; made up of only one kind of atom
Molecule—a group of atoms bound together in a group
Compound—substances whose molecules have more than one kind of atom

8. Chemical Bonding Chemical bonds form to make atoms more stable
Atoms react with one another in ways that make their outermost energy level full
Atoms may share electrons or donate or borrow them to become stable
What is a chemical bond? The energy that holds two atoms together.

9. Chemical Bonding Ionic bonds
Ions form when an atom gains or loses electrons in its outer energy level to become stable
Positive ion—has lost electrons; indicated by superscript positive sign(s), as in Na+ or Ca++
Negative ion—has gained electrons; indicated by superscript negative sign(s), as in Cl–
How is an ion formed? Ions form when an atom gains or loses an electron to become stable.

10. Chemical Bonding Ionic bonds
Ionic bonds form when positive and negative (oppositely charged) ions attract each other
Electrolyte—molecule that dissociates (breaks apart) in water to form individual ions; an ionic compound
What is dissociation? Occurs when molecules dissolve in water because water molecules wedge between the ions and force them apart.

11. Ionic bonding. A, The sodium atom donates the single electron in its outer energy level to a chlorine atom having seven electrons in its outer level. Then both have eight electrons in their outer shells. Because the electron/proton ratio changes, the sodium atom becomes a positive sodium ion. The chlorine atom becomes a negative chloride ion. The positive-negative attraction between these oppositely charged ions is called an ionic bond. B, A cube-shaped crystal of sodium chloride (table salt).

12. Chemical Bonding Covalent bonds
Covalent bonds form when atoms share their outer energy ions to complete the energy level and thus become stable
Covalent bonds do not ordinarily easily dissociate in water
Covalent bonding is used to form all of the major organic compounds found in the body
What is covalent chemical bonding? Bonding that forms when atoms fill up their energy levels by sharing electrons rather than donating or receiving them.

13. Covalent bonding. Two hydrogen atoms move together, overlapping their energy levels. Although neither gains nor loses an electron, the atoms share the electrons, thereby forming a covalent bond.

14. Chemical Bonding Hydrogen bonds
Hydrogen bonds do not create new molecules
Hydrogen bonds weakly bond to neighboring molecules
Hydrogen bonds are present in water, DNA, and proteins

15. Inorganic Chemistry
Organic molecules contain carbon-carbon covalent bonds and/or carbon-hydrogen covalent bonds; inorganic molecules do not
Organic molecules are generally larger and more complex than inorganic molecules
Does the human body contain organic or inorganic molecules? The human body contains both kinds of compounds.

16. Inorganic Chemistry Water
Water is an inorganic compound essential to life
Water is a solvent (liquid into which solutes are dissolved), forming aqueous solutions in the body
What is an aqueous solution? An aqueous solution is one in which water is the solvent for a mixture.

17. Inorganic Chemistry Water is involved in chemical reactions
Dehydration synthesis—chemical reaction in which water is removed from small molecules so they can be strung together to form a larger molecule
Hydrolysis—chemical reaction in which water is added to the subunits of a large molecule to break it apart into smaller molecules
Chemical reactions always involve energy transfers, as when energy is used to build ATP molecules
Chemical equations show how reactants interact to form products; arrows separate the reactants from the products

18. Water-based chemistry
Water-based chemistry. Dehydration synthesis (on the left) is a reaction in which small molecules are assembled into large molecules by removing water (H and O atoms). Hydrolysis (on the right) operates in the reverse direction; H and O from water are added as large molecules are broken down into small molecules.

19. Inorganic Chemistry Acids, bases, and salts
Water molecules dissociate to form equal amounts of H+ (hydrogen ion) and OH– (hydroxide ion)
Acid—substance that shifts the H+/OH– balance in favor of H+; opposite of base
Base—substance that shifts the H+/OH– balance against H+; also known as an alkaline; opposite of acid
What solutions have an excess of hydrogen ions? Acids

20. Inorganic Chemistry Acids, bases, and salts
pH—Mathematical expression of relative H+ concentration in an aqueous solution
7 is neutral (neither acid nor base)
pH values above 7 are basic; pH values below 7 are acidic
What is pH? pH—mathematical expression of relative H+ concentration in an aqueous solution

21. The pH scale. The H+ concentration is balanced with the OH− concentration at pH 7. At values above 7 (low H+), the scale tips in the basic direction. At values below 7 (high H+), the scale tips toward the acid side.

22. Inorganic Chemistry Acids, bases, and salts
Neutralization occurs when acids and bases mix and form salts
Buffers form chemical systems that absorb excess acids or bases and thus maintain a relatively stable pH

23. Organic Chemistry Carbohydrates—sugars and complex carbohydrates
Contain carbon (C), hydrogen (H), oxygen (O)
Monosaccharides— basic unit of carbohydrate molecules (e.g., glucose)
Disaccharide—double sugar made up of two monosaccharide units (e.g., sucrose, lactose)
What is the name of a substance in which many saccharides join together? Polysaccharide

24. Organic Chemistry Carbohydrates—sugars and complex carbohydrates
Polysaccharide—complex carbohydrate made up of many monosaccharide units (e.g., glycogen; stored by the body)
Function of carbohydrates is to store energy for later use

25. Carbohydrates. Monosaccharides are single carbohydrate units joined by dehydration synthesis to form disaccharides and polysaccharides. The detailed chemical structure of the monosaccharide glucose is shown in the inset.

26. Organic Chemistry Lipids—fats and oils Triglycerides
Formed by a glycerol unit and joined to three fatty acids
Store energy for later use
What are the three main types of lipids in the body? Triglycerides, phospholipids, and cholesterol

27. Triglyceride. Each triglyceride is composed of three fatty acid units attached to a glycerol unit.

28. Organic Chemistry Lipids—fats and oils Phospholipids
Similar to triglyceride structure, but have phosphorus-containing units—each with a head and two tails
The head attracts water and the double tail does not, thus forming stable double layers (bilayers) in water
Form membranes of cells

29. Phospholipids. A, Each phospholipid molecule has a phosphorus-containing “head” that attracts water and a lipid “tail” that repels water. B, Because the tails repel water, phospholipid molecules often arrange themselves so that their tails face away from water. The stable structure that results is a bilayer sheet forming a small bubble.

30. Organic Chemistry Lipids—fats and oils Cholesterol
Molecules have a steroid structure made up of multiple rings
Cholesterol stabilizes the phospholipid tails in cellular membranes and is also converted into steroid hormones by the body

31. Organic Chemistry Proteins
Very large molecules made up of amino acids held together in long, folded chains by peptide bonds
Structural proteins
Form essential structures of the body
Collagen is a fibrous protein that holds many tissues together
Keratin forms tough, waterproof fibers in the outer layer of the skin

32. Protein. Protein molecules are large, complex molecules formed by one or more strands of amino acids. Each amino acid is connected to the next by a type of covalent bond called a peptide bond. Additional weak forces between atoms of the larger molecule then cause the strand to twist or fold into a secondary (second-level) protein structure. New relationships among the atoms then cause the molecule to fold again on itself to form a three-dimensional tertiary (third level) protein structure. Several tertiary proteins may join to form a quaternary (fourth-level) protein structure.

33. Organic Chemistry Proteins Functional proteins
Participate in chemical processes of the body
Examples include hormones, cell membrane channels and receptors, and enzymes
Enzymes—chemical catalysts
Help chemical reactions occur
Enzyme action sometimes called lock-and-key model
What is the lock-and-key model and how does it relate to enzymes? Each enzyme has a shape that “fits” with the specific molecules it works on, similar to the way a key fits specific locks.

34. Organic Chemistry Proteins
Proteins can combine with other organic molecules to form “mixed” molecules such as glycoproteins or lipoproteins

35. Enzyme action. Enzymes are functional proteins whose molecular shape allows them to catalyze chemical reactions. Molecules A and B are brought together by the enzyme to form a larger molecule, AB.

36. Organic Chemistry Nucleic acids Made up of nucleotides
A phosphate unit
A sugar (ribose or deoxyribose)
A nitrogen base (adenine, thymine or uracil, guanine, cytosine)
What are the two forms of nucleic acid? The two forms are deoxyribonucleic acid [DNA] and ribonucleic acid (RNA).

37. Organic Chemistry Nucleic acids DNA (deoxyribonucleic acid)
Used as the cell’s “master code” for assembling proteins
Uses deoxyribose as the sugar and A, T (not U), C, and G as bases
Forms a double helix shape
What is the role of DNA in the body? It makes up the genetic code in the body.

38. DNA. Deoxyribonucleic acid (DNA), like all nucleic acids, is composed of units called nucleotides. Each nucleotide has a phosphate, a sugar, and a nitrogen base. In DNA, the nucleotides are arranged in a double helix formation.

39. Organic Chemistry Nucleic acids RNA (ribonucleic acid)
Used as a temporary “working copy” of a gene (portion of the DNA code)
Uses ribose as the sugar and A, U (not T), C, and G as bases

40. Organic Chemistry Nucleic acids
By directing the formation of structural and functional proteins, nucleic acids ultimately direct overall body structure and function