Transcript 01 Chemical bases of life

Chemical bases of life
Matter, Mass, and Weight
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All living and nonliving things are composed of
matter, which is anything that occupies space
and has mass.Mass is the amount of matter in
an object, and weight is the gravitational force
acting on an object of a given mass.
An element is the simplest type of matter with
unique chemical properties. The characteristics
of living and nonliving matter result from the
structure, organization, and behavior of atoms.
Electrons and Chemical Bonding
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The outermost electrons of an atom determine its
chemical behavior. When these outermost electrons are
transferred or shared between atoms, chemical bonding
occurs. Two major types of chemical bonding are ionic
and covalent bonding.
An atom is electrically neutral because it has an equal
number of protons and electrons. If an atom loses or
gains electrons, thenumber of protons and electrons are
no longer equal, and a charged particle called an ion is
formed.
Covalent Bonding
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Covalent bonding results when atoms share one or more
pairs of electrons. The resulting combination of atoms is
called a molecule. An example is the covalent bond
between two hydrogen atoms to form a hydrogen
molecule.
Each hydrogen atom has one electron. As the two
hydrogen atoms get closer together, the positively
charged nucleus of each atom begins to attract the
electron of the other atom. At an optimal distance, the
two nuclei mutually attract the two electrons, and each
electron is shared by both nuclei. The two hydrogen
atoms are now held together by a covalent bond.
Ions
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Positively charged ions are called cations, and negatively
charged ions are called anions. Because oppositely
charged ions are attracted to each other, cations and
anions tend to remain close together, which is called
ionic bonding. For example, sodium and chloride ions are
held together by ionic bonding to form an array of ions
called sodium chloride,or table salt.
A sodium atom loses an electron to become a smallersized positively charged ion, and a chlorine atom gains
an electron to become a larger-sized negatively charged
ion. The attraction between the oppositely charged ions
results in an ionic bond and the formation of sodium
chloride.
Intermolecular Forces
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Intermolecular forces result from the weak
electrostatic attractions between the oppositely
charged parts of molecules, or between ions and
molecules. Intermolecular forces are much
weakerthan the forces producing chemical
bonding.
The positive hydrogen part of one water
molecule forms a hydrogen bond (red dotted
line) with the negative oxygen part of another
water molecule. As aresult, hydrogen bonds hold
the water molecules together.
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In water,
amphipathic
molecules
aggregate
into spherical
clusters. Their
polar regions
form
hydrogen
bonds with
water
molecules at
the surface of
the cluster.
Chemical Reactions and Energy
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In a chemical reaction, atoms, ions,
molecules, or compounds interact either to
form or to break chemical bonds.
The substances that enter into a chemical
reaction are called the reactants, and the
substances that result from the chemical
reaction are called the products.
Three important points can be made about
chemical reactions
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First, in some reactions, less complex reactants are
combined to form a larger, more complex product. An
example is the synthesis of the complex molecules of the
human body from basic “building blocks” obtained in food.
Second, in other reactions, a reactant can be broken
down, or decomposed, into simpler, less complex
products. An example is the breakdown of food molecules
into basic building blocks.
Third, atoms are generally associated with other atoms
through chemical bonding or intermolecular forces;
therefore, to synthesize new products or break down
reactants it is necessary to change the relationship
between atoms.
Synthesis Reactions
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When two or more reactants chemically combine to form
a new and larger product, the process is called a
synthesis reaction. An example of a synthesis reaction is
the combination of two amino acids to form a dipeptide.
In this particular synthesis reaction, water is removed
from the amino acids as they are bound together.
Synthesis reactions in which water is a product are called
dehydration (water out) reactions. Note that old
chemical bonds are broken and new chemical bonds are
formed as the atoms rearrange as a result of a synthesis
reaction.
Decomposition Reactions
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The term decompose means to break down into smaller
parts. A decomposition reaction is the reverse of a
synthesis reaction— a larger reactant is chemically
broken down into two or more smaller products.
The breakdown of a disaccharide (a type of
carbohydrate) into glucose molecules is an example.
Note that this particular reaction requires that water be
split into two parts and that each part be contributed to
one of the new glucose molecules. Reactions that use
water in this manner are called hydrolysis reactions.
Reversible Reactions
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A reversible reaction is a chemical reaction in which the
reaction can proceed from reactants to products or from
products to reactants. When the rate of product
formation is equal to the rate of the reverse reaction, the
reaction system is said to be at equilibrium.
At equilibrium the amount of reactants relative to the
amount of products remains constant. An important
reversible reaction in the human body involves carbon
dioxide and hydrogen ions.
Major Categories of Organic
Molecules in the Body
Adenosine Triphosphate
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Adenosine triphosphate (ATP) is an especially
important organic molecule found in all living
organisms. It consists of adenosine and three
phosphate groups.
Adenosine is the sugar ribose with the organic
base adenine. The potential energy stored in the
covalent bond between the second and third
phosphate groups is important to living organisms
because it provides the energy used in nearly all
of the chemical reactions within cells.
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Sucrose (table sugar) is
a disaccharide formed
by the linking together
of two
monosaccharides,
glucose and fructose.
Many molecules of
glucose linked end-toend and at branch
points form the
branched-chain
polysaccharide
glycogen, shown in
diagrammatic form in
(a). The four red
subunits in (b)
correspond to the four
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(a) Steroid ring
structure, shown with
all the carbon and
hydrogen atoms in the
rings and again
without these atoms to
emphasize the overall
ring structure of this
class of lipids. (b)
Different steroids have
different types and
numbers of chemical
groups attached at
various locations on
the steroid ring, as
shown by the structure
of cholesterol.
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Structures of
8 of the 20
amino acids
found in
proteins.
Note that
proline does
not have a
free amino
group, but it
can still
form a
peptide bond.