The Chemistry of Life: Study Guide for General Biology

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Chapter 2: The Chemistry of Life

Chemistry and Matter

Chemistry is the study of matter and the chemical reactions that govern its behavior. Matter is anything that occupies space and has mass, and it exists in three states: solid, liquid, and gas.

Matter: Anything that occupies space and has mass.
States of Matter: Solid, liquid, gas.
Mass: For practical purposes on Earth, mass is equivalent to weight.

Elements and Atoms

Elements are pure substances that cannot be broken down into simpler substances by ordinary chemical means. There are 94 naturally occurring elements on Earth, and they serve as the building blocks of all matter. Most elements are found as compounds, such as CO2 (carbon dioxide) and HCl (hydrochloric acid), while few exist in pure form (e.g., gold).

Element: A substance that cannot be broken down by ordinary chemical means.
Atom: The smallest unit of an element, consisting of a nucleus (protons and neutrons) and electrons orbiting the nucleus.

Atomic Structure

Nucleus: Contains protons (positive charge, mass = 1) and neutrons (neutral, mass = 1).
Electrons: Negatively charged, negligible mass, orbit the nucleus in shells.
Atomic Number: Number of protons; defines the element.
Atomic Mass: Sum of protons and neutrons.
Electron Configuration: Electrons fill shells in order (first shell: 2, second: 8, third: 8, etc.).
Valence Electrons: Electrons in the outermost shell; involved in chemical bonding.

Example: Sodium (Na), atomic number 11: 2 electrons in the first shell, 8 in the second, 1 in the third (valence electron). Carbon (C), atomic number 6: 2 in the first shell, 4 in the second (4 valence electrons).

Types of Chemical Bonds

Atoms combine to form molecules through chemical bonds, which are interactions between valence electrons.

Ionic Bonds: One atom donates an electron to another, forming ions (charged atoms). Example: NaCl (sodium chloride).
Covalent Bonds: Atoms share electrons.
- Nonpolar Covalent Bonds: Equal sharing of electrons (e.g., methane, CH4); hydrophobic.
- Polar Covalent Bonds: Unequal sharing of electrons (e.g., water, H2O); hydrophilic.
Hydrogen Bonds: Weak attractions between a hydrogen atom in one molecule and an electronegative atom (e.g., oxygen) in another. Responsible for many unique properties of water.

Properties of Water Due to Hydrogen Bonds

Cohesion: Water molecules stick to each other (surface tension).
Adhesion: Water molecules stick to other substances.
High Heat Capacity: Water absorbs heat, moderating Earth's temperature.
Ice Floats: Solid water is less dense than liquid water, insulating aquatic life.

Organic Molecules and Macromolecules

Organic molecules contain a carbon backbone and at least one carbon-hydrogen (C-H) bond. They are the basis of life and include four major classes of macromolecules.

Macromolecule	Monomer	Function	Example
Carbohydrates	Monosaccharide (e.g., glucose)	Energy source	Starch (polymer of glucose)
Proteins	Amino acids (20 types)	Structural support, enzymes, antibodies, transport	Enzymes, membrane receptors
Nucleic Acids	Nucleotide (phosphate + 5C sugar + nitrogenous base)	Genetic information storage	DNA, RNA
Lipids	None (not true polymers)	Long-term energy storage, membranes	Triglycerides, phospholipids

Carbohydrates: Composed of carbon and water; main energy source.
Proteins: Made of amino acids; function depends on 3D shape.
Nucleic Acids: DNA and RNA; store and transmit genetic information.
Lipids: Hydrophobic molecules; include fats (triglycerides) for energy storage and phospholipids for cell membranes.

pH and Acidity

The pH scale measures the concentration of hydrogen ions (H+) in a solution, indicating its acidity or basicity. The scale is logarithmic, so each unit represents a tenfold change in H+ concentration.

pH Definition:
Acid: Increases H+ concentration (pH < 7).
Base: Decreases H+ concentration (pH > 7).
Neutral: pH = 7.
Logarithmic Scale: Each pH unit = 10x change in H+. For example, a solution with pH 2 is 1,000,000 times more acidic than one with pH 8.

Properties of Living Things

All living organisms share several fundamental properties:

Reproduction: Ability to produce new individuals.
Growth: Increase in size and complexity.
Response to Stimuli: Reacting to environmental changes.
Homeostasis: Maintaining a stable internal environment.
Energy Requirement: All living things require a source of energy.