1. Cell Biology: Cell Compounds and Biological Molecules
Lesson 1 – Chemistry of Water

2. Today’s Objectives
Describe characteristics of water and its role in biological systems, including:
Describe the role of water as a solvent, temperature regulator, and lubricant
Describe how the polarity of the water molecule results in hydrogen bonding

3. Chemistry of Water
Why do we care about water chemistry in a Biology course?
1) The first cells evolved in water
2) All living things are 70-90% water
3) Due to hydrogen bonding, water molecules cling together which raises the melting and boiling points. Without hydrogen bonding, most water on the Earth would be steam, making life unlikely
These and other unique properties of water make it essential to the existence of life

4. Cell Compounds
In the formation of chemical compounds, both organic or inorganic, atoms must bond together to form stable structures
In order to do this, the electron configurations around the various nuclei in the compound must also be stable.
The stability can be achieved through some degree of sharing electrons between the atoms

5. Review of Bonding
If the atoms in a compound share their electrons reasonably equally, their association is termed a covalent bond
The alternative is a very unequal sharing. This is called Ionic bonding where it seems as though one atom actually gives away its electrons
Neutral atoms become positive ions when they lose electrons and negative ions when they gain electrons

6. Polar Covalent Bonding
In between these two extremes is what is termed polar covalent bonding
Polar covalent molecules have dipoles
A dipole is a region with a slight positive or negative charge. A water molecule is an example of this kind of molecule
If the electrons in a covalent bond are not shared equally by the two atoms, then this is a polar covalent bond
Larger atoms have a greater electronegativity than smaller atoms. Electronegativity is the ability of an atom to attract electrons.

7. Bonding in Water
In a water molecule, each hydrogen atom shares an electron with the oxygen atom
The oxygen atom is much larger and tends to attract the shared electrons more (higher electronegativity). Therefore, oxygen has a partial negative charge while the hydrogen's have a partial positive charge

8. Polarity of Water
Because water molecules have dipoles, water molecules are loosely attracted to one another
The negative charge on the oxygen attracts the positive charge on the hydrogen
This produces a weak bond called a hydrogen bond
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9. Polarity of Water
Each water molecule can form up to 4 hydrogen bonds at a time with its neighbors
Although the hydrogen bond is weak, the vast number of these bonds gives water it’s unique properties

10. Water is a solvent
Due to it’s polarity, water facilitates chemical reactions
It dissolves a great number of substances, which are then called solutes
Solutes that are polar dissolve much more easily than non- polar solutes
For example, when ionic salts (such as NaCl), are put into water, the negative ends of the water molecules are attracted to the sodium ions, and the positive ends are attracted to the chloride ions
This causes the sodium ions and chloride ions to separate, or dissolve, in water

11. Water is a solvent
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Since blood is mainly water, the ability of water to dissolve and transport substances greatly aids in bringing about necessary chemical reactions in the body.

12. Water regulates temperature
Water can help regulate temperature because of its high heat capacity (ability to absorb heat energy without a great change in temperature)
This is due to the many hydrogen bonds that link water molecules together
In hot weather, water warms slowly, and in winter it releases heat slowly
This property helps us regulate body temperature in different temperatures (our body is over 70% water!)

13. Water regulates temperature
As a liquid evaporates, the surface of the liquid that remains behind cools – called evaporative cooling
This occurs because the most energetic molecules are the most likely to evaporate, leaving the lower kinetic energy molecules behind
Example: oceans keep surrounding land masses cool in summer, warm in winter
Example: Body systems are mainly water, so bodies tend to keep fairly constant temperature despite changing external conditions

14. Ice is less dense than liquid water
Therefore, ice forms on top of water, insulating lower levels. This allows living things to function beneath the ice
If ice was more dense than liquid water, it would sink to the bottom and push more water upwards to freeze
If ice sank, all water would eventually freeze solid

15. Water is transparent
Allows light to penetrate well into water bodies to the organisms below

16. Water is cohesive and adhesive
Cohesion means water molecules stick to each other
This is evident because water flows freely, yet water molecules don’t separate from each other
Cohesion plays a major role in the transport of water against gravity in plants
Adhesion means that water molecules stick to other surfaces
This keeps surfaces moist and lubricated
Diffusion of gases occurs much more efficiently across moist surfaces
Adhesion also assists in the transport of water against gravity in plants as water adheres to the walls of vessels

17. Water has a high surface tension
Surface tension is a measure of the force required to stretch or break the surface of a liquid
Is related to cohesion
Water has a higher surface tension than most other liquids because hydrogen bonds between molecules resist stretching or breaking
Some animals can stand, walk, or run on the surface of the water because of the surface tension

18. Homework Read pg. 24-28 – Chemistry/properties of Water
Read pg – Acids, bases, buffers