6.1 Biological Macromolecules & Polymers

Biological macromolecules are large, carbon-based organic molecules essential for cellular life. They are categorized into four major classes: carbohydrates, lipids, proteins, and nucleic acids. Except for lipids, these macromolecules exist as polymers—long chains constructed from smaller, repeating structural units called monomers. The assembly and breakdown of biological polymers depend on two fundamental chemical reactions: dehydration synthesis and hydrolysis.

During dehydration synthesis (a condensation reaction), two monomers are covalently bonded together with the simultaneous removal of a water molecule ($H_2O$). One monomer contributes a hydroxyl group ($-OH$) while the other contributes a hydrogen atom ($-H$). Conversely, hydrolysis breaks down polymers into individual monomers by inserting a water molecule, cleaving the covalent bond linking the subunits. Digestive enzymes utilize hydrolysis to break down ingested food into absorbable units.

Carbohydrates: Monosaccharides, Disaccharides, and Polysaccharides

Carbohydrates serve as the primary source of immediate cellular energy and provide structural components. Chemically, carbohydrates consist of carbon, hydrogen, and oxygen in a characteristic 1:2:1 ratio, represented by the empirical formula $(CH_2O)_n$.

Monosaccharides and Disaccharides

Monosaccharides are the simplest carbohydrate monomers. Key hexoses ($C_6H_{12}O_6$) include glucose, fructose, and galactose. Glucose is the principal fuel utilized in cellular respiration to generate ATP. Glucose, fructose, and galactose share the same chemical formula but are structural isomers with distinct atomic arrangements.

Disaccharides consist of two monosaccharides linked by a covalent glycosidic bond formed through dehydration synthesis:

• Sucrose (table sugar): composed of glucose and fructose.
• Lactose (milk sugar): composed of glucose and galactose.
• Maltose (malt sugar): composed of two glucose molecules.

Polysaccharides

Polysaccharides are long polymers of monosaccharides functioning in energy storage or structural support:

• Glycogen: A highly branched glucose polymer serving as the primary short-term energy storage molecule in animal liver and skeletal muscle.
• Starch: The glucose storage polysaccharide in plants, existing as amylose and amylopectin. Amylase enzymes hydrolyze starch into glucose for energy.
• Cellulose: A rigid structural polymer in plant cell walls built from $\beta$-glucose monomers linked by $\beta$-1,4 glycosidic bonds. Human digestive enzymes cannot hydrolyze $\beta$-1,4 bonds, making cellulose indigestible dietary fiber (roughage).
• Chitin: A structural polysaccharide containing nitrogen-modified glucose units (N-acetylglucosamine) forming fungal cell walls and arthropod exoskeletons.

Lipids: Fats, Phospholipids, and Steroids

Lipids are hydrophobic, nonpolar organic molecules composed primarily of carbon, hydrogen, and minimal oxygen. Lipids are not true polymers because they lack repeating monomer units.

Fatty Acids and Triglycerides

Triglycerides consist of a glycerol backbone attached to three fatty acid chains via ester bonds.

• Saturated Fatty Acids: Contain only single carbon-carbon bonds ($C-C$) in hydrocarbon chains. They pack tightly, making saturated fats solid at room temperature (e.g., butter, lard).
• Unsaturated Fatty Acids: Contain one or more double carbon-carbon bonds ($C=C$), creating kinks that prevent tight packing and keeping them liquid at room temperature (e.g., plant oils).

Phospholipids and Steroids

Phospholipids are amphipathic molecules containing a hydrophilic phosphate head and two hydrophobic fatty acid tails. In aqueous environments, they spontaneously arrange into a phospholipid bilayer, forming the structural foundation of plasma membranes.

Steroids are lipids characterized by four fused carbon rings. Cholesterol is the essential precursor for synthesizing steroid hormones (estrogen, testosterone, cortisol), vitamin D, and bile salts, while also regulating cell membrane fluidity.

Proteins: Structure, Folding, and Enzymatic Catalysts

Proteins perform structural, transport, signaling, and catalytic functions. They are polymers of amino acids linked by peptide bonds.

Amino Acid Structure and Peptide Bonds

Each of the 20 standard amino acids contains a central $\alpha$-carbon bonded to an amino group ($-NH_2$), a carboxyl group ($-COOH$), a hydrogen atom, and a variable side chain (R-group). The R-group determines the amino acid's specific chemical properties. Peptide bonds form via dehydration synthesis between carboxyl and amino groups.

Four Levels of Protein Structure

1. Primary Structure: The unique linear sequence of amino acids encoded by DNA.
2. Secondary Structure: Local folding stabilized by hydrogen bonds along the polypeptide backbone, forming $\alpha$-helices and $\beta$-pleated sheets.
3. Tertiary Structure: The overall 3D shape of a single polypeptide, driven by R-group interactions (hydrophobic interactions, ionic bonds, hydrogen bonds, and covalent disulfide bridges).
4. Quaternary Structure: The association of multiple polypeptide subunits into a functional protein (e.g., hemoglobin).

Enzymes as Biological Catalysts

Enzymes are protein catalysts that accelerate chemical reactions by lowering the activation energy ($E_a$). Enzymes bind specific substrates at their active site via an induced fit model. Extreme temperatures or non-optimal pH levels cause denaturation, altering protein shape and destroying catalytic activity.

Nucleic Acids: DNA vs. RNA

Nucleic acids store and transmit genetic information. Their monomer units are nucleotides, consisting of a 5-carbon sugar, a phosphate group, and a nitrogenous base.

• DNA (Deoxyribonucleic Acid): Uses the sugar deoxyribose and forms a double-stranded double helix. Bases are Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). Complementary pairing: A pairs with T (2 hydrogen bonds) and C pairs with G (3 hydrogen bonds).
• RNA (Ribonucleic Acid): Uses the sugar ribose, is single-stranded, and replaces Thymine with Uracil (U). Types include mRNA, tRNA, and rRNA, which direct protein synthesis.