Transcript Chemistry of Life - juan

A
knowledge of chemistry is
essential for understanding
organisms
 Important to biology are
inorganic compounds,
including water, simple acids
and bases, and simple salts
 Elements
◦Substances that cannot be
broken down into simpler
substances by ordinary
chemical reactions
◦Each has a chemical symbol
 Four
elements comprise the
mass of most organisms
◦Oxygen, carbon, hydrogen,
and nitrogen
 In addition, other elements,
such as calcium, and trace
elements are present
Functions of elements
 Atom
◦ The smallest portion of an
element that retains its chemical
properties
 Subatomic particles include
◦ Electron—carries a negative
charge
◦ Proton—carries a positive charge
◦ Neutron—uncharged particle
 Every
element has
◦A fixed number of protons in
the atomic nucleus, known as
the atomic number
 The
periodic table is a chart of
the elements arranged by
atomic number
 The
atomic mass of an atom
◦ Is a number that indicates how
much matter it contains.
◦ Is expressed by the atomic
mass unit (amu), also known
as the dalton.
◦ The atomic mass= number of
protons + number of neutrons
Characteristics of protons,
neutrons and electrons
 Isotopes
◦Are two or more forms of
atoms of the same element
◦Contain the same number of
protons and electrons, but
the number of neutrons
varies
 Radioisotopes break down
and emit radiation
Carbon Isotopes
 Electrons
move through
orbitals
 Electrons at the same
principal energy level make
up an electron shell
 Electrons
in a shell distant from
the nucleus have greater energy
and are called Valence Electrons
 Valence electrons occupy the
valence shell (outermost shell)
 Changes in electron energy
levels are important in energy
conversions in organisms
Atomic orbitals
 The
chemical behavior of an
atom is determined by the
number and arrangement of
its valence electrons
 When
the valence shell is not
full, the atom tends to lose,
gain, or share electrons
A
chemical compound
consists of atoms of two or
more elements
 Atoms combine in a fixed
ratio
 Atoms may join to form a
molecule
A
chemical formula describes
the chemical composition of a
substance
◦ Simplest formula
◦ Molecular formula
◦ Structural formula
 Molecular
Mass
◦ Sum of the atomic masses of the
component atoms of a single
molecule
 Chemical
reactions in an
organism:
◦ Described by chemical equations
◦ Reactants are written on the left &
products are written on the right
◦ Reactions can proceed
simultaneously in both directions
◦ At dynamic equilibrium, forward and
reverse rates of reaction are equal
Chemical bonds
◦ Forces of attraction that hold atoms
of a compound together
◦ The two principal types are
 Covalent bonds
 Ionic bonds
Bond Energy
◦ Energy necessary to break a
chemical bond
 Covalent
bonds
 Covalent
compound
◦ Share electrons between atoms
◦ Each atom has a filled valence shell
◦ Compound consisting mainly of
covalent bonds
◦ Example is hydrogen gas molecule
◦ Bond can be single, double, or triple
Covalent bonds
Number of Covalent bonds
Covalent bonds can be
nonpolar or polar
Ion
◦ Particle with one or more units of
electrical charge
◦ Results when an atom gains or loses
electrons
 Cations—positively charged ions (Na+)
 Anions—negatively charged ions ( Cl- )
◦ Cations and anions are involved in
biological processes, such as muscle
contraction
Sodium, potassium, and chloride ions are essential
for this nerve cell to stimulate these muscle fibers
Ioninc bonds
◦ Formed due to attraction between a
cation and an anion
◦ An example of ionic bond is the
attraction between sodium ions and
chloride ions
Ionic bonding
Hydrogen Bonds
◦ Form between an atom with partial
negative charge and a hydrogen
atom covalently bonded to oxygen or
nitrogen
◦ Readily formed and broken
◦ While individually weak, hydrogen
bonds are strong when present in
large numbers
Hydrogen bonding
 Many
energy conversions in a
cell involve an electron
transfer from one substance to
another
 Known
as oxidation-reduction,
or redox reaction
Water
 Water is Polar (due to its
uneven distribution of charges)
 Large
part of the mass of most
organisms is water
 Water
is important as internal
constituent and environmental
factor
Water facilitates chemical reactions:
◦ Hydrophilic substances—interact
readily with water, such as table salt
◦ Hydrophobic substances—not
disrupted or dissolved by water, such
as fats
 Water
exists as gas, liquid, or
solid
 Hydrogen bonds are formed
or broken as water changes
state
 Acid
◦ Substance that dissociates in
solution to yield hydrogen ions and
an anion
 Base
◦ Substance that dissociates to yield a
hydroxide ion and a cation when
dissolved in water
 The
degree of a solution’s
acidity is expressed in pH
 Definition
of pH
◦ Measure of how acidic or basic a
substance is
◦ The negative logarithm of the
hydrogen ion concentration
◦ Expressed in moles per liter

Neutral solution

Acidic solution

Basic solution

An acid and a base react to form a salt
plus water

A buffer is a substance that resists pH
changes in a solution.

Buffers a important substances in
biological systems.
◦ pH of 7
◦ pH value of less than 7
◦ pH greater than 7
pH values
of common
solutions