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.
Elements and Compounds
Elements
Element: A substance that cannot be broken down into other substances by chemical means.
There are 92 naturally occurring elements, but only 20-25% are essential for life.
Essential elements are required for an organism to survive, grow, and reproduce.
Trace elements are required in minute quantities (e.g., iodine for thyroid function in vertebrates).
Compounds
Compound: A substance consisting of two or more elements in a fixed ratio.
Compounds have emergent properties that differ from those of their constituent elements.
Evolution of Tolerance to Toxic Elements
Some elements are toxic, but certain species have adapted to survive in toxic environments.
Phytoremediation: The use of plants (e.g., sunflowers) to absorb and detoxify heavy metals from contaminated soils.
Example: Sunflowers were used to clean up soils after Hurricane Katrina.
Atomic Structure and Properties
Subatomic Particles
Atoms are composed of subatomic particles: protons (positive), neutrons (neutral), and electrons (negative).
The atomic nucleus contains protons and neutrons; electrons form a cloud around the nucleus.
Atomic Number and Atomic Mass
Atomic number: Number of protons in the nucleus (defines the element).
Mass number: Sum of protons and neutrons in the nucleus.
Atomic mass is approximately equal to the mass number.
Isotopes
Atoms of the same element with different numbers of neutrons are called isotopes.
Radioactive isotopes have unstable nuclei that decay, emitting particles and energy.
Applications: Dating fossils, tracing metabolic pathways, medical diagnostics (e.g., PET scans).
Hazards: Radiation can damage cellular molecules; radioactive fallout is a serious environmental threat.
Energy and Electrons
Potential Energy and Electron Shells
Energy: The capacity to cause change.
Potential energy: Energy due to location or structure; electrons have potential energy based on their distance from the nucleus.
Electrons occupy electron shells; they can move to higher or lower shells by absorbing or releasing energy.
Chemical Bonds and Interactions
Valence Electrons and Reactivity
The chemical behavior of an atom is determined by the number of electrons in its outermost (valence) shell.
Atoms with full valence shells are inert; those with incomplete shells are reactive and form bonds.
Covalent Bonds
Covalent bond: Sharing of a pair of valence electrons between two atoms.
A single bond involves one pair of shared electrons; a double bond involves two pairs.
Molecules are formed by atoms held together by covalent bonds.
Electronegativity and Covalent Bonds
Electronegativity: The tendency of an atom to attract electrons in a covalent bond.
Unequal sharing of electrons leads to polar covalent bonds (e.g., in water).
Ionic Bonds
Formed when one atom transfers electrons to another, creating oppositely charged ions (cations and anions).
Ionic bond: Attraction between a cation and an anion.
Ionic compounds are called salts.
Weak Chemical Interactions
Many biological molecules are stabilized by weak interactions, including hydrogen bonds and van der Waals interactions.
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 charges; significant when many occur together (e.g., gecko feet adhesion).
Chemical Reactions
Making and Breaking Bonds
Chemical reactions involve the making and breaking of chemical bonds.
Reactants: Starting substances; Products: Ending substances.
Example: Photosynthesis –
Water and Life
Hydrogen Bonding in Water
Water is a polar molecule; hydrogen bonds hold water molecules together (cohesion).
Properties of water essential for life: cohesion, temperature moderation, expansion upon freezing, and versatility as a solvent.
Cohesion and Adhesion
Cohesion: Water molecules stick together due to hydrogen bonding.
Adhesion: Water molecules stick to other substances.
Both are critical for water transport in plants against gravity (with transpiration).
Surface Tension
Surface tension: Measure of how difficult it is to stretch or break the surface of a liquid.
Water has high surface tension due to hydrogen bonding at the air-water interface.
Moderation of Temperature by Water
Water absorbs heat from warmer air and releases heat to cooler air, moderating temperature changes.
Water's high specific heat allows it to buffer temperature fluctuations in organisms and environments.
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 life during cold periods.
Water as the Solvent of Life
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.
Hydrophilic and Hydrophobic Substances
Hydrophilic: Substances with an affinity for water (ionic or polar).
Hydrophobic: Substances that do not interact with water (nonpolar, e.g., oils).
Acids, Bases, and pH
Definitions and Biological Importance
Acids: Increase H+ concentration in solution (pH < 7).
Bases: Decrease H+ concentration (pH > 7).
Most biological fluids have pH between 6 and 8; internal pH of cells is close to 7.
Buffers: Substances that minimize changes in H+ or OH- concentration, maintaining stable pH in biological systems.
