WATER

* WATER MOLECULES ARE POLAR AND HYDROGEN BONDS FORM BETWEEN THEM.

- The covalent bond between the hydrogen and oxygen involves unequal sharing of electrons (oxygen atom shares 2 elctrons, whereas hydrogen atoms share 1), making it a polar molecule.

- There is a small positive charge on each hydrogen atom and there is a small negative charge on each oxygen atom.

- Water molecules are bent, not linear, so the hydrogen atoms are on the same side to form one pole, while the oxygen atom forms the other.

- The hydrogen bond is an intermolecular force where a hydrogen atom is attracted to a slightly negative atom of another polar covalent molecule.

- Water molecules are small enough that the "hydrogen bond" allows them to cohere.

* USE THEORIES TO EXPLAIN NATURAL PHENOMENA: THE THEORY THAT HYDROGEN BONDS FORM BETWEEN WATER MOLECULES EXPLAIN WATER'S PROPERTIES.

- Hydrogen bonds can't be seen directly so there is no proof which states they exist.

- However, hydrogen bonds can explain the properties of water, how they cohere and adhere, their thermal properties and ability to serve as a solvent. These distinctive properties make water so useful to living organisms.

- We can assume this theory is correct if there is evidence for it, if it helps to predict behaviour, if it has not been falsified and if it helps to explain natural phenomena.

* HYDROGEN BONDING AND DIPOLARITY EXPLAIN THE ADHESIVE, COHESIVE, THERMAL AND SOLVENT PROPERTIES OF WATER.

Cohesive properties:

- Water molecules are cohesive and this is important for water transport in plants.

- Water is sucked in through xylem vessels at a low pressure. The intermolecular forces results in a continuous string of water molecules which can be pulled over a hundred metres upwards.

Adhesive properties:

- When water sticks to other polar molecules due to the hydroen bond, this can be seen as adhesion.

- Water adheres to the cellulose molecules in the cell wall of leaves.

- When water evaporates, adhesive forces cause water to be drawn out of the nearest xylem vessel to keep the cell walls moist. It helps to absorb carbon dioxide for photosynthesis.

Thermal properties:

- Water has a high specific heating capacity because hydrogen bonds restrict the motion of water molecules so more energy is needed to break the bonds. Water's temperature is relatively stable to the air and land, making it a thermally stable habitat for aquatic organisms.

- Water has a high latent heat of vaporization meaning that a lot of energy is lost when water evaporates. Therefore, water is a good evaporative coolant and it can be seen through the example of sweating.

- Water has a high boiling point and remains a liquid over a broad range of temperatures, from 0 - 100 degrees celcius. This temperature range can be found in most habitats on earth.

Solvent properties:

- Water can form shells around charged and polar molecules due to it polar nature. This prevents substances from clumping and keeps them in solution.

- Positive hydrogen pole is attracted to negatively charged electrons and negative oxygen pole is attracted to positively charged ions so both dissolve. (Remember to use "partially").

* SUBSTANCES CAN BE HYDROPHILIC OR HYDROPHOBIC.

- Substances that dissolve in water are hydrophilic.

- Things that are insoluble in water (but soluble in others) are hydrophobic, though they are not actually water-fearing.

- Substances that water adheres to are hydrophobic.

- Molecules that are not charged and are nonpolar are hydrophobic. All lipids, fats and oils are hydrophobic.

- If a nonpolar molecule is surrouded by water molecules, hydrogen bonds form within the water molecules.

- If there are two nonpolar molecule surrounded by water, they will behave as if they were attracted to each other, especially if they are in contact. This is because more hydrogen bonds can form between water molecules as they are more attracted to each other than nonpolar molecules. Nonpolar molecules will tend to join together in groups in water, this is known as hydrophobic interactions.

* COMPARISON OF THE THERMAL PROPERTIES OF WATER WITH THOSE OF METHANE.

- Methane is a waste product of anaerobic respiration in certain prokaryotes that live in habitats which lack oxygen. This includes swamps, wetlands and waste dumps.

- Methane can be used as fuel but can also contribute to the greenhouse effect if it escapes into the atmosphere.

- Water and methane are both small molecules with single covalent bonds. Water is polar and forms hydrogen bonds, while methane is nonpolar. Their physical properties are therefore very different.

- Water has a higher specific heat capacity, higher latent heat of vaporization, higher melting point and higher boiling point. Water is liquid over a broader temperature range.

* USE OF WATER AS A COOLANT IN SWEAT.

- Sweat is carried along narrow duct to to the surface of the skin where it is secreted.

- The heat needed for the evaporation of water in sweat is taken from the skin tissues, reducing their temperature. Blood flowing through the skin is cools as a result. Solutes in sweat like sodium are left on the skin.

- The hypothalumus in the brain has receptors to monitor blood temperature and also recieves sensory inputs from temperature receptors in the skin.

- The hypothalumus stimulated the sweat glands to secrete sweat when the body is overheated or when adrenaline is secreted (brain anticipates a period of intense activity).

- Other forms of heat loss usually rely on the evaporation of water such as panting in dogs, fur licking in kangaroos and transpiration in plants under hot environments.

* METHODS OF TRANSPORT OF GLUCOSE, AMINO ACIDS, CHOLESTEROL, FATS, OXYGEN AND SODIUM CHLORIDE IN BLOOD IN RELATION TO THEIR SOLUBILITY IN WATER.

- Blood transports a wide variety of substances in large enough quantities for the body's needs.

- Sodium chloride is an inorganic compund that is freely soluble in water, dissolving to form sodium and chloride ions, which are carried in blood plasma.

- Amino acids have negative nad positive charges so they are soluble in water. Their solubility depends on the R group as some are hydrophilic or hydrophobic. They are all soluble enough to be carried dissolved in blood plasma.

- Glucose is a polar moleclue, freely soluble in water and is carried dissolved in blood plasma.

- Oxygen is nonpolar, but the molecules are small enough to dissolve in water. (Water saturates with oxygen at low concentrations). The solubility of oxygen decreases as the water temperature rises so at 37 degrees celcius, the blood plasma cannot transport sufficient amounts of it. Haemoglobin has binding sites for oxygen which greatly increases the capacity of blood for oxygen transport.

- Fat molecules are completely nonpolar, larger than oxygen and insoluble in water. Lipids are carried in lipoprotein complexes. The surface is made of a phopholipid monolayer and proteins, encasing the cholesterol and triglycerides inside.

- Cholesterol molecules are hydrophobic except for one end of it. This is not enough to make them dissolve in water and are carried through the blood plasma in lipoproteins.