The Chemical Context of Life

Introduction

Understanding the chemical basis of life is essential for the study of Anatomy & Physiology. Biological processes depend on the properties and interactions of chemical elements and compounds. This section introduces the fundamental chemical concepts that underpin the structure and function of 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 their atomic number and chemical properties.

• Elements are grouped into categories such as metals, nonmetals, and metalloids.

• Each element is identified by a unique symbol (e.g., H for hydrogen, O for oxygen).

Major Elements in Biological Molecules (CHONPS)

Six elements are especially important in the molecules that make up living things:

• Carbon (C): Forms the backbone of organic molecules.

• Hydrogen (H): Found in water and organic compounds.

• Oxygen (O): Essential for cellular respiration and water.

• Nitrogen (N): Key component of proteins and nucleic acids.

• Phosphorus (P): Important in nucleic acids and energy transfer (ATP).

• Sulfur (S): Found in some amino acids and vitamins.

CHONPS elements are the primary building blocks of biological macromolecules.

Elemental Composition of the Human Body

Distribution of Elements

• Oxygen: ~62% (mostly in water)

• Carbon: ~18%

• Hydrogen: ~9.5%

• Nitrogen: ~3.2%

• Calcium: ~1.5%

• Phosphorus: ~1%

• Other elements: Sodium, Chlorine, Magnesium, Sulfur (trace amounts)

These elements combine to form the molecules necessary for life, such as water, proteins, carbohydrates, lipids, 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.

Example: Carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Isotopes

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

• Example: Carbon-12 (6 neutrons), Carbon-13 (7 neutrons), Carbon-14 (8 neutrons; radioactive).

Molecules, Compounds, and Ions

Molecules

• Molecule: Two or more atoms held together by chemical bonds.

• Examples: O2 (oxygen gas), N2 (nitrogen gas), H2O (water).

Compounds

• Compound: A substance formed by the chemical combination of two or more different elements.

• Examples: NaCl (table salt), C6H12O6 (glucose).

Ions

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

• Examples:

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

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

Complex Ions

• Examples: OH- (hydroxide), HCO3- (bicarbonate), NO3- (nitrate), PO43- (phosphate), NH4+ (ammonium), C2H3O2- (acetate).

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 in one molecule and an electronegative atom (such as oxygen or nitrogen) in another molecule.

Hydrogen bonds are crucial for the properties of water and the structure of biological molecules like DNA and proteins.

Organic Molecules

Major Classes

• Carbohydrates: Energy source and structural components (e.g., glucose, starch).

• Lipids: Energy storage, cell membrane structure (e.g., fats, oils).

• Proteins: Enzymes, structural support, transport (e.g., hemoglobin, collagen).

• Nucleic Acids: Genetic information storage and transfer (e.g., DNA, RNA).

Chemical Reactions in Biology

Types of Chemical Reactions

• Synthesis (Anabolism): Combining smaller molecules to form larger ones.

  • Example:

• Decomposition (Catabolism): Breaking down larger molecules into smaller ones.

  • Example:

• Exchange (Rearrangement): Atoms are exchanged between molecules.

  • Example:

Energy in Chemical Reactions

• Exergonic Reaction: Releases energy; spontaneous.

• Endergonic Reaction: Absorbs energy; nonspontaneous.

Example of exergonic reaction: Cellular respiration

Example of endergonic reaction: ATP synthesis

ATP: The Energy Currency of the Cell

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.

• High heat of vaporization: Important for cooling mechanisms (e.g., sweating).

• Polarity: Water is a universal solvent due to its polar nature.

• Reactivity: Participates in hydrolysis and condensation reactions.

Acids, Bases, and pH

Definitions

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

• Acid: Proton donor; increases H+ concentration.

• Base: Proton acceptor; decreases H+ concentration.

• Neutralization: Reaction between acid and base to form water and a salt.

pH Scale

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

Examples of pH Values

Buffers

• Buffer: Substance that resists changes in pH by absorbing or releasing H+ ions.

• Example: Bicarbonate buffer system in blood maintains pH around 7.4.

Buffer reactions:

• If blood is too basic: (pH drops)

• If blood is too acidic: (pH rises)

Practice Questions

• How many neutrons in N?

• Which of the following pH’s is most acidic? 3, 7, 6, 2

• The symbols K, Na, C, and S are:

• Which of the following are elements?

• Which of the following are pure substances?

Additional info: This guide expands on fragmented notes to provide a coherent overview of chemical principles relevant to Anatomy & Physiology, including atomic structure, chemical bonding, water properties, and acid-base balance in the human body.