Chapter 2 Chemical components of the cell

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Chapter 2Chemical components of the cell

• Outline
• Chemical bonds
• Molecules in cells
• Macromolecules in cells

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Nucleus of an atom contains most of the mass (
charge). Electrons are much lighter
(-charge). Entire atom is electrically
uncharged (Fig. 2-1).
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Examples of 2 atoms carbon and hydrogen
(Fig. 2-2). Atomic number corresponds to the
number of charged protons. Isotopes
are chemically identical. They differ by the
number of neutrons. The number of protons remains
the same. Isotopes differ in atomic weight, their
atomic number stays the same. Unstable atomic
nuclei and energy emission
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Isotopes are chemically identical. They differ by
the number of neutrons. The number of protons
remains the same. Isotopes differ in atomic
weight, their atomic number stays the same.
Unstable atomic nuclei and energy
emission. Carbon dating
5
There are 92 elements but only 4 contribute to
96.5 of all living organisms. Carbon Oxygen Nitr
ogen Hydrogen (highest abundance) Figure 2-3 2-4
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Figure 2- 5 The atoms are non reactive if the
outer most shell is filled helium, neon,
argon The atoms are reactive if the outer shell
is not filled. These atoms are reactive and will
try to gain or loose electrons to complete outer
shell. Transferring or sharing electrons leads
to formation of a chemical bond. Question 2-2
(Book) Other examples
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Electronegativity

• The ability of the atom to attract to itself an
  electron pair that is shared with another atom
• Figure 2-6 Figure 2-7

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DECREASE
INCREASE
Within a period, elements become more
electronegative from left to right. In the second
period the trend is completely regular increasing
by about 0.5 per element as we move from lithium
(left) to fluorine (right). Within a group,
electronegativity decreases from top to bottom.
Chlorine is less electronegative than fluorine
and sulfur is less electronegative than oxygen.
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Polar covalent bonds
Ionic bonds
Nonpolar covalent bonds
Increasing ionic character
Increasing covalent character
0 1
2 3
4
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Ionic bond formation Sodium atom reacts with
chlorine atom. In this reaction a single electron
is transferred. As a result, two electrically
charged ions with complete set of electrons in
the outermost shell are formed. Figure 2 - 8
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Covalent bond formation Each of the hydrogen
atoms has one electron, the outer shell is
incompletely filled. The length of covalent bond
is defined, 0.074 nm. Figure 2 9
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Typical covalent bond is stronger than thermal
energy so they are resistant to heating. This
bond can be broken by specific and controlled
reactions involving enzymes. Single bond is
longer and weaker compared to a double bond which
is shorter and stronger. Figure 2 10 Figure 2 11
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In a polar covalent bond electrons are shared
unequally. Negative charge is concentrated
towards one end (negative pole). Positive charge
is concentrated towards the other end (positive
pole). In water molecule oxygen has a strong
attraction for electrons, hydrogen-weaker. Figure
2 12
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Polar covalent bonds are very important because
they allow for attraction and interaction between
two molecules of opposite charges. Figure 2
13 Functional groups Phosphate, aldehyde,
hydroxyl, amido, carboxyl, amino, methyl
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Figure 2 15 Carbohydrates/sugars General formula
(CH2O)n Examples of simple sugars glucose or
mannose. Glucose and mannose differ by the
position of hydroxyl groups (structure)
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Glucose serves as an energy source Figure 2
16 Long term storage of glucose
-in animal cells glycogen

-in plant cells starch Cellulose plant cell
wall Chitin insect exoskeleton
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Monosaccharides joined by a covalent bond form
polysaccharides Figure 2 17 Condensation
reaction (example) Hydrolysis reaction Glycoprotei
ns-sugars covalently bound to proteins Glycolipids
-sugars covalently bound to lipids
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Fatty acid Palmitic acid Figure 2 18 Carboxyl
group is ionized to chemically active COO- group
(hydrophilic head wants to react with
water) Palmitic acid is an example of amphipathic
molecule it has both, hydrophilic and
hydrophobic group
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Triglycerol Three fatty acid chains joined to a
glycerol molecule Triglycerol is predominantly
hydrophobic Figure 2 19
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Saturated fatty acids have only single bonds
between carbons. Saturated fatty acid chains are
tight (examples lard, butter) Unsaturated fatty
acids have one or more double bonds between
carbons. The presence of double bonds creates
kinks in fatty acid chains (corn oil, olive
oil) Fatty acids, such as triglycerol are
examples of lipids. Another example of a lipid is
a phospholipid.
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Phospholipids are amphipathic molecules hydrophil
ic heads consists of choline and a phosphate
group hydrophobic tail is composed of fatty
acids Phospholipids contribute to formation of
phospholipid bilayer-a structural basis for every
cell membrane Figure 2 20
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Every amino acid has an amino group and a
carboxyl group, both linked to the same
carbon Amino acids joined by peptide bonds build
larger molecules called proteins List names and
structures of amino acids Figure 2 21
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Each protein polymer is joined by a peptide
bond. Peptide bonds are formed by the reaction of
condensation. This reaction takes place on a
ribosome when protein is being synthesized from
messenger RNA (mRNA). The same 20 amino acids are
building blocks for proteins in bacteria, plants,
and animals. Figure 2 22
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Nucleotide Figure 2 23 Base Purine (adenine,
guanine) Pirimidine (thymine, cytosine) Sugar

ribose-ribonucleic acid (RNA)
deoxyribose-deoxiribonucleic acid (DNA)
Phosphate group

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Adenosine triphosphate (ATP) is an energy carrier
in a cell. Breaking of a phosphoanhydride bond
releases energy Figure 2 24
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Nucleotides in RNA and DNA are linked by
phosphodiester bonds DNA is double stranded. The
two strand are linked by hydrogen bonds. The
double stranded DNA is stable and serves as a
long term storage of genetic information. mRNA
is single stranded and less stable. It is a
template for protein synthesis. Figure 2 25
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Macromolecules are formed by reaction of
condensation (loss of water) Hydrolysis is the
opposite reaction Figure 2 26 Figure 2
27 Figure 2 28
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One protein molecule can be one large chain or
several smaller chains held together by weak
bonds. Figure 2 29
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Molecules can be folded into stable
conformations. They are stabilized by the
noncovalent bonds Figure 2 31
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Examples of weak, noncovalent bonds (Table
2-1) Ionic bonds Hydrogen bonds Van der Waals
bonds Figure 2 32 Figure 2 33 Homework
Questions
2-11 2-13 2-14 2-19 2-21