The Chemical Context of Life: Elements, Atoms, and Chemical Bonds

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The Chemical Context of Life

Introduction to Chemistry in Biology

Biology is fundamentally linked to chemistry, as all living organisms are composed of matter and are governed by the laws of physics and chemistry. Understanding the chemical basis of life is essential for studying biological processes.

Matter: Anything that takes up space and has mass.
Organisms are composed of matter, which exists as elements and compounds.
Elements: Substances that cannot be broken down into other substances by chemical reactions.
Compounds: Substances consisting of two or more elements in a fixed ratio, with properties different from their constituent elements.

Sodium and chlorine combine to form sodium chloride (table salt)

Elements Essential to Life

Major and Trace Elements in Living Organisms

Of the 92 naturally occurring elements, only a small fraction are essential for life. These elements are required for the structure and function of biomolecules.

Essential elements: About 20–25% of elements are required for life.
Major elements: Carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) make up about 96% of living matter.
Other important elements: Calcium (Ca), phosphorus (P), potassium (K), sulfur (S), sodium (Na), chlorine (Cl), and magnesium (Mg) make up most of the remaining 4%.
Trace elements: Required in minute quantities (e.g., iron, zinc, iodine).

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%

Table of elements in the human body

Atoms and Subatomic Particles

Structure of Atoms

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of subatomic particles:

Protons: Positively charged particles found in the nucleus.
Neutrons: Electrically neutral particles found in the nucleus.
Electrons: Negatively charged particles that orbit the nucleus in electron shells.

The number of protons determines the atomic number and the identity of the element. The sum of protons and neutrons gives the mass number.

Isotopes and Atomic Mass

Isotopes: Atoms of the same element with different numbers of neutrons.
Radioactive isotopes: Unstable isotopes that decay, releasing energy and particles.

Electron Configuration and Chemical Properties

Energy Levels and Electron Shells

Electrons occupy specific energy levels or shells around the nucleus. The arrangement of electrons determines the chemical behavior of an atom.

Valence electrons: Electrons in the outermost shell, crucial for chemical bonding.
Atoms with full valence shells are chemically inert (unreactive).

Chemical Bonds

Covalent Bonds

Covalent bonds involve the sharing of pairs of valence electrons between atoms. These bonds can be single, double, or triple, depending on the number of shared electron pairs.

Single bond: Sharing of one pair of electrons.
Double bond: Sharing of two pairs of electrons.
Molecule: Two or more atoms held together by covalent bonds.

Molecular diagrams of H2, O2, H2O, and CH4

Electronegativity and Bond Polarity

Electronegativity is an atom's attraction for electrons in a covalent bond. Differences in electronegativity lead to:

Nonpolar covalent bonds: Electrons are shared equally.
Polar covalent bonds: Electrons are shared unequally, resulting in partial charges (δ+ and δ−) on atoms.

Polarity of water molecule

Ionic Bonds

Ionic bonds form when one atom transfers electrons to another, resulting in oppositely charged ions that attract each other.

Cation: Positively charged ion (loss of electrons).
Anion: Negatively charged ion (gain of electrons).
Ionic compound (salt): Compound formed by ionic bonds, often forming crystalline structures.

Electron transfer and formation of sodium and chloride ions Crystal lattice structure of sodium chloride

Weak Chemical Interactions

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom. These bonds are crucial for the structure and function of biological molecules, such as DNA and proteins.

Hydrogen bond between water and ammonia molecules

Van der Waals Interactions

Van der Waals interactions are weak attractions that occur when transient, uneven electron distributions create temporary dipoles in molecules that are close together. These interactions are important for the three-dimensional structure of large biological molecules.

Molecular Shape and Function

Importance of Molecular Shape

The shape of a molecule is determined by the positions of its atoms' orbitals and is critical for its function. Molecular shape determines how biological molecules recognize and interact with each other, influencing processes such as enzyme-substrate binding and signal transduction.