Chapter 2: Chemistry Comes Alive – Study Notes for Human Anatomy & Physiology

Chapter 2: Chemistry Comes Alive

Introduction

Chemistry is fundamental to understanding physiological processes in the human body. All body functions, from movement to digestion, are governed by chemical and biochemical reactions. This chapter introduces the basic principles of chemistry as they relate to anatomy and physiology, focusing on matter, energy, atomic structure, chemical bonds, and reactions.

2.1 Matter and Energy

Matter

Matter is anything that has mass and occupies space. It can exist in three states:

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

• Liquid: Changeable shape, definite volume (e.g., blood plasma).

• Gas: Changeable shape and volume (e.g., air in lungs).

Weight is the effect of gravity on mass.

Energy

Energy is the capacity to do work or put matter into motion. It does not have mass or occupy space. Energy exists in two main forms:

• Kinetic energy: Energy in action (e.g., muscle contraction).

• Potential energy: Stored energy (e.g., energy stored in chemical bonds).

Energy can be transformed from one form to another, but some energy is always lost as heat during conversion.

• Chemical energy: Stored in bonds of chemical substances.

• Electrical energy: Movement of charged particles (e.g., nerve impulses).

• Mechanical energy: Directly involved in moving matter (e.g., muscle movement).

• Radiant energy: Travels in waves (e.g., light, X-rays).

2.2 Atoms and Elements

Elements

Elements are substances that cannot be broken down into simpler substances by ordinary chemical methods. The human body is composed mainly of four elements: carbon, oxygen, hydrogen, and nitrogen, which make up about 96% of body mass.

Element	Symbol	Approx. % Body Mass	Functions
Oxygen	O	65.0	Component of organic/inorganic molecules; needed for ATP production
Carbon	C	18.5	Component of all organic molecules
Hydrogen	H	9.5	Component of organic molecules; influences pH
Nitrogen	N	3.2	Component of proteins and nucleic acids

Other elements are present in lesser or trace amounts and are essential for various physiological functions.

Atoms

Atoms are the smallest units of elements that retain the properties of that element. Atoms consist of:

• Protons (p+): Positive charge, found in nucleus

• Neutrons (n0): No charge, found in nucleus

• Electrons (e-): Negative charge, orbit nucleus

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

Atomic Number, Mass Number, Isotopes, and Atomic Weight

• Atomic number: Number of protons in the nucleus.

• Mass number: Total number of protons and neutrons.

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

• Atomic weight: Average of mass numbers of all isotopes of an element.

Radioisotopes

Radioisotopes are unstable isotopes that decompose to more stable forms, emitting radiation. They are used in medical diagnosis and treatment but can also be harmful to living tissue.

2.3 Combining Matter

Molecules and Compounds

Molecules are formed when two or more atoms bond together. Compounds are molecules composed of two or more different elements (e.g., H2O, C6H12O6).

Mixtures

Most matter exists as mixtures, which are physical combinations of two or more substances. There are three main types:

• Solutions: Homogeneous mixtures; solute particles are very small and do not settle out (e.g., mineral water).

• Colloids: Heterogeneous mixtures; solute particles are larger and scatter light but do not settle out (e.g., Jell-O, cytosol).

• Suspensions: Heterogeneous mixtures; large particles settle out (e.g., blood).

Mixtures vs. Compounds:

• Mixtures do not involve chemical bonding; compounds do.

• Mixtures can be separated physically; compounds require chemical means.

• Mixtures can be homogeneous or heterogeneous; compounds are always homogeneous.

2.4 Chemical Bonds

Role of Electrons in Chemical Bonding

Electrons occupy energy levels called electron shells. The outermost shell is the valence shell, which determines chemical reactivity. Atoms are most stable when their valence shell is full (usually 8 electrons, known as the octet rule).

Types of Chemical Bonds

• Ionic bonds: Formed by the transfer of electrons from one atom to another, creating ions (cations and anions). Opposite charges attract, forming an ionic bond (e.g., NaCl).

• Covalent bonds: Formed by sharing electrons between atoms. Can be single, double, or triple bonds.

• Nonpolar covalent bonds: Equal sharing of electrons (e.g., CO2).

• Polar covalent bonds: Unequal sharing of electrons, resulting in partial charges (e.g., H2O).

• Hydrogen bonds: Weak attractions between a hydrogen atom in one molecule and an electronegative atom in another. Important in water and biological molecules.

2.5 Chemical Reactions

Chemical Equations

Chemical reactions involve the formation, rearrangement, or breaking of chemical bonds. They are represented by chemical equations, showing reactants and products.

Example equations:

• $\mathrm{H} + \mathrm{H} \rightarrow \mathrm{H}_2$

• $4\mathrm{H} + 1\mathrm{C} \rightarrow \mathrm{CH}_4$

Types of Chemical Reactions

• Synthesis (Combination) reactions: Atoms or molecules combine to form larger, more complex molecules. Important in anabolic processes.

• Decomposition reactions: Molecules are broken down into smaller molecules or atoms. Important in catabolic processes.

• Exchange (Displacement) reactions: Involve both synthesis and decomposition; bonds are made and broken.

In biological systems, many reactions are redox (reduction-oxidation) reactions, where electrons are transferred between molecules.

Energy Flow in Chemical Reactions

• Exergonic reactions: Release energy; products have less potential energy than reactants (e.g., catabolic reactions).

• Endergonic reactions: Absorb energy; products have more potential energy than reactants (e.g., anabolic reactions).

Reversibility of Chemical Reactions

All chemical reactions are theoretically reversible, but many biological reactions are not due to high energy requirements or removal of products.

Rate of Chemical Reactions

The speed of chemical reactions is influenced by:

• Temperature (higher temperature increases rate)

• Concentration of reactants (higher concentration increases rate)

• Particle size (smaller particles increase rate)

• Catalysts: Substances that increase reaction rate without being changed (e.g., enzymes in the body)

Additional info: Understanding these chemical principles is essential for grasping more advanced topics in anatomy and physiology, such as cellular metabolism, nerve impulse transmission, and muscle contraction.