Biomolecules and Cellular Chemistry — Comprehensive Study Notes Summary & Study Notes

🧩 What is an isomer?

Isomer: molecules with the same molecular formula but different arrangements of atoms. Short examples and types:

• Structural (constitutional) isomers: different connectivity of atoms (different bonding order).
• Cis-trans (geometric) isomers: different spatial arrangement around a double bond or ring (e.g., substituents on same vs opposite sides).
• Stereoisomers (enantiomers): non-superimposable mirror images; important in biology because enzymes and receptors are often chiral and selective.

🔬 Molecular shape and biological activity

The three-dimensional shape of a molecule strongly influences its biological function. Many drugs work because they mimic the shape of natural ligands or substrates, allowing binding to the same receptor or enzyme. Small changes in shape or stereochemistry (e.g., enantiomers) can produce large differences in activity, potency, or safety.

🧪 Major classes of biomolecules — form and function

Overview of the four major classes and their basic roles:

• Carbohydrates: energy storage and structural roles (cell recognition, extracellular matrix).
• Proteins: catalysis (enzymes), structural support, transport, signaling, and regulation.
• Nucleic acids: storage and transmission of genetic information; instructions for building proteins.
• Lipids: energy storage, membrane structure, and signaling molecules.

🧩 Monomers and polymers

• Proteins
  • Monomer: Amino acids. Each amino acid contains an amino group $-NH_2$, a carboxyl group $-COOH$, a side chain (R group), and the central alpha-carbon.
  • Polymer: Polypeptides (chains of amino acids linked by peptide bonds).
• Carbohydrates
  • Monomer: Monosaccharides ("one" sugar).
  • Polymers: Disaccharides (two sugars) and polysaccharides (many sugars; complex carbohydrates).
• Nucleic acids
  • Monomer: Nucleotide (sugar + phosphate + nitrogenous base).
  • Polymer: DNA / RNA polynucleotides.
• Lipids
  • No true monomer; built from components like fatty acids and glycerol but often considered a diverse class rather than a single polymeric type.

🔗 Linkages and reactions

• Carbohydrates form glycosidic linkages between sugar units.
• Proteins form peptide bonds (also called polypeptide bonds) between amino acids.
• Nucleic acids form phosphodiester bonds linking nucleotides along the backbone.
• Polymerization typically occurs by dehydration (condensation) reactions, where a molecule of water ($H_2O$) is released when two monomers join.
• Polymers are broken down by hydrolysis, where $H_2O$ is used to cleave the bond.

🧬 Major differences between DNA and RNA

• Sugar: $DNA$ contains deoxyribose; $RNA$ contains ribose.
• Strandedness: $DNA$ is usually double-stranded; $RNA$ is usually single-stranded.
• Bases: $DNA$ uses thymine ($T$); $RNA$ uses uracil ($U$) instead of $T$.
• Stability and function: $DNA$ is more chemically stable and stores genetic information; $RNA$ is versatile (mRNA, tRNA, rRNA) and participates in expression and regulation.

🧬 Central dogma of molecular biology

The basic flow of genetic information: DNA → RNA → Protein. DNA is transcribed into RNA (messenger RNA), and RNA is translated into protein (polypeptide) by the ribosome.

🧪 Hydrocarbons and polarity

• Hydrocarbon chains are hydrophobic because C–H bonds are largely nonpolar ($C-H$). They do not interact favorably with water.
• Functional groups that contain oxygen (e.g., $-OH$, carbonyls, carboxyl) create polar regions because oxygen is highly electronegative and draws electron density toward itself, enabling hydrogen bonding with water.

🌿 Elements of life

Most organic molecules in living systems are composed mainly of four elements: carbon, hydrogen, oxygen, and nitrogen—often abbreviated as $CHON$. These account for roughly 96% of living matter.

🧾 Phospholipids and membranes

Phospholipids have two distinct halves: a hydrophilic phosphate head (polar) and hydrophobic fatty-acid tails (nonpolar). In aqueous environments, phospholipids self-assemble into bilayers, with heads facing the water on each side and tails sequestered inside—forming the basis of cellular membranes.

🧬 Amino acid structure

All amino acids share a common backbone: an amino group $-NH_2$, a carboxyl group $-COOH$, and an R group (side chain) attached to the central alpha-carbon. The properties of the R group (polar, nonpolar, charged) determine the amino acid's behavior in proteins.

🧩 Four levels of protein structure

• Primary structure: the amino acid sequence of the polypeptide.
• Secondary structure: regular local folding patterns stabilized by hydrogen bonds between backbone groups; common motifs include alpha-helix and beta-pleated sheet.
• Tertiary structure: the overall three-dimensional folding of a single polypeptide, stabilized by interactions among R-groups (hydrophobic interactions, hydrogen bonds, ionic bonds, disulfide bridges).
• Quaternary structure: the assembly of multiple polypeptide chains into a functional protein complex.

⚡ ATP — cellular energy carrier

ATP ($ATP$, adenosine triphosphate) stores and transports chemical energy in the cell in the form of its phosphate groups. Hydrolysis of the terminal (third) phosphate releases energy and yields $ADP$ + $P_i$, which powers many cellular processes.

✅ Key takeaways (quick)

• Isomers = same formula, different arrangement; three common types: structural, geometric, stereoisomers.
• Shape matters: molecular 3D structure dictates biological activity.
• Know the four major biomolecule classes, their monomers/polymers, and the linkages that join them.
• Polymers form by dehydration (release $H_2O$) and break down by hydrolysis (use $H_2O$).
• DNA vs RNA: sugar, bases, strandedness, and function differ.
• Hydrocarbon tails are hydrophobic; oxygen-containing groups are polar.
• Life is mainly built from $CHON$.
• Phospholipids self-assemble into bilayers—fundamental to membranes.
• Amino acids contain $-NH_2$, $-COOH$, and an R group; proteins have four structural levels.
• $ATP$ releases energy via phosphate hydrolysis to drive cellular work.