BackChapter 2: The Chemical Basis of Life – Elements, Atoms, and Compounds
Study Guide - Smart Notes
Tailored notes based on your materials, expanded with key definitions, examples, and context.
Elements, Atoms, and Compounds
Organisms Are Composed of Elements, Usually Combined into Compounds
All living organisms are made up of matter, which consists of chemical elements. These elements combine in specific ways to form compounds, which are essential for life.
Chemical elements are substances that cannot be broken down into other substances by chemical means.
About 25 elements are essential for human life.
Four elements—oxygen (O), carbon (C), hydrogen (H), and nitrogen (N)—make up about 96% of the weight of most living organisms.
A compound is a substance consisting of two or more different elements combined in a fixed ratio.
Example: Table salt (sodium chloride, NaCl) is a compound formed from sodium (Na) and chlorine (Cl). Both elements have very different properties, but when combined, they form a substance with new characteristics, illustrating the concept of emergent properties.
Major and Trace Elements in Living Organisms
Elements are classified based on their abundance and necessity in living organisms. Major elements are required in large amounts, while trace elements are needed in minute quantities but are still essential for health.
Major elements: Oxygen, carbon, hydrogen, and nitrogen.
Trace elements: Elements such as iodine, iron, and zinc are required in very small amounts.
Trace elements are often added to food to prevent diseases and improve nutrition. For example, iodine is added to table salt to prevent goiter, a thyroid gland disorder.
Element | Percentage in Human Body | Role in Organism |
|---|---|---|
Oxygen (O) | 65% | Component of water and organic molecules; essential for cellular respiration |
Carbon (C) | 18.5% | Backbone of organic molecules |
Hydrogen (H) | 9.5% | Component of water and organic molecules |
Nitrogen (N) | 3.3% | Component of proteins and nucleic acids |
Trace Elements | <0.01% | Vital for specific biological functions (e.g., iodine for thyroid hormones) |
Additional info: The concept of emergent properties refers to new characteristics that arise when elements combine to form compounds, which are not present in the individual elements themselves.
