Ch 2 The Chemical Context of Life

Concept 2.3: The formation and function of molecules depend on chemical bonding between atoms
•	Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms
•	This usually results in atoms staying close together, held by attractions called chemical bonds

Covalent Bonds
•	A covalent bond is the sharing of a pair of valence electrons by two atoms
•	In a covalent bond, the shared electrons count as part of each atom’s valence shell
•	Two or more atoms held together by valence bonds constitute a molecule
•	Figure 2.7
•	Figure 2.8
•	Figure 2.9
•	Figure 2.10

Ionic Bonds
•	Atoms sometimes strip electrons from their bonding partners
•	A cation is a positively charged ion
•	An anion is a negatively charged ion
•	An ionic bond is an attraction between an anion and a cation
•	Figure 2.11

Hydrogen Bonds
•	A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom

Weak Chemical Bonds
•	Most of the strongest bonds in organisms are covalent bonds that form a cell’s molecules
•	Weak chemical bonds, such as ionic bonds and hydrogen bonds, are also important
•	Many large biological molecules are held in their functional form by weak bonds

Van der Waals Interactions
•	If electrons are distributed asymmetrically in molecules or atoms, they can result in “hot spots” 
of positive or negative charge
•	Van der Waals interactions are attractions between molecules that are close together because of these “Hot Spot” charges
•	Van der Waals interactions are individually weak 
•	But together, such interactions can be strong, as between molecules of a gecko’s toe hairs and a wall surface

Concept 2.5: Hydrogen bonding gives water properties that help make life possible on Earth
•	All organisms are made mostly of water and live in an environment dominated by water4 Emergent 

Properties of Water Contribute to Earth’s Suitability for life:
1.	Cohesive behavior 
2.	Ability to moderate temperature
3.	Expansion upon freezing
4.	Versatility as a solvent

Cohesive Behavior
•	Due to multiple hydrogen bonds of water molecules
•	H-bonds cause H2O molecules to stay close together = Cohesion,
•	 H-bonds also cause H2O molecules  to clinging to another substance = Adhesion, 
•	tends to be difficult to break the surface = Surface Tension

Ability to moderate temperature
•	The specific heat of water is
•	The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1 degree C
•	 1 cal/g/C
•	Water resists changing its temperature because of its high specific heat.  Why?
•	Water’s high specific heat can be traced to hydrogen bonding
•	Heat is released when hydrogen bonds form
•	Liquid state to SOLID (Ice) : bonds formed
•	Heat is absorbed when hydrogen bonds break
•	Liquid to GAS (water vapor/steam): bonds break
•	Evaporative cooling: cools remaining surface water
•	The high specific heat of water keeps temperature fluctuations within limits that permit life
•	Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water
•	Figure 2.19
•	Water absorbs heat from warmer air and releases stored heat to cooler air and it can do this  absorbing/releasing of large amount of heat with only a slight change in its own temperature
•	This allows oceans to moderate coastal climates, 

Expansion upon freezing
•	Water reaches its greatest density at 4deg C, then get less dense as it freezes
•	Ice floats in liquid water because hydrogen bonds in ice are more “ordered,”
•	 (the bonds held at arm’s length apart so there are less of them packed together) making ice less dense
•	If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth
•	Instead ice floats and insulates the water below allowing for life under the water surface to survive
•	Figure 2.20

Water: The Solvent of Life
•	A solution = homogeneous mixture of substances
•	A solvent = dissolving agent of a solution
•	The solute = substance that is dissolved
•	An aqueous solution = water is the solvent
•	Figure 2.21
•	Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily
•	When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell
•	Neg O regions attract to Pos Na cations, & Pos H regions attract to Neg Cl anions 
•	Figure 2.22 A water-soluble protein
•	Water can also dissolve compounds made of nonionic polar molecules
•	Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions

Hydrophilic and Hydrophobic Substances
•	A hydrophilic substance is one that has an affinity for water
•	A hydrophobic substance is one that does not have an affinity for water
•	A colloid is a stable suspension of fine particles in a liquid