AP Biology 1.1 Structure of Water and Hydrogen Bonding Study Notes - New Syllabus Effective 2025
AP Biology 1.1 Structure of Water and Hydrogen Bonding Study Notes- New syllabus
AP Biology 1.1 Structure of Water and Hydrogen Bonding Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.
Key Concepts:
- Structure & Properties of Water
- Hydrogen Bonding in Water
1.1.A.1
Living systems depend on the properties of water to sustain life.
i. Water has polarity, because of the formation of polar covalent bonds between hydrogen and oxygen within water molecules. This polarity contributes to hydrogen bonding between and within biological molecules.
ii. Water has a high specific heat capacity, which allows for the maintenance of homeostatic body temperature within living organisms.
iii. Water has a high heat of vaporization, which allows for the evaporative cooling of the surrounding environment. In living organisms, this property allows for body temperature to be maintained.
- Ice floats. Unlike most substances that contract and become more dense when they freeze, water expands as it freezes, becomes less dense than its liquid form, and, as a result, floats in liquid water. Hydrogen bonds are typi cally weak, constantly breaking and reforming, allowing molecules to periodically approach one another. In the solid state of water, the weak hydrogen bonds between water molecules become rigid and form a crystal that keeps the molecules separated and less dense than its liquid form. If ice did not float, it would sink and remain frozen due to the insulating protection of the overlaying water.
1.1.A.2
Water: The Versatile Molecule
➢ In water, electrons are not shared equally in the bonds between hydrogen and oxygen
- Hydrogen atoms have a partial positive charge while oxygen atoms has a partial negative charge
■ Water is polar
➢ Hydrogen bonds (The hydrogen bonds between adjacent polar water molecules result in cohesion, adhesion, and surface tension)
- Weak attractions that result of water’s polarity
■ Positive end of another polar molecule attracted to oxygen negative charge, and vice versa with the hydrogen end
■ Hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom
■ Weak Individually, but strong on a larger scale
- Lends watermany special properties
■ Cohesion
● Tendency for water to stick to water
● Important during transpiration - Water evaporates, pulls other water molecules with it, pulling all the way down from leaves to roots
■ Adhesion
● Tendency of water to stick to other substances
● $\text{Cohesion + Adhesion =}$ capillary action - Allows water to flow up roots/trunks/branches of trees in thin vessels
■ Surface tension
● Results from cohesion of water molecules
● Ex. water striders can sit on top of water without sinking
■ High heat capacity
● Heat Capacity=ability of a substance to resist temperature changes
● Keeps ocean temperatures stable
● Allows organisms to keep constant body temperature, since most life
forms are mostly made up of water
● Heat is absorbed when hydrogen bonds break, released when hydrogen bonds form
■ High heat of vaporization
● Heat a liquid must absorb for 1g to be converted to gas
● Evaporative cooling - As a liquid evaporates, its remaining surface cools
■ How sweat works to cool body down
■ Expansion on freezing
● Lattice structure of ice causes water to expand on freezing
● Allows ice to float on top of lakes in winter - Animal life can live beneath ice
■ Versatility as a solvent
● Solution is a liquid that is a homogenous mix of substances
● Solvent is the dissolving agent of a solution
● Solute is the substance that is dissolved
● Aqueous solution is one where water is the solvent
● Polarity of water allows it to be a versatile solvent - Can form hydrogen bonds easily
● Hydrophobic substances do not dissolve in water, but hydrophilic ones will