BackChapter 2: The Chemical Context of Life – Study Notes
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The Chemical Context of Life
Introduction to Chemistry in Biology
All living organisms are composed of matter, which consists of elements and compounds. Understanding the chemical basis of life is essential for studying biological processes, as the structure and behavior of atoms and molecules determine the properties of cells and organisms.
Matter: Anything that takes up space and has mass.
Element: A substance that cannot be broken down into other substances by chemical means.
Atom: The smallest unit of an element that retains its chemical properties.
Compound: A substance consisting of two or more elements in a fixed ratio, exhibiting emergent properties distinct from its constituent elements.
Elements Essential for Life
Major and Trace Elements
Of the 92 naturally occurring elements, only a small fraction are essential for life. These elements are required for organisms to survive, grow, and reproduce. Trace elements are needed in minute quantities but are still vital for biological functions.
Essential elements: Oxygen, carbon, hydrogen, and nitrogen make up about 96% of living matter.
Trace elements: Required in very small amounts (e.g., iodine for thyroid function in vertebrates).
Deficiency: Lack of essential or trace elements can lead to health problems (e.g., goiter from iodine deficiency).
Element | Symbol | Percentage of Body Mass (including water) |
|---|---|---|
Oxygen | O | 65.0% |
Carbon | C | 18.5% |
Hydrogen | H | 9.5% |
Nitrogen | N | 3.3% |
Calcium | Ca | 1.5% |
Phosphorus | P | 1.0% |
Potassium | K | 0.4% |
Sulfur | S | 0.3% |
Sodium | Na | 0.2% |
Chlorine | Cl | 0.2% |
Magnesium | Mg | 0.1% |
Adaptation to Toxic Elements
Some elements are toxic, but certain organisms have evolved mechanisms to tolerate or even utilize these elements. For example, sunflowers can absorb heavy metals from contaminated soils, a process known as phytoremediation.
Phytoremediation: The use of plants to remove contaminants from the environment.
Application: Sunflowers were used to detoxify soils after environmental disasters.
Atomic Structure and Properties
Subatomic Particles
Atoms are composed of three types of subatomic particles: protons, neutrons, and electrons. The arrangement and number of these particles determine the atom's properties and behavior in chemical reactions.
Proton: Positively charged particle found in the nucleus.
Neutron: Electrically neutral particle found in the nucleus.
Electron: Negatively charged particle found in a cloud around the nucleus.
Atomic Number, Mass Number, and Isotopes
The atomic number is the number of protons in an atom, while the mass number is the sum of protons and neutrons. Isotopes are atoms of the same element with different numbers of neutrons.
Atomic number (Z): Number of protons in the nucleus.
Mass number (A): Number of protons plus neutrons.
Isotopes: Atoms with the same number of protons but different numbers of neutrons.
Radioactive isotopes: Unstable isotopes that decay, emitting radiation; used in dating fossils, tracing atoms, and medical diagnostics.
Medical Applications of Radioactive Isotopes
Radioactive isotopes are used in medical imaging, such as PET scans, to diagnose and monitor diseases.
Energy Levels and Electron Arrangement
Electron Shells and Energy
Electrons occupy specific energy levels or shells around the nucleus. The arrangement of electrons determines how atoms interact and form bonds.
Potential energy: Energy due to position or structure; electrons farther from the nucleus have higher potential energy.
Electrons can move between shells by absorbing or releasing energy.
Chemical Bonds and Interactions
Covalent Bonds
Covalent bonds involve the sharing of electron pairs between atoms. Molecules are formed when two or more atoms are held together by covalent bonds.
Single bond: Sharing of one pair of electrons.
Double bond: Sharing of two pairs of electrons.
Electronegativity: The tendency of an atom to attract electrons in a covalent bond.
Polar covalent bond: Unequal sharing of electrons due to differences in electronegativity.
Ionic Bonds
Ionic bonds are formed when electrons are transferred from one atom to another, resulting in the formation of oppositely charged ions. The electrostatic attraction between these ions forms an ionic bond.
Cation: Positively charged ion (lost electron).
Anion: Negatively charged ion (gained electron).
Ionic compound: A compound formed by ionic bonds, often called a salt (e.g., NaCl).
Weak Chemical Interactions
Weak interactions, such as hydrogen bonds and van der Waals interactions, play crucial roles in the structure and function of biological molecules.
Hydrogen bond: Attraction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.
Van der Waals interactions: Weak attractions due to transient local partial charges; significant when many such interactions occur together.
Chemical Reactions in Biology
Making and Breaking Bonds
Chemical reactions involve the making and breaking of chemical bonds, transforming reactants into products. Photosynthesis is a key example of a biological chemical reaction.
Reactants: Starting substances in a chemical reaction.
Products: Substances formed as a result of the reaction.
Photosynthesis equation:
Water and Life
Hydrogen Bonding and Water Properties
Water's unique properties arise from its ability to form hydrogen bonds. These properties are essential for life and include cohesion, adhesion, surface tension, temperature moderation, and its role as a solvent.
Cohesion: Attraction between water molecules due to hydrogen bonding.
Adhesion: Attraction between water molecules and other substances.
Surface tension: Measure of how difficult it is to stretch or break the surface of a liquid.
Moderation of Temperature by Water
Water can absorb or release large amounts of heat with only slight changes in its own temperature, helping to stabilize environmental and organismal temperatures.
High specific heat: Water resists temperature changes due to hydrogen bonding.
Evaporative cooling: As water evaporates, it removes heat, cooling the surface.
Floating of Ice on Liquid Water
Ice is less dense than liquid water because hydrogen bonds in ice are more ordered, causing it to float. This property insulates aquatic environments, allowing life to persist under ice-covered surfaces.
Water as the Solvent of Life
Water's polarity allows it to dissolve a wide variety of substances, making it the principal solvent in biological systems. Solutions in which water is the solvent are called aqueous solutions.
Solvent: The dissolving agent (water).
Solute: The substance dissolved.
Hydrophilic: Substances with an affinity for water (ionic or polar).
Hydrophobic: Substances that do not interact with water (nonpolar).
Acids, Bases, and pH
pH and Biological Buffers
The pH scale measures the concentration of hydrogen ions in a solution. Acids increase H+ concentration, while bases decrease it. Most biological fluids maintain a pH between 6 and 8, with buffers helping to stabilize pH by accepting or donating H+ ions as needed.
Acid: Substance that increases H+ concentration (pH < 7).
Base: Substance that decreases H+ concentration (pH > 7).
Buffer: Substance that minimizes changes in pH.
Example: The bicarbonate buffer system in blood helps maintain a stable pH necessary for life.
