Chapter 2: The Chemical Context of Life – Study Notes

Chapter 2: The Chemical Context of Life

Learning Objectives

This chapter introduces the chemical principles that underlie biological processes. Students should be able to:

• Describe matter, chemical elements, and chemical compounds.

• Explain how the structure of an element's atoms determines its properties.

• Identify and describe types of strong and weak chemical bonds, and use examples to show how bonds affect molecular shape.

Overview: A Chemical Connection to Biology

Biology and Chemistry

Biology is deeply connected to the laws of physics and chemistry. Living organisms are composed of matter and are subject to chemical principles.

• Biology is a multidisciplinary science, integrating knowledge from chemistry and physics.

• Life is organized into a hierarchy of structural levels:

  • Atoms → Molecules → Cells

  • Emergent properties arise at each level of organization.

  • The transition from molecules to cells marks the boundary between nonlife and life.

CONCEPT 2.1: Matter Consists of Chemical Elements in Pure Form and in Combinations Called Compounds

Matter, Elements, and Compounds

All living organisms are composed of matter, which is anything that takes up space and has mass.

• 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. Compounds have emergent properties different from those of their constituent elements.

Example: Sodium Chloride (NaCl)

• Table salt (NaCl) is a compound formed from sodium (a metal) and chlorine (a gas).

• When combined, they form an edible compound with properties distinct from the individual elements.

• This illustrates emergent properties—new characteristics that arise when elements combine.

The Elements of Life

Only a subset of the 92 naturally occurring elements are essential for life.

• About 20–25% are essential elements.

• Carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) make up 96% of living matter.

• The remaining 4% includes calcium (Ca), phosphorus (P), potassium (K), and sulfur (S).

• Trace elements are required in minute quantities (e.g., iron (Fe), iodine (I)).

• Some trace elements are required by all organisms; others are species-specific (e.g., iodine for vertebrates).

Table: Elements in the Human Body

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%

Trace elements (less than 0.01%): Boron (B), chromium (Cr), cobalt (Co), copper (Cu), fluorine (F), iodine (I), iron (Fe), manganese (Mn), molybdenum (Mo), selenium (Se), silicon (Si), tin (Sn), vanadium (V), zinc (Zn)

CONCEPT 2.2: An Element's Properties Depend on the Structure of Its Atoms

Atomic Structure

Each element consists of unique atoms, which are the smallest units of matter retaining the properties of the element.

• Atoms are composed of subatomic particles:

  • Neutrons: No electrical charge

  • Protons: Positive charge; located in the atomic nucleus

  • Electrons: Negative charge; form a cloud around the nucleus

• Neutron and proton mass are nearly identical and measured in daltons.

• Electrons are much smaller and their mass is usually ignored in atomic mass calculations.

Atomic Models

• Electrons are visualized as a cloud of negative charge due to their motion around the nucleus.

• Simplified models may show electrons as discrete spheres orbiting the nucleus.

Atomic Number and Atomic Mass

• Atomic number: Number of protons in the nucleus (also equals number of electrons in a neutral atom).

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

• Atomic mass: Approximate total mass of the atom (close to mass number).

Formula:

• $\text{Mass number} = \text{Number of protons} + \text{Number of neutrons}$

• $\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$

Example: Sodium (Na)

• Atomic number = 11 (protons)

• Mass number = 23

• Number of neutrons = 23 - 11 = 12

Additional info:

• Isotopes are atoms of the same element with different numbers of neutrons.

• Radioactive isotopes decay spontaneously, emitting particles and energy.