Atoms and Molecules: The Chemical Basis of Life

1. Atoms and Molecules:The Chemical Basis of Life

2. A knowledge of chemistry is essential for understanding organisms • Important to biology are inorganic compounds, including water, simple acids and bases, and simple salts

3. Elements • Substances that cannot be broken down into simpler substances by ordinary chemical reactions • Each has a chemical symbol

4. Four elements comprise the mass of most organisms • Oxygen, carbon, hydrogen, and nitrogen • In addition, other elements, such as calcium, and trace elements are present

5. Functions of elements

6. Atom • The smallest portion of an element that retains its chemical properties • Subatomic particles include • Electron—carries a negative charge • Proton—carries a positive charge • Neutron—uncharged particle

7. Every element has • A fixed number of protons in the atomic nucleus, known as the atomic number • The periodic table is a chart of the elements arranged by atomic number

8. Periodic chart (including Bohr models)

9. The atomic mass of an atom • Is a number that indicates how much matter it contains • Is expressed by the atomic mass unit (amu), also known as the dalton • Is indicated by a superscript to the left of the chemical symbol

10. Characteristics of protons, neutrons, and electrons

11. Isotopes • Are two or more forms of atoms of the same element • Contain the same number of protons and electrons, but the number of neutrons varies • Radioisotopes break down and emit radiation

12. Carbon Isotopes

13. Electrons move through orbitals • Electrons at the same principal energy level make up an electron shell

14. Electrons in a shell distant from the nucleus have greater energy • Valence electrons occupy the valence shell • Changes in electron energy levels are important in energy conversions in organisms

15. Atomic orbitals

16. The chemical behavior of an atom is determined by the number and arrangement of its valence electrons • When the valence shell is not full, the atom tends to lose, gain, or share electrons

17. A chemical compound consists of atoms of two or more elements • Atoms combine in a fixed ratio • Atoms may join to form a molecule

18. A chemical formula describes the chemical composition of a substance • Simplest formula • Molecular formula • Structural formula

19. Molecular mass • Sum of the atomic masses of the component atoms of a single molecule • One mole is the amount of an element or compound whose mass in grams is equivalent to its atomic or molecular mass • The mole allows for comparison of atoms and molecules of very different mass

20. Chemical reactions in an organism • Described by chemical equations • Reactants are written on the left • Products are written on the right • Reactions can proceed simultaneously in both directions • At dynamic equilibrium, forward and reverse rates of reaction are equal

21. Chemical bonds • Forces of attraction that hold atoms of a compound together • The two principal types are • Covalent bonds • Ionic bonds • Bond energy • Energy necessary to break a chemical bond

22. Covalent bonds • Share electrons between atoms • Each atom has a filled valence shell • Covalent compound • Compound consisting mainly of covalent bonds • Example is hydrogen gas molecule • Bond can be single, double, or triple

23. Covalent bonds

24. Number of Covalent bonds

25. Covalent bonds can be nonpolar or polar

26. Ion • Particle with one or more units of electrical charge • Results when an atom gains or loses electrons • Cations—positively charged ions • Anions—negatively charged ions • Cations and anions are involved in biological processes, such as muscle contraction

27. Sodium, potassium, and chloride ions are essential for this nerve cell to stimulate these muscle fibers

28. Ion bonds • Formed due to attraction between a cation and an anion • An ionic compound is a substance consisting of cations and anions bonded together • An example of ionic bond is the attraction between sodium ions and chloride ions

29. Ionic bonding

30. Hydrogen bonds • Tend to form between an atom with partial negative charge and a hydrogen atom covalently bonded to oxygen or nitrogen • Readily formed and broken • While individually weak, hydrogen bonds are strong when present in large numbers

31. Hydrogen bonding

32. Many energy conversions in a cell involve an electron transfer from one substance to another • Known as oxidation-reduction, or redox reaction

33. Large part of the mass of most organisms is water • Water is important as internal constituent and environmental factor

34. Water facilitates chemical reactions • Hydrophilic substances—interact readily with water, such as table salt • Hydrophobic substances—not disrupted or dissolved by water, such as fats

35. Water exists as gas, liquid, or solid • Hydrogen bonds are formed or broken as water changes state

36. Acid • Substance that dissociates in solution to yield hydrogen ions and an anion • Base • Substance that dissociates to yield a hydroxide ion and a cation when dissolved in water

37. The degree of a solution’s acidity is expressed in pH • Definition of pH • The negative logarithm of the hydrogen ion concentration • Expressed in moles per liter

39. Neutral solution • pH of 7 • Acidic solution • pH value of less than 7 • Base solution • pH greater than 7 • An acid and a base react to form a salt plus water

40. pH valuesof commonsolutions