The Chemical Level of Organization: Foundations for Anatomy and Physiology

The Chemical Level of Organization

Introduction to Chemistry in Anatomy and Physiology

Understanding chemistry is essential for studying anatomy and physiology because physiological processes are driven by chemical reactions. The first three levels of biological organization—atoms, molecules, and macromolecules—are fundamentally chemical in nature.

• Atom: The smallest unit of an element, retaining its properties.

• Molecule: Two or more atoms bonded together.

• Macromolecule: Large, complex molecules such as proteins, carbohydrates, DNA, and fats.

• Matter: Anything that occupies space and has mass.

• Mass: The amount of matter in an object.

• Weight: The force of gravity acting on mass.

• Volume: The space matter occupies.

States of Matter

Matter exists in three primary states, all of which are interchangeable through the addition or removal of energy (heat):

• Solid: Definite shape and volume (e.g., teeth, bones).

• Liquid: Definite volume, no definite shape (e.g., blood).

• Gas: No definite shape or volume (e.g., air we breathe).

Forms of Energy in the Body

Types of Energy

Energy is the mover of matter and exists in several forms within the body:

• Chemical Energy: Stored in chemical bonds; released or absorbed during reactions.

• Electrical Energy: Movement of charged particles (ions).

• Mechanical Energy: Directly involved in moving matter.

• Radiant Energy: Energy traveling in waves (e.g., light, UV rays).

Adenosine Triphosphate (ATP): The primary energy currency in living systems. Energy conversions are inefficient, with heat always being released, which helps maintain body temperature (98.6°F).

Elements and Atoms

Elements in the Human Body

Elements are pure substances made of one type of atom and cannot be broken down by chemical means. The human body is composed mainly of:

• Oxygen (O): 65%

• Carbon (C): 18.6%

• Hydrogen (H): 9.7%

• Nitrogen (N): 3.2%

• Other elements include calcium, phosphorus, potassium, sodium, chlorine, magnesium, iron, iodine, and trace elements.

There are 92 naturally occurring elements and additional synthetic ones.

Atomic Structure

• Atoms: Composed of protons (+), neutrons (0), and electrons (−).

• Atomic Number: Number of protons in the nucleus (also equals number of electrons in a neutral atom).

• Atomic Mass (amu): Sum of protons and neutrons; electrons have negligible mass.

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Nucleus: Central region containing protons and neutrons.

Subatomic Particles

• Protons: Positive charge, located in the nucleus.

• Neutrons: No charge, located in the nucleus.

• Electrons: Negative charge, orbit the nucleus in electron shells.

Electron Shells and Chemical Bonds

Electron Shells

Electrons occupy shells (energy levels) around the nucleus. The arrangement of electrons determines chemical reactivity:

• First shell: Holds 2 electrons (lowest energy).

• Second shell: Holds 8 electrons.

• Third shell: Holds 18 electrons.

• Valence Electrons: Electrons in the outermost shell; determine chemical behavior.

• Stable Atoms: Atoms with full outer shells (noble gases) are chemically inert.

• Unstable Atoms: Atoms with incomplete outer shells are reactive.

The Periodic Table of Elements

The periodic table organizes elements by atomic number (number of protons) and groups them by similar chemical properties. Columns (groups) indicate the number of valence electrons, while rows (periods) indicate the number of electron shells.

Types of Chemical Bonds

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

• Ionic Bonds: Formed by the transfer of electrons from one atom to another, creating charged ions (cations and anions) that attract each other. Example: NaCl (table salt).

• Covalent Bonds: Formed when two atoms share electrons. Can be polar (unequal sharing, creating partial charges) or nonpolar (equal sharing).

• Hydrogen Bonds: Weak bonds between a hydrogen atom and an electronegative atom (e.g., between water molecules).

Chemical Reactions

Types of Chemical Reactions

• Synthesis (Anabolic): Building larger molecules from smaller ones. $A + B \rightarrow AB$

• Decomposition (Catabolic): Breaking down molecules into smaller units. $AB \rightarrow A + B$

• Exchange: Both synthesis and decomposition occur. $AB + CD \rightarrow AD + CB$

Metabolism: The sum of all chemical reactions in the body.

Energy in Chemical Reactions

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy.

• Exergonic Reactions: Release energy (catabolic, oxidative).

• Endergonic Reactions: Absorb energy (anabolic).

Factors Affecting Reaction Rates

• Temperature (higher increases rate)

• Particle size (smaller increases rate)

• Concentration (higher increases rate)

• Catalysts/Enzymes (increase rate without being consumed)

Water and Solutions in the Body

Importance of Water

• Makes up about 2/3 of body weight

• Acts as a lubricant, solvent, and medium for chemical reactions

• Has high heat capacity and is polar (dissolves many substances)

Solutions, Solvents, and Solutes

• Solution: Uniform mixture of two or more substances.

• Solvent: The dissolving medium (water is the universal solvent).

• Solute: The dissolved substance.

Electrolytes

Electrolytes are inorganic compounds that dissociate into ions in water, conducting electricity. Examples include sodium chloride (NaCl), which is essential for muscle and nerve function.

Acids, Bases, Salts, and pH

Acids and Bases

• Acid: Releases hydrogen ions (H+) in solution; lowers pH.

• Base: Accepts hydrogen ions; increases pH.

• Salt: Formed when acids and bases react; dissociates into ions other than H+ or OH−.

pH Scale: Measures hydrogen ion concentration; ranges from 0 (acidic) to 14 (basic), with 7 being neutral.

• Blood pH: 7.35–7.45 (slightly basic)

• Acidosis: Blood pH below 7.35

• Alkalosis: Blood pH above 7.45

Buffers: Substances that resist changes in pH by absorbing or releasing H+ ions.

Organic Compounds

Major Classes of Organic Compounds

• Proteins: Made of amino acids; contain C, H, O, N. Functions include structure, enzymes, and transport.

• Carbohydrates: Sugars and starches; main energy source. Monosaccharides (glucose), disaccharides (sucrose), polysaccharides (glycogen).

• Lipids: Fats, oils, cholesterol; energy storage, cell membranes, hormones. Includes triglycerides, phospholipids, steroids.

• Nucleic Acids: DNA and RNA; store and transfer genetic information. Made of nucleotides (adenine, guanine, cytosine, thymine [DNA], uracil [RNA]).

Protein Denaturation

Proteins lose their structure and function when exposed to high heat or acidity (e.g., cooking an egg).

Nucleic Acids

• DNA: Deoxyribonucleic acid; found in the nucleus; stores genetic information.

• RNA: Ribonucleic acid; involved in protein synthesis; found outside the nucleus.

• Nucleotides: Building blocks of nucleic acids; consist of a sugar, phosphate, and nitrogenous base.

• Pyrimidines: Cytosine, thymine (DNA), uracil (RNA).

• Purines: Adenine, guanine (both DNA and RNA).

Base pairing in DNA: Adenine pairs with thymine, guanine pairs with cytosine. In RNA, uracil replaces thymine.