Fundamentals of Atoms and Chemical Bonding
All matter is composed of atoms, the smallest units that retain the properties of an element. While each atom of a given element shares the same number of protons, the number of neutrons can vary, creating isotopes. The chemical reactivity of an atom is largely determined by its valence electrons, the electrons in the outermost shell. Atoms tend to achieve a stable configuration when their outer shell contains eight electrons, a principle known as the octet rule. Hydrogen is an exception; its single shell can hold only two electrons.
- Atoms with a full outer shell are most stable.
- Hydrogen achieves stability with two electrons, not eight.
Types of Chemical Bonds
Atoms can join together through two primary bonding mechanisms:
- Ionic bonds involve the transfer of electrons from one atom to another, creating oppositely charged ions that attract each other.
- Covalent bonds involve the sharing of one or more pairs of valence electrons between atoms.
When covalent bonds form between polar molecules, a weaker interaction called a hydrogen bond can occur. Hydrogen bonds are crucial in many biological structures, including the network of water molecules.
Water, pH, and Buffers
Water is a polar molecule because oxygen is more electronegative than hydrogen, pulling shared electrons toward itself and creating an uneven charge distribution. This polarity allows water molecules to form extensive hydrogen bonds, giving water its unique physical properties.
The acidity or basicity of a solution is measured by the pH scale, which reflects the concentration of hydrogen ions (H⁺). Pure water has a neutral pH of 7. Substances that maintain a relatively constant pH are called buffers. Buffers work by either releasing H⁺ when a base is added or absorbing H⁺ when an acid is added, thereby stabilizing the solution’s pH.
Macromolecules and Their Building Blocks
Living organisms rely on four major classes of organic molecules:
- Carbohydrates
- Lipids
- Proteins
- Nucleic acids
Carbohydrates
A carbohydrate is an organic compound composed of one or more simple sugars. These sugars serve as quick energy sources and structural components.
Proteins
Proteins are built from amino acids. Each amino acid contains a central carbon atom, an amino group, and a carboxyl group, plus a unique side chain known as the R group. The sequence of amino acids defines the protein’s primary structure, while the three‑dimensional folded shape represents its tertiary structure. Interactions among R groups drive the folding process.
Lipids
Phospholipids are a key lipid class that form cell membranes. Their amphiphilic nature—having a hydrophilic head and hydrophobic tails—causes them to arrange into a bilayer, with heads facing outward toward water and tails tucked inward.
Nucleic Acids
DNA and RNA are polymers of nucleotides. DNA contains the bases adenine (A), thymine (T), cytosine (C), and guanine (G); RNA replaces thymine with uracil. The sugar in RNA is ribose, whereas DNA contains deoxyribose. Base pairing follows specific rules: A pairs with T (or U in RNA) via two hydrogen bonds, while G pairs with C via three hydrogen bonds, making G‑C pairs stronger.
Energy Transformations in Biochemistry
Cellular energy is often stored in the high‑energy phosphate bonds of ATP. These bonds are unstable because the negatively charged phosphate groups repel each other; breaking a terminal bond releases usable energy.
Two fundamental reactions govern the assembly and disassembly of macromolecules:
- Dehydration synthesis joins monomers by removing a water molecule, forming covalent bonds.
- Hydrolysis adds a water molecule to break covalent bonds, separating polymers into monomers.
Special Topics
Nitrogen Fixation
Some bacteria, such as rhizobia, convert atmospheric N₂ into ammonia, a process called nitrogen fixation. This capability is limited to certain leguminous plants that host rhizobia in root nodules; most plants rely on soil nitrogen.
Fatty Acids
Fatty acids differ based on the presence of double bonds:
- Saturated fatty acids have no double bonds, resulting in straight chains that pack tightly.
- Unsaturated fatty acids contain one or more double bonds, introducing kinks that prevent tight packing.
Protein Denaturation
Proteins can lose their functional shape (denature) not only at extremely high temperatures above 100 °C but also under extreme pH conditions or in the presence of certain solvents.
Understanding these foundational concepts provides a solid base for exploring more complex biological systems and biochemical pathways.
