College Board Review Notes

Water: Unique Properties

  • Definition of Water: Defined as polar, meaning it has an unequal sharing of electrons.

    • Polarity: The term "polar" indicates that in water, electrons are shared unequally between the hydrogen and oxygen atoms.

    • Reason for Unequal Sharing: Some elements, like oxygen, attract electrons more strongly than hydrogen due to having more protons than electrons, which results in a higher electronegativity.

  • Electronegativity:

    • Defined as the tendency of an atom to attract electrons. Oxygen is noted as one of the most electronegative elements.

    • The uneven distribution of charge creates a slight negative charge around the oxygen and a slight positive charge around the hydrogens, leading to unique properties of water.

  • Consequences: These properties are critical for life; no life forms are known to exist without water.

    • Space Exploration Thought: Evidence of water on Mars suggests the possibility of past life due to similar conditions necessary for life.

Hydrogen Bonding

  • Definition: Hydrogen bonds form when the slightly positive hydrogen from one water molecule is attracted to the slightly negative oxygen of another water molecule.

  • Properties of Water due to Hydrogen Bonds:

    • Unique properties such as cohesion and adhesion.

    • Water models could be utilized to demonstrate how water molecules interact.

Unique Properties of Water

Cohesion and Adhesion:

  • Cohesion:

    • Definition: The tendency of water molecules to stick to each other due to hydrogen bonding.

    • Example: The surface tension that allows some insects to walk on water.

  • Adhesion:

    • Definition: The tendency of water molecules to stick to other substances.

    • Example: Water sticking to the walls of a shower demonstrating adhesion.

Movement of Water in Plants:

  • The movement of water up a tree relies on cohesion and adhesion. As water evaporates from leaves, it pulls additional water upward from below through a process known as negative pressure.

    • Example: Drinking through a straw involves applying negative pressure to draw the liquid upward.

High Specific Heat and High Heat of Vaporization:

  • High Specific Heat:

    • Definition: Water's ability to resist changes in temperature due to the hydrogen bonds that require significant energy to break.

    • Example: Lakes take longer to warm than the surrounding air, maintaining stable environments for aquatic life.

    • Important for maintaining the body temperature in organisms, due to the high water content in living beings.

  • High Heat of Vaporization:

    • Definition: Refers to the energy required for water to evaporate. The fastest-moving molecules escape first, cooling the remaining liquid.

    • Example: Sweating cools the body by allowing the fastest-moving water molecules to evaporate.

    • Implications for temperature regulation in animals.

Low Density of Ice:

  • Water is unique as it expands when it freezes, meaning ice is less dense than liquid water.

    • This allows ice to float on water, creating an insulating effect for aquatic life in cold environments.

Water as a Solvent:

  • Water is known as a universal solvent because it can dissolve a wide range of substances, particularly those that are ionic or polar.

  • Nonpolar substances like oil do not dissolve in water, which demonstrates the principle of "like dissolves like."

Acids and Bases

  • Dissociation of Water:

    • A very small fraction of water molecules dissociate into hydroxide ( ext{OH}^- ) and hydronium ({H_3O}^+) ions, an equilibrium condition.

  • pH Scale:

    • Neutral solutions have equal concentrations of hydroxide and hydronium ions (pH = 7).

    • Acids: Solutions with more hydronium ions are acidic (pH < 7).

    • Bases: Solutions with more hydroxide ions are basic (pH > 7).

    • Importance: pH levels affect enzyme activity and biological processes.

Chemical Composition of Life

Key Elements:

  • The fundamental elements in life are typically summarized in the acronym CHONPS, which stands for:

    • C - Carbon

    • H - Hydrogen

    • O - Oxygen

    • N - Nitrogen

    • P - Phosphorus

    • S - Sulfur

Macromolecules:

  1. Carbohydrates (CHO):

    • Composition: Consist primarily of carbon, hydrogen, and oxygen (with a hydrogen to oxygen ratio of 2:1, similar to water).

    • Function: Provide energy; examples include simple sugars and polysaccharides like starch.

    • Cellulose, a type of fiber, cannot be digested by humans, but is crucial for digestive health.

  2. Lipids (CHO + sometimes P):

    • Composition: Made primarily of carbon, hydrogen, and oxygen; phospholipids contain phosphorus.

    • Function: Store energy long-term; include fats, oils, and steroids which play roles in cell membrane structure.

    • Difference between Saturated vs. Unsaturated: Saturated fats are solid at room temperature due to single bonds, while unsaturated fats are liquid due to double bonds which create kinks in the structure.

  3. Proteins (CHON + sometimes S):

    • Composition: Consist of amino acids, containing carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.

    • Function: Structural components and enzymes that perform biological functions. The sequence of amino acids determines the protein's structure and function.

    • Structures include:

      • Primary Structure: Sequence of amino acids.

      • Secondary Structure: Alpha helices and beta-pleated sheets formed through hydrogen bonding.

      • Tertiary Structure: The overall 3D shape of the protein due to interactions among R groups of amino acids.

      • Quaternary Structure: Multiple polypeptide chains bonded together.

  4. Nucleic Acids (CHONP):

    • Composition: Include DNA and RNA, consisting of carbon, hydrogen, oxygen, nitrogen, and phosphorus.

    • Function: Store and transmit genetic information.

    • Difference between RNA (single-stranded, contains uracil) and DNA (double-stranded, contains thymine).

Chemical Reactions:

  • Dehydration Synthesis: A reaction that builds larger molecules by removing water to form bonds (important in constructing macromolecules).

  • Hydrolysis: The process of breaking down macromolecules by adding water, facilitated by enzymes.

Conclusion

  • Understanding these principles is crucial as they not only form the basis of biological molecules but also their functions are integral to life processes. The properties of water play a particularly significant role in supporting life as we know it, affecting biological interactions and environmental conditions.