Biochemistry Study Guide: Cell Structure, Water, pH, Nucleic Acids, and Proteins

Cellular Organization

Levels of Molecular Hierarchy in Cells

Cells are organized into a hierarchy of molecular structures, each with distinct functions and properties. Understanding these levels is fundamental to biochemistry.

• Monomers: Small molecules such as amino acids, nucleotides, and monosaccharides.

• Polymers: Macromolecules formed by linking monomers, e.g., proteins, nucleic acids, polysaccharides.

• Supramolecular Assemblies: Complexes of macromolecules, such as ribosomes and membranes.

• Organelles: Specialized cellular compartments (e.g., nucleus, mitochondria).

Prokaryotic vs. Eukaryotic Cells: Prokaryotes lack membrane-bound organelles and a nucleus, while eukaryotes possess these structures. Eukaryotic cells are generally larger and more complex.

• Domains: Bacteria and Archaea are prokaryotic; Eukarya are eukaryotic.

• Examples: Escherichia coli (prokaryote), Homo sapiens (eukaryote).

Cellular Structures: Organelles such as mitochondria, endoplasmic reticulum, and cytoskeleton are unique to eukaryotes and are involved in energy production, protein synthesis, and structural support.

Carbon Chemistry

Bonding and Structure

Carbon's ability to form four covalent bonds allows for a diversity of organic molecules essential for life.

• Tetrahedral Geometry: Carbon forms four single bonds in a tetrahedral arrangement (bond angle ~109.5°).

• Hybridization: sp3 (single bonds), sp2 (double bonds), sp (triple bonds).

• Functional Groups: Common groups include hydroxyl, carboxyl, amino, and phosphate.

Isomerism: Structural isomers differ in connectivity; stereoisomers differ in spatial arrangement.

Molarity

Calculating Concentration

Molarity (M) is a measure of solute concentration in a solution, defined as moles of solute per liter of solution.

• Formula: $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Application: Used to prepare solutions for biochemical experiments.

Chemical Bonding & Properties of H2O

Hydrogen Bonding and Water's Unique Properties

Water's structure and hydrogen bonding confer unique physical and chemical properties essential for life.

• Hydrogen Bonds: Weak interactions between a hydrogen atom covalently bonded to an electronegative atom (like O or N) and another electronegative atom.

• Cohesion and Adhesion: Water molecules stick to each other (cohesion) and to other surfaces (adhesion).

• High Specific Heat: Water absorbs significant heat before changing temperature, stabilizing biological systems.

• Solvent Properties: Water dissolves many ionic and polar substances, facilitating biochemical reactions.

Hydrophobic Effect: Nonpolar molecules aggregate in water, minimizing their exposure to the polar solvent. This drives the folding of proteins and formation of membranes.

pH & Buffers

Acidity, Alkalinity, and Buffer Systems

pH measures the hydrogen ion concentration in a solution, influencing biochemical processes and molecular stability.

• Definition: $\text{pH} = -\log_{10}[\text{H}^+]$

• Acid: Donates protons (H+); Base: Accepts protons.

• Buffer: A solution that resists changes in pH upon addition of acid or base, typically consisting of a weak acid and its conjugate base.

• Henderson-Hasselbalch Equation: $\text{pH} = \text{p}K_a + \log_{10}\left(\frac{[\text{A}^-]}{[\text{HA}]}\right)$

Biological Buffers: Phosphate and bicarbonate systems help maintain physiological pH.

Nucleic Acids: DNA, RNA, & Translation of Proteins

Structure and Function of Nucleic Acids

Nucleic acids store and transmit genetic information. DNA and RNA differ in structure and function.

• Nucleotides: Building blocks composed of a nitrogenous base, a pentose sugar, and a phosphate group.

• DNA: Double-stranded helix, deoxyribose sugar, bases A, T, C, G.

• RNA: Single-stranded, ribose sugar, bases A, U, C, G.

• Base Pairing: A-T (DNA), A-U (RNA), C-G (both), via hydrogen bonds.

• Central Dogma: DNA → RNA → Protein (transcription and translation).

Genetic Code: Triplet codons specify amino acids. There are 64 possible codons for 20 amino acids.

Protein Structure

Levels of Protein Structure

Proteins are polymers of amino acids with complex structures that determine their function.

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Local folding into α-helices and β-sheets, stabilized by hydrogen bonds.

• Tertiary Structure: 3D folding of a single polypeptide chain, stabilized by various interactions (hydrophobic, ionic, disulfide bonds).

• Quaternary Structure: Association of multiple polypeptide chains.

Peptide Bond: Covalent bond between amino acids, exhibiting partial double-bond character due to resonance (conjugated system), restricting rotation.

Protein Folding and Domains

• Domains: Independently folding units within a protein, often associated with specific functions.

• Denaturation: Loss of structure and function due to disruption of non-covalent interactions.

Amino Acids

Properties and Classification

Amino acids are the building blocks of proteins, each with a central α-carbon, amino group, carboxyl group, hydrogen, and unique side chain (R group).

• Classification: Nonpolar, polar uncharged, acidic, basic.

• Essential Amino Acids: Cannot be synthesized by the body and must be obtained from the diet.

• Isoelectric Point (pI): The pH at which an amino acid has no net charge.

Table: Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double-stranded	Single-stranded
Bases	A, T, C, G	A, U, C, G
Function	Genetic information storage	Protein synthesis, gene regulation

Table: Levels of Protein Structure

Level	Description	Stabilizing Forces
Primary	Sequence of amino acids	Peptide bonds
Secondary	α-helix, β-sheet	Hydrogen bonds
Tertiary	3D folding of polypeptide	Hydrophobic interactions, ionic bonds, disulfide bridges
Quaternary	Association of multiple polypeptides	Non-covalent interactions, sometimes disulfide bonds

Additional info:

• Some explanations and context were expanded for clarity and completeness.

• Tables were inferred and formatted for comparison and classification purposes.