GACE Biology (6–12) (710)

All matter consists of atoms, the fundamental units of chemical elements. Understanding atomic structure is essential for comprehending biological molecules and chemical reactions within living systems.

An atom contains three subatomic particles: protons, neutrons, and electrons. Protons carry a positive charge and reside in the nucleus. Neutrons carry no charge and also reside in the nucleus. Electrons carry a negative charge and orbit the nucleus in electron shells or energy levels.

The atomic number equals the number of protons in an atom's nucleus and defines the element. The mass number equals the sum of protons and neutrons. Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers but identical chemical properties.

Electrons occupy discrete energy levels around the nucleus. The first shell holds a maximum of 2 electrons. The second shell holds a maximum of 8 electrons. The third shell holds a maximum of 18 electrons, though it typically fills to 8 before the fourth shell begins filling.

Valence electrons occupy the outermost shell and determine an atom's chemical reactivity. Atoms tend toward stability by filling their valence shell, either by gaining, losing, or sharing electrons with other atoms.

Ionic bonds form when one atom transfers electrons to another atom. The atom losing electrons becomes a positively charged cation. The atom gaining electrons becomes a negatively charged anion. Electrostatic attraction holds these oppositely charged ions together. Sodium chloride (NaCl) exemplifies ionic bonding.

Covalent bonds form when atoms share electron pairs. Nonpolar covalent bonds involve equal sharing of electrons between atoms with similar electronegativity. Polar covalent bonds involve unequal sharing, creating partial positive and negative charges on different regions of the molecule. Water (H₂O) contains polar covalent bonds.

Hydrogen bonds are weak attractions between a hydrogen atom covalently bonded to an electronegative atom (oxygen, nitrogen, or fluorine) and another electronegative atom. Though individually weak, hydrogen bonds collectively provide significant structural stability in biological molecules like DNA and proteins.

Van der Waals forces are weak, temporary attractions resulting from momentary uneven electron distribution. These forces contribute to molecular interactions in biological systems.

Water's polar nature produces unique properties essential for life. Cohesion refers to water molecules' attraction to each other through hydrogen bonding. Adhesion refers to water molecules' attraction to other polar surfaces. These properties enable capillary action in plants.

Water has high specific heat capacity, meaning it resists temperature changes. This property stabilizes environmental and internal body temperatures. Water has high heat of vaporization, meaning significant energy is required to convert liquid water to vapor. This property enables evaporative cooling in organisms.

Water serves as an excellent solvent for polar and ionic substances. Hydrophilic substances dissolve readily in water. Hydrophobic substances do not dissolve in water and tend to aggregate. Amphipathic molecules contain both hydrophilic and hydrophobic regions, important for cell membrane structure.

The pH scale measures hydrogen ion concentration on a scale from 0 to 14. A pH of 7 is neutral. Values below 7 indicate acidic conditions (higher H⁺ concentration). Values above 7 indicate basic or alkaline conditions (lower H⁺ concentration).

Acids release hydrogen ions in solution. Bases accept hydrogen ions or release hydroxide ions. Buffers resist pH changes by accepting or donating hydrogen ions as needed. Biological systems rely on buffer systems to maintain pH homeostasis. The bicarbonate buffer system maintains blood pH near 7.4.