Chemistry Comes Alive: Foundations for Anatomy & Physiology

Chemistry Comes Alive

Introduction to Chemistry in Physiology

Chemistry is fundamental to understanding physiological processes in the human body. All body functions depend on chemical reactions, from movement and digestion to nerve impulses and cellular respiration.

Matter and Energy

Definitions and Properties

• Matter: Anything that occupies space and has mass. Mass is the amount of matter in an object and remains constant, while weight varies with gravity.

• Energy: The capacity to do work or put matter into motion. Energy exists in two main forms: kinetic (energy in action) and potential (stored energy).

Kinetic vs Potential Energy

• Kinetic Energy: Energy of motion; increases with mass or velocity.

• Potential Energy: Stored energy due to position or structure; can be converted to kinetic energy.

Forms of Energy

Major Types

• Chemical Energy: Stored in the bonds of chemical substances; released during chemical reactions (e.g., ATP hydrolysis).

• Electrical Energy: Results from the movement of charged particles (ions); essential for nerve impulses and muscle contraction.

• Mechanical Energy: Directly involved in moving matter (e.g., muscle contraction, heart beating).

• Radiant (Electromagnetic) Energy: Energy that travels in waves (e.g., light, X-rays, ultraviolet rays).

Elements and Atoms

Chemical Elements

Elements are unique substances that cannot be broken down by ordinary chemical means. The four major elements forming the bulk of body matter are:

• Oxygen (O)

• Carbon (C)

• Hydrogen (H)

• Nitrogen (N)

Atomic Structure

• Atoms: Smallest units of elements, composed of subatomic particles:

  • Protons: Positive charge, mass = 1 amu, located in nucleus

  • Neutrons: No charge, mass = 1 amu, located in nucleus

  • Electrons: Negative charge, negligible mass, orbit nucleus in electron cloud

Atomic Number, Mass, and Isotopes

• Atomic Number: Number of protons in the nucleus

• Atomic Mass: Number of protons plus neutrons

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

• Radioisotopes: Unstable isotopes that decay, emitting radiation (radioactivity)

• Atomic Weight: Average of the mass numbers of all isotopes

Molecules, Compounds, and Mixtures

Definitions

• Molecule: Two or more atoms held together by chemical bonds

• Compound: Molecule containing two or more different elements

• Mixture: Physical combination of two or more substances; no chemical bonding

Types of Mixtures

• Solutions: Homogeneous mixtures; solute particles are very small

• Colloids: Heterogeneous mixtures; solute particles are larger and do not settle out

• Suspensions: Heterogeneous mixtures with large, visible solutes that settle out

Concentration of Solutions

• Percent (%): Parts per 100 parts of solution

• mg/dL: Milligrams per deciliter

• Molarity (M): Moles of solute per liter of solution

• Mole: Equal to the molecular weight in grams; contains $6.02 \times 10^{23}$ molecules (Avogadro's number)

Chemical Bonds

Role of Electrons and the Octet Rule

• Electrons occupy energy levels (shells) around the nucleus.

• The octet rule states that atoms tend to gain, lose, or share electrons to achieve 8 electrons in their valence shell (except H and He, which require 2).

Types of Chemical Bonds

• Ionic Bonds: Formed by the transfer of electrons from one atom to another, creating ions (cations and anions).

• Covalent Bonds: Formed by sharing pairs of electrons between atoms; can be polar (unequal sharing) or nonpolar (equal sharing).

• Hydrogen Bonds: Weak attractions between a hydrogen atom (covalently bonded to an electronegative atom) and another electronegative atom.

Chemical Reactions

Types of Chemical Reactions

• Synthesis (Combination): Atoms or molecules combine to form a larger, more complex molecule. $A + B \rightarrow AB$

• Decomposition: Molecule is broken down into smaller molecules or atoms. $AB \rightarrow A + B$

• Exchange (Displacement): Bonds are both made and broken; atoms are rearranged. $AB + C \rightarrow AC + B$

Oxidation-Reduction (Redox) Reactions

• Oxidation: Loss of electrons (electron donor is oxidized)

• Reduction: Gain of electrons (electron acceptor is reduced)

• Redox reactions are essential for energy transfer in metabolism (e.g., cellular respiration)

Energy Flow in Chemical Reactions

• Exergonic Reactions: Release energy; products have less potential energy than reactants (catabolic, oxidative reactions)

• Endergonic Reactions: Absorb energy; products have more potential energy than reactants (anabolic reactions)

Irreversibility of Reactions in the Body

• Many biological reactions are irreversible due to large energy release or removal of products.

• Metabolism is driven forward to sustain life (e.g., glucose oxidation to CO2 and H2O).

Factors Affecting Reaction Rates

• Temperature: Higher temperature increases reaction rate (up to a point).

• Concentration: Higher concentration of reactants increases rate.

• Particle Size: Smaller particles react faster.

• Catalysts/Enzymes: Speed up reactions without being consumed.

Summary Table: Major Chemical Bond Types

Type	Description	Strength
Covalent bonds	Sharing of pairs of electrons. May be polar (not equally shared) or nonpolar (equally shared).	Strongest
Ionic bonds	Attraction between two oppositely charged ions.	Intermediate
Hydrogen bonds	Attraction between a hydrogen atom carrying a partial positive charge (δ+) and an electronegative atom with a slightly negative charge (δ-).	Weakest

Additional info: For further reading, see Marieb & Hoehn, Human Anatomy & Physiology, 11th ed., Pearson, 2018.