The Chemical Basis of Life Chapter 2 The Chemical Basis of Life

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.

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.

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

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)

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

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

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

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

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

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