General Biology: Macromolecules, Proteins, and Nucleic Acids – Study Guide

Macromolecules: Structure and Function

Pyrimidines vs. Purines

Purines and pyrimidines are nitrogenous bases found in nucleic acids. They differ in structure and function:

• Pyrimidines have a single six-membered ring (e.g., cytosine, thymine, uracil).

• Purines have a fused double ring structure (a six-membered and a five-membered ring; e.g., adenine, guanine).

• Purines contain two rings, while pyrimidines contain one.

Example: Adenine (purine) pairs with thymine (pyrimidine) in DNA.

Starch and Cellulose: Structure and Function

Both starch and cellulose are polysaccharides composed of glucose monomers, but they differ in structure and biological role:

• Starch is used by plants for energy storage; it is digestible by humans.

• Cellulose is a structural component in plant cell walls; it is not digestible by humans due to the orientation of glycosidic bonds.

• Starch is important for energy storage, while cellulose is important for structural support.

Example: Humans can digest starch but not cellulose.

Carbohydrates: Monomers and Polymers

Carbohydrates are composed of simple sugars (monosaccharides) that can be linked to form polysaccharides.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, cellulose, glycogen).

Example: Glycogen is a storage polysaccharide in animals.

Oxygen and Cellular Respiration

Oxygen is essential for aerobic respiration, serving as the final electron acceptor in the electron transport chain.

• Oxygen is not used in glycolysis but is required for the complete oxidation of glucose in mitochondria.

• Without oxygen, cells may undergo fermentation instead of aerobic respiration.

Biological Polymers

Polymers are large molecules made by joining many smaller units (monomers) via covalent bonds.

• Examples: Proteins (polymers of amino acids), nucleic acids (polymers of nucleotides), polysaccharides (polymers of sugars).

Energy Storage in Plants

Plants store energy primarily as starch, a polysaccharide composed of glucose units.

• Starch is synthesized in plastids and can be broken down to glucose when energy is needed.

Hydrolysis of Polysaccharides

Hydrolysis is the process of breaking down polymers into monomers by adding water molecules.

• To break an n-monomer polymer, (n-1) water molecules are required.

Example: To break a 50-monomer sugar molecule, 49 water molecules are needed.

Proteins: Structure and Function

Levels of Protein Structure

• Primary structure: Sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding into alpha-helices and beta-sheets, stabilized by hydrogen bonds.

• Tertiary structure: Overall 3D shape of a single polypeptide chain.

• Quaternary structure: Arrangement of multiple polypeptide subunits.

All proteins have at least primary structure.

Bonds in Proteins

• Peptide bonds: Covalent bonds linking amino acids in a protein.

• Hydrogen bonds: Stabilize secondary and tertiary structures.

• Disulfide bridges: Covalent bonds between cysteine residues, stabilizing tertiary structure.

Protein Folding and Hydrophobicity

• Non-polar amino acids are typically found in the interior of proteins, away from water.

• Polar and charged amino acids are often on the protein surface, interacting with the aqueous environment.

Nucleic Acids: Structure and Function

DNA and RNA Structure

• DNA is composed of two antiparallel strands forming a double helix.

• Each nucleotide consists of a phosphate group, a deoxyribose sugar, and a nitrogenous base.

• In DNA, the sugar-phosphate backbone faces outward, while nitrogenous bases face inward and pair via hydrogen bonds.

Central Dogma of Molecular Biology

• Information flow: DNA → RNA → Protein

Phosphodiester Bonds

• Nucleotides are joined by phosphodiester bonds between the phosphate group of one nucleotide and the sugar of the next.

Table: Comparison of Starch and Cellulose

Property	Starch	Cellulose
Monomer	Glucose	Glucose
Bond Type	α(1→4) glycosidic bonds	β(1→4) glycosidic bonds
Function	Energy storage	Structural support
Digestibility (by humans)	Digestible	Indigestible

Key Equations

• Number of water molecules required to hydrolyze a polymer of n monomers: $n - 1$

Additional info:

• Enzymes are biological catalysts that speed up chemical reactions, including the breakdown of macromolecules.

• Cellulose digestion in cows is possible due to symbiotic bacteria in their digestive tract.