Cell Biology:Cell Compounds and Biological Molecules

Lesson 1 – Chemistry of Water

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

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

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

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

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.

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

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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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

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

Water is a solvent

http://programs.northlandcollege.edu/biology/Biology1111/animations/dissolve.html

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.

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!)

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

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

Water is transparent Allows light to penetrate well into

water bodies to the organisms below

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

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

Homework Read pg. 24-28 – Chemistry/properties of

Water Read pg. 29-30 – Acids, bases, buffers