Basic Chemistry for Biology: Elements, Compounds, and Trace Elements

Chemical Composition of Living Organisms

Introduction to Matter and Elements

All living things are composed of matter, which is anything that occupies space and has mass. Matter is made up of elements, fundamental substances that cannot be broken down chemically into simpler substances.

• Examples of matter: rock, water, grass, animals, human beings

• Elements: There are 92 naturally occurring elements found in nature, such as carbon (C), nitrogen (N), oxygen (O), hydrogen (H), gold (Au), and copper (Cu).

• Each element is represented by a unique chemical symbol (e.g., C for carbon, O for oxygen, Na for sodium).

Elements Essential for Life

Although many elements exist, only a subset are essential for human life. These elements are crucial for the structure and function of biological molecules.

• There are 25 elements necessary for human life.

• 96% of the human body’s weight is due to four main elements: carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).

• These four elements are the main components of sugars, fats, proteins, and nucleic acids.

• The remaining 4% of body weight is made up of elements such as:

  • Calcium (Ca)

  • Magnesium (Mg)

  • Chlorine (Cl)

  • Sodium (Na)

  • Potassium (K)

  • Phosphorus (P)

  • Sulfur (S)

• Trace elements are required in very small amounts but are essential for life.

Table: Major Elements in the Human Body

Element	Symbol	Percentage of Body Weight
Oxygen	O	65%
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%
Other elements (trace)	-

Trace Elements

Definition and Importance

Trace elements are elements required by an organism in only minute quantities, but they are vital for proper physiological functioning.

• Iron (Fe): Accounts for only 0.004% of body mass, but is essential as a component of hemoglobin, the molecule responsible for oxygen transport in blood.

• Iodine (I): Required in very small amounts (about 0.15 mg per day). It is a key component of hormones produced by the thyroid gland.

• Iodine deficiency can lead to an enlarged thyroid gland, a condition known as goiter.

• To prevent deficiency, table salt is often supplemented with iodine ("iodized salt").

Compounds

Formation and Properties

Compounds are substances consisting of two or more elements combined in a fixed ratio. Most substances found in living organisms are compounds rather than pure elements.

• Examples: Sodium chloride (NaCl), water (H2O)

• Compounds often have properties very different from the elements that compose them.

• Example: Sodium (Na) is a highly reactive metal, and chlorine (Cl) is a poisonous gas. When combined, they form sodium chloride (NaCl), which is edible table salt.

Table: Comparison of Elements and Compounds

Substance	Type	Properties
Sodium (Na)	Element	Soft, reactive metal
Chlorine (Cl)	Element	Greenish, poisonous gas
Sodium chloride (NaCl)	Compound	Edible, crystalline solid (table salt)

Key Points

• Elements combine to form compounds through chemical bonds.

• Compounds are more common in living organisms than pure elements.

• The properties of compounds are distinct from those of their constituent elements.

Example: Formation of Sodium Chloride

• Sodium (Na): A soft, silvery metal that reacts violently with water.

• Chlorine (Cl): A toxic, yellow-green gas.

• Sodium chloride (NaCl): When sodium and chlorine react, they form a stable, edible compound—table salt.

Additional info: Understanding the chemical composition of living organisms is foundational for studying biological processes, as the structure and function of biomolecules depend on the elements and compounds present in cells.