The Chemical Context of Life: Study Notes for General Biology

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Chapter 2: The Chemical Context of Life

Section 2.1: Matter, Elements, and Compounds

Understanding the chemical basis of life begins with the study of matter, elements, and compounds. These are the fundamental building blocks of all living and nonliving things.

Matter: Anything that takes up space and has mass.
Element: A substance that cannot be broken down into other substances by chemical reactions. Each element consists of unique atoms.
Compound: A substance consisting of two or more elements combined in a fixed ratio. Compounds have characteristics different from those of their constituent elements.
Example: Table salt (NaCl) is a compound formed from sodium (Na) and chlorine (Cl).

Common Elements of Life

Life depends on a limited number of elements, with only a few making up the majority of living matter.

About 96% of living matter is composed of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).
Other essential elements include phosphorus (P), sulfur (S), calcium (Ca), and potassium (K).
Trace elements are required in minute quantities (e.g., iron, iodine).
Example: Iodine is necessary for thyroid hormone production; deficiency can cause goiter.

Toxic Elements and Tolerance

Some elements are toxic to organisms, but some species have adapted to tolerate or even require them.

Organisms have evolved mechanisms to tolerate or detoxify certain toxic elements.
Example: Some plants can tolerate soils with high concentrations of heavy metals.
Radioactive isotopes can be used to detect contaminants and study biological processes.

Section 2.2: Elemental Properties and Atomic Structure

Atoms are the smallest units of matter that retain the properties of an element. Understanding atomic structure is essential for studying chemical behavior.

Each element consists of a certain type of atom, defined by the number of protons in its nucleus.
Atoms are composed of subatomic particles: protons, neutrons, and electrons.

Subatomic Particles

Proton: Positive charge (+1), located in the nucleus.
Neutron: No charge (neutral), located in the nucleus.
Electron: Negative charge (-1), orbits the nucleus in electron shells.

Atomic Number and Mass Number

Atomic number: Number of protons in the nucleus; defines the element.
Mass number: Sum of protons and neutrons in the nucleus.
Atomic mass: Approximate total mass of an atom, measured in daltons (Da).

Isotopes

Isotopes: Atoms of the same element with different numbers of neutrons.
Some isotopes are radioactive and decay spontaneously, emitting energy.
Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon.

Applications of Isotopes and Radioactivity

Radioactive isotopes are used in biological research and medicine (e.g., PET scans, radiometric dating).
Exposure to high levels of radiation can be hazardous to living organisms.

Potential Energy and Electrons

Electrons have potential energy due to their position relative to the nucleus. The arrangement of electrons in shells determines an atom's chemical properties.

Potential energy: Energy that matter possesses because of its location or structure.
Electrons in outer shells have more potential energy than those closer to the nucleus.
Energy is absorbed or lost as electrons move between shells.

Electron Distribution and Chemical Properties

The chemical behavior of an atom is determined by the distribution of electrons in electron shells, especially the outermost shell (valence shell).

Valence electrons: Electrons in the outermost shell; determine reactivity.
Atoms with full valence shells are chemically inert (e.g., noble gases).
Atoms with incomplete valence shells tend to form chemical bonds.

Section 2.3: Formation and Function of Molecules

Atoms interact to form molecules through chemical bonds, which are essential for the structure and function of biological molecules.

Covalent Bonds

Covalent bond: The sharing of a pair of valence electrons between two atoms.
Single bond: Sharing of one pair of electrons.
Double bond: Sharing of two pairs of electrons.
Nonpolar covalent bond: Electrons are shared equally.
Polar covalent bond: Electrons are shared unequally, leading to partial charges.
Example: In water (H2O), oxygen is more electronegative than hydrogen, resulting in a polar covalent bond.

Electronegativity

Electronegativity: The attraction of an atom for the electrons in a covalent bond.
Differences in electronegativity determine bond polarity.

Ionic Bonds

Ionic bond: Formed when one atom transfers an electron to another, resulting in oppositely charged ions that attract each other.
Cation: Positively charged ion.
Anion: Negatively charged ion.
Example: Sodium chloride (NaCl) forms when sodium donates an electron to chlorine.

Ionic Compounds and Weak Chemical Interactions

Ionic compounds (salts): Compounds formed by ionic bonds; often form crystalline structures.
Weak interactions, such as hydrogen bonds and van der Waals interactions, are crucial for the structure and function of biological molecules.

Hydrogen Bonds

Hydrogen bond: A weak bond formed when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom.
Important in stabilizing the structures of proteins and nucleic acids.

Van der Waals Interactions

Weak attractions between molecules or parts of molecules that result from transient local partial charges.
Significant when many such interactions occur simultaneously.

The Importance of Molecular Shape

The shape of a molecule is determined by the positions of its atoms' orbitals.
Molecular shape is crucial for the function of molecules, especially in biological recognition (e.g., enzyme-substrate interactions).

Section 2.4: Chemical Reactions Make and Break Chemical Bonds

Chemical reactions involve the making and breaking of chemical bonds, leading to changes in the composition of matter.

Reactants: Starting materials in a chemical reaction.
Products: Substances formed as a result of the reaction.
Chemical equilibrium: The point at which the forward and reverse reactions occur at the same rate.
Example: Photosynthesis and respiration are key chemical reactions in biology.

Key Equations

Photosynthesis:

Respiration:

Table: Comparison of Bond Types

Bond Type	Strength	Example	Biological Importance
Covalent	Strong	H2O, CH4	Stable backbone of biological molecules
Ionic	Moderate (weaker in water)	NaCl	Formation of salts, nerve function
Hydrogen	Weak (individually)	Between water molecules	Stabilizes DNA, proteins
Van der Waals	Very weak	Gecko feet adhesion	Temporary interactions, molecular recognition

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