IB Biology - Curriculum Notes 

​2.2 Water  

Understandings: ∑

∑ - Water molecules are polar and hydrogen bonds form between them.

A water molecule consists of an oxygen atom covalently bound to two hydrogen atoms
Since O is more electronegative than H, an unequal sharing of electrons occurs
This creates a polar covalent bond, with H having a partial positive charge and O having a partial negative charge
Water is also bent so the positive charge exists more or less on one side and the negative charge from the O exists on the opposite side
The partial +ve charge is attracted to the partial –ve charge creating an intermolecular attraction between the water molecules called a “Hydrogen bond.”
H-bonds are the strongest of the intermolecular bonding, but is still considered a weak bond; however, since there are so many H2O molecules they give water its unique properties and make it essential to life on this planet

∑ - Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water.

Thermal Property

Water has a high specific heat capacity (amount of energy needed to raise the temperature of a substance by a certain temperature level). Basically, water can absorb a lot of heat and give off a lot of heat without drastically changing the temperature of the water.
Water’s high specific heat capacity results from the extensive hydrogen bonding between the water molecules.
Water also has a high latent heat of vaporization which means it takes a lot of heat to evaporate water from a liquid to a vapour. This is very important as a cooling mechanism for humans. As we sweat, the water droplets absorb heat from our skin causing the water to evaporate and our bodies to cool down. Interesting Documentary on the 

ICEMAN: Man who can control his core body temperature (relates to metabolism as well) https://www.youtube.com/watch?v=VaMjhwFE1Zw

Cohesive Properties

Water is a polar molecule, with a negative oxygen end and a positive hydrogen end.
Hydrogen bonds that exist between water molecules create a high level of attraction linking water molecules together. This attraction between two of the same molecules is called cohesion.
These cohesive forces allow water to move up vascular tissue in plants against gravity. It also creates surface tension on the water that allows some organisms to walk on water.

Adhesive Properties

Not only does water bind strongly to itself, but it also forms H-bonds with other polar molecules. This is called adhesion.
This is an important property in transpiration as well, as water adheres to the cellulose in the walls of the xylem vessels
As water is evaporated from the stomata, the adhesion can help the water move up through the xylem

Solvent Properties

Water is known as the “universal solvent” because of its ability to dissolve many substances because of its polarity.
Water is able to dissolve other polar molecules such as many carbohydrates, proteins and DNA; and positively and negatively charged ions such as Na+.
This is essential because it allows water to act as a transport medium (blood and cytoplasm) of important molecules in biological organisms.

∑ - Substances can be hydrophilic or hydrophobic.

Essentially hydrophilic means “water-loving”
Any substances that dissolve in water including charged ions such as Na+ or polar molecules such as glucose and fructose are hydrophilic. Molecules that are attracted to water like phospholipid heads are also hydrophilic
Hydrophobic molecules are kind of “water-fearing” but basically, these are non-polar, insoluble in water or non-charged substances, such as lipids 
Note: Don't use the terms "water-loving or water-fearing" to describe these properties on an exam, it should simply be a way to remember what hydrophilic and hydrophobic mean.

Lab Activity – Water stations to demonstrate the different properties of water.

 β - Applications and skills:

 β - Application: Comparison of the thermal properties of water with those of methane.

Methane’s formula is CH4
Water’s formula is H2O


Single covalent bonds
Single covalent bonds
Since water is polar it has stronger intermolecular attraction (H-bonds) and therefore has a much greater specific heat capacity, latent heat of vaporization, melting point and boiling point
SHC = 2.2 J per g per ͦ C
4.2 J per g per ͦ C
LH of V =  760 J/g
2257 j/g
BP = -160  ͦ  C
100  ͦ C
MP = -182  ͦ  C
     0  ͦ C



β - Application: Use of water as a coolant in sweat.

Water is essential to living organisms.
Water has a high latent heat of vaporization which means it takes a lot of heat to evaporate water from a liquid to a vapour.
This is very important as a cooling mechanism for living organisms. As humans sweat, the water droplets absorb heat from the blood flowing under our skin causing the water to evaporate and our blood to cool down. This will, in turn, cool our whole body down.
This cooling is controlled by negative feedback through receptors in the hypothalamus
If the body is overheated, receptors in the hypothalamus sense this and stimulate the sweat glands to secrete sweat
Some reptiles such as crocodiles cool by opening their mouths (gaping). Dogs also pant which causes water to evaporate from their upper respiratory tract.

β - Application: Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in relation to their solubility in water.

Blood transport many different substances to different parts of the body using a variety of methods
Water is critical both as a solvent in which many of the body's solutes dissolve
Due to its polarity water is a great solvent of other polar molecules and ions. This is vital because it allows water to act as a transport medium (blood and cytoplasm) of important molecules in biological organisms.
NaCl is an ionic compound that is very soluble in water. Na+ and Cl- dissolve and are carried in the blood plasma
Glucose is polar and is soluble in water and is therefore transported in the plasma
Amino acids have both a negative and a positive charge, but their “R” groups vary, therefore they can be hydrophilic or hydrophobic. They are all soluble enough to be carried in the plasma
Fats are non-polar and therefore insoluble in water. They are transported in a single layer sphere of phospholipids called a lipoprotein complex. The hydrophilic heads face outwards towards the water in the plasma and the tails face inwards towards the fats. Proteins are also embedded in the phospholipid layer.
Finally, cholesterol, which is mostly hydrophobic because it is a lipid, is also transported inside the lipoprotein complex with the small hydrophilic end facing the phospholipid heads

Video on Transportation of Oxygen https://www.youtube.com/watch?v=GVU_zANtroE 
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