2.1.2 Biological Molecules

What is the structure of water?

Water consists of 2 hydrogen atoms covalently bonded to one oxygen atom

How does hydrogen bonding occur in water?

Each hydrogen shares a pair of electrons with the oxygen (covalent bonding).

Oxygen has a greater affinity for electrons than hydrogen, so it ‘pulls’ the electrons closer

What are the properties of water?

• High specific heat capacity

• High latent heat of vaporisation

• Surface tension

• A solvent

• An insulator

• Higher density than ice

• A transport medium

What is surface tension and how could it benefit organisms?

The hydrogen bonds mean the water is ‘sticky’ by forces of adhesion & cohesion.

Molecules in the bulk of liquid are not affected by molecules above, therefore ‘pull’ more strongly to form a stretched membrane → a habitat can exist on water (pond skaters)

How can being a solvent be beneficial for organisms?

• dissolves most organic & inorganic substances

• needed for biochemical reactions

• removes excretory products e.g. urea

• in plants, root hairs absorb mineral salts present in soil

How can being an insulator be beneficial for organisms?

• ice is less dense than water, therefore floats above it → insulates the water below the surface of ponds & lakes

• water won’t freeze, so aquatic organisms can survive

How can a high latent heat of vaporisation be beneficial for organisms?

• loss of heat through vaporisation → sweating, which helps us cool down & prevents overheating

How does being a transport medium be beneficial for organisms?

• human blood plasma consists maily of water (90%)

• carries many dissolved substances like excretory waste, hormones & gases around the body

• in plants, sugar & mineral salts are transported in solution in vascular bundles