Biochemistry Foundations: Elements, Atoms, and Chemical Bonds in the Human Body

Biochemistry and the Human Body

Elements: The Building Blocks of Matter

Elements are pure substances that cannot be broken down into simpler substances by chemical reactions. They are fundamental to all matter, including the human body.

• Definition: An element is a substance consisting of only one type of atom, distinguished by its atomic number.

• Occurrence: 94 elements occur naturally, while 24 have been synthesized in laboratories.

• Examples: Gold (Au), Copper (Cu), Oxygen (O)

Elemental Composition of the Human Body

The human body is primarily composed of a few key elements, with the majority of body mass coming from just four elements.

• Major Elements: Oxygen (O), Carbon (C), Hydrogen (H), Nitrogen (N)

• Distribution: These four elements make up over 96% of the human body by mass.

Element	Percent of Body Mass	Approximate Weight (160 lbs)
Oxygen (O)	65.0%	104.0 lbs
Carbon (C)	18.5%	29.6 lbs
Hydrogen (H)	9.5%	15.2 lbs
Nitrogen (N)	3.3%	5.3 lbs
Other Elements	3.7%	5.9 lbs

Essential and Trace Elements

Some elements are required in large amounts (essential elements), while others are needed only in minute quantities (trace elements).

• Essential Elements: Oxygen, Carbon, Hydrogen, Nitrogen, Calcium, Phosphorus, Potassium, Sodium, Magnesium, etc.

• Trace Elements: Iron (Fe), Iodine (I), and others are required in very small amounts but are vital for health.

Example: Iodine deficiency can cause goiter, a swelling of the thyroid gland. Iodized salt helps prevent this condition.

Atoms: Structure and Properties

Atomic Structure

Atoms are the smallest units of elements that retain their chemical properties. They are composed of subatomic particles:

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Neutral particles also located in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus in energy shells.

Neutral Atom: The number of protons equals the number of electrons, resulting in no overall charge.

Electron Shells and Orbitals

Electrons occupy energy levels or shells around the nucleus. The arrangement of electrons determines an atom's chemical behavior.

• First shell: Holds up to 2 electrons.

• Subsequent shells: Hold up to 8 electrons each.

• Electrons fill the lowest energy shells first (from the inside out).

Example: Neon (Ne) has 10 electrons: 2 in the first shell, 8 in the second shell (full outer shell, making it chemically inert).

Chemical Reactions and Bonds

Chemical Reactions

Chemical reactions involve the making or breaking of bonds between atoms, primarily determined by the behavior of electrons in the outermost shells (valence electrons).

• Atoms tend to react to achieve a full outer shell, which is a stable configuration.

Molecules and Compounds

Atoms combine to form molecules and compounds through chemical bonds.

• Molecule: Two or more atoms bonded together (e.g., O2, N2).

• Compound: A substance composed of two or more different elements (e.g., NaCl, H2O, C6H12O6).

Chemical Bonds: Types and Properties

Chemical bonds are forces that hold atoms together in molecules and compounds. The type of bond affects the properties of the resulting substance.

• Ionic Bonds

• Covalent Bonds

• Polar Covalent Bonds

Ionic Bonds

An ionic bond forms when one atom donates an electron to another, resulting in the formation of charged ions. Opposite charges attract, holding the ions together.

• Cation: Positively charged ion (lost electron)

• Anion: Negatively charged ion (gained electron)

Example: Sodium (Na) donates an electron to chlorine (Cl), forming Na+ and Cl-, which combine to make sodium chloride (NaCl).

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms. The shared electrons allow each atom to achieve a full outer shell.

• Single bond: One pair of electrons shared

• Double bond: Two pairs of electrons shared

• Triple bond: Three pairs of electrons shared

Example: In a water molecule (H2O), each hydrogen shares one electron with oxygen, forming two single covalent bonds.

Polar Covalent Bonds

Polar covalent bonds occur when electrons are shared unequally between two atoms, resulting in partial charges on the atoms.

• Electronegativity: The ability of an atom to attract electrons in a bond. Oxygen is more electronegative than hydrogen.

• In water (H2O), electrons spend more time near the oxygen atom, giving it a partial negative charge and hydrogen a partial positive charge.

Example: The polarity of water molecules is essential for many biological processes, including hydrogen bonding and solubility of substances in water.

Summary Table: Types of Chemical Bonds

Bond Type	Mechanism	Example	Properties
Ionic	Electron transfer	NaCl	Forms ions, strong in solid state, dissolves in water
Covalent	Electron sharing	H2O, O2	Strong, forms molecules
Polar Covalent	Unequal electron sharing	H2O	Partial charges, leads to polarity

Key Equations and Concepts

• Atomic Number (Z): Number of protons in the nucleus

• Mass Number (A): Number of protons + neutrons

• Electron Shell Capacity:

  • First shell: $2n^2$ where $n=1$ (max 2 electrons)

  • Second shell: $2n^2$ where $n=2$ (max 8 electrons)

Example Equation:

$\text{Number of electrons in shell} = 2n^2$

Additional info: Understanding the chemical basis of life is foundational for further study in anatomy and physiology, as all physiological processes are ultimately chemical in nature.