Chapter 2: The Chemical Level of Organization – Study Notes

Chapter 2: The Chemical Level of Organization

Atoms and Atomic Structure

The atom is the fundamental unit of matter, composed of subatomic particles: protons, neutrons, and electrons. Understanding atomic structure is essential for grasping the chemical basis of anatomy and physiology.

• Protons (p+): Positively charged particles found in the nucleus.

• Neutrons (n): Neutral particles also located in the nucleus.

• Electrons (e-): Negatively charged particles that orbit the nucleus in an electron cloud.

• Atomic number: The number of protons in an atom, unique to each element.

• Electron cloud: The region around the nucleus where electrons are likely to be found.

Elements and Isotopes

Elements are pure substances consisting of only one type of atom, defined by their atomic number. Isotopes are variants of elements with different numbers of neutrons.

• Element: Substance with atoms of the same number of protons; cannot be broken down by ordinary means.

• Chemical symbol: Abbreviation for each element (e.g., O for oxygen, Na for sodium).

• Isotopes: Atoms of the same element with different numbers of neutrons, thus different mass numbers.

• Radioisotopes: Unstable isotopes that emit radiation as they decay; used in medical diagnostics (e.g., PET scans).

• Half-life: The time required for half of a radioactive isotope to decay.

Electrons and Energy Levels

Electrons occupy specific energy levels or shells around the nucleus. The arrangement of electrons determines the chemical properties and reactivity of an atom.

• Electron shells: Energy levels where electrons reside; the first shell holds up to 2 electrons, the second and third up to 8 each.

• Valence shell: The outermost electron shell; determines chemical reactivity.

• Stable atoms: Atoms with filled valence shells are chemically inert; those with unfilled shells are reactive.

Molecules and Compounds

Atoms combine to form molecules and compounds through chemical bonds. The type of bond and the elements involved determine the properties of the resulting substance.

• Molecule: Two or more atoms held together by covalent bonds (e.g., H2, O2, H2O).

• Compound: Substance composed of atoms of two or more different elements (e.g., NaCl, C6H12O6).

• Chemical bonds: Forces holding atoms together, including ionic and covalent bonds.

Ions and Ionic Bonds

Ions are charged atoms or groups of atoms formed by the loss or gain of electrons. Ionic bonds result from the electrostatic attraction between oppositely charged ions.

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

• Ionic bond: Bond formed by the attraction between cations and anions (e.g., NaCl).

Covalent Bonds

Covalent bonds are formed when atoms share electrons. These bonds can be single, double, or triple, depending on the number of shared electron pairs.

• Nonpolar covalent bond: Equal sharing of electrons (e.g., H2, O2).

• Polar covalent bond: Unequal sharing of electrons, resulting in partial charges (e.g., H2O).

Hydrogen Bonds

Hydrogen bonds are weak attractions between the partial positive charge of a hydrogen atom and the partial negative charge of another atom (often oxygen or nitrogen). They are crucial in stabilizing the structures of proteins and nucleic acids.

• Example: Hydrogen bonds between water molecules and within DNA.

Chemical Reactions

Chemical reactions involve the making or breaking of bonds between atoms. They are represented by chemical equations and are essential for all physiological processes.

• Decomposition reaction: AB → A + B

• Hydrolysis: AB + H2O → AH + BOH

• Synthesis reaction: A + B → AB

• Dehydration synthesis: AH + BOH → AB + H2O

• Exchange reaction: AB + CD → AD + CB

• Reversible reaction: A + B $\leftrightarrow$ AB

pH and Homeostasis

pH measures the concentration of hydrogen ions in a solution. Maintaining pH within a narrow range is vital for cellular function and overall homeostasis.

• Neutral pH: 7 (equal H+ and OH- ions)

• Acidic pH: < 7 (more H+ ions)

• Basic (alkaline) pH: > 7 (fewer H+ ions)

• Blood pH: Normally 7.35–7.45; deviations can cause acidosis or alkalosis.

• Regulation: Achieved via respiratory and renal mechanisms.

Macromolecules

Macromolecules are large, complex molecules essential for life. They include carbohydrates, lipids, proteins, and nucleic acids, each with specific monomers and functions.

• Carbohydrates: Energy sources; monomer is the monosaccharide (e.g., glucose).

• Lipids: Energy storage, insulation, and membrane structure; monomers include fatty acids and glycerol.

• Proteins: Structure, transport, enzymes, and defense; monomer is the amino acid.

• Nucleic acids: Information storage and transfer; monomer is the nucleotide.

Carbohydrates

Carbohydrates are organic molecules with a 1:2:1 ratio of carbon, hydrogen, and oxygen. They serve as primary energy sources.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

• Isomers: Molecules with the same formula but different structures (e.g., glucose and fructose).

Lipids

Lipids are hydrophobic molecules with diverse functions, including energy storage and membrane structure.

• Fatty acids: Saturated (no double bonds) or unsaturated (one or more double bonds).

• Glycerides: Fatty acids attached to glycerol (mono-, di-, triglycerides).

• Steroids: Four-ring structures (e.g., cholesterol, hormones).

• Phospholipids: Major components of cell membranes.

Proteins

Proteins are polymers of amino acids and perform a wide range of functions in the body.

• Functions: Support, movement, transport, buffering, metabolic regulation, coordination, control, and defense.

• Structure: Four levels—primary (sequence), secondary (alpha helix, beta sheet), tertiary (3D folding), quaternary (multiple polypeptides).

• Enzymes: Proteins that catalyze biochemical reactions by lowering activation energy.

Nucleic Acids

Nucleic acids store and transmit genetic information. DNA and RNA are the two main types, composed of nucleotide monomers.

• DNA: Double-stranded, contains A, T, C, G; stores genetic information.

• RNA: Single-stranded, contains A, U, C, G; involved in protein synthesis.

• Nucleotide: Composed of a pentose sugar, phosphate group, and nitrogenous base.

High-Energy Compounds

Cells use high-energy compounds, such as ATP, to perform work. These compounds store energy in covalent bonds, especially those involving phosphate groups.

• ATP (Adenosine triphosphate): The primary energy carrier in cells.

• Phosphorylation: Addition of a phosphate group to a molecule.

• ATPase: Enzyme that catalyzes the breakdown of ATP to ADP, releasing energy.

• Equation: $\mathrm{ADP} + \text{phosphate group} + \text{energy} \leftrightarrow \mathrm{ATP} + \mathrm{H}_2\mathrm{O}$