Chapter 2 Flashcards
Why is water the medium of life?
Life evolved in water due to protection it provides from UV light.
Organisms typically contain 70 - 90% water.
Biochemical reactions occur in aqueous solutions.
Critical determinant of the structure and function of proteins, nucleic acids, and membranes.
What is a hydrogen bond?
Weak electrostatic attraction between one electronegative atom and a hydrogen atom covalently linked to a second electronegative atom.
What are hydrogen bonds not unique to water?
Hydrogen bonds form between the hydrogen acceptor (electronegative atom, usually O or N) and a hydrogen atom covalently bonded to another electronegative atom named hydrogen donor.
Hydrogen atoms covalently bonded to carbon DO NOT participate in hydrogen bonding (C-H bond is not very polar).
What does not dissolve in water?
Things that do not make hydrogen bonds, like oils.
Nonpolar gases are poorly soluble in water. Biologically important gases are O2, N2, CO2.
What are hydrophilic compounds?
Dissolve easily in water.
Ionic compounds (NaCl).
Polar organic compounds (alcohols, ketones, carbonyls).
Weak acids (phosphates, amino acids).
Sugars.
What are hydrophobic compounds?
Not soluble in water.
Non-polar hydrocarbons (hexane).
Lipids such as fats.
What do amphipathic compounds contain?
Polar and nonpolar domains.
What is an example of amphipathic compounds?
Palmitic acid.
What is the hydrophobic effect?
Clustering of hydrophobic molecules in water.
Powerful organizing force in biological systems.
If you put nonpolar molecule, it disrupts hydrogen bonding; tendency to stick together to minimize number of water molecules.
Compare bond energy.
Covalent bonds are stronger than noncovalent bonds.
Explain the ionization of water.
Pure water is slightly ionized.
O-H bonds are polar and can dissociate.
Products are a proton (H+) and a hydroxide ion (OH-).
Dissociation of water is a rapid reversible process.
Most water molecules remain un-ionized.
Equilibrium is strongly to the left (low Keq).
Extent of dissociation depends on the temperature.
What is pH?
Defined as the negative logarithm of the hydrogen ion concentration.
Simplifies equations.
pH and pOH must always add up to 14.
In neutral solution, [H+] = [OH-] and the pH is 7.
Why do acids ionize?
To form a proton and base.
The acid is a proton donor and base is proton acceptor.
Chemical formed upon ionization of an acid is called its conjugate base.
Acid formed when a base binds a proton is called its conjugate acid.
HA and A- are conjugated acid-base pair.
Are strong acids or weak acids completely ionized in water?
Strong acids are completely ionized in water.
Weak acids are not completely ionized in water.
Explain the principle about the dissociation of weak acids.
Weak acids dissociate only partially in water.
Extent of dissociation is determined by the acid dissociation constant Ka.
Stronger the tendency to dissociate a proton, the stronger is the acid and lower its pKa.
