Biochemical Molecules: Organic and Inorganic Compounds, and the Properties of Water

Classification of Biochemical Molecules

The human body and all living organisms are composed of two fundamental classes of biochemical molecules: organic molecules and inorganic molecules.

Inorganic biochemical molecules are defined as chemical compounds that do not contain both carbon and hydrogen within the same molecule. Typically, inorganic molecules are not produced through biological processes, with specific exceptions such as oxides of carbon (for example, carbon dioxide, $CO_2$) and carbonate ions. Inorganic compounds can contain hydrogen alone or carbon alone. For instance, water ($H_2O$) and stomach-generated hydrochloric acid ($HCl$) contain hydrogen atoms but no carbon, whereas carbon dioxide ($CO_2$) contains carbon but no hydrogen atoms.

Organic biological molecules, by contrast, are strictly defined by the presence of both carbon and hydrogen atoms in their molecular structure. Living creatures, including humans, utilize strong covalent bonds to construct organic biochemical compounds. Within the human body, carbon is the second most abundant element, and hydrogen is the third most abundant element. These two primary elements form the fundamental backbones of bodily structures, the nutrients consumed in food, and the essential chemicals that sustain biological operations and living systems.

Inorganic Molecule: Water

Water is one of the most essential inorganic molecules required for the existence and survival of living organisms. Life on Earth cannot exist without access to water.

Water serves two primary structural and functional roles in living systems:

1. It forms the foundational solvent and component of biological cells, comprising between $70\%$ and $95\%$ of total cellular mass.
2. It constitutes the majority of multi-cellular organisms, representing approximately $60\%$ of total human body weight.

Chemical Structure and Molecular Bonding of Water

Water is an inorganic compound composed of two distinct chemical elements: hydrogen and oxygen. The chemical formula for water is $H_2O$. Each individual water molecule consists of two hydrogen atoms bound to a single central oxygen atom.

At the atomic level, each hydrogen atom shares an electron pair with the oxygen atom to form covalent bonds. Within these $O-H$ covalent bonds, oxygen exhibits a significantly higher electronegativity than hydrogen. Consequently, oxygen exerts a stronger pull on the shared electron pairs, causing unequal electron sharing across the bond and creating polar molecular characteristics.

Learning Activities and Conceptual Exercises

Questions & Discussion

Question: Why is water considered a biochemical molecule despite being inorganic?

Response: Water is classified as a biochemical molecule because it is a critical, indispensable constituent of living organisms. It forms up to $70\%$ to $95\%$ of cell mass and $60\%$ of the human body, serving as the essential matrix and medium for life processes.

Activity 3.1: Writing Biochemical Molecules Based on background knowledge, reference texts, and biological research, analyze and categorize:

1. Inorganic compounds
2. Organic compounds
3. Both organic and inorganic molecules contained within living organisms

Activity 3.2: Group Work on Water Properties Form a group of 3 to 4 students to identify and enumerate the chemical and physical properties of water, establishing explicit links between each property and its specific biological importance for supporting life.