3.1: Organic Molecules

Learning Objectives

• Identify common elements and structures found in organic molecules
• Distinguish between macronutrients and micronutrients (trace elements)
• Differentiate inorganic from organic molecules

Biochemistry is the discipline that studies the chemistry of life, and its objective is to explain form and function based on chemical principles. Organic chemistry is the discipline devoted to the study of carbon-based chemistry, which is the foundation for the study of biomolecules and the discipline of biochemistry. Both biochemistry and organic chemistry are based on the concepts of general chemistry, some of which are presented in Appendix A.

Elements in Living Cells

The most abundant element in cells is hydrogen (H), followed by carbon (C), oxygen (O), nitrogen (N), phosphorous (P), and sulfur (S). We call these elements macronutrients, and they account for about 99% of the dry weight of cells. Some elements, such as sodium (Na), potassium (K), magnesium (Mg), zinc (Zn), iron (Fe), calcium (Ca), molybdenum (Mo), copper (Cu), cobalt (Co), manganese (Mn), or vanadium (Va), are required by some cells in very small amounts and are called micronutrients or trace elements. All of these elements are essential to the function of many biochemical reactions, and, therefore, are essential to life.

The four most abundant elements in living matter (C, N, O, and H) have low atomic numbers and are thus light elements capable of forming strong bonds with other atoms to produce molecules (Figure \(\PageIndex{1}\)). Carbon forms four chemical bonds, whereas nitrogen forms three, oxygen forms two, and hydrogen forms one. When bonded together within molecules, oxygen, sulfur, and nitrogen often have one or more “lone pairs” of electrons that play important roles in determining many of the molecules’ physical and chemical properties (see Appendix A). These traits in combination permit the formation of a vast number of diverse molecular species necessary to form the structures and enable the functions of living organisms.

Figure \(\PageIndex{1}\): Some common molecules include carbon dioxide, ammonia, and oxygen, which consist of combinations of oxygen atoms (red spheres), carbon atoms (gray spheres), hydrogen atoms (white spheres), or nitrogen atoms (blue spheres).

Living organisms contain inorganic compounds (mainly water and salts; see Appendix A) and organic molecules. Organic molecules contain carbon; inorganic compounds do not. Carbon oxides and carbonates are exceptions; they contain carbon but are considered inorganic because they do not contain hydrogen. The atoms of an organic molecule are typically organized around chains of carbon atoms.

Inorganic compounds make up 1%–1.5% of a living cell’s mass. They are small, simple compounds that play important roles in the cell, although they do not form cell structures. Most of the carbon found in organic molecules originates from inorganic carbon sources such as carbon dioxide captured via carbon fixation by microorganisms.

Query \(\PageIndex{1}\)

Query \(\PageIndex{1}\)

 

Interactive Element

Case Study Preview: “Hot Tub Trouble”

When 16-year-old Penny shows up with an itchy rash, her doctor chalks it up to sunscreen allergies and prescribes a hydrocortisone cream. But when the rash gets worse - bumpy, blistered, and full of pus - Penny reveals she recently lounged in a hot tub. That changes everything.

In this case, you’ll explore how a common water-loving bacterium, Pseudomonas aeruginosa, can turn a relaxing soak into an opportunistic infection. Learn how it’s identified in the lab, why it resists many antibiotics, and how it hides in biofilms - making treatment tricky and prevention critical.

Think it’s just a rash? Dive deeper. There’s more brewing beneath the surface.

Chapter 7 Case Study - Hot Tub Trouble

 

Key Concepts and Summary

• The most abundant elements in cells are hydrogen, carbon, oxygen, nitrogen, phosphorus, and sulfur.