Chapter 2: The Chemical Basis of Life – Elements, Atoms, and Compounds

Elements, Atoms, and Compounds

Introduction to the Chemical Basis of Life

All living organisms are composed of matter, which itself is made up of chemical elements. Understanding the nature of elements, atoms, and compounds is fundamental to the study of biology, as these components form the basis of all biological molecules and processes.

• Element: A pure substance that cannot be broken down into other substances by chemical means. Each element is defined by its number of protons.

• Atom: The smallest unit of an element that retains the properties of that element.

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

Example: Water (H2O) is a compound made from the elements hydrogen and oxygen in a 2:1 ratio.

Essential Elements in Living Organisms

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

• Major Elements: Oxygen (O), Carbon (C), Hydrogen (H), and Nitrogen (N) make up about 96% of the mass of most living organisms.

• Other Important Elements: Calcium (Ca), Phosphorus (P), Potassium (K), Sulfur (S), Sodium (Na), Chlorine (Cl), Magnesium (Mg), and trace elements.

• Trace Elements: Elements required in minute quantities, such as iron (Fe), iodine (I), and zinc (Zn), are vital for specific biological functions.

Example: Iodine is a trace element added to table salt to prevent thyroid disease.

Classification of Elements in the Human Body

The human body contains a variety of elements, each contributing to different physiological roles. The following table summarizes the major and trace elements found in the human body:

Element	Symbol	Role in the Body
Oxygen	O	Component of water and organic molecules; required for cellular respiration
Carbon	C	Backbone of organic molecules
Hydrogen	H	Component of water and organic molecules
Nitrogen	N	Component of proteins and nucleic acids
Calcium	Ca	Bone formation, nerve signaling
Phosphorus	P	Component of nucleic acids and ATP
Potassium	K	Nerve function, fluid balance
Sulfur	S	Component of some amino acids
Sodium	Na	Nerve function, fluid balance
Chlorine	Cl	Fluid balance
Magnesium	Mg	Enzyme cofactor
Trace Elements	Fe, I, Zn, etc.	Various specialized functions

Additional info: Trace elements, though present in very small amounts, are critical for health. For example, iron is essential for oxygen transport in blood, and zinc is important for immune function.