AP BIO CHAP 3 UNIT 1

Water and the Fitness of the Environment

Overview: The Molecule That Supports All of Life

Concept 3.1: The polarity of water molecules results in hydrogen bonding

The water molecule is a polar molecule: The opposite ends have opposite charges
Polarity allows water molecules to form hydrogen bonds with each other
- ex. water dripping from fingers

Concept 3.2: Four emergent properties of water contribute to Earth’s fitness for life

Four of water’s properties that facilitate an environment for life are:
- Cohesive behavior (stick together)
- Ability to moderate temperature
- Expansion upon freezing
- Versatility as a solvent

COHESION
Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion
- Cohesion helps the transport of water against gravity in plants
- Adhesion is an attraction between different substances, for example, between water and plant cell walls
- Surface tension is a measure of how hard it is to break the surface of a liquid
  - think of belly flopping, it hurts because of the surface tension of the water
- Surface tension is related to cohesion
  - water molecules are tightly held together (cohesion) = greater surface tension

MODERATION OF TEMP.
Water absorbs heat from warmer air and releases stored heat to cooler air
- Water can absorb or release a large amount of heat with only a slight change in its own temperature

Kinetic energy is the energy of motion
Heat is a measure of the total amount of kinetic energy due to molecular motion

Temperature measures the intensity of heat due to the average kinetic energy of molecules

The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1ºC
The specific heat of water is 1 cal/g/ºC
Water resists changing its temperature because of its high specific heat
Water’s high specific heat can be traced to hydrogen bonding
- Heat is absorbed when hydrogen bonds break
  - boiling water (hydrogen bonds break to give molecules enough kinetic energy, absorbing heat, to go into the gaseous state)
- Heat is released when hydrogen bonds form
  - freezing water (hydrogen bonds form, losing kinetic energy to settle into a more structure state, solid, releasing heat)
The high specific heat of water minimizes temperature fluctuations to within limits that permit life

Evaporation is transformation of a substance from liquid to gas

Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to gas
As a liquid evaporates, its remaining surface cools, a process called evaporative cooling
Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water

Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense

Water reaches its greatest density at 4°C
If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth

Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily
When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell
- Water can also dissolve compounds made of nonionic polar molecules
- Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions

Hydrophilic and Hydrophobic Substances

A hydrophilic substance is one that has an affinity for water (like)
A hydrophobic substance is one that does not have an affinity for water (no like)
Oil molecules are hydrophobic because they have relatively nonpolar bonds
A colloid is a stable suspension of fine particles in a liquid

Concept 3.3: Acidic and basic conditions affect living organisms

A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other:
- The hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H⁺)
- The molecule with the extra proton is now a hydronium ion (H₃O⁺), though it is often represented as H⁺
- The molecule that lost the proton is now a hydroxide ion (OH^–)

Concentrations of H⁺ and OH^– are equal in pure water

Adding certain solutes, called acids and bases, modifies the concentrations of H⁺ and OH^–
Biologists use something called the pH scale to describe whether a solution is acidic or basic (the opposite of acidic)

An acid is any substance that increases the H⁺ concentration of a solution

A base is any substance that reduces the H⁺ concentration of a solution

The pH Scale

In any aqueous solution at 25°C the product of H⁺and OH^– is constant and can be written as: [H⁺][OH^–] = 10^–14
The pH of a solution is defined by the negative logarithm of H⁺concentration, written as: pH = –log [H⁺]
For a neutral aqueous solution : [H⁺] is 10^–7 = –(–7) = 7
- Acidic solutions have pH values less than 7
- Basic solutions have pH values greater than 7
- Most biological fluids have pH values in the range of 6 to 8

Buffers

The internal pH of most living cells must remain close to pH 7
Buffers are substances that minimize changes in concentrations of H⁺ and OH^– in a solution
Most buffers consist of an acid-base pair that reversibly combines with H⁺
- Acid precipitation refers to rain, snow, or fog with a pH lower than 5.6
  - not good