Introduction to Chemistry in Biology

The Chemical Basis of Life

All living organisms are composed of chemicals, which are organized into atoms, molecules, organelles, and cells. Understanding the structure and behavior of atoms and molecules is fundamental to biology, as these principles underlie the structure and function of biomolecules and cells.

• Atoms are the basic units of matter, composed of protons, neutrons, and electrons.

• Molecules are formed when atoms bond together through chemical interactions.

• Biological molecules, such as proteins, lipids, carbohydrates, and nucleic acids, are essential for life.

Chemical Elements in Living Organisms

Major and Trace Elements

Living organisms are primarily composed of a few major elements, with trace elements required in smaller amounts. These elements are essential for the structure and function of biomolecules.

Atomic Structure

Subatomic Particles

Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in shells. The number of protons defines the element, while the arrangement of electrons determines chemical behavior.

• Proton: Positively charged particle in the nucleus.

• Neutron: Neutral particle in the nucleus.

• Electron: Negatively charged particle in electron shells around the nucleus.

Atomic Number and Mass Number

• Atomic number is the number of protons in an atom and defines the element.

• Mass number is the sum of protons and neutrons in the nucleus.

For example, carbon has 6 protons and typically 6 neutrons, giving it a mass number of 12.

Electron Shells and Valence Electrons

Electrons are arranged in shells around the nucleus. The outermost shell, called the valence shell, determines the chemical reactivity of the atom.

• The first shell holds up to 2 electrons.

• The second shell holds up to 8 electrons.

• Atoms are most stable when their valence shell is full (the "octet rule").

Chemical Bonds

Types of Chemical Bonds

Atoms interact to achieve stable electron configurations, forming chemical bonds. The main types of bonds in biology are ionic, covalent, and hydrogen bonds.

• Ionic Bonds: Formed when electrons are transferred from one atom to another, creating ions with opposite charges that attract each other.

• Covalent Bonds: Formed when two atoms share one or more pairs of electrons. Can be polar (unequal sharing) or nonpolar (equal sharing).

Ionic Bond Example: Sodium Chloride (NaCl)

Sodium (Na) donates an electron to chlorine (Cl), forming Na+ and Cl- ions, which are held together by ionic bonds to form table salt.

Covalent Bonding and the HONC Rule

The number of covalent bonds an atom can form is determined by the number of electrons needed to fill its valence shell. The "HONC" rule summarizes the typical bonding patterns of hydrogen, oxygen, nitrogen, and carbon:

• Hydrogen (H): 1 bond

• Oxygen (O): 2 bonds

• Nitrogen (N): 3 bonds

• Carbon (C): 4 bonds

Polarity and Electronegativity

Polarity arises when atoms in a covalent bond have different electronegativities, causing unequal sharing of electrons. This results in partial charges (δ+ and δ-) on the atoms.

• Nonpolar covalent bond: Electrons are shared equally (e.g., H2).

• Polar covalent bond: Electrons are shared unequally (e.g., H2O).

• Electronegativity: The ability of an atom to attract shared electrons. Oxygen and nitrogen are highly electronegative.

Redox Reactions

Redox (reduction-oxidation) reactions involve the transfer of electrons between atoms or molecules. These reactions are fundamental to energy transfer in biological systems.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Mnemonic: O.I.L. R.I.G. (Oxidation Is Loss, Reduction Is Gain)

Biological Molecules and Their Components

Major Classes of Biomolecules

Biomolecules are built from atoms and are essential for life. The four major classes are:

• Proteins: Polymers of amino acids; perform structural, enzymatic, and regulatory functions.

• Lipids: Hydrophobic molecules; important for membranes and energy storage.

• Carbohydrates: Sugars and polymers of sugars; provide energy and structural support.

• Nucleic Acids: DNA and RNA; store and transmit genetic information.

Cellular Organization

Cell Structure and Organelles

Cells are the basic units of life, containing specialized structures called organelles that perform distinct functions. Eukaryotic cells have membrane-bound organelles, while prokaryotic cells do not.

• Nucleus: Contains DNA and controls cellular activities.

• Mitochondria: Site of cellular respiration and energy production.

• Endoplasmic Reticulum (ER): Synthesizes proteins and lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes and Peroxisomes: Involved in digestion and detoxification.

Summary Table: Key Chemical Concepts in Biology

Additional info: Understanding atomic structure, chemical bonding, and the properties of biomolecules is foundational for all topics in General Biology, including cell structure, metabolism, genetics, and physiology.