Chapter 2: Chemistry and Molecules – Foundations for Anatomy & Physiology

Basic Chemistry and Chemical Reactions

Chemistry and Physiological Reactions

Chemistry is fundamental to understanding physiological processes in the human body. All living organisms are composed of chemicals, and chemical reactions underlie essential functions such as movement, digestion, and cellular respiration.

• Human Body Composition: The body is made up of various elements, with oxygen, carbon, hydrogen, and nitrogen being the most abundant.

• Role of Chemistry: Chemical reactions drive processes like muscle contraction, nerve signaling, and energy production.

Molecules and Compounds

Atoms combine to form molecules and compounds, which are the building blocks of matter in biological systems.

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

• Molecule: Two or more atoms bonded together. Example: $H_2$, $O_2$

• Compound: A molecule containing two or more different kinds of atoms. Example: Glucose ($C_6H_{12}O_6$)

Chemical Reactions

Chemical reactions occur when chemical bonds are formed, rearranged, or broken. These reactions are represented by chemical equations.

• Reactants: Substances entering into a reaction.

• Products: Resulting chemical end products.

Example Equations:

• $H + H \rightarrow H_2$ (hydrogen gas)

• $4H + 1C \rightarrow CH_4$ (methane)

Primary Types of Chemical Reactions

There are three main types of chemical reactions important in physiology:

• Synthesis (Combination) Reactions: Atoms or molecules combine to form a larger, more complex molecule. General Equation: $A + B \rightarrow AB$ Role: Anabolic (building) processes, e.g., amino acids forming proteins. Example: Rapidly growing tissues.

• Decomposition Reactions: Breakdown of a molecule into smaller molecules or its constituent atoms. General Equation: $AB \rightarrow A + B$ Role: Catabolic (bond-breaking) processes, e.g., glycogen breaking down to release glucose. Example: Digestion.

• Exchange (Displacement) Reactions: Involve both synthesis and decomposition; bonds are made and broken. General Equations: $AB + C \rightarrow AC + B$ $AB + CD \rightarrow AD + CB$ Example: ATP transfers its terminal phosphate group to glucose to form ADP and glucose-phosphate.

Other Reactions: Redox (Reduction-Oxidation)

Redox reactions are vital in living systems, especially for energy production.

• Role: Basis for breakdown of food fuels into energy (e.g., cellular respiration).

• Example Equation:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP$

• Reactants: Sugar, Oxygen

• Products: Carbon Dioxide, Water, Usable Energy (ATP)

Factors Influencing Rate of Chemical Reactions

The speed of chemical reactions in the body can be affected by several factors:

• Temperature: Increased temperature raises reaction rate.

• Concentration: Higher concentration of reactants increases rate.

• Particle Size: Smaller particles increase rate.

• Catalysts: Substances that increase the rate of reaction without being consumed. Enzymes: Biological catalysts essential for life.

Biochemistry

Introduction to Biochemistry

Biochemistry is the study of the chemical composition and reactions of living matter. All chemicals in the body are classified as either organic or inorganic compounds.

• Inorganic Compounds: Do not contain carbon (C). Examples: Water, salts, many acids and bases.

• Organic Compounds: Contain carbon; usually large and covalently bonded. Examples: Carbohydrates, fats, proteins, nucleic acids.

Both types are essential for life.

Inorganic Compounds

Overview of Inorganic Compounds

Inorganic compounds are vital for physiological processes and include water, salts, acids, and bases.

• Water: Most abundant inorganic compound; excellent solvent, high heat capacity, involved in chemical reactions.

• Salts: Ionic compounds that dissociate in water; important for nerve function and muscle contraction.

• Acids and Bases: Substances that release hydrogen ions (acids) or hydroxide ions (bases); crucial for maintaining pH balance.

Major Elements in the Human Body

The following table summarizes the major elements found in the human body, their percentage by body mass, and their physiological roles.

Element	Symbol	Body Mass (%)	Function
Oxygen	O	65	Component of water and organic molecules; essential for cellular respiration
Carbon	C	18.5	Forms backbone of all organic molecules
Hydrogen	H	9.5	Component of water and most organic molecules
Nitrogen	N	3.2	Component of proteins and nucleic acids
Calcium	Ca	1.5	Structural component of bones and teeth; involved in muscle contraction
Phosphorus	P	1.0	Component of nucleic acids and ATP; structural role in bones and teeth
Potassium	K	0.4	Necessary for nerve function and muscle contraction
Sulfur	S	0.3	Component of proteins
Sodium	Na	0.2	Important for water balance, nerve function, and muscle contraction
Chlorine	Cl	0.2	Helps maintain water balance
Magnesium	Mg	0.1	Required for enzyme activity