CHEMISTRY OF CELLS

1
CHAPTER 7

• CHEMISTRY OF CELLS
• PART 2

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THE PROPERTIES OF WATER

• WATER IS A POLAR MOLECULE
• Atoms in a covalently bonded molecule may share
  electrons equally, creating a nonpolar molecule
• If electrons are shared unequally, a polar
  molecule is created

3
In a water molecule, oxygen exerts a stronger
pull on the shared electrons than hydrogen

• This makes the oxygen end of the molecule
  slightly negatively charged
• The hydrogen end of the molecule is slightly
  positively charged
• Water is therefore a polar molecule

_
O
H
H


4
Overview Waters polarity leads to hydrogen
bonding and other unusual properties

• The charged regions on water molecules are
  attracted to the oppositely charged regions on
  nearby molecules
• This attraction forms weak bonds called hydrogen
  bonds

Hydrogen bond
5
Like no other common substance, water exists in
nature in all three physical states

• as a solid
• as a liquid
• as a gas

6
Hydrogen bonds make liquid water cohesive

• Due to hydrogen bonding, water molecules can move
  from a plants roots to its leaves
• Insects can walk on water due to surface tension
  created by cohesive water molecules

7
Waters hydrogen bonds moderate temperature

• It takes a lot of energy to disrupt hydrogen
  bonds
• Therefore water is able to absorb a great deal of
  heat energy without a large increase in
  temperature
• As water cools, a slight drop in temperature
  releases a large amount of heat

8
A water molecule takes a large amount of energy
with it when it evaporates

• This leads to evaporative cooling

9
Ice is less dense than liquid water

• Molecules in ice are farther apart than those in
  liquid water

Hydrogen bond
ICE Hydrogen bonds are stable
LIQUID WATER Hydrogen bonds constantly break and
re-form
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Ice is therefore less dense than liquid water,
which causes it to float

• If ice sank, it would seldom have a chance to
  thaw
• Ponds, lakes, and oceans would eventually freeze
  solid

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Water is a versatile solvent

• Solutes whose charges or polarity allow them to
  stick to water molecules dissolve in water
• They form
• aqueous
• solutions

Na

_
Na
Cl
Cl
_

Ions in solution
Salt crystal
12
The chemistry of life is sensitive to acidic and
basic conditions

• A compound that releases H ions in solution is
  an acid, and one that accepts H ions in solution
  is a base
• Acidity is measured on the pH scale
• 0-7 is acidic
• 8-14 is basic
• Pure water and solutions that are neither basic
  nor acidic are neutral, with a pH of 7

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pH scale

• The pH scale

H
OH
Lemon juice gastric juice
Increasingly ACIDIC (Higher concentration of H)
Grapefruit juice
Acidic solution
Tomato juice
Urine
NEUTRAL H OH
PURE WATER
Human blood
Seawater
Neutral solution
Increasingly BASIC (Lower concentration of H)
Milk of magnesia
Household ammonia
Household bleach
Oven cleaner
Basic solution
Figure 2.15
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• Cells are kept close to pH 7 by buffers

• Buffers are substances that resist pH change
• They accept H ions when they are in excess and
  donate H ions when they are depleted
• Buffers are not foolproof

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Connection Acid precipitation threatens the
environment

• Some ecosystems are threatened by acid
  precipitation
• Acid precipitation is formed when air pollutants
  from burning fossil fuels combine with water
  vapor in the air to form sulfuric and nitric acids

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These acids can kill fish, damage buildings, and
injure trees

• Regulations, new technology, and energy
  conservation may help us reduce acid
  precipitation

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REARRANGEMENTS OF ATOMS

• In a chemical reaction
• reactants interact
• atoms rearrange
• products result

2 H2
O2
2 H2O
REACTANTS
PRODUCTS
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THE DIFFERENT ARRANGEMENTS OF ATOMS GIVE US

• CARBOHYDRATES
• PROTEINS
• FATS,OILS,LIPIDS
• NUCLEIC ACIDS

19
Spider Silk Stronger than Steel

• Lifes diversity results from the variety of
  molecules in cells
• A spiders web-building skill depends on its DNA
  molecules
• DNA also determines the structure of silk
  proteins
• These make a spiderweb strong and resilient

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The capture strand contains a single coiled silk
fiber coated with a sticky fluid

• The coiled fiber unwinds to capture prey and
  then recoils rapidly

21
Cells make a huge number of large molecules from
a small set of small molecules

• Most of the large molecules in living things are
  macromolecules called polymers
• Polymers are long chains of smaller molecular
  units called monomers
• A huge number of different polymers can be made
  from a small number of monomers

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• Cells link monomers to form polymers by
  dehydration synthesis

1
2
3
Unlinked monomer
Short polymer
Removal ofwater molecule
1
2
3
4
Longer polymer
Figure 3.3A
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DEHYDRATION SYNTHESIS

• Removing water putting together

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• Polymers are broken down to monomers by the
  reverse process, hydrolysis

1
2
3
4
Addition ofwater molecule
1
2
3
Coating of capture strand
Figure 3.3B
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HYDROLYSIS

• Add Water splitting apart

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CARBOHYDRATES

• Carbohydrates are a class of molecules
• They range from small sugars to large
  polysaccharides
• Polysaccharides are long polymers of monomers

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Monosaccharides are the simplest carbohydrates

• Monosaccharides are single-unit sugars
• These molecules typically have a formula that is
  a multiple of CH2O
• Monosaccharides are the fuels for cellular work

28

• Many monosaccharides form rings, as shown here
  for glucose

Abbreviatedstructure
Figure 3.4C
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Cells link single sugars to form disaccharides

• Monosaccharides can join to form disaccharides,
  such as sucrose (table sugar) and maltose
  (brewing sugar)

Glucose
Glucose
Sucrose
Figure 3.5
Maltose
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How Sweet is Sweet????Various types of
molecules, including non-sugars, taste sweet
because they bind to sweet receptors on the
tongue
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Polysaccharides are long chains of sugar units

• These large molecules are polymers of hundreds or
  thousands of monosaccharides linked by
  dehydration synthesis

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• Starch and glycogen are polysaccharides that
  store sugar for later use

• Cellulose is a polysaccharide in plant cell walls

Starch granules in potato tuber cells
Glucosemonomer
STARCH
Glycogen granules in muscle tissue
GLYCOGEN
Cellulose fibrils ina plant cell wall
CELLULOSE
Cellulosemolecules
Figure 3.7
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Lipids include fats, which are mostly
energy-storage molecules

• These compounds are composed largely of carbon
  and hydrogen
• They are not true polymers
• They are grouped together because they do not
  mix with water

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• The fatty acids of unsaturated fats (plant oils)
  contain double bonds

• These prevent them from solidifying at room
  temperature
• Saturated fats (lard) lack double bonds
• They are solid at room temperature

Figure 3.8C
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Phospholipids, waxes, and steroids are lipids
with a variety of functions

• Phospholipids are a major component of cell
  membranes
• Waxes form waterproof coatings
• Steroids are often hormones

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Anabolic steroids are usually synthetic variants
of testosterone-a lipid

• Use of these substances can cause serious health
  problems