Learning Objectives

What is an element, and which four elements form the bulk of body matter?

An element is a pure substance that cannot be broken down by ordinary chemical methods. Oxygen, carbon, hydrogen, and nitrogen make up about 96% of body weight.

How are elements and atoms related?

An atom is the smallest unit of an element that retains its chemical properties. All atoms of an element have the same number of protons.

What are the subatomic particles, their relative masses, charges, and positions?

Protons: positive charge, about 1 amu, in the nucleus. Neutrons: no charge, about 1 amu, in the nucleus. Electrons: negative charge, about 1/1836 amu, in the electron cloud outside the nucleus.

What is a radioisotope, and how is it used to diagnose and treat disease?

A radioisotope is an isotope with an unstable nucleus that releases radiation. Radioisotopes serve as tracers in imaging, such as PET and thyroid scans, and can deliver radiation to destroy cancer cells.

What is a molecule, and how are molecules related to compounds?

A molecule consists of two or more atoms joined by covalent bonds. A compound contains two or more different elements chemically combined. O₂ is a molecule but not a compound; H₂O is both. Ionic compounds, such as NaCl, do not consist of separate molecules.

How do electrons participate in chemical reactions?

Chemical reactions rearrange valence electrons through sharing or transfer, breaking existing bonds and forming new ones.

How do ionic, polar covalent, and nonpolar covalent bonds differ?

Ionic bonds involve attraction between oppositely charged ions. Polar covalent bonds share electrons unequally. Nonpolar covalent bonds share electrons equally or nearly equally.

Why are hydrogen bonds important?

Hydrogen bonds are weak attractions involving a partially positive hydrogen atom and an electronegative atom nearby. They help give water its properties and stabilize protein shapes and DNA’s double helix.

How do synthesis, decomposition, and exchange reactions differ?

Synthesis joins smaller substances: A + B → AB. Decomposition breaks substances apart: AB → A + B. Exchange reactions swap components: AB + CD → AD + CB.

Why is water important to body homeostasis, and what roles does it perform?

Water helps maintain stable temperature, dissolves and transports substances, provides a medium for chemical reactions, participates in hydrolysis, lubricates joints, and cushions organs. Sweat evaporation also removes heat.

Which salts or ions are important to body functioning?

Sodium and potassium support nerve impulses, muscle function, and fluid balance. Calcium supports bones, muscle contraction, and blood clotting. Chloride helps fluid balance and stomach acid formation. Phosphate supports bones, ATP, and nucleic acids. Magnesium supports enzyme activity.

How do a salt, an acid, and a base differ?

A salt is an ionic compound that separates into positive and negative ions in water. An acid donates hydrogen ions (H⁺). A base accepts hydrogen ions; some bases release hydroxide ions (OH⁻) in water.

What is pH, and what is the normal pH of blood?

pH measures hydrogen ion concentration. Below 7 is acidic, 7 is neutral, and above 7 is basic. Each pH unit represents a tenfold change in hydrogen ion concentration. Normal blood pH is about 7.35–7.45.

How do dehydration synthesis and hydrolysis form and break down organic molecules?

Dehydration synthesis joins smaller molecules by removing water. Hydrolysis adds water to break bonds and split larger molecules into smaller ones.

How do carbohydrates and lipids compare in building blocks, structures, and functions?

Both contain carbon, hydrogen, and oxygen. Carbohydrates consist of simple sugars, often joined into chains, and provide quick energy and short-term storage as glycogen. Neutral fats consist of glycerol and three fatty acids and provide long-term energy storage, insulation, and cushioning. Other lipids form membranes or act as hormones.

How do fibrous proteins differ from globular proteins?

Fibrous proteins are elongated and provide strength and structural support, as collagen and keratin do. Globular proteins are compact and perform functions such as catalysis, transport, and defense; examples include enzymes, hemoglobin, and antibodies.

What is an enzyme, and what role do enzymes perform?

An enzyme is a biological catalyst, usually a protein, that speeds up a chemical reaction by lowering activation energy. Enzymes bind specific substrates and are not consumed during the reaction.

How do DNA and RNA compare in structure and function?

Both are nucleic acids made of nucleotides. DNA usually has two strands, contains deoxyribose sugar and the bases A, T, C, and G, and stores hereditary information. RNA usually has one strand, contains ribose sugar and the bases A, U, C, and G, and helps use genetic instructions to make proteins.

Why is ATP important in the body?

ATP, or adenosine triphosphate, transfers energy for cellular work. Its hydrolysis to ADP and phosphate releases usable energy that powers processes such as muscle contraction, active transport, and molecule synthesis.