The Chemical Context of Life

Introduction

Understanding the chemical basis of life is essential for the study of Anatomy & Physiology. Biological processes are governed by the properties and interactions of chemical elements and compounds, especially those that make up living organisms.

Elements and the Periodic Table

Definition of Elements

• Element: A pure substance that cannot be broken down into simpler chemical entities by ordinary chemical reactions.

• There are 118 known elements, each represented on the Periodic Table.

The Periodic Table

• The Periodic Table organizes elements by increasing atomic number and groups elements with similar chemical properties.

• Elements are classified as metals, nonmetals, and metalloids.

• Each element is identified by its symbol, atomic number, and atomic mass.

Major Elements in Biological Molecules (CHONPS)

Six elements are most abundant in living organisms and are essential for the structure and function of biological molecules:

• Carbon (C)

• Hydrogen (H)

• Oxygen (O)

• Nitrogen (N)

• Phosphorus (P)

• Sulfur (S)

These elements form the backbone of carbohydrates, lipids, proteins, and nucleic acids.

Atoms and Subatomic Particles

Structure of Atoms

• Atom: The smallest unit of an element, composed of subatomic particles.

• Subatomic particles:

  • Proton (p+): Positively charged, found in the nucleus.

  • Neutron (n0): Neutral charge, found in the nucleus.

  • Electron (e-): Negatively charged, orbits the nucleus.

• Atomic number (Z): Number of protons in the nucleus; determines the element.

• Atomic mass: Sum of protons and neutrons in the nucleus.

Isotopes

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

• Some isotopes are stable, while others are radioactive (unstable).

• Example: Carbon-12, Carbon-13 (stable), Carbon-14 (radioactive).

Molecules, Compounds, and Ions

Molecules and Compounds

• Molecule: Two or more atoms held together by chemical bonds (e.g., O2, N2, H2O).

• Compound: A substance formed by binding two or more different kinds of elements (e.g., NaCl, C6H12O6).

Ions

• Ion: An atom or molecule that has gained or lost electrons, resulting in a net charge.

• Cation: Positively charged ion (e.g., Na+, K+).

• Anion: Negatively charged ion (e.g., Cl-, HCO3-).

• Complex ions: Groups of atoms with a net charge (e.g., OH-, PO43-).

Chemical Bonds

Types of Chemical Bonds

• Ionic Bonds: Formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions that attract each other.

• Covalent Bonds: Formed by the sharing of electrons between atoms.

• Hydrogen Bonds: Weak attractions between a hydrogen atom and an electronegative atom (such as oxygen or nitrogen), important in water and biological molecules.

Chemical Reactions

Types of Chemical Reactions

• Synthesis (Combination): Two or more reactants combine to form a larger product.

• Decomposition: A compound breaks down into simpler substances.

• Exchange (Rearrangement): Parts of molecules are exchanged to form new products.

Energy in Chemical Reactions

• Exergonic Reaction: Releases energy; spontaneous.

• Endergonic Reaction: Absorbs energy; nonspontaneous.

• Example: Cellular respiration is exergonic; ATP synthesis is endergonic.

ATP: The Energy Currency

Structure and Function

• Adenosine triphosphate (ATP): Molecule that stores and transfers energy in cells.

• Hydrolysis of ATP releases energy:

• Dehydration (synthesis) of ATP stores energy:

Properties of Water

Physical Properties

• Boiling Point: 100°C

• Freezing Point: 0°C

• Density: 1 g/cm3

Chemical Properties

• High heat capacity: Absorbs and releases large amounts of heat with little temperature change.

• High heat of vaporization: Requires significant energy to change from liquid to gas; important for cooling (sweat).

• Polarity: Water molecules have a partial positive and negative charge, making water a universal solvent.

• Reactivity: Involved in hydrolysis and condensation (dehydration) reactions.

Acids, Bases, and pH

Definitions

• pH: Measures the potential of a substance to attract hydrogen ions (H+).

• Acid: Proton donor; increases H+ concentration (e.g., HCl).

• Base: Proton acceptor; decreases H+ concentration (e.g., NaOH).

• Neutralization: Acid and base react to form water and a salt.

• Buffer: Resists changes in pH; important in maintaining homeostasis (e.g., bicarbonate buffer in blood).

pH Scale

The pH scale is logarithmic; each unit represents a tenfold change in H+ concentration.

Examples of pH Values

Buffer Systems in the Body

• Bicarbonate Buffer: Maintains blood pH around 7.4.

• When blood is too basic: (pH drops)

• When blood is too acidic: (pH rises)

Summary Table: Key Chemical Concepts

Additional info: These chemical principles form the foundation for understanding physiological processes such as metabolism, cellular respiration, and homeostasis in the human body.