1. Water

2. Water (H2O) Most abundant substance in living systems .
Essential for all forms of life
Makes up 70 – 90% of the weight of most living organisms.
H and O atoms in a water molecule are linked by covalent bonds.
Each water molecule is linked to others by hydrogen bonds ,which is responsible for many of the unique properties of water (high melting, , high heat of vaporization and cohesion adhesion properties.
All the major components in cells (proteins, DNA, RNA
and polysaccharides) can dissolve in water .
Water and its ionization products OH- and H3O+ ions are important determinants of the characteristic structures and properties of different biomolecules.

3. Properties of Water Polar molecule Cohesion and adhesion
High specific heat
High heat of vaporization
Density – greatest at 4oC
Universal solvent of life

4. 1-Polarity of Water
In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. Gives water more structure than other liquids
Because oxygen is more electronegative, the region around oxygen has a partial negative charge.
The region near the two hydrogen atoms has a partial positive charge.
A water molecule is a polar molecule with opposite ends of the molecule with opposite charges.
2 corners are orbitals with unshared electrons and a weak negative charge
2 are occupied by hydrogen atoms that have polar covalent bonds

5. Water has a variety of unusual properties because of attractions between these polar molecules.
The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond.
Each water molecule can form hydrogen bonds with up to four neighbors.
Fig. 3.1
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6. HYDROGEN BONDS
Extraordinary Properties that are a result of hydrogen bonds.
Cohesive behavior
Resists changes in temperature
High heat of vaporization, high melting temperatures.
Expands when it freezes
Versatile solvent
Hold water molecules together
Each water molecule can form a maximum of 4 hydrogen bonds
The hydrogen bonds joining water molecules are weak, about 1/20th as strong as covalent bonds.
They form, break, and reform with great frequency in liquid form

7. 2-Cohesion and adhesion
Cohesion ; Attraction between particles of the same substance
( why water is attracted to itself)
Results in Surface tension (a measure of the strength of water’s surface)
Produces a surface film on water that allows insects to walk on the surface of water
Cohesion is responsible for the transport of the water column in plants
Cohesion among water molecules plays a key role in the transport of water against gravity in plant.

8. Surface tension, a measure of the force necessary to stretch or break the surface of a liquid.
Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface.
Water behaves as if covered by an invisible film.
Some animals can stand, walk, or run on water without breaking the surface.
Fig. 3.3
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9. Helps insects walk across water
Cohesion …
Helps insects walk across water

10. Adhesion Adhesion ; Attraction between two different substances
Adhesion Adhesion ; Attraction between two different substances. Water will make hydrogen bonds with other surfaces such as glass, soil, plant tissues, and cotton. Capillary action-water molecules will “tow” each other along when in a thin glass tube. Example: transpiration process which plants and trees remove water from the soil, and paper towels soak up water.

11. Adhesion Also Causes Water to …
Attach to a silken spider web
Form spheres & hold onto plant leaves

12. Water Chemistry adhesion
Capillary action-water molecules will “tow” each other along when in a thin glass tube.
Capillary action is facillitated by both the adhesion and cohesion properties of water
adhesion

13. 3-High Heat of Vaporization
Amount of energy to convert 1g or a substance from a liquid to a gas
The heat of vaporization is a direct measure of the amount of energy required to overcome the attractive forces between adjacent molecules in a liquid so that individual molecules can escape from each other and enter the gaseous state.
In order for water to evaporate, hydrogen bonds must be broken.
As water evaporates, it removes a lot of heat with it. 1

14. High Heat of Vaporization
Water's heat of vaporization is 540 cal/g.
In order for water to evaporate, each gram must GAIN 540 calories .
Water has a high heat of vaporization.
- The evaporation of water from a surface causes cooling of that surface. 1

15. Evaporative Cooling
The cooling of a surface occurs when the liquid evaporates
This is responsible for:
Moderating earth’s climate
Stabilizes temperature in aquatic ecosystems
Preventing organisms from overheating

16. 4-Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by 1oC.
Three-fourths of the earth is covered by water .The high Specific Heat of water is responsible for:
Prevention of temperature fluctuations that are outside the range suitable for life.
Coastal areas having a mild climate
A stable marine environment

17. 5-Density of water
Ice water is less dense than liquid water that is why it floats, thus Bodies of water freeze from the top down.
Liquid water has hydrogen bonds that are constantly being broken and reformed.
Frozen water forms a crystal-like lattice whereby molecules are set at fixed distances.

18. Ice is about 10% less dense than water at 4oC.
When water reaches 0oC, water becomes locked into a crystalline lattice with each molecule bonded to to the maximum of four partners.
As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state.
Ice is about 10% less dense than water at 4oC.
Fig. 3.5
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19. 1

20. 6-Solvent for Life The polarity and hydrogen bonding
properties of the water molecule
makes it a potent solvent for many
ionic compounds and neutral molecules .
Water also disperses amphipathic
molecules such as soaps to form
micelles , which are clusters of
molecules arranged such that the
hydrophobic groups are hidden from exposure to water and the hydrophilic (polar) groups are on the external surface exposed to water .

21. Hydrophobic and hydrophilic
interactions

22. Solvent properties
Water is a good solvent for polar and ionic substances.
The polarity and hydrogen-bonding capability of water makes it an excellent solvent for polar molecules. The reason is that water greatly weakens electrostatic
forces and hydrogen bonding between ionic and polar molecules by competing for their attractions. .

23. Solvent properties
The existence of life on Earth depends critically on the capacity of water to dissolve a remarkable array of polar molecules that serve as fuels, building blocks, catalysts, and information carriers. High concentrations of these polar molecules can coexist in water, where they are free to diffuse and interact with one another.

24. Dissociation of Water Molecules
Occasionally, a hydrogen atom shared by two water molecules shifts from one molecule to the other.
The hydrogen atom leaves its electron behind and is transferred as a single proton - a hydrogen ion (H+).
The water molecule that lost a proton is now a hydroxide ion (OH-).
The water molecule with the extra proton is a hydronium ion (H3O+).
Unnumbered Fig. 3.47
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

25. This reaction is reversible.
A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion:
H2O <=> H+ + OH-
This reaction is reversible.
At equilibrium the concentration of water molecules greatly exceeds that of H+ and OH-.
In pure water only one water molecule in every 554 million is dissociated.
At equilibrium, the concentration of H+ or OH- is 10-7M (25°C) .
Only a hydrogen ion is actually transferred
The transferred proton binds to an unshared orbital of the second water molecule.
The water molecule that loses the proton