Chapter 2: The Chemical Context of Life – Study Notes

Chapter 2: The Chemical Context of Life

Overview: A Chemical Connection to Biology

Biology is deeply intertwined with chemistry, as living organisms are composed of chemical elements and depend on chemical reactions for life processes. Understanding the chemical context of life is essential for grasping biological structure and function.

• Living organisms are subject to the basic laws of physics and chemistry.

• Biology is a multidisciplinary science, drawing on insights from other sciences.

• Life is organized into a hierarchy of structural levels, from molecules to cells to organisms.

Concept 2.1: Matter consists of chemical elements in pure form and in combinations called compounds

Matter is anything that takes up space and has mass. It is composed of elements, which can combine to form compounds with unique properties.

• Element: A substance that cannot be broken down to other substances by chemical reactions.

• Compound: A substance consisting of two or more elements in a fixed ratio.

• There are 92 naturally occurring elements; about 25 are essential for life.

• Essential elements: Elements required for an organism to survive, grow, and reproduce.

• Trace elements: Required by organisms in minute quantities (e.g., iron, iodine).

Element	Symbol	Role in Life
Oxygen	O	Major component of water and organic molecules
Carbon	C	Forms backbone of organic molecules
Hydrogen	H	Component of water and organic molecules
Nitrogen	N	Component of proteins and nucleic acids
Calcium	Ca	Required for bone formation, signaling
Phosphorus	P	Component of nucleic acids, ATP
Iron	Fe	Required for oxygen transport in blood
Iodine	I	Required for thyroid hormone production

Additional info: The four elements oxygen, carbon, hydrogen, and nitrogen make up about 96% of living matter.

Concept 2.2: An element’s properties depend on the structure of its atoms

Atoms are the smallest units of matter that retain the properties of an element. The structure of an atom determines its chemical behavior.

• Atom: Composed of protons, neutrons, and electrons.

• 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 the nucleus.

• Mass number: Sum of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Radioactive isotopes: Decay spontaneously, giving off particles and energy.

Example: Carbon-12 and Carbon-14 are isotopes of carbon; Carbon-14 is radioactive and used in dating fossils.

Additional info: The atomic mass is measured in daltons (Da).

Electron Configuration and Chemical Behavior

The arrangement of electrons in an atom determines its chemical properties and reactivity.

• Electrons are found in energy levels called electron shells.

• The valence shell is the outermost shell; electrons here are involved in chemical reactions.

• Valence electrons: Electrons in the outermost shell.

• Atoms are most stable when their valence shell is full.

• Orbital: The three-dimensional space where an electron is found 90% of the time.

Shell	Maximum Electrons
First	2
Second	8
Third	18

Example: Oxygen has 8 electrons: 2 in the first shell, 6 in the second shell.

Concept 2.3: The formation and function of molecules depend on chemical bonding between atoms

Atoms interact through chemical bonds to form molecules. The type of bond affects the properties of the resulting compound.

• Covalent bond: Sharing of a pair of valence electrons between atoms.

• Molecule: Two or more atoms held together by covalent bonds.

• Single bond: Sharing one pair of electrons ($H-H$).

• Double bond: Sharing two pairs of electrons ($O=O$).

• Electronegativity: Atom’s attraction for shared electrons.

• Polar covalent bond: Electrons are shared unequally (e.g., $H_2O$).

• Nonpolar covalent bond: Electrons are shared equally (e.g., $O_2$).

• Ionic bond: Transfer of electrons from one atom to another, resulting in charged ions.

• Cation: Positively charged ion.

• Anion: Negatively charged ion.

• Hydrogen bond: Weak bond between a hydrogen atom and an electronegative atom.

• Van der Waals interactions: Weak attractions between molecules or parts of molecules.

Example: Table salt ($NaCl$) is formed by ionic bonding between sodium and chloride ions.

Concept 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: Resulting materials from a chemical reaction.

• Chemical equilibrium: Point at which forward and reverse reactions occur at the same rate.

Example: Photosynthesis is a chemical reaction: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Additional info: Chemical reactions in living organisms are often catalyzed by enzymes.