The Chemical Basis of Life

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The Chemical Basis of Life

• Why are we studying chemistry in a biology
  course???
• At the base of the
• hierarchy of life are
• atoms and molecules!
• Many biological processes
• take place on a molecular
• level

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• The food we eat is made up
• of chemical elements, many
• of which are essential to life

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Elements, Atoms and Molecules

• Living organisms are made up of 25 chemical
  elements
• An element is a substance that cannot be broken
  down to any other substance by ordinary chemical
  means
• Remember your Periodic Table of Elements?
• Oxygen (O), Carbon (C), Nitrogen (N), Hydrogen
  (H) 96 of the weight of the human body
• Main ingredients of proteins, sugars and fats

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Elements, Atoms and Molecules

• The next 3.99 is made up of
• Calcium (Ca) ? bone formation
• Phosphorus (P) ? DNA synthesis
• Potassium (K) ? cell signaling, nervous system
• Sulfur (S)
• Sodium (Na)
• Chlorine (Na)
• Magnesium (Mg)

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Elements, Atoms and Molecules

• The remaining 0.01 are represented as trace
  elements
• very small quantities, but extremely important
• Examples of trace elements include
• Iron (Fe) hemoglobin, binds oxygen
• Copper (Cu) enzyme function
• Fluorine (F) prevents tooth decay
• Iodine (I) thyroid hormones

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Elements, Atoms, and Molecules

• An element is a unique substance that cannot be
  broken down to into smaller substances
• An atom is the smallest unit of matter (stuff
  that the universe is made of) that still retains
  the properties of an element
• A molecule is a substance consisting of 2 or more
  elements (either the same or different)
• Sodium chloride (NaCl table salt)
• Water (H2O)
• O2 (oxygen)

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• The Periodic Table of Elements lists all of the
  known elements and their atomic numbers

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Atoms

• An atom is made up of
• Proton () charge
• Electron (-) charge
• Neutron neutral, no charge
• Each elements differs by the number of protons,
  neutrons and electrons found in their respective
  atoms

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Elements

• An element is an atom with a specific number of
  protons in its nucleus
• Every element has an atomic number. The atomic
  number is the number of protons in the nucleus of
  the atom

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Structure of an Atom

• An atom is made up of a nucleus which contains
  protons (positive charge) and neutrons (no
  charge) and is surrounded by a cloud of electrons
  (negative charge)

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An Introduction to Chemistry

• The electrons are much, much smaller than the
  nucleus and circle around the atom at the speed
  of light in energy layers, or shells

ATOM
proton
neutron












nucleus
electron
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• The first shell of an atom always contains up to
  two electrons
• The outer shells always contain up to eight
  electrons.
• An atom is stable when the outermost shell of
  electrons is complete

ATOM












electron
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• Lets take a look at Oxygen
• Oxygen has an atomic number
  of 8 so it has
• 8 protons
• 8 neutrons
• 8 electrons
• 2 of the 8 electrons are in the first electron
  shell, leaving 6 for the outermost electron shell
  (8-2 6)
• Its outermost electron shell is not complete it
  wants 8 electrons and only has 6 ( so it needs 2
  more)

O
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• Now lets look at Hydrogen
• Hydrogen has an atomic number of 1 so it has only
  1 proton and 1 electron
• With only 1 electron, it has only 1 electron in
  its first electron shell
• Like oxygen, is outermost electron shell is not
  complete it wants 2 electrons and only has 1 (
  so it needs 1 more)

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• But what if oxygen which needs 2 more electrons
  could share an electron with 2 hydrogen atoms?
  The shared electrons would orbit around the atoms
  completing the outermost electron shells for all
  3 atoms

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• This is exactly what happens.when oxygen shares
  its electrons with 2 hydrogen atoms, theresulting
  molecule is water, or H2O

WATER
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Covalent Bonds

• The sharing of outer shell electrons is called a
  covalent bond. The resulting bonding of atoms to
  one another forms a molecule.
• But covalent bonds are not the only types of
  bonds that atoms can form.
• Lets look at another example this time with
  Sodium and Chlorine

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• Sodium (Na) has 11 electrons 2 in the first
  shell, 8 in the second shell and 1 in the
  outermost shell

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• Chlorine (Cl) has 17 electrons 2 in the first
  shell, 8 in the second shell and 7 in the
  outermost shell

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• If Sodium donated its extra electron to Chlorine,
  its outermost electron shell would be complete

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• When Chlorine accepts this extra electron, its
  outer electron shell will also be complete

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• Since Sodium lost an electron, it has one more
  proton than it does electrons (11 protons, only
  10 electrons) this gives Sodium a positive ()
  charge

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• Since Chlorine gained an electron, it has one
  more electron than it does protons (17 protons,
  and 18 electrons) this gives Chlorine a negative
  (-) charge

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• The resulting atoms are now called ions, since
  they have an electrical charge. When two atoms
  exchange electrons, they form an ionic bond

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• Na (sodium ion) is attracted to the Cl- ion
  (chloride ion) and they bond to one another,
  forming NaCl (sodium chloride)

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Hydrogen Bonds

• Covalent bonds exist between atoms sharing
  electrons
• Ionic bonds exist between ions, atoms that have
  exchanged (gained or lost) electrons, and as a
  result have a positive or negative charge
• Hydrogen bonds are a type of bond that exist
  between molecules, and result from an unequal
  sharing of electrons within the molecule

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• Recall that in a water molecule, two hydrogen
  atoms share electrons with an oxygen atom,
  completing the outer shell of all 3 atoms

H
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• But the electrons do not spend an equal amount of
  time with hydrogen as they do oxygen

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• This is because oxygen has 8 protons in its
  nucleus,
  while hydrogen only has 1
• Remember that opposites attract. The difference
  in positive charges pulls the
  shared electrons
  toward oxygen,
  and
  away from the two
  hydrogen atoms

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• The abundance of electrons near oxygen makes the
  oxygen atom in a water molecule slightly negative
• Likewise, the 2
  hydrogen atoms
  become slightly
  positive
  since their
  shared electrons
  spend so little
  time
  near them

(-)










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• A water molecule is said to be polar in that it
  has two ends of opposing charges a slightly
  positive charge near the hydrogen atoms, and a
  slightly negative charge near the oxygen atom

(-)
O
H
H
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• The bonds that form between the positive and the
  negative ends of two or more polar molecules are
  called hydrogen bonds

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Water is the solvent of life

• Because of its polar properties, water is an
  incredible solvent
• A solvent is an agent that dissolves substances
  or solutes, together making up a solution

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• The positive and negative ends of a water
  molecule attract ions, even those once joined by
  ionic bonds

• For example, table salt, NaCl is readily
  separated back into Na and Cl- ions in water

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Water is essential to life

• Because of its polar properties, water is also
  cohesive, or sticky
• Cohesion exists because the positive (hydrogen)
  end of one water molecule is drawn
  towards the negative (oxygen) end of
  another water
  molecule

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Water Heat Capacity

• Because of its hydrogen bonds, water is said to
  have a high heat capacity, or ability to absorb a
  large amount of heat energy without an increase
  in its own temperature
• To increase the temperature of water, heat energy
  must first disrupt the hydrogen bonds that have
  formed between water molecules

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Water Heat Capacity

• We experience the benefits of waters high heat
  capacity every time we sweat.
• Our body provides the heat to evaporate water
  from a liquid to a gas (water vapor), keeping us
  cool
• This is the same reason why dogs pant and pigs
  wallow in the mud!

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Waters amazing properties

• Water is unique in that it naturally exists in
  all phases (gas, liquid, solid) on Earth
• Water is also unique in that its solid phase
  (ice) is less dense than its liquid phase (water)

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Waters amazing properties

• At low temperatures, the hydrogen bonds between
  water molecules stabilize
• This creates a 3-dimensional shape, or crystal
  that traps air between the water molecules
• The trapped air causes ice to take up more space
  than liquid water, making it less dense and
  causing ice to
  float

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