Biological Macromolecules Overview

Macromolecules

• Macromolecules are large molecules with complex structures
• They are composed of thousands of covalently connected atoms
• Small organic molecules are joined together to form larger molecules within cells
• All living organisms are made up of 4 classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids
• Molecular structure and function of macromolecules are closely related

4 Major Categories of Biological Macromolecules

• Carbohydrates
• Lipids
• Proteins
• Nucleic acids

Carbohydrates

• Carbohydrates serve as fuel and building material
• Carbohydrates are composed of carbon, hydrogen, and oxygen
• The molecular formula is multiples of the unit (CH₂O)ₙ
• Examples include pentoses (C₅H₁₀O₅) like ribose and deoxyribose, and hexoses (C₆H₁₂O₆) like glucose and fructose.
• Biologically important carbohydrates are also called sugars
• Monosaccharides are the simplest carbohydrates, single sugars (monomers)
• Polysaccharides are carbohydrate polymers composed of many sugar building blocks
• There are four categories of carbohydrates:
  • Monosaccharides (e.g., glucose, fructose)
  • Disaccharides (e.g., maltose, sucrose, lactose)
  • Oligosaccharides (composed of 20–30 monosaccharides)
  • Polysaccharides (e.g., starch, glycogen, cellulose, chitin)
• Monosaccharides serve as fuel for cells and raw materials for building molecules
• Monosaccharides are classified by:
  • The location of the carbonyl group (as aldose or ketose)
  • The number of carbons in the carbon skeleton
• Monosaccharides can be linear but in aqueous solutions many form rings
• A disaccharide is formed when a dehydration reaction joins two monosaccharides; this covalent bond is called a glycosidic linkage. Common examples include maltose (glucose + glucose), lactose (glucose + galactose), and sucrose (glucose + fructose).
• Polysaccharides include storage polysaccharides (starch and glycogen) and structural polysaccharides (cellulose and chitin).
  • Starch, the major storage polysaccharide in plants, consists of amylose (20-30%) and amylopectin (70-80%).
  • Glycogen, the storage polysaccharide in animals, is highly branched and functions to store glucose.
  • Cellulose, a structural polysaccharide in plant cell walls, is a glucose polymer with different glycosidic linkages from starch. Cellulose is difficult to digest for humans.
  • Chitin, a structural polysaccharide in fungal cell walls and arthropod exoskeletons, is similar in structure to cellulose but contains nitrogen.

Lipids

• Lipids are hydrophobic - they do not mix with water
• Lipids are a diverse group of hydrophobic molecules that do not consist of polymers
• Biologically important lipids: fats, phospholipids, steroids
  • Fats are triglycerides: glycerol + 3 fatty acids (important energy source)
  • Phospholipids have a glycerol backbone, two fatty acid "tails", and a phosphate group "head". Phospholipids are amphipathic (hydrophilic head and hydrophobic tails)
  • Steroids have four fused carbon rings (e.g., cholesterol, hormones). Cholesterol is important for cell membranes in animals.

Proteins

• Proteins have many structures, thus a wide range of functions
• Proteins account for more than 50% of the dry mass of most cells
• Protein functions include structural support, storage, transport, cellular communication, movement, and defense against foreign substances)
• Proteins are polymers of amino acids
• Polypeptides range in length from a few to more than a thousand monomers
• Amino acids have carboxyl and amino groups that differ in properties due to differing R-groups
• The 20 amino acids are categorized as nonpolar, polar, or electrically charged.
• Proteins have four levels of structure:
  • Primary structure: the unique sequence of amino acids forming a polypeptide chain
  • Secondary structure: coils and folds in the polypeptide chain (e.g., α-helices and β-pleated sheets)
  • Tertiary structure: the 3D structure of the polypeptide chain determined by interactions among various side chains (R groups)
  • Quaternary structure: results when a protein consists of multiple polypeptide chains (subunits)
• Enzymes are specific types of proteins that act as catalysts, speeding up chemical reactions
• Protein conformation depends on physical and chemical conditions of the protein's environment
• Denaturation is the loss of a protein's native conformation due to the unraveling of the protein, causing it to lose its function. Denaturation can result from pH changes, salt concentration changes, temperature changes, and other environmental factors.

Nucleic Acids

• Nucleic acids store and transmit hereditary information
• The amino acid sequence of a polypeptide is encoded by a unit of inheritance called a gene. Genes are made of DNA. DNA is a nucleic acid.
• DNA stores information for protein synthesis.
• DNA directs RNA synthesis.
• DNA directs protein synthesis through mRNA (messenger RNA)
• Nucleic acids exist as polymers called polynucleotides
• Each polynucleotide consists of monomers called nucleotides
• A nucleotide = nitrogenous base + pentose sugar + phosphate group
• There are two types of nucleic acids: DNA and RNA.
• DNA is double-stranded while RNA is single-stranded.
• The sequence of bases along a nucleotide polymer is unique for each gene and specifies the amino acid sequence of a protein.
• DNA replication is the copying of all DNA molecules into a second set, required before cell division.

Protein-Folding & Chaperones

• Most proteins fold into a specific 3D structure to become fully functional.
• Chaperones are proteins that help other proteins fold correctly, assist as needed.
• Chaperones locate in cytosol, mitochondria, chloroplasts, and ER.

Protein Structure Determination

• X-ray crystallography is used to determine a protein's three-dimensional structure.

Explore the fascinating world of biological macromolecules in this quiz. Learn about the four major categories: carbohydrates, lipids, proteins, and nucleic acids, and their structures and functions. Understand how these molecules are essential to all living organisms.