Biology Flashcards: Acidity, Buffers, Carbon-Based Molecules, and Functional Groups (Chapters 1–7)

Acidity, pH, and Buffers

• Water dissociation basics

  • Pure water (H₂O) dissociates into hydrogen ions (H⁺) and hydroxide ions (OH⁻).
  • In pure water, concentrations of H⁺ and OH⁻ are equal, as are H₂O molecules.
  • This balance underpins the concept of acidity and basicity in solutions.

• What defines acidity or basicity

  • Acidity: solution has more hydrogen ions (H⁺) than hydroxide ions (OH⁻).
  • Basicity (alkalinity): solution has more hydroxide ions (OH⁻) than hydrogen ions (H⁺).
  • The relative concentrations of these ions determine the pH of the solution.

• The pH scale and its meaning

  • pH scale ranges from 0 to 14; pH 7 is neutral (equal H⁺ and OH⁻).
  • pH < 7 is acidic (higher H⁺ concentration); pH > 7 is basic (higher OH⁻ concentration).
  • The pH scale is logarithmic, not linear.
  • Relationship between pH and hydrogen ion concentration:
  • $pH = -\log_{10}[H^+]$
  • $[H^+] = 10^{-pH}$

• Interpreting changes in pH with examples

  • A difference of 1 pH unit represents a 10-fold change in [H⁺].
  • Example: a solution with pH 5 has 10 times more H⁺ than a solution with pH 6.
  • Example: a solution with pH 3 has 100 times more H⁺ than a solution with pH 5 (since 10² = 100).
  • Conversely, a solution with pH 5 has 1/100 the [H⁺] of a solution with pH 3.

• How acids and bases alter pH

  • Adding an acid to a neutral solution increases H⁺, lowers pH (more acidic).
  • Adding a base to a neutral solution increases OH⁻, lowers H⁺ relative to OH⁻, raises pH (more basic).
  • Some molecules act by either releasing H⁺ or removing H⁺ to shift the balance.

• Buffer systems and why buffers matter

  • Buffers minimize changes in pH when small amounts of acid or base are added.
  • Example discussed: bicarbonate buffering system involving carbonic acid (H₂CO₃) and bicarbonate (HCO₃⁻).
  • Equilibrium concept: H₂CO₃ ⇌ H⁺ + HCO₃⁻.
  • If pH tends to rise (more basic), the buffer provides H⁺ to counteract the base, partly by shifting the equilibrium toward H₂CO₃.
  • If pH tends to drop (more acidic), the buffer consumes free H⁺ by forming H₂CO₃, or more H⁺ is released to form HCO₃⁻, helping to raise pH.
  • In biological systems like blood, buffers maintain homeostasis by resisting drastic pH changes, enabling proper cellular function.

• Practical intuition and homeostatic relevance

  • The concept of buffering is central to maintaining stable internal conditions for cellular processes, membrane potentials, and metabolic reactions.
  • Buffers act as a first line of defense against pH fluctuations caused by metabolic activities or external factors.

• Quick practice framing for exams

  • When given a molecule, identify whether it increases or decreases H⁺ or OH⁻ in solution.
  • Determine whether the molecule behaves as an acid, base, or neutral species in that context.

• Carbon-centered biology: why carbon matters in life

• Carbon and organic molecules

  • Carbon is the backbone of life; organic molecules are carbon-based.
  • Carbon is tetravalent: it has four valence electrons in its outer shell and can form up to four covalent bonds.
  • This tetravalency drives immense molecular diversity and hence functional variety in biology.
  • Visualizing carbon’s valence and bonding patterns helps explain structure-function relationships in biomolecules.

• Carbon chains, hydrocarbons, and scaffolding

  • Hydrocarbons: carbon-hydrogen skeletons (e.g., ethane C₂H₆, propane C₃H₈).
  • General hydrocarbon pattern: CnH₂n+2 for saturated alkanes.
  • Branching and double bonds introduce diversity in structure and function.
  • Carbon chains can be straight or branched; branching changes how enzymes and receptors fit substrata.
  • Rings are also possible, adding cyclic structures to the diversity.
  • These structural variations underlie differences in physical properties and biological roles.

• Isomers: same formula, different structure

  • Isomers are molecules with the same molecular formula but different structures.
  • Structural (constitutional) isomers: same atoms, different covalent bond connectivity (e.g., different branching pattern).
  • Stereoisomers: same covalent connectivity but different spatial arrangement around one or more bonds.
  • Cis/trans (geometric) isomers: around a double bond, cis means substituents on the same side; trans means opposite sides. Rotation around a double bond is not allowed.
  • Enantiomers: non-superimposable mirror images around a chiral center (often a carbon atom with four different substituents).
  • The same molecular formula can yield a wide range of properties (e.g., taste, smell, metabolism, pharmacological activity), illustrating the phrase “structure leads to function.”

• Why isomerism matters in biology and medicine

  • Different isomers can have dramatically different biological activities:
  • One may be pharmacologically active while another is inactive or harmful.
  • In metabolism, one isomer may be a usable substrate while another is not.
  • Understanding isomerism is essential for drug design, metabolic studies, and predicting molecule behavior in cells.

• Practice cue: recognizing isomers

  • Be able to identify structural isomers by comparing connectivity.
  • Be able to distinguish cis/trans by which substituents lie on the same or opposite sides of a double bond.
  • Be able to recognize enantiomers as non-superimposable mirror images, reflecting different spatial arrangements around a chiral center.

• Functional groups: key players in biochemistry

  • Functional groups are specific clusters of atoms that confer characteristic properties and reactivity to molecules.
  • For introductory biology, seven major functional groups are emphasized.
  • In practice, these groups are attached to an R group, which represents the rest of the molecule (the hydrocarbon skeleton or other substituents).
  • Polarity and hydrophilicity/hydrophobicity depend on the functional group and its electronegativity distribution.

• Hydroxyl group (–OH)

  • Also called alcohols when part of a larger molecule.
  • Structure: a hydroxyl group attached to the carbon skeleton (R–OH).
  • Polarity: high electronegativity of oxygen; the group is polar.
  • Hydrophilicity: hydrophilic (water-loving).
  • Acid-base character: generally neutral, but can act as a weak acid by donating a proton (rarely).
  • Relevance: common in carbohydrates, proteins, nucleic acids, lipids; contributes to polarity and hydrogen bonding.

• Carbonyl group (C=O) with two variants

  • General: carbon double-bonded to oxygen (R–C(=O)–R’).
  • Polarity: highly polar due to the strong electronegativity of oxygen.
  • Hydrophilicity: generally hydrophilic.
  • Acidity: carbonyls themselves do not typically ionize; they are neutral under most conditions.
  • Aldehyde vs. ketone
  • Aldehyde: carbonyl carbon at the end of the carbon chain (R–CHO).
  • Ketone: carbonyl carbon in the middle of the chain (R–CO–R’).
  • Note: carbonyls are central to many biochemical reactions and metabolic pathways.

• Carboxyl group (–COOH)

  • Structure: carbon double-bonded to oxygen and single-bonded to a hydroxyl group (–C(=O)OH).
  • Polarity: highly polar.
  • Hydrophilicity: hydrophilic.
  • Acidity: true acids; can readily ionize to form carboxylate (–COO⁻) with H⁺ loss.
  • Relevance: essential in amino acids, fatty acids, and many metabolic intermediates.

• Amino group (–NH₂)

  • Structure: nitrogen with two hydrogens attached (–NH₂).
  • Polarity: polar.
  • Hydrophilicity: hydrophilic.
  • Acidity/basicity: acts as a base; readily accepts a proton to become –NH₃⁺.
  • Relevance: fundamental in amino acids and many biomolecules; contributes to buffering capacity.

• Sulfhydryl group (–SH)

  • Structure: sulfur attached to hydrogen (–SH).
  • Polarity: polar due to electronegativity of sulfur.
  • Hydrophilicity: hydrophilic system, but less polar than –OH.
  • Acidity/basicity: can act as a weak acid; may donate a proton to form –S⁻ under certain conditions.
  • Relevance: important in protein structure (disulfide bonds) and in some metabolic contexts.

• Phosphate group (–PO₄) and related forms

  • Structure: phosphorus atom bound to four oxygens (one or more may be protonated depending on pH).
  • Polarity: highly polar and typically hydrophilic.
  • Acidity/basicity: acidic nature; phosphate groups can donate protons and exist in ionized forms (e.g., –PO₄³⁻, –HPO₄²⁻, –H₂PO₄⁻) depending on pH; often acts as an acid in biological contexts.
  • Relevance: central to energy transfer (ATP), nucleic acids, and signaling molecules.

• Methyl group (–CH₃)

  • Structure: carbon attached to three hydrogens.
  • Polarity: nonpolar; hydrophobic.
  • Acidity/basicity: non-ionizing; typically neutral.
  • Relevance: commonly used as a simple nonpolar substituent; influences hydrophobicity and molecular folding.

• Putting functional groups together

  • The presence and arrangement of these groups determine the molecule’s polarity, solubility, reactivity, and overall biological role.
  • The same functional group can confer different properties depending on its position and the rest of the molecule (R group).

• Practical takeaway for biology students

  • Recognize that polarity and hydrophilicity/hydrophobicity influence where a molecule will localize inside a cell or organism.
  • Understand that functional groups can act as acids or bases, contributing to buffering and reaction chemistry in biological systems.
  • Use the worksheet and practice questions to reinforce the recognition and application of functional groups in context.

• Connections to broader biology themes

  • Structure leads to function: the arrangement of atoms in carbon-based molecules underpins their roles in metabolism, signaling, and genetic information.
  • Homeostasis and biotechnology: buffers, isomerism, and functional groups all influence drug design, nutrient transport, and metabolic efficiency.

• Quick recap of key terms to remember

  • pH, acid, base, buffer, H⁺, OH⁻, isomer, structural isomer, stereoisomer, cis/trans, enantiomer, functional group, R group, hydrophilic, hydrophobic, electronegativity.

• Encouraged study strategies

  • Recall information multiple times, space out study sessions, and revisit concepts with alternate representations (textbook, slides, practice questions).
  • Use real-world contexts (e.g., blood buffering, drug design) to anchor abstract concepts to tangible scenarios.