BackChapter 2: The Chemical Context of Life – Study Notes
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Chapter 2: The Chemical Context of Life
Introduction
This chapter explores the fundamental chemical principles that underlie biological processes. Understanding the nature of matter, elements, and compounds is essential for studying how living organisms function at the molecular level.
Concept 2.1: Matter Consists of Chemical Elements in Pure Form and in Combinations Called Compounds
Definition of Matter and Elements
Matter is anything that takes up space and has mass.
Elements are pure substances that cannot be broken down into other substances by chemical reactions.
Compounds
A compound is a substance consisting of two or more elements combined in a fixed ratio.
Compounds have emergent properties that are different from those of their constituent elements.
Example: Sodium (Na) is a reactive metal, and chlorine (Cl) is a poisonous gas, but together they form sodium chloride (NaCl), or table salt, which is edible and essential for life.
Elements of Life
Essential Elements
About 20–25% of the 92 naturally occurring elements are essential for life. These are called essential elements.
Four elements—oxygen (O), carbon (C), hydrogen (H), and nitrogen (N)—make up approximately 96% of living matter.
The remaining 4% of an organism’s mass consists mainly of calcium (Ca), phosphorus (P), potassium (K), sulfur (S), sodium (Na), chlorine (Cl), and magnesium (Mg).
Trace Elements
Trace elements are required by organisms in only minute quantities (less than 0.01% of body mass).
Examples include iron (Fe), iodine (I), and zinc (Zn).
Table: Elements in the Human Body
Element | Symbol | Percentage of Body Mass |
|---|---|---|
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 | - | <0.01% |
Additional info: Trace elements include 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), and zinc (Zn).
Emergent Properties of Compounds
The properties of a compound depend on the types of atoms involved and how they are bonded together.
Example: Formic acid (HCOOH) is used by wood ants (Formica rufa) as a chemical defense mechanism.
Atoms and Their Structure
Atoms
An atom is the smallest unit of matter that retains the properties of an element.
Atoms are composed of subatomic particles: protons (positive charge), neutrons (no charge), and electrons (negative charge).
Protons and neutrons are located in the atomic nucleus; electrons form a cloud around the nucleus.
Atomic Number and Mass
The atomic number is the number of protons in the nucleus of an atom and defines the element.
The mass number is the sum of protons and neutrons in the nucleus.
The atomic mass is the atom’s total mass, which can be approximated by the mass number (measured in daltons).
Isotopes
Isotopes are atoms of the same element that have different numbers of neutrons.
Radioactive isotopes decay spontaneously, releasing particles and energy.
Radioactive isotopes are used as tracers in medical diagnostics and in radiometric dating to determine the age of fossils and rocks.
Conclusion
Understanding the chemical context of life is foundational for studying biology. The unique properties of elements and compounds, as well as their interactions, form the basis for the structure and function of all living organisms.
