BackChapter 2: The Chemical Context of Life – Study Notes
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Chemistry of Life: Creating Compounds
Elements, Atoms, and Compounds
All living organisms are composed of matter, which consists of elements and compounds. Understanding the basic chemical building blocks is essential for studying biological processes.
Element: A substance that cannot be broken down or converted 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 of Life
Essential and Trace Elements
Only a fraction of the 92 naturally occurring elements are essential for life. These elements are required for survival, growth, and reproduction. Trace elements are needed in minute quantities but are vital for proper physiological function.
Essential elements: 20-25% of natural elements are required for life.
Trace elements: Needed in small amounts; e.g., iodine is necessary for thyroid function, and its deficiency can cause goiter.
Evolution of Tolerance to Toxic Elements
Phytoremediation
Some organisms have evolved mechanisms to tolerate or detoxify toxic elements. Sunflowers, for example, can absorb heavy metals from contaminated soils, a process known as phytoremediation.
Phytoremediation: The use of plants to remove contaminants from the environment.
Example: Sunflowers were used to detoxify soils after Hurricane Katrina.
Element Properties Depend on Atomic Structure
Subatomic Particles
The properties of elements are determined by the structure of their atoms, which are composed of subatomic particles.
Neutrons: No electrical charge.
Protons: Positive charge.
Electrons: Negative charge.
Atomic nucleus: Contains protons and neutrons; electrons form a cloud around the nucleus.
Atomic Number and Atomic Mass
Defining Atoms
Atoms of different elements are distinguished by their number of protons, neutrons, and electrons.
Atomic number: Number of protons in the nucleus.
Mass number: Sum of protons and neutrons in the nucleus.
Atomic mass: Approximated by the mass number.
Isotopes
Variation in Neutron Number
Isotopes are atomic forms of an element with different numbers of neutrons. Some isotopes are radioactive and have important biological and medical applications.
Isotopes: Atoms of the same element with varying neutron numbers.
Radioactive isotopes: Nuclei decay spontaneously, emitting particles and energy.
Applications: Dating fossils, tracing metabolic processes, medical diagnostics (e.g., PET scans).
Energy and Electrons
Electron Shells and Potential Energy
Electrons possess potential energy based on their distance from the nucleus. Their arrangement in shells influences chemical reactivity.
Energy: Capacity to cause change.
Potential energy: Energy due to position or structure.
Electron shells: Electrons occupy shells with varying energy levels; movement between shells involves energy absorption or release.
Electrons & Chemical Bonds
Valence Shells and Reactivity
The chemical behavior of atoms is determined by the electrons in their outermost shell (valence shell). Atoms with incomplete valence shells tend to form chemical bonds.
Inert atoms: Completed valence shells; unreactive.
Reactive atoms: Incomplete valence shells; form bonds by sharing or transferring electrons.
Covalent Bonds
Bond Formation by Electron Sharing
Covalent bonds involve the sharing of valence electrons between atoms, resulting in the formation of molecules.
Covalent bond: Sharing of a pair of electrons.
Single bond: One pair of shared electrons.
Double bond: Two pairs of shared electrons.
Electronegativity & Covalent Bonds
Polarity in Molecules
Electronegativity is the tendency of an atom to attract electrons. Differences in electronegativity lead to polar covalent bonds, where electrons are shared unequally.
Electronegativity: Measure of an atom's ability to attract electrons.
Polar covalent bond: Unequal sharing of electrons, resulting in partial charges.
Ionic Bonds
Electron Transfer and Ion Formation
Ionic bonds are formed when electrons are transferred from one atom to another, resulting in oppositely charged ions (cations and anions) that attract each other.
Cation: Positively charged ion (lost electron).
Anion: Negatively charged ion (gained electron).
Ionic bond: Attraction between cation and anion.
Ionic compounds: Also called salts.
Weak Chemical Interactions
Hydrogen Bonds and Van der Waals Interactions
Weak chemical interactions, such as hydrogen bonds and van der Waals forces, play crucial roles in maintaining 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 asymmetrical electron distributions.
Example: Geckos use van der Waals interactions to adhere to surfaces.
Chemical Reactions
Making and Breaking Bonds
Chemical reactions involve the making and breaking of covalent bonds, transforming reactants into products. Photosynthesis is a key biological reaction.
Reactants: Starting molecules.
Products: Final molecules.
Photosynthesis equation:
Hydrogen Bonding & Water
Properties of Water
Water's unique properties arise from hydrogen bonding, which is essential for life. These properties include cohesion, adhesion, surface tension, temperature moderation, and the ability to float ice.
Cohesion: Water molecules stick together due to hydrogen bonding.
Adhesion: Water molecules cling to other substances.
Surface tension: Difficulty in breaking the surface of water.
Temperature moderation: Water absorbs/releases heat with minimal temperature change.
Floating ice: Ice is less dense than liquid water, allowing it to float.
Cohesion and Adhesion in Water
Transport in Plants
Cohesion and adhesion are vital for the transport of water and nutrients in plants, especially against gravity.
Cohesion: Hydrogen bonding keeps water molecules together.
Adhesion: Water adheres to cell walls, countering gravity.
Surface Tension
Biological Importance
Surface tension is a result of cohesion at the air-water interface, giving water an unusually high surface tension. This property allows certain organisms to walk on water.
Surface tension: Related to cohesion; important for aquatic life.
Moderation of Temperature by Water
Heat Absorption and Release
Water's ability to absorb and release heat helps regulate temperature in organisms and environments, contributing to climate stability.
High specific heat: Water can absorb/release large amounts of heat with little temperature change.
Water: The Solvent of Life
Solutions, Solvents, and Solutes
Water is an excellent solvent, capable of dissolving a wide range of substances due to its polarity. Biological fluids are often aqueous solutions.
Solution: Homogeneous mixture of substances.
Solvent: Dissolving agent (water in biological systems).
Solute: Substance dissolved in the solvent.
Aqueous solution: Solution where water is the solvent.
Water as a Solvent
Hydrophilic and Hydrophobic Substances
Water dissolves ionic and polar substances (hydrophilic), but not nonpolar substances (hydrophobic). This distinction is important for biological molecules and cell membranes.
Hydrophilic: Affinity for water; dissolves easily.
Hydrophobic: Repels water; does not dissolve (e.g., oils).
Acids, Bases, & pH
Regulation of Hydrogen Ion Concentration
Acids and bases alter the concentration of hydrogen ions in water, affecting pH and biological processes. Buffers help maintain stable pH in living systems.
Acids: Increase H+ concentration (pH < 7).
Bases: Decrease H+ concentration (pH > 7).
Buffers: Minimize changes in H+ or OH- concentration.
Biological fluids: pH typically ranges from 6 to 8; internal cell pH is close to 7.
