The Chemical Level of Organization: Elements, Atoms, and Chemical Bonds in Human Anatomy & Physiology

The Chemical Level of Organization

Introduction

The chemical level of organization is the foundational level in the hierarchy of biological organization, underlying all physiological processes in the human body. Understanding the nature of elements, atoms, and chemical bonds is essential for grasping how molecules and compounds form and interact in living systems.

Elements and Atoms

Definition and Properties of Matter

• Matter is anything that takes up space and has mass.

• All living things are composed of matter, which is made up of various elements.

• An element is a substance that cannot be broken down into simpler substances by chemical reactions.

• The Periodic Table organizes all known elements by their properties.

Elements Essential to Human Life

• The human body is primarily composed of a small number of elements, including oxygen (O), carbon (C), hydrogen (H), and nitrogen (N).

• Elements can be classified as organic (carbon-containing) or inorganic (not containing carbon).

• Elements must be obtained from the environment; the body cannot create them.

Atoms: Structure and Properties

• An atom is the smallest unit of an element that retains its unique properties.

• Atoms are composed of three subatomic particles:

  • Protons: Positively charged, located in the nucleus.

  • Neutrons: No charge, located in the nucleus.

  • Electrons: Negatively charged, orbiting the nucleus.

• The atomic number is the number of protons in an atom and defines the element.

• The mass number is the sum of protons and neutrons in the nucleus.

• The number of neutrons can be determined by subtracting the atomic number from the mass number:

$\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$

• Atoms are electrically neutral when the number of protons equals the number of electrons.

Table: Major Elements in the Human Body

Element	Symbol	Approximate % of Body Weight	Function/Location
Oxygen	O	65%	Component of water, needed for cellular respiration
Carbon	C	18%	Backbone of organic molecules
Hydrogen	H	10%	Component of water and most organic molecules
Nitrogen	N	3%	Component of proteins and nucleic acids
Additional info: Other elements include calcium, phosphorus, potassium, sulfur, sodium, chlorine, magnesium, and trace elements.

Electron Behaviour

Electron Shells and Energy Levels

• Electrons are located in energy levels called electron shells around the nucleus.

• Each shell has a specific capacity:

  • First shell: up to 2 electrons

  • Second shell: up to 8 electrons

  • Third shell: up to 18 electrons (for larger atoms)

• The valence shell is the outermost electron shell and determines the chemical reactivity of the atom.

• Atoms are most stable when their valence shell is full.

Valence Electrons and Chemical Reactivity

• Atoms with incomplete valence shells tend to interact with other atoms to achieve stability.

• The number of valence electrons influences how atoms bond and form molecules.

• If an atom has a full valence shell, it is inert and does not readily react (e.g., noble gases).

Chemical Bonds

Overview of Chemical Bonds

• Chemical bonds are forces that hold atoms together in molecules and compounds.

• Atoms form bonds to achieve a full valence shell and greater stability.

Covalent Bonds

• Covalent bonds are formed when two atoms share one or more pairs of electrons.

• These are the strongest type of chemical bond found in biological molecules.

• Sharing can be equal (nonpolar covalent bond) or unequal (polar covalent bond).

• Polar molecules have regions of partial positive and negative charge due to unequal sharing.

Hydrogen Bonds

• A hydrogen bond is a weak attraction between a hydrogen atom (already covalently bonded to another atom) and an electronegative atom (such as oxygen or nitrogen) in another molecule.

• Hydrogen bonds are important in stabilizing the structure of water and biological macromolecules (e.g., DNA, proteins).

• They are responsible for many unique properties of water, such as high surface tension and boiling point.

Ionic Bonds

• Ionic bonds are formed when one atom donates an electron to another, resulting in the formation of oppositely charged ions.

• A cation is a positively charged ion; an anion is a negatively charged ion.

• The electrostatic attraction between cations and anions forms the ionic bond.

• Ionic compounds (salts) are formed from these bonds and can exist in various shapes and crystalline structures.

Table: Comparison of Chemical Bonds

Bond Type	Mechanism	Strength	Example
Covalent	Electron sharing	Strong	Water (H2O), Glucose (C6H12O6)
Ionic	Electron transfer	Moderate	Sodium chloride (NaCl)
Hydrogen	Weak attraction between polar molecules	Weak	Between water molecules

Summary and Applications

• Understanding the chemical level of organization is essential for studying physiology, as all biological processes depend on molecular interactions.

• Key organic compounds in humans include carbohydrates, lipids, proteins, and nucleic acids, all formed by specific chemical bonds.

• Inorganic compounds such as water and salts are also vital for physiological functions.

Additional info: These notes expand on the basic concepts presented in the slides, providing definitions, examples, and tables for clarity and exam preparation.