1. The Chemical Basis of Life
Chapter 2
The Chemical Basis of Life
Modified by C. Kerins

2. NATURE’S CHEMICAL LANGUAGE
Chemicals play an important role in all organisms.
Hormones send messages within an organism
Other chemicals signal between organisms
The rattlebox moth provides a good example of chemicals used in mating and defense.

3. Rattlebox Moth
Froth found on moth is a noxious chemical that is distasteful to the spiders that prey on this moth.
A male moth passes an extra dose of this defensive chemical to the female to supply additional protection for her and their offspring.
Chemicals play many more roles in life than signaling…they are the very stuff making up our bodies, those of other organisms, and the physical environment.

5. 2.1 ELEMENTS, ATOMS, AND MOLECULES
At the base of the biological hierarchy are elements, atoms, and molecules. The properties of life emerge from the arrangement of its chemical parts into higher and higher levels of biological organization.

6. ELEMENTS, ATOMS, AND MOLECULES
Living organisms are composed of about 25 chemical elements
Elements are the basic chemical units that cannot be broken apart by typical chemical processes
There are 92 naturally occurring elements
25 are required by living organisms
4 make up 96 % of the human body
Oxygen, carbon, hydrogen, and nitrogen
Main ingredients in biological molecules like proteins, carbohydrates, and lipids.

7. ELEMENTS, ATOMS, AND MOLECULES
Other 4 %
Calcium
Phosphorus
Potassium
Sulfur
Sodium
Chlorine
Magnesium
These elements are involved in bone formation, nerve signaling, and DNA synthesis.

9. Trace elements are common additives to food and water
Trace elements are essential in minute quantities for proper biological functioning
Iodine is a trace element that prevents goiter.
Many foods are fortified with trace elements and vitamins (which consist of two or more elements—they’re compounds…we’ll get there…).

11. TRACE ELEMENTS
Fluoride is a form of fluorine added to water to reduce tooth decay
Iron and other minerals are added to food to boost nutritional value

13. Elements can combine to form compounds
Compounds contain two or more elements in a fixed ratio
Different arrangements of the atoms of elements determine the unique properties of each compound
The smallest unit of an element is an atom
The physical and chemical properties of compounds are usually VERY different from those of their constituent atoms.

14. LE 2-3
Sodium
Chlorine
Sodium Chloride

15. Atoms consist of protons, neutrons, and electrons
Subatomic particles
Protons and neutrons occupy the central region (nucleus) of an atom—this is also where the mass of the atoms is located
A proton has a positive charge
A neutron is electrically neutral
Electrons surround the nucleus
An electron has a negative charge

16. Determining the number of P, N, and e- in an atom
Differences in Elements
All the atoms of a particular element have the same number of protons
The atomic number (number of protons) defines the element's unique properties
Atoms are neutral; therefore, the number of electrons is equal to the number of protons.
An atom's mass number is the sum of its protons and neutrons
Mass number – atomic number = # neutrons

17. LE 2-4a Electron cloud 6e– 2e– Nucleus 2 Protons 6 Protons Mass
number = 4
Mass
number = 12 2
Neutrons 6
Neutrons 2
Electrons 6
Electrons
Helium atom
Carbon atom

18. ISOTOPES
Isotopes
Isotopes are atoms with the same atomic number but different mass numbers (different number of neutrons)
An element’s atomic mass is the weighted average of all of an element’s naturally occurring isotopes…usually NOT a whole number.
ROUND the atomic mass to the nearest whole number and use it as the atom’s mass number.

19. Radioactive isotopes have nuclei that break down at a constant rate over time.
They pose serious risk to living organisms
They have many uses in biological research and medicine

20. Radioactive isotopes can help or harm us
Radioactive isotopes are useful as tracers to study the fate of elements and molecules in living systems
Radioactive tracer isotopes are often used in combination with sophisticated imaging instruments for medical diagnosis
Uncontrolled exposure to radioactive material can harm living organisms

21. PET scanner

22. PET image of throat cancer

23. 2.6 ELECTRON ARRANGMENTS IN ATOMS
Electron arrangement determines the chemical properties of an atom
Electrons in an atom are arranged in electron shells, which may contain different numbers of electrons
The farther an electron is from the nucleus, the greater its energy.
It is the number of electrons in the outermost shell (valence electrons) that determines the chemical properties of an atom.

24. Atoms want to have full or empty valence shells.
1st orbital holds two electrons
2nd orbital holds eight electrons
3rd orbital holds eight electrons
Atoms form chemical bonds to complete their valence shells.

25. UNHAPPY ATOMS…. 
Atoms whose outer shells are not full share or transfer electrons to other atoms, forming molecules
Two major types of chemical bonds between atoms form compounds
Ionic bonds
Covalent bonds

26. 2.7 IONIC BONDS
Ionic bonds are formed when one atom transfers one or more electrons to another atom.
An ion is a charged atom that has lost or gained electrons in its outer shell
A positively charged ion (cation) is an atom that has lost an electron
A negatively charged ion (anion) is an atom that has gained an electron

27. Animation: Ionic Bonds
SODIUM CHLORIDE
An electrical attraction between ions with opposite charges results in an ionic bond
Example: sodium chloride (table salt) results from an ionic bond between sodium and chlorine
Animation: Ionic Bonds

28. Na Sodium atom Cl Chlorine atom
LE 2-7a-1
Transfer of
electron Na
Sodium atom Cl
Chlorine atom

29. Sodium chloride (NaCl)
LE 2-7a-2
Note that the overall charge on the MOLECULE is neutral!
Na+
Sodium ion
Cl-
Chloride ion
Sodium chloride (NaCl)

30. LE 2-7b
Na+
Cl-

31. Animation: Covalent Bonds
Covalent bonds join atoms into molecules through sharing one or more PAIRS of electrons
Covalently bonded atoms share one or more pairs of outer shell electrons, forming a molecule
In a double bond, two pairs of electrons are shared…in a triple bond, three pairs of electrons are shared
Covalent bonds can be represented in various ways
Animation: Covalent Bonds

33. Unequal electron sharing creates polar molecules
A molecule whose covalently bonded atoms share electrons equally is nonpolar (fats, oils, and waxes)
A molecule whose covalently bonded atoms share electrons unequally is polar
One part of the molecule is slightly positive, and one part is slightly negative…
…like our friend water!

34. Polar Molecules
Atoms in a covalently bonded molecule are in a constant tug-of-war for the shared electrons of their covalent bond. An atom’s attraction for its electrons, including shared electrons, is called its electronegativity. The more electronegative an atom, the more strongly it pulls shared electrons toward its nucleus.

35. Electronegativity
Electronegativity increases as you move from left to right on the periodic table.
Electronegativity decreases as you move down the periodic table.

36. LE 2-9
A water molecule

37. Hydrogen bonds are weak bonds important in the chemistry of life
The attraction between slightly positive regions (hydrogen atoms) and slightly negative regions of polar molecules creates hydrogen bonds
Hydrogen bonding occurs in many biologically important compounds
Water
DNA
Proteins

38. LE 2-10
Hydrogen bond

39. 2.11 Water’s Life-Supporting Properties
Hydrogen bonds make liquid water cohesive
Cohesion is the tendency of molecules to stick together
Surface tension results from the cohesion of water molecules
Animation: Water Transport

41. 2.12 Water's hydrogen bonds moderate temperature
When water is heated, the heat energy is absorbed, disrupting hydrogen bonds
The water stores a large amount of heat while warming only a few degrees
When water is cooled, heat energy is released as hydrogen bonds are formed
The temperature of the water is lowered slowly

42. Water also moderates temperature by evaporative cooling
The surface cools as the hottest molecules leave

44. 2.13 Ice is less dense than liquid water
Hydrogen bonds in ice create a stable, three-dimensional structure
Ice is less dense than water, because it has fewer molecules in the same volume

45. Hydrogen bonds are stable constantly break and re-form
Ice
Liquid water
Hydrogen bonds are stable
Hydrogen bonds
constantly break and re-form

46. 2.14 Water is the solvent of life
A solution is a homogeneous mixture of a liquid solvent and one or more dissolved solutes
Because water is a polar molecule, it readily forms solutions with many other polar and ionic compounds
A solution in which water is the solvent is an aqueous solution

47. LE 2-14
Ion in
solution
Salt
crystal

48. 2.15 The chemistry of life is sensitive to acidic and basic conditions
A compound that releases H+ ions in solution is an acid
A compound that accepts H+ ions in solution is a base
Acidity is measured on the pH scale from 0 (most acidic) to 14 (most basic)
The pH of most cells is kept close to 7 (neutral) by buffers that resist pH change

49. (Higher concentration of H+) (Lower concentration of H+)
LE 2-15
pH scale H+ H+
OH H+ H+
Lemon juice, gastric juice
OH H+ H+ H+ H+
Grapefruit juice, soft drink
(Higher concentration of H+)
Increasingly ACIDIC
Acidic solution
Tomato juice
Human urine
OH
OH
NEUTRAL
[HOH-]
Pure water H+ H+
OH
OH
OH
Human blood H+ H+ H+
Seawater
Neutral solution
(Lower concentration of H+)
Increasingly BASIC
Milk of magnesia
Household ammonia
OH
OH
OH H+
OH
Household bleach
OH
OH H+
Oven cleaner
Basic solution

50. 2.16 Acid precipitation threatens the environment
CONNECTION
2.16 Acid precipitation threatens the environment
Acid precipitation is formed when air pollutants from burning fossil fuels combine with water vapor in the air to form sulfuric and nitric acids
Some ecosystems and structures are threatened by acid precipitation

53. 2.17 Chemical reactions change the composition of matter
In a chemical reaction, reactants interact, leading to products
Atoms are rearranged, but the number of atoms stays constant on both sides of the equation

54. LE 2-17a +
2 H2 + O2
2 H2O

55. Living cells carry out thousands of chemical reactions that rearrange matter in significant ways

56. LE 2-17b
Beta-carotene
Vitamin A
(2 molecules)