Chapter 2
Chemical Composition of the Body
Dr M Hassanpour
Basic Science School
Biology Department
PBS course
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Objectives



Explain how covalent bonds are formed
and distinguish between nonpolar and
polar covalent bonds.
Describe the nature of hydrogen bonds
and explain their significance.
Describe the structure of DNA and RNA,
and explain the law of complementary
base pairing.
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Chemical Bonds, Molecules, and
Ionic Compounds

Chemical bonds:


Interaction of valence electrons between 2
or more atoms.
Number of bonds determined by
number of electrons needed to
complete outermost shell.
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Covalent Bonds


Atoms share their
valence electrons.
Nonpolar covalent
bonds:


Electrons are equally
distributed between the
two identical atoms.
Strongest bond.
 H2 and 02.
Figure 2.2
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Covalent Bonds

(continued)
Polar bonds:

Electrons are shared
between two
different atoms.



Electrons may be
pulled more toward
one atom.
Has + and – poles.
Oxygen, nitrogen,
phosphorous have
tendency to pull
electrons towards
themselves.
Figure 2.4
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Ionic Bonds


One or more valence electrons
from one atom are completely
transferred to a second atom.
Cation and anion attract, form
ionic compound.


Weaker than polar covalent
bonds.
Dissociate easily when
dissolved in H20.

Form hydration spheres.

Make an ion/molecule more
soluble.

NaCl
Figure 2.5
Na++ Cl-
Figure 2.6
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Ionic Bonds

Glucose, amino acids, are H20 soluble.

Hydration spheres form around atoms of
oxygen, nitrogen, phosphorous.


(continued)
Hydrophilic molecules.
Molecules composed of nonpolar
covalent bonds are not H20 soluble.

Cannot form hydration spheres.

Hydrophobic molecules.
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Hydrogen Bond


Hydrogen forms a
polar bond with
another atom,
giving hydrogen
has a slight +
charge.
Weak attraction
for a second
electronegative
atom.

Insert fig. 2.7
Surface tension.
Figure 2.7
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Acid/Base

Acid:

Molecule that can release protons (H+).


Proton donor.
Base:

Negatively charged ion that can combine
with H+, and remove it from solution.

Proton acceptor.
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pH

pH = log _1__
[H+]



[H+] = molar concentration of H+.
pH inversely related to [H+].
Because of logarithmic relationship, a
solution with 10 times [H+] of H20 has
a pH = 6; solution with 0.1 the [H+]
has a pH = 8.
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Buffers




System of molecules and ions that act
to prevent changes in [H+].
Stabilizes pH of a solution.
In blood:
H20 + C02
H2C03
H+ + HC03
Reaction can proceed in either direction
(depending upon the concentration of
molecules and ions).
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Blood pH


7.35 – 7.45
Constancy achieved in part by HC03-.



H20 + C02
Acidosis:


HC03- and H2C03 act as buffer pair.
pH < 7.35.
Alkalosis:

pH > 7.45.
H2C03
H+ + HC03-
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Organic Molecules



Molecules that contain
carbon and hydrogen.
Carbon has 4 electrons in
outer shell and
covalently bonds to fill its
outer shell.
Functional groups:


Carbonyl group:


Aldehydes and ketones.
Carboxyl group:


Inactive “backbone” to
which more reactive atoms
are attached.
Organic acids (lactic and
acetic acids).
Hydroxyl group:

Alcohol.
Figure 2.10
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Carbohydrates


Organic molecules that
contain carbon, hydrogen
and oxygen.

CnH2n0n.
Monosaccharides:

Simple sugars.


Disaccharide:

2 monosaccharides joined
covalently.


Glucose, fructose, galactose.
Sucrose (glucose and fructose),
lactose (glucose and galactose),
maltose (2 glucose).
Polysaccharide:

Numerous monosaccharides
joined covalently.


Starch (thousands of glucose
joined), glycogen (repeating
glucose joined that are highly
branched).
Mechanism for storing energy
with less osmotic H20
movement.
Figure 2.13
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Lipids




Diverse group of molecules.
Differ greatly in chemical structure.
Insoluble in polar solvents (H20).
Consist primarily of hydrocarbon chains and
rings.

Hydrophobic.
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Triglycerides (triacylglycerol)

Formed by
condensation of glycerol
and 3 fatty acids.


Figure 2.18
Saturated:


Fatty acids consist of
nonpolar hydrocarbon
chain with carboxyl end.
Hydrocarbon chains
joined by single covalent
bonds.
Unsaturated:

Double covalent bonds
within hydrocarbon
chain.
Figure 2.17
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Ketone Bodies

Hydrolysis of
triglycerides in adipose
tissue release free fatty
acids.


Free fatty acids can be
converted in the liver to
ketone bodies.
Ketoacidosis:

Increased ketone bodies
in the blood which lowers
pH.
Figure 2.19
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Phospholipids

Phospholipids:

Number of different
categories of lipids
that contain
phosphate group.


Nonpolar end is
hydrophobic, polar
end is hydrophilic.
Lecithin:

Phosphate attached
to a nitrogencontaining choline
molecule.
Figure 2.20
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Steroids

All have same basic
structure; three 6carbon rings joined
to a 5-carbon ring.


Nonpolar and
insoluble in H20.
Cholesterol is
precursor for steroid
hormones.
Figure 2.22
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Prostaglandins

Fatty acid with cyclic
hydrocarbon group.


Derived from
arachidonic acid.
Serve a variety of
regulatory functions.

Blood vessel
diameter, ovulation,
uterine contractions,
inflammation, blood
clotting.
Figure 2.23
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Proteins

Large molecules
composed of long chains
of amino acids.



20 different amino acids
can be used in
constructing a given
protein.
Each amino acid
contains an amino group
(NH2) at one end and
carboxyl group (COOH)
at the other end.
Differences between
amino acids are due to
differences in functional
groups (“R”).
Figure 2.24
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Protein Structure Level

Primary structure:


Secondary structure:


Weak hydrogen bonds form
between hydrogen of 1 amino
acid and the and oxygen of a
different amino acid nearby.

a-helix or b-sheet.
Tertiary structure:



Sequence of the amino acids in
the protein is described.
Polypeptide chains bend and
fold to produce 3 -dimensional
shape.
Formed and stabilized by weak
chemical bonds between
functional groups.
Quaternary structure:

Number of polypeptide chains
covalently linked together.
Figure 2.26
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Nucleic Acids


Include DNA and RNA.
Nucleotides:

Subunits of nucleic acids
bonded together to form
long polynucleotide chains.

Each composed of 3
smaller units:




5-carbon sugar.
Phosphate group attached
to one end of sugar.
Nitrogenous base
attached to other end of
sugar.
Nitrogenous bases:


Pyrimidines: single ring of
carbon and nitrogen.
Purines: two rings of
carbon and nitrogen.
Figure 2.29
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DNA and RNA



DNA:
Basis of genetic code.
Deoxyribose covalently bonded
to 1 of 4 bases:




Each base can form hydrogen
bonds with other bases.



Purines: guanine and adenine.
Pyrimidines: cytosine and
thymine.
Sugar-phosphate bonds form
the chain.
Two strands are are produced
by hydrogen bonding.
RNA:
Consists of a single long chain
of nucleotides joined together
by sugar-phosphate bonds.

Ribose covalently bonds to 4
bases.

Uracil replaces thymine.
Figure 2.32