Biology Unit 1.3 Water

• The cohesion and adhesion of water molecules, these properties allow water to move up the trunks of really tall trees
• Although water as a whole is electrically neutral, the sharing of the electrons is uneven between the oxygen and hydrogen atoms
• The oxygen atom attracts the electrons more strongly than the hydrogen atoms, resulting in a weak negatively charged region on the oxygen atom and a weak positively charged region on the hydrogen atom
• When a molecule has one end that is negatively charged and one end that is positively charged it is also a polar molecule
• Water is a polar molecule
• Hydrogen bonds are weak, when there are few, so they are constantly breaking and reforming
• Hydrogen bonds cause many of the properties of water molecules that make them so important to living organisms
• Water is also able to bond via hydrogen atoms to other molecules which are polar or charged, such as cellulose, which is known as adhesion
• Adhesion: the action or process of adhering to a surface or object
• Hydrogen bonds within water molecules allows for strong cohesion between water molecules
• Cohesion: the tendency of water molecules to stick together, which is important for maintaining the shape of a cell
• Adhesion  enables water to move up the xylem during transpiration
• The cohesive forces between water molecules allow it to form capillaries in narrow spaces like xylem vessels.
• Transpiration: The loss of water vapour from plants through their leaves.
• Hydrophilic: water-loving
• Hydrophobic: water-hating
• Polar molecules and molecules with positive or negative charges can form hydrogen bonds with water (and dissolve) so are generally hydrophilic
• Non-polar molecules with no positive or negative charge, cannot form hydrogen bonds with water so are generally hydrophobic
• Because most biological molecules are hydrophilic and can be dissolved, water is regarded as the universal solvent
• Different solutes have different hydrophobic and hydrophilic properties which affect their solubility in water
• Highly soluble molecules can be easily transported in solution within organisms
• Non-polar, hydrophobic molecules cannot dissolve in water
• Most enzymes require water in order to hold its shape and improve its stability
• Hydrogen bonds will often facilitate the binding of the enzyme active site and its substrate molecule
• Specific heat capacity is a measure of the energy required to raise the temperature of 1 kg of a substance by 1oC
• Water has a higher specific heat capacity compared to air, meaning a relatively large amount of energy is required to raise its temperature
• The high specific heat capacity is due to the many hydrogen bonds present in water
• Specific heat capacity provides suitable, stable aquatic habitats since water temperatures will change more slowly than air temperatures
• Specific heat capacity is able to maintain a constant temperature
• The density of ice is lower than the density of liquid water, which means that ice floats on water
• The thermal conductivity of water is almost 30 times higher than that of air, which makes air a very good insulator for organisms living in colder climates
• Ice in its environment will also form an insulating layer above the water, since the thermal conductivity of ice is much lower than liquid water
• Buoyancy refers to the ability of an object to float in water
• Viscosity refers to the resistance of a fluid to flow
• Surface tension allows insects like water striders to walk across the surface of still water without sinking