Basic Chemistry for Biology: Chemical Composition of Living Organisms

Lecture #2: Basic Chemistry

Chemical Composition of Living Organisms

Understanding the chemical basis of life is essential in biology. All living things are composed of matter, which is anything that occupies space and has mass.

• Matter: Always occupies space and has mass.

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

Elements

• All matter is composed of elements.

• An element is a substance that cannot be broken down chemically into simpler substances.

• There are 92 naturally occurring elements found in nature.

• Each element is given a symbol (e.g., Carbon: C, Oxygen: O, Sodium: Na).

• Examples of elements: carbon, nitrogen, oxygen, hydrogen, gold, copper.

Chemical Composition of Living Organisms: Elements Essential for Life

Of the 92 naturally occurring elements, only a subset is essential for life. These elements are found in all living organisms and are required for biological processes.

• There are 25 elements necessary for human life.

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

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

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

  • Calcium (Ca)

  • Phosphorus (P)

  • Potassium (K)

  • Sulfur (S)

  • Sodium (Na)

  • Chlorine (Cl)

  • Magnesium (Mg)

  • Iron (Fe)

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

Element	Symbol	Approximate % of Human Body Weight
Oxygen	O	65%
Carbon	C	18.5%
Hydrogen	H	9.5%
Nitrogen	N	3.3%
Other elements (Ca, P, K, S, Na, Cl, Mg, Fe, etc.)	-	~3.7%
Trace elements	-	<0.01%

Trace Elements

Trace elements are required by organisms in minute quantities but are vital for proper physiological functioning.

• Iron (Fe): Accounts for only 0.004% of body mass but is essential for life.

• Iron is a key component of hemoglobin, which is used to transport oxygen in blood.

• Iodine (I): Another trace element required for human life; about 0.15 mg must be acquired each day.

• Iodine is a key component of hormones produced by the thyroid gland, which regulate metabolism and development.

• To avoid iodine deficiency (which can cause goiter), table salt is often supplemented with iodine (iodized salt).

Compounds

Elements rarely exist in pure form in living organisms. Instead, they combine to form compounds, which are substances consisting of two or more elements in a fixed ratio.

• A compound is a substance consisting of two or more elements combined in a fixed proportion.

• Compounds are much more common than pure elements in nature.

• Examples of compounds: sodium chloride (NaCl), water (H2O).

• Compounds have entirely different properties than their constituent elements.

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

Key Equation: Chemical Formula of Water

• The chemical formula for water is: $H_2O$

• The chemical formula for sodium chloride is: $NaCl$

Summary Table: Elements in the Human Body

Category	Examples	Role
Major Elements	O, C, H, N	Main components of organic molecules
Minor Elements	Ca, P, K, S, Na, Cl, Mg	Structural and functional roles (bones, nerve function, etc.)
Trace Elements	Fe, I, Zn, Cu, etc.	Enzyme function, hormone production, oxygen transport

Additional info: Understanding the chemical composition of living organisms is foundational for studying biological processes such as metabolism, cellular respiration, and genetics.