1 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.

2 3 rd 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.

3 Electrons spend more time around oxygen. Protons of hydrogen atom are exposed. Polar molecules

4 H-bonds hold large molecules together Example: A-T base pair in DNA

5 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

6 Hydrophobic molecules are mostly carbon and hydrogen and repel water Surfaces made of molecules without polar groups (e.g. CH 3 CH 2 CH 2 CH 2.) repel water.

7 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

8 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.

9 Water participates in chemical reactions: dehydration synthesis - combines atoms with the removal of water hydrolysis – breaks apart molecules with the addition of water.

10 Water breaks up into hydrogen ions (H+) and hydroxide ions (OH-) Chemical equilibrium = balance H 2 O  H + + OH - K = (H + + OH - )/ H 2 O = 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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12 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.

13 A salt ionizes into anions and cations, neither H + nor OH -

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15 pH regulation (homeostatsis)in humans 7.35 – 7.45 CO 2 + H 2 O ↔ H 2 CO 3 ↔ HCO H + Buffer systems: act to maintain constant pH Respiratory system Excretion by kidney

16 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

17 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.

18 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

19 Carbohydrates: CH 2 O Monosaccharides: glucose, fructose, many others Disaccharides: sucrose, lactose Polysaccharides: starch, cellulose, glycogen, agar, chitin, xanthan gum

20 Simple sugars

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22 A nucleotide: a monomer of DNA

23 Nucleic acids are the polymers made from nucleotides. DNAtRNA

24 Structure of DNA