Download Chemical Foundations

Chapter 2
Chemical Foundations
The Chemicals of Life
The Chemicals of Life
(b) Macromolecules (23%)
Covalent bonds
and noncovalent interactions
The “glue” that holds macromolecules together:
a Covalent bonds
a Noncovalent interactions:
ionic bonds
hydrogen bonds
van der Waals interactions
hydrophobicity-driven interactions
Covalent bonds
a Formed when two different atoms share electrons in the
outer atomic orbitals
a Each atom can make a characteristic number of bonds
(e.g., carbon is able to form 4 covalent bonds)
a Covalent bonds in biological systems are typically single
(one shared electron pair) or double (two shared electron
pairs) bonds
Asymmetric carbon atoms are present in
most biological molecules
a Carbon atoms that are bound to four different atoms or groups are said
to be asymmetric
a The bonds formed by an asymmetric carbon can be arranged in two
different mirror images (stereoisomers) of each other
a Stereoisomers are either right-handed or
left-handed and typically have completely
different biological activities
a Asymmetric carbons are key features of
amino acids and carbohydrates
Water is a polar molecule
a
a
a
a
Electrons are not distributed equally
Electrons have higher affinity for the oxygen
Polar molecules have a dipole moment
Essential for hydration of ions
α and β glycosidic bonds link monosaccharides
Noncovalent bonds
a Several types: hydrogen bonds, ionic bonds, van der
Waals interactions, hydrophobic bonds
a Noncovalent bonds require less energy to break than
covalent bonds
a The energy required to break noncovalent bonds is only
slightly greater than the average kinetic energy of
molecules at room temperature
a Noncovalent bonds are required for maintaining the threedimensional structure of many
macromolecules and for stabilizing
specific associations between
macromolecules
Multiple noncovalent bonds can confer
binding specificity
The hydrogen bond underlies water’s
chemical and biological properties
Molecules with polar bonds that form
hydrogen bonds with water can
dissolve in water and are termed
hydrophilic
Ionic bonds
a Ionic bonds result from the attraction of a positively
charged ion (cation) to a negatively charged ion (anion)
a The atoms that form the bond have very different
electronegativity values and the electron is completely
transferred to the more electronegative atom
a Ions in aqueous solutions are surrounded
by water molecules, which interact via
the end of the water dipole carrying the
opposite charge of the ion
Van der Waals interactions are caused by
transient dipoles
When any two atoms approach each other closely, a weak nonspecific attractive
force (the van der Waals force) is created due to momentary random
fluctuations that produce a transient electric dipole
Hydrophobic bonds cause nonpolar
molecules to adhere to one another
Nonpolar molecules (e.g., hydrocarbons) are insoluble in water and are termed
hydrophobic
Since these molecules cannot form hydrogen bonds with water, it is energetically
favorable for such molecules to interact with other hydrophobic molecules
This force that causes hydrophobic molecules to interact is termed a hydrophobic
bond
Phospholipids are amphipathic molecules
Phospholipids spontaneously assemble via
multiple noncovalent interactions to form different
structures in aqueous solutions
Chemical balance determines which
bonds can be formed
Copyright (c) by W. H. Freeman and Company
Energy in biological systems
a Most important forms of energy in a cell:
`Chemical potential energy
`Energy of concentration gradient
`Electric potential energy
a All forms of energy are interconvertable
a Energy unit in biochemistry: kcal = Cal = 1000 cal
a Direction of chemical reaction: towards minimal free energy
ΔG [cal/mole] = Gproducts – Greactants
Rate of a chemical reaction depends on the activation
energy
Enzymes accelerate biochemical reactions by reducing transition-state free energy
Cellular processes are driven by ATP
hydrolysis
a All forms of energy are interconvertable
a Many chemical reactions are
energetically unfavorable (ΔG > 0) and
will not proceed spontaneously
a Cells can carry out such a reaction by
coupling it to a reaction that has a
negative ΔG of larger magnitude
a Energetically unfavorable reactions in
cells are often coupled to the hydrolysis
of adenosine triphosphate (ATP), which
has a ΔGº′ = -7.3 kcal/mol
a The useful free energy in an ATP
molecule is contained in
phosphoanhydride bonds
ATP is used to fuel many cell processes
The ATP cycle
At the completion of this lecture
you should be able to:
a Describe the different kinds of bonds that hold biological
macromolecules together
a Understand how the variability and complexity of
organic carbon-based molecules provided (evolution)
the basis for the development of life
a Describe and draw the structure of micelles, liposomes,
and bilayer sheets
a Declare how enzymes accelerate biochemical processes