Chapter 2: The Chemical Context of Life – General Biology Study Notes

Chapter 2: The Chemical Context of Life

Learning Objectives

This chapter introduces the chemical principles essential for understanding 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 illustrate 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. Understanding life requires knowledge of chemical principles, as living organisms are composed of chemical substances and undergo chemical reactions.

• 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

Elements and Compounds

All organisms are composed of matter, which is anything that takes up space and has mass. Matter is made up of elements and compounds.

• 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 characteristics (emergent properties) different from those of their constituent elements.

Example: Sodium (Na) is a metal, and chlorine (Cl) is a gas. When combined, they form sodium chloride (NaCl), or table salt, which is edible. This illustrates emergent properties.

The Elements of Life

Of the 92 natural elements, about 20–25% are essential for life. Four elements—carbon, hydrogen, oxygen, and nitrogen—make up 96% of living matter. The remaining 4% consists mainly of calcium, phosphorus, potassium, and sulfur.

• Trace elements are required in minute quantities (e.g., iron for all organisms, iodine for vertebrates).

• Iodine is necessary for thyroid function; a daily intake of 0.15 mg is sufficient for humans.

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. An atom is the smallest unit of matter that retains the properties of an element.

• Neutrons: No electrical charge.

• Protons: Positive charge; together with neutrons, form 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.

Additional info: The arrangement of these subatomic particles determines the chemical behavior of the atom.