The Chemical Basis of Life

Introduction to Chemistry in Biology

Chemicals form the foundation of all biological structures and processes. Understanding the basic concepts of chemistry is essential for comprehending the organization and function of living organisms.

• Biological Organization: Atoms are ordered into molecules, representing the lowest level of biological organization.

• Importance: Chemistry underpins the structure and function of cells, tissues, and organs.

Elements, Atoms, and Molecules

Living organisms are composed of approximately 25 essential chemical elements, which are organized into higher levels of structural complexity.

• Element: A substance that cannot be broken down into other substances by ordinary chemical means.

• Matter: Anything that occupies space and has mass.

• Trace Elements: Elements required in minute amounts for proper physiological function.

Trace Elements and Their Biological Importance

Trace elements are common additives to food and water and are vital for preventing diseases.

• Iron (Fe): Essential for oxygen transport in blood.

• Iodine (I): Required for thyroid hormone production; deficiency can cause goiter.

Compounds

A compound is a substance consisting of two or more different elements combined in a fixed ratio. Many compounds in living organisms contain carbon, hydrogen, oxygen, and nitrogen.

• Example: Table salt (NaCl) is formed from sodium (a metal) and chlorine (a poisonous gas), but together they form an edible compound.

• Biological Compounds: DNA contains carbon, hydrogen, oxygen, and nitrogen.

Atomic Structure and Properties

Atoms and Subatomic Particles

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of protons, neutrons, and electrons.

• Proton: Positive charge

• Neutron: No charge (neutral)

• Electron: Negative charge

Element Information: Atomic Number and Mass

Each element is defined by its atomic number (number of protons) and atomic mass (sum of protons and neutrons).

• Atomic Number: Number of protons in the nucleus; also equals the number of electrons in a neutral atom.

• Atomic Mass: Weighted average mass of an atom, based on isotopic abundance.

• Element Symbol: Derived from the element’s common or Latin name.

Isotopes and Atomic Mass

Isotopes are variants of elements with the same number of protons but different numbers of neutrons. Atomic mass reflects the average mass of all isotopes.

• Example: Hydrogen has three isotopes: 1H (protium), 2H (deuterium), and 3H (tritium).

• Atomic Mass Calculation: Weighted average based on natural abundance.

Isotopes of Carbon

Carbon exists as several isotopes, including stable (12C, 13C) and radioactive (14C) forms.

• Radioactive Isotopes: Unstable isotopes that emit energy; used in medical imaging (e.g., PET scans).

Electron Arrangement and Chemical Bonds

Electron Shells and Chemical Properties

Electrons are arranged in energy levels called shells. The number of electrons in the outermost shell determines an atom’s chemical properties.

• First shell: Holds up to 2 electrons

• Second and third shells: Hold up to 8 electrons each

• Atoms seek to fill their outer shells by sharing, donating, or receiving electrons, forming chemical bonds.

Ionic Bonds

An ion is an atom or molecule with an electrical charge resulting from the gain or loss of electrons. Ionic bonds form between oppositely charged ions.

• Example: Sodium (Na) donates an electron to chlorine (Cl), forming Na+ and Cl– ions, which attract to form NaCl.

Covalent Bonds

Covalent bonds result when atoms share outer-shell electrons. Molecules are formed by covalent bonds, following the octet rule for stability.

• Nonpolar Covalent Bonds: Equal sharing of electrons (e.g., H2, O2).

• Polar Covalent Bonds: Unequal sharing of electrons, leading to partial charges (e.g., H2O).

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom (in a polar covalent bond) and an electronegative atom (such as oxygen or nitrogen) in another molecule. These bonds are crucial for the properties of water and biological molecules.

• Cohesion: Hydrogen bonding causes water molecules to stick together, important for transport in plants.

• Surface Tension: Hydrogen bonds create a strong surface tension in water.

Water's Unique Properties

Hydrogen bonds give water unique properties, such as high cohesion, surface tension, and temperature moderation.

• Temperature Moderation: Water resists temperature changes due to hydrogen bonding.

• Density: Ice is less dense than liquid water because hydrogen bonds form a stable crystal structure with space between molecules.

Water as the Solvent of Life

Water is a universal solvent, dissolving many substances due to its polarity. Solutions consist of a solute (dissolved substance) and a solvent (dissolving agent).

• Example: Water dissolves table salt (NaCl) by attracting Na+ and Cl– ions.

Chemical Reactions and Energy

Types of Chemical Reactions

Chemical reactions involve making and breaking bonds, changing the composition of matter. Types include synthesis, decomposition, exchange, and reversible reactions.

• Synthesis:

• Decomposition:

• Exchange:

• Reversible:

Energy in Chemical Reactions

Energy is required to initiate chemical reactions (activation energy). Chemical energy is stored in bonds and released during reactions.

• Potential Energy: Stored energy, such as in chemical bonds.

• Kinetic Energy: Energy of motion, used to perform work.

Acidic and Basic Conditions

Acids, Bases, and pH

Acids increase the concentration of hydrogen ions (H+) in solution, while bases decrease it by accepting H+ or providing hydroxide ions (OH–). The pH scale measures the acidity or basicity of a solution.

• Acidic Solution: Higher concentration of H+ than OH–.

• Basic Solution: Higher concentration of OH– than H+.

• Neutral Solution: Equal concentrations of H+ and OH–.

Summary Table: Elements in the Human Body

Trace elements (less than 0.01%): boron (B), chromium (Cr), cobalt (Co), copper (Cu), fluorine (F), iodine (I), iron (Fe), manganese (Mn), molybdenum (Mo), selenium (Se), silicon (Si), tin (Sn), vanadium (V), and zinc (Zn).

Conclusion

The chemical basis of life is fundamental to understanding anatomy and physiology. Elements, atoms, molecules, and their interactions through chemical bonds and reactions form the structural and functional foundation of living organisms. Mastery of these concepts is essential for further study in biological sciences.