1. The Chemical Basis of Life
Chapter 2
The Chemical Basis of Life

2. Key Concepts
Molecules form when atoms bond to each other. Chemical bonds are based on electron sharing. The degree of electron sharing varies from nonpolar covalent bonds, to polar covalent bonds, to ionic bonds.
Water is essential for life. Water is highly polar and readily forms hydrogen bonds. Hydrogen bonding makes water an extremely efficient solvent.
Energy is the capacity to do work or supply heat, and can be (1) a stored potential or (2) an active motion. Chemical energy is a form of potential energy, stored in chemical bonds.

3. Key Concepts, cont’d
Chemical reactions tend to be spontaneous if they lower potential energy and increase entropy (disorder). An input of energy is required for nonspontaneous reactions to occur.
Most of the important compounds in organisms contain carbon. Key carbon-containing molecules formed early in Earth’s history.

4. The Atom
The middle of an atom is the nucleus. Two particles are found in the nucleus:
Protons are positively charged
Neutrons are neutral
The outside of an atom contains electrons
Electrons travel in orbitals
Electrons on the outside orbital are responsible for bonding
Helium and hydrogen are stable with two outside electrons
Other elements are stable with eight outside electrons
Two or more different elements share electrons (chemical bond) to achieve stability

5. Sharing of Electrons Types of bonding
Nonpolar Covalent Bonding - Sharing
No charge
Example: Glucose (sugar)
Polar Covalent Bonding - Incomplete Sharing
Partial positive, partial negative
Example: Water
Ionic Bonding - Transferring of electrons
Complete positive, complete negative
Example: Sodium Chloride (table salt)

6. Importance of Water in Biology
Electronegativity: the degree of attraction an element has to electrons
Water has one oxygen atom
Oxygen has more electronegativity
Oxygen has more attraction to electrons than hydrogen
Oxygen is partially negative
Water has two hydrogen atoms
Hydrogen has less electronegativity
Hydrogen is partially positive

7. Hydrogen Bonding in Water
Hydrogen Bond (water): The positive attraction of hydrogen of one water molecule with the negative attraction of oxygen of another water molecule
Special properties of water due to its polarity:
Adhesion- ability to H bond with other substances
Universal Solvent- ability to dissolve substances easily
Cohesion- ability to H bond with itself
High Specific Heat- ability to be resistant to temperature change
High Heat of Vaporization- ability to be resistant to vaporizing

8. Energy Energy is the capacity to do work or supply heat
Two types of energy:
Stored potential (potential energy) - chemical energy stored in bonds
Active motion (kinetic energy)
Production of heat with movement of molecules
High kinetic, Low potential - High potential, Low kinetic

9. Spontaneous Reactions
Entropy - the amount of disorder
Low entropy = organization in bonds
High entropy = random movement of molecules
Spontaneous Reactions - low potential energy and high entropy
No energy input required
Nonspontaneous Reactions - high potential energy and low entropy
Requires an input of energy

10. Important Carbon Molecules
Carbon has 6 protons, 6 neutrons, and 6 electrons
Carbon’s 4 valence electrons give is versatile bonding properties
Bonding of carbon
Can form up to 4 stable covalent bonds
Can form up to 2 stable covalent double bonds
Can form up to 1 stable covalent triple bonds

11. Molecule Formation Early in Earth’s History
1. Simples molecules were present in the atmosphere of ancient Earth
Carbon Monoxide (CO), Carbon Dioxide (CO2), Hydrogen, Ammonia (NH3), Water, and Nitrogen
2. The energy in sunlight drove reactions among the simple molecules.
Formaldehyde (H2CO) and Hydrogen Cyanide (HCN)
3. Complex molecules are formed.
Molecules with carbon-carbon bonds
Acetaldehyde, Glycine, and Ribose