BackChapter 2: The Chemical Basis of Life – Structured Study Notes
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Elements, Atoms, and Compounds
Organisms Are Composed of Elements, Usually Combined into Compounds
Living organisms are made of matter, which consists of chemical elements. About 25 elements are essential for human life, with four elements—oxygen, carbon, hydrogen, and nitrogen—making up about 96% of the weight of most living organisms. A compound is a substance consisting of two or more different elements in a fixed ratio, often exhibiting emergent properties not found in the individual elements.
Element: A pure substance consisting of only one type of atom.
Compound: A substance formed from two or more elements chemically combined in a fixed ratio.
Emergent Properties: New characteristics that arise when elements combine to form compounds.
Example: Table salt (NaCl) is formed from sodium (a reactive metal) and chlorine (a toxic gas), but together they form a safe, edible compound.
Additional info: The four main elements in the human body are oxygen, carbon, hydrogen, and nitrogen. Trace elements, though present in small amounts, are vital for health.
Trace Elements Are Common Additives to Food and Water
Trace elements are required in minute quantities to prevent disease and maintain health. For example, fluoride is added to water to reduce tooth decay, and other chemicals are added to food for preservation, nutrition, and appearance.
Trace Element: An element required by organisms in small amounts (e.g., iron, iodine, fluoride).
Application: Fluoride in water prevents dental cavities.
Atoms: Structure and Properties
Atoms Consist of Protons, Neutrons, and Electrons
An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of three subatomic particles: protons (positive charge), neutrons (no charge), and electrons (negative charge). Protons and neutrons are located in the nucleus, while electrons orbit the nucleus. The atomic number is defined by the number of protons.
Proton: Positively charged particle in the nucleus.
Neutron: Neutral particle in the nucleus.
Electron: Negatively charged particle orbiting the nucleus.
Atomic Number: Number of protons in an atom.
Mass Number: Sum of protons and neutrons.
Isotopes and Radioactive Isotopes
Isotopes are variants of an element with the same number of protons but different numbers of neutrons. Some isotopes are radioactive, meaning their nuclei decay over time, emitting radiation. Radioactive isotopes are used as tracers in biological research and medical imaging, but they can also pose health risks.
Isotope: Atoms of the same element with different neutron numbers.
Radioactive Isotope: An isotope whose nucleus decays, releasing energy.
Application: PET scans use radioactive isotopes to monitor brain activity.
Isotope | Protons | Neutrons | Electrons | Mass Number |
|---|---|---|---|---|
Carbon-12 | 6 | 6 | 6 | 12 |
Carbon-13 | 6 | 7 | 6 | 13 |
Carbon-14 | 6 | 8 | 6 | 14 |
Chemical Bonds
The Distribution of Electrons Determines an Atom’s Chemical Properties
Electrons occupy specific shells around the nucleus. Atoms with incomplete outer shells tend to interact with other atoms, forming chemical bonds by sharing, gaining, or losing electrons. The main types of chemical bonds are ionic, covalent, and hydrogen bonds.
Electron Shell: Energy levels where electrons reside.
Chemical Bond: Attraction between atoms due to electron interactions.
Covalent Bonds
In a covalent bond, atoms share pairs of electrons. If electrons are shared equally, the bond is nonpolar; if unequally, the bond is polar. Water is an example of a polar covalent molecule.
Covalent Bond: Bond formed by sharing electrons.
Nonpolar Covalent Bond: Electrons shared equally.
Polar Covalent Bond: Electrons shared unequally, creating partial charges.
Ionic Bonds
An ionic bond forms when one atom transfers an electron to another, resulting in oppositely charged ions that attract each other. Ionic compounds, such as salt (NaCl), are formed by these bonds.
Ion: Atom or molecule with a net electrical charge.
Ionic Bond: Attraction between oppositely charged ions.
Example: Sodium donates an electron to chlorine, forming Na+ and Cl-.
Hydrogen Bonds
Hydrogen bonds are weak attractions between the slightly positive hydrogen atom of one molecule and the slightly negative atom (often oxygen or nitrogen) of another. These bonds are crucial in water and biological molecules.
Hydrogen Bond: Weak bond between polar molecules.
Polar Molecule: Molecule with uneven charge distribution.
Chemical Reactions
Chemical reactions involve breaking and forming chemical bonds, converting reactants into products. Matter is not created or destroyed, only rearranged.
Reactant: Starting substance in a chemical reaction.
Product: Substance formed as a result of a chemical reaction.
Example: Formation of water from hydrogen and oxygen.
Water’s Life-Supporting Properties
Hydrogen Bonds Make Liquid Water Cohesive
Water molecules stick together due to hydrogen bonding, a property called cohesion. Adhesion is the clinging of water to other substances. Cohesion contributes to surface tension, allowing small insects to walk on water.
Cohesion: Attraction between molecules of the same substance.
Adhesion: Attraction between different substances.
Surface Tension: Difficulty in breaking the surface of a liquid.
Water’s Hydrogen Bonds Moderate Temperature
Water absorbs and releases heat slowly due to hydrogen bonding, moderating temperature changes. Evaporative cooling occurs when the surface of a liquid cools as molecules evaporate.
Thermal Energy: Energy from random movement of molecules.
Heat: Transfer of thermal energy.
Evaporative Cooling: Cooling effect as molecules evaporate.
Ice Floats Because It Is Less Dense than Liquid Water
Water is less dense as a solid than as a liquid due to hydrogen bonding. When water freezes, molecules form stable hydrogen bonds, creating a less dense structure. This allows ice to float, which is crucial for aquatic life.
Density: Mass per unit volume.
Hydrogen Bonding: Creates open structure in ice.
Water is the Solvent of Life
Water’s polarity makes it an excellent solvent, dissolving many substances to form aqueous solutions. Polar or charged solutes dissolve as water molecules surround them.
Solution: Uniform mixture of substances.
Solvent: Dissolving agent (water).
Solute: Substance dissolved.
The Chemistry of Life Is Sensitive to Acidic and Basic Conditions
The pH scale measures how acidic or basic a solution is. Acids increase hydrogen ion (H+) concentration, while bases increase hydroxide ion (OH-) concentration. Buffers help maintain stable pH in biological systems.
Acid: Substance that increases H+ concentration.
Base: Substance that increases OH- concentration.
Buffer: Minimizes changes in pH.
pH Equation:
Impact of Rising CO2 on Coral Reefs
Ocean Acidification and Coral Reefs
Rising atmospheric CO2 from fossil fuel combustion is absorbed by oceans, lowering pH through ocean acidification. Extra hydrogen ions combine with carbonate ions, reducing their availability for coral and shell-building organisms, slowing calcification rates and threatening reef ecosystems.
Ocean Acidification: Lowering of ocean pH due to CO2 absorption.
Calcification: Process by which corals build calcium carbonate skeletons.
Equation:
Evolution Connection: Water and Extraterrestrial Life
The Search for Water in the Universe
The unique properties of water are essential for life on Earth and are central to the search for life on other planets. Water’s ability to support life is a key factor in astrobiology.
Emergent Properties: Water’s cohesion, temperature moderation, solvent abilities, and density changes are crucial for life.
Astrobiology: The study of life in the universe, focusing on water as a prerequisite.
Summary Table: Types of Chemical Bonds
Bond Type | Mechanism | Example | Strength |
|---|---|---|---|
Covalent | Electron sharing | H2O, CH4 | Strong |
Ionic | Electron transfer | NaCl | Strong (in dry conditions) |
Hydrogen | Attraction between polar molecules | Water, DNA | Weak |
Additional info: Hydrogen bonds, though weak individually, are collectively strong and essential for the structure of biological molecules.
