The Chemical basis for Life (continued)

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The Chemical basis for Life (continued)

• What holds atoms together?
• Ionic bonds
• Attraction between oppositely charged ions (atoms
  or molecules)
• Weak bonds easily disrupted by water
• Covalent bonds
• Sharing of electrons between atoms
• Strong bonds.

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3rd kind of bond Hydrogen bond

• Attractions between atoms in polar molecules
• Polar molecules have ends which vary in how
  negative or positive they are.
• Best example water
• Hydrogen bonds very important in biologically
  important molecules.

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Polar molecules
Electrons spend more time around oxygen. Protons
of hydrogen atom are exposed.
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H-bonds hold large molecules together
Example A-T base pair in
DNA
http//www.mun.ca/biology/scarr/Fg10_16a.gif
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Polar vs. non-polar the basis for hydrophilic
and hydrophobic

• When one end of molecule is slightly positive and
  the other end is slightly negative, the molecule
  is polar.
• When molecules have a more uniform distribution
  of charges they are called nonpolar molecules.
• For example hydrocarbons

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Hydrophobic molecules are mostly carbon and
hydrogen and repel water
Surfaces made of molecules without polar groups
(e.g. CH3CH2CH2CH2.) repel water.
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Water

• Water is denser as a liquid than a solid
• Ice floats
• Water has a high heat capacity
• Allows for heat loss by sweating or panting
• Water is cohesive
• Water strider good
• Lungs bad holds surfaces closed
• Surfactant reduces the surface tension of water

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Water continued

• Water clings to surfaces adhesion
• This is how plants bring water up to their
  leaves transpiration
• Water is a good solvent
• Substances dissolved in water are solutes
• Most important biological molecules are
  hydrophilic and dissolve in water.

http//www.mccullagh.org/db9/10d-18/redwood-beach.
jpg
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Water participates in chemical reactions dehydra
tion synthesis - combines atoms with the removal
of water hydrolysis breaks apart molecules
with the addition of water.
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Water breaks up into hydrogen ions (H) and
hydroxide ions (OH-) Chemical equilibrium
balance
H2O ? H OH- K (H OH-)/ H2O
10-7 pH The negative log of the hydrogen ion
concentration. The more hydrogen ion a solution
has, the lower its pH is. power of hydrogen
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In water Acids - Molecules that release one
or more hydrogen ions (H) and one or more
anions when they dissolve in water. Bases -
hydroxide ion (OH-) and one or more
cations. Salts are neither acidic nor basic.
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A salt ionizes into anions and cations, neither
H nor OH-
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pH regulation (homeostatsis)in humans

• 7.35 7.45
• CO2 H2O ? H2CO3 ? HCO3- H
• Buffer systems act to maintain constant pH
• Respiratory system
• Excretion by kidney

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Molecules of life

• Molecules in organisms are made of chains of
  carbon atoms.
• All carbon and hydrogen containing compounds are
  called organic molecules.
• Organic chemistry study of organic molecules
• Biochemistry study of molecules made by organisms

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Small vs. large

• Biological molecules tend to be fairly small or
  very large.
• Large molecules are called macromolecules.
• Macromolecules are long chains of small units,
  called monomers, linked together to form long
  chains called polymers.

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Four Classes of Biological Molecules

• Carbohydrates
• Sugars and their polymers
• Nucleic acids and nucleotides
• DNA, RNA, ATP
• Lipids
• Various hydrophobic molecules
• Proteins and amino acids

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Carbohydrates CH2O
Monosaccharides glucose, fructose, many
others Disaccharides sucrose,
lactose Polysaccharides starch, cellulose,
glycogen, agar, chitin, xanthan gum
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Simple sugars
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A nucleotide a monomer of DNA
http//www.emc.maricopa.edu/faculty/farabee/BIOBK/
nucleotide.gif
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Nucleic acids are the polymers made from
nucleotides.
DNA tRNA
http//www.biochem.uwo.ca/meds/medna/IMG/tRNA.GIF
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Structure of DNA
http//academic.brooklyn.cuny.edu/biology/bio4fv/p
age/molecular20biology/16-05-doublehelix.jpg