AP Biology Unit 1

Essential elements

Carbon, oxygen, and hydrogen are the most prevalent elements in biological molecules.

• Additionally, sulfur is used for proteins, phosphorus is used in phospholipids and nucleic acids, and nitrogen is used in nucleic acids and proteins.

CHOPN(S)

Polar covalent bond

Happens when electrons are not shared equally.

• In water, the oxygen is more electronegative so it pulls electrons closer making it partially negative. Hydrogen is partially positive.

Hydrogen bond

Partially positive hydrogen in one polar covalent molecule will be attracted to an electronegative atom (O, N, F) in another polar covalent bond.

• Happens between water molecules

Polarity

Water is polar because of the uneven sharing of electrons between oxygen and hydrogens.

Cohesion

Attraction of water molecules to each other

• Enables surface tension: Surface H₂O molecules experience greater inward pull due to unbalanced forces. This causes them to form stronger bonds with their neighbours.

Adhesion

Attraction of water molecules to other molecules that are polar or have a charge

Capillary action

Movement of water against gravity in narrow spaces

• Occurs when forces of adhesion are stronger than cohesion

• In plants water moves from roots to leaves through the xylem tubes

High specific heat

Water resists changes in temperature due to hydrogen bonds.

• Water can absorb/release large amounts of heat with minimal temperature changes.

• Large amounts of heat must be absorbed to break hydrogen bonds, after which the molecules can start gaining kinetic energy.

• Energy is released when hydrogen bonds form.

Important for moderating air temperature: absorbs heat making surrounding cooler.

Stabilizes ocean temperature.

High heat of vaporization

Water absorbs a large amount of heat before evaporating.

• Evaporative cooling: the surface water molecules evaporate from gets cooler.

• Moderates earths climate, stabilizes temperature in aquatic environments, prevents terrestrial organisms from overheating (sweating), and leaves from becoming too warm in the sun.

Density

As water freezes, the molecules expand forming a crystalline structure. This makes ice less dense than water.

• Allows marine life to live under ice sheets

Solvent

Water is a versatile solvent due to its polar nature. Water molecules are attracted to other polar molecules.

• Oxygen and hydrogens in water form hydrogen bonds with the solutes.