Molecules, Chemistry, and the Cell: Essential Study Notes for Anatomy & Physiology

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Molecules of the Body

Principal Elements in the Human Body

The human body is composed of various elements, each playing a critical role in physiological processes. Understanding these elements is foundational for studying anatomy and physiology.

Element	% of Total Body Weight	Significance
Oxygen (O)	65	Component of water and other compounds; essential for respiration
Carbon (C)	18.6	Found in all organic molecules
Hydrogen (H)	9.7	Component of water and most other compounds
Nitrogen (N)	3.2	Found in proteins, nucleic acids, and other organic compounds
Calcium (Ca)	1.8	Bones and teeth; membrane function, nerve impulses, muscle contraction, blood clotting
Phosphorus (P)	1.0	Bones and teeth, nucleic acids, high-energy compounds
Potassium (K)	0.4	Membrane function, nerve impulses, muscle contraction
Sodium (Na)	0.2	Blood volume, membrane function, nerve impulses, muscle contraction
Chlorine (Cl)	0.2	Membrane function, water absorption
Magnesium (Mg)	0.06	Cofactor for many enzymes
Sulfur (S)	0.04	Found in many proteins
Iron (Fe)	0.007	Oxygen transport and energy capture
Iodine (I)	0.0002	Hormones of the thyroid gland
Trace elements	-	Some function as cofactors; many functions are poorly understood

Table of principal elements in the human body Continuation of principal elements table

Organic and Inorganic Compounds

Definitions and Classifications

Compounds in the body are classified as organic or inorganic based on their chemical structure and function.

Organic Compounds: Molecules based on carbon and hydrogen. Includes carbohydrates, proteins, lipids, and nucleic acids.
Inorganic Compounds: Molecules not based on carbon and hydrogen. Includes water, carbon dioxide, oxygen, acids, bases, and salts.
Nutrients: Essential molecules obtained from food.
Metabolites: Molecules made or broken down in the body.

Properties of Water and Electrolytes

Water and Solutions

Water is the most abundant compound in the body, accounting for up to two-thirds of total body weight. It acts as a solvent, forming solutions with solutes dispersed within it.

Electrolytes in Body Fluids

Electrolytes are inorganic ions that conduct electricity in solution. Their balance is vital for normal body function.

Electrolyte	Ions Released
NaCl (sodium chloride)	Na+ + Cl-
KCl (potassium chloride)	K+ + Cl-
CaPO4 (calcium phosphate)	Ca2+ + PO42-
NaHCO3 (sodium bicarbonate)	Na+ + HCO3-
MgCl2 (magnesium chloride)	Mg2+ + 2Cl-
Na2HPO4 (sodium hydrogen phosphate)	2Na+ + HPO42-
Na2SO4 (sodium sulfate)	2Na+ + SO42-

Table of important electrolytes that dissociate in body fluids

pH and Homeostasis

pH Scale and Body Fluids

The pH scale measures the concentration of hydrogen ions (H+) in a solution. It is inversely related to H+ concentration: more H+ means lower pH (acidic), less H+ means higher pH (basic).

Neutral pH: 7.0 (pure water)
Acidic pH: < 7.0 (high H+, low OH-)
Basic pH: > 7.0 (low H+, high OH-)
Human blood pH: 7.35–7.45

pH scale and hydrogen ion concentration

Carbohydrates

Structure and Function

Carbohydrates are organic molecules containing carbon, hydrogen, and oxygen in a 1:2:1 ratio. They are classified by the number of sugar units:

Monosaccharides: Simple sugars (glucose, fructose)
Disaccharides: Two sugars (sucrose, lactose, maltose)
Polysaccharides: Many sugars (glycogen, starch, cellulose)

Structural Class	Examples	Primary Function	Remarks
Monosaccharides	Glucose, fructose	Energy source	Manufactured in the body and obtained from food
Disaccharides	Sucrose, lactose, maltose	Energy source	Must be broken down to monosaccharides before absorption
Polysaccharides	Glycogen	Storage of glucose	Glycogen in animal cells; starch and cellulose in plant cells

Table of carbohydrates in the body

Lipids

Types and Functions

Lipids are hydrophobic molecules, mainly composed of carbon and hydrogen. They include fatty acids, glycerides, steroids, phospholipids, and glycolipids.

Fatty Acids: Long chains with a carboxyl group; can be saturated or unsaturated.
Glycerides: Fatty acids attached to glycerol; triglycerides serve as energy source, insulation, and protection.
Steroids: Four rings of carbon and hydrogen; includes cholesterol, estrogens, testosterone, corticosteroids, and bile salts.
Phospholipids and Glycolipids: Structural lipids with hydrophilic heads and hydrophobic tails; key components of cell membranes.

Steroid structures: cholesterol, estrogen, testosterone Phospholipids and glycolipids forming micelles

Lipid Type	Examples	Primary Functions	Remarks
Fatty acids	Lauric acid	Energy source	Absorbed from food or synthesized in cells
Eicosanoids	Prostaglandins, leukotrienes	Chemical messengers	Produced in most body tissues
Glycerides	Mono-, di-, triglycerides	Energy, insulation, protection	Stored in fat deposits
Steroids	Cholesterol	Structural component, regulation	All have the same carbon ring framework
Phospholipids/glycolipids	Lecithin	Structural components	Derived from fatty acids and nonlipid components

Table of representative lipids and their functions

Nucleic Acids

Structure and Function

Nucleic acids are large organic molecules that store and process information at the molecular level. DNA and RNA are composed of nucleotides, each with a pentose sugar, phosphate group, and nitrogenous base.

DNA: Double-stranded, forms a double helix; stores genetic information.
RNA: Single-stranded; involved in protein synthesis.
Nucleotides: Building blocks; bases include adenine (A), guanine (G), cytosine (C), thymine (T, DNA only), uracil (U, RNA only).

Purine bases: adenine and guanine Structure of nucleic acids: DNA and RNA

Characteristic	RNA	DNA
Sugar	Ribose	Deoxyribose
Nitrogenous bases	A, G, C, U	A, G, C, T
Number of nucleotides	100–50,000	Always more than 45 million
Shape	Varies; three main types	Paired strands, double helix
Function	Protein synthesis	Stores genetic information

Table comparing RNA and DNA

Classes of Inorganic and Organic Compounds

Overview Table

The body contains both inorganic and organic compounds, each with distinct building blocks, sources, and functions.

Class	Building Blocks	Sources	Functions
Water	H, O	Diet/metabolism	Solvent, transport medium
Acids, bases, salts	H+, OH-, ions	Diet/metabolism	Regulate pH, sources of ions
Dissolved gases	C, N, O	Atmosphere/metabolism	Cellular metabolism
Carbohydrates	C, H, O	Diet/manufactured	Energy, structure
Lipids	C, H, O (sometimes P)	Diet/manufactured	Energy, structure, regulation
Proteins	C, H, O, N	Diet/manufactured	Structure, enzymes, regulation
Nucleic acids	C, H, O, N, P	Manufactured	Storage/processing of genetic info
High-energy compounds	Nucleotides, phosphate groups	Synthesized	Storage/transfer of energy

Table of classes of inorganic and organic compounds

Cell Chemistry and Turnover

Turnover Times

Cell components are recycled at different rates depending on cell type and function.

Cell Type	Component	Average Recycling Time
Liver	Total protein	5–6 days
Liver	Enzymes	1 hour to several days
Liver	Glycogen	1–2 days
Liver	Cholesterol	5–7 days
Muscle cell	Total protein	30 days
Muscle cell	Glycogen	12–24 hours
Neuron	Phospholipids	200 days
Neuron	Cholesterol	100+ days
Fat cell	Triglycerides	15–20 days

Table of turnover times for cell components