Water: The Medium of Life

Thermal, cohesive and solvent properties of water. Cohesive Properties – water has a very high cohesion as you learned in chemistry. Thermal – water has an extremely high specific heat (1 cal/gram or 4.184 Joules/g). Compare with gold at 0.129 J/g. Polarity of water molecule causes it to dissolve other polar substances readily.

The properties of water can be explained by the theory that hydrogen bonds form between the molecules. Water is a polar molecule and hydrogen bonds form between different molecules. http://upload.wikimedia.org/wikipedia/commons/b/b5/Hydrogen-bonding-in-water-2D.png http://biomodel.uah.es/en/water/index.htm

The structure of water molecules showing polarity and hydrogen bond formation. A type of dipole force that occurs between molecules with a Hydrogen and a highly electronegative atom with at least one pair of electrons (F, O, N) (why H2O is liquid at room temp.) Picture: http://academic.brooklyn.cuny.edu/biology/bio4fv/page/image12.gif

Hydrogen Bond and the structure of Ice. Note that hydrogen bonds are intermolecular attractions and not true chemical bonds. Picture From: http://onramp.nsdl.org/eserv/onramp:1054/web_water2.jpg

Cohesion of water – this allows other objects to be supported: https://www.youtube.com/watch?v=w7QNcqeKSQs Okay, so maybe that was a hoax, but this one isn’t: https://www.youtube.com/watch?v=Qhsxo7vY8ac Cohesion of water allows water to move together up through xylem tubes in plants to the leaves for photosynthesis.

Thermal properties of water… The specific heat, melting point, and boiling point of water are all extremely high.

Contrast with Methane Picture: http://commons.wikimedia.org/wiki/File:Methane-2D-square.png

Comparison of the Thermal Properties of Water and Methane Melting Point 0.0 ºC -182.5 ºC Boiling Point 100.0 ºC -161.5 ºC Specific Heat 4.184 J g-1 ºC−1 2.20 J g-1 ºC−1 Polarity Polar Non-Polar The Difference is because of Hydrogen bonding!

Water is important for the maintenance of homeostasis The human body is about 70 % water. Water is used to maintain homeostasis. If the balance is lost, the risk of heat stroke, frostbite, hypothermia, etc. is greatly increased. Similarly, it takes energy to evaporate water, so sweating cools you down as energy is expended to remove the water.

Hydrophobic vs. Hydrophilic Hydrophobic – literally afraid of water Hydrophilic – literally loves water https://www.youtube.com/watch?v=nB9n_nQhmmg

Water dissolves many substances. For this reason, it is an essential medium for transport and metabolism in organisms.

Application: Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in relation to their solubility in water. Amino acids have positive and negative regions of charge and are soluble in water – carried in plasma R – Group determines solubility. http://upload.wikimedia.org/wikipedia/commons/a/a5/L-alpha-amino-acid-2D-skeletal.png Glucose is a polar molecule. It dissolves and is carried in the blood plasma. http://upload.wikimedia.org/wikipedia/commons/6/64/Glucose_Haworth.png

Oxygen gas is non-polar. It is small so has some solubility in water. Most is carried by the erythrocyte (red blood cells). Each hemoglobin has 4 bonding sites for oxygen. NaCl is an ionic compound Dissolves in water Carried in the plasma http://www.verticallearning.org/curriculum/science/gr7/student/unit01/page30.html

Cholesterol has a small hydrophilic portion. Not enough to be water soluble Carried by a lipoprotein complex in the plasma. Fats are large and non polar Not water soluble Carried by lipoprotein complex in the plasma. Lipoprotein complex: has phospholipids on surface that anneal to water Hydrophobic fats on inside; cholesterol is positioned so that hydrophilic portion faces outward. Picture: http://eatingacademy.com/wp-content/uploads/2012/05/lipoprotein2.jpg