Comprehensive theory, key formulas, diagrams, and memory aids for Biomolecules.
Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield them upon hydrolysis. They serve as primary energy sources, structural components, and cellular recognition markers.
Monosaccharides are the simplest sugars and cannot be hydrolyzed into smaller units. They are classified by the number of carbon atoms (triose, pentose, hexose) and their functional group (aldose or ketose). - Glucose: An aldohexose, the primary fuel source for cellular respiration. - Fructose: A ketohexose found in fruits and honey, isomeric to glucose. - Galactose: An aldohexose, a component of milk sugar (lactose).
In solution, pentoses and hexoses spontaneously cyclize to form hemiacetal or hemiketal rings: - Pyranose ring: A six-membered ring (e.g., glucopyranose). - Furanose ring: A five-membered ring (e.g., fructofuranose).
Cyclization creates a new chiral center at the carbonyl carbon (anomeric carbon), resulting in two anomers: $\alpha$ (hydroxyl group pointing down in Haworth projection) and $\beta$ (hydroxyl group pointing up).
Disaccharides consist of two monosaccharide units joined by a covalent glycosidic bond formed via a condensation reaction. - Maltose: Two $\alpha$-D-glucose units joined by an $\alpha(1\rightarrow4)$ glycosidic bond. - Lactose: $\beta$-D-galactose and $\beta$-D-glucose joined by a $\beta(1\rightarrow4)$ bond. - Sucrose: $\alpha$-D-glucose and $\beta$-D-fructose joined by an $\alpha(1\rightarrow2)\beta$ glycosidic bond. Since both anomeric carbons participate in the bond, sucrose is a non-reducing sugar.
Polysaccharides are long chains of monosaccharide units. They function in energy storage or structural support. - Starch: Plant storage polysaccharide consisting of amylose (unbranched, $\alpha(1\rightarrow4)$ linkages) and amylopectin (branched, $\alpha(1\rightarrow4)$ with $\alpha(1\rightarrow6)$ branches every 24–30 residues). - Glycogen: Animal storage polysaccharide, structurally similar to amylopectin but much more highly branched (branches every 8–12 residues), allowing rapid mobilization of glucose. - Cellulose: Structural component of plant cell walls, linear polymer of $\beta$-D-glucose units with $\beta(1\rightarrow4)$ linkages. The $\beta$-linkage allows straight, extended chains to pack tightly into rigid microfibrils held together by hydrogen bonds, which animals cannot digest without specialized cellulase enzymes.
Lipids are a heterogeneous group of hydrophobic organic molecules soluble in nonpolar solvents. They play key roles in energy storage, membrane structure, and signaling.
Fatty acids are carboxylic acids with long hydrocarbon chains. - Saturated fatty acids: No double bonds between carbon atoms (e.g., palmitic acid). Typically solid at room temperature. - Unsaturated fatty acids: One or more double bonds (e.g., oleic acid). The double bonds are usually in the cis conformation, introducing a kink that prevents tight packing, making them liquid at room temperature.
Triglycerides (triacylglycerols) are composed of a glycerol molecule esterified to three fatty acids. They are the primary long-term energy storage molecules in animals due to their highly reduced state and anhydrous storage format.
Phospholipids are amphipathic molecules composed of: - A glycerol backbone (or sphingosine in sphingomyelin). - Two hydrophobic fatty acid tails. - A hydrophilic head group containing a phosphate group and a polar molecule (e.g., choline, ethanolamine).
In water, phospholipids spontaneously organize into bilayers, forming the structural foundation of all biological membranes.
graph TD
A[Phospholipid Molecule] --> B[Hydrophilic Head: Phosphate + Polar Group]
A --> C[Hydrophobic Tails: Two Fatty Acid Chains]
B -->|Exposed to| D[Aqueous Environment]
C -->|Aggregated in| E[Membrane Core]
D & E --> F[Forms Semi-Permeable Lipid Bilayer]
Proteins are linear polymers of amino acids that carry out almost all cellular functions.
All amino acids share a common structure: a central carbon ($\alpha$-carbon) bonded to a hydrogen atom, an amino group ($-NH_2$), a carboxyl group ($-COOH$), and a variable side chain ($R$-group). The $R$-group determines the physical and chemical properties of each amino acid (polar, nonpolar, acidic, basic).
At physiological pH (~7.4), amino acids exist as zwitterions, where the amino group is protonated ($-NH_3^+$) and the carboxyl group is deprotonated ($-COO^-$).
graph LR
A[Amino Acid at pH 7] --> B[Zwitterionic Form]
B --> C["Carboxyl Group: -COO⁻ (Negative)"]
B --> D["Amino Group: -NH₃⁺ (Positive)"]
B --> E["R-Group: Variable Side Chain"]
Amino acids are linked by peptide bonds (amide bonds) formed between the carboxyl carbon of one amino acid and the nitrogen of the next via a condensation reaction. Protein structure is organized into four levels:
Nucleic acids (DNA and RNA) are polymers of nucleotides that store, transmit, and express genetic information.
A nucleotide consists of three components: 1. A pentose sugar: ribose in RNA, 2'-deoxyribose in DNA. 2. A phosphate group attached to the 5'-carbon of the sugar. 3. A nitrogenous base attached to the 1'-carbon.
Nitrogenous bases are divided into: - Purines: Double-ring structures — Adenine (A) and Guanine (G). - Pyrimidines: Single-ring structures — Cytosine (C), Thymine (T, found in DNA), and Uracil (U, found in RNA).
Nucleotides are linked by phosphodiester bonds between the 3'-hydroxyl of one sugar and the 5'-phosphate of the next, creating a sugar-phosphate backbone with a 5' end and a 3' end.
DNA exists as a double-stranded helix of anti-parallel chains held together by hydrogen bonds between complementary base pairs (Watson-Crick base pairing): - Adenine pairs with Thymine (2 hydrogen bonds). - Guanine pairs with Cytosine (3 hydrogen bonds).
The higher G-C content of a DNA molecule increases its thermal stability due to the extra hydrogen bonds.
Vitamins are essential organic micronutrients that the body cannot synthesize in sufficient quantities. - Water-soluble vitamins: Vitamin B complex and Vitamin C. They act primarily as precursors for coenzymes in metabolic pathways. For example, Thiamine ($B_1$) is converted to thiamine pyrophosphate (TPP) for decarboxylation reactions, and Riboflavin ($B_2$) forms FAD. - Fat-soluble vitamins: Vitamins A, D, E, and K. They are absorbed along with dietary fats and stored in the liver and adipose tissue. Vitamin A is essential for vision; Vitamin D regulates calcium homeostasis; Vitamin E acts as an antioxidant; Vitamin K is required for blood clotting factor synthesis.
Minerals are inorganic ions required in small quantities for critical physiological processes: - Calcium ($Ca^{2+}$): Structural component of bones and teeth; essential for muscle contraction, blood clotting, and intracellular signaling. - Iron ($Fe^{2+}/Fe^{3+}$): Oxygen binding center in hemoglobin and myoglobin; electron carrier in cytochromes. - Sodium ($Node^+$) and Potassium ($K^+$): Maintenance of osmotic balance, fluid volume, and generation of action potentials across neuronal membranes. - Magnesium ($Mg^{2+}$): Crucial cofactor for over 300 enzymes, particularly those utilizing ATP (since ATP binds to magnesium as $Mg\text{-}ATP^{2-}$).