CAPE Bio (P2) - Aspects of Biochemistry

Briefly describe the importance of water to animals and plants (3)

In plants, water is important in transpiration and the transport of solutes and products of photosynthesis around the plant. It makes up approximately 90% of the structure of plants.

In animals, water is important in excretion and the composition of blood which transports nutrients, gases and metabolic water to/away from organs. It makes up 70% of the human body and 90% of blood.

Water is also important to both organisms as it is the site for many chemical reactions and acts as a coolant or habitat.

Explain why water is a polar molecule

The high electronegativity of oxygen pulls the electrons from the hydrogen atoms closer to the oxygen atoms — unevenly distributing the charge in each atom creating dipoles which make water polar

Explain how the hydrogen bonds in water give way to its cohesive nature and high surface tension including the organisms it benefits

Each water molecule can form hydrogen bonds with up to four other water molecules. In its liquid state, this allows water to constantly form and re-establish hydrogen bonds, the rate of which increasing as temperature decreases. This allows water molecules to be cohesive with each other and is key to the transportation of water in the xylem and transpiration in plants

Its high surface tension arises due to the amount of energy needed to dispel the hydrogen bonds which allows small animals to walk on water surfaces and increases the resistance of water molecules during transportation in the xylem

Explain how pure water is less dense than pure ice including the organisms it benefits

Ice molecules form a hexagonal shape with fixed hydrogen bonds and a resultant fixed shape whilst liquid water takes the shape of its container and has no fixed hydrogen bonds, allowing it to arrange itself in a formation more dense than the one ice possesses with the maximum density of water occurring at 4 degrees Celsius.

Ice being less dense than water allows it to float to the surface which is particularly beneficial to organisms in freshwater habitats as the ice layer insulates the habitat from heat loss and allows organisms to survive in the water below. In marine habitats, a similar dynamic is maintained however its freezing point is much lower as seawater is impure. However, it provides benefits to aquatic predators suited to colder regions which require both land and water for their survival.

Explain how the high specific heat capacity is beneficial including coral as an example

It makes aquatic habitats more resistance to seasonal temperature change and also regulates the temperature of coastlines. The resistance to temperature change reduces the rate at which dissolved gases, such as oxygen, which are necessary for aquatic life, escape the water.

The level of oxygen dissolved in the water is particularly important to cold-blooded organisms such as coral because their rate of metabolism increases with temperature which also increases the amount of oxygen it uses which can become detrimental at night when photosynthesis is impossible. Even slight temperature change kills the algae that photosynthesize for the coral and causes the coral to bleach