Biological Macromolecules: Properties, Structures, and Functions

Biological Macromolecules

Introduction

Biological macromolecules are large, complex molecules essential for life. They include carbohydrates, proteins, nucleic acids, and lipids. Each type of macromolecule has unique properties, structures, and functions that are critical for cellular processes and organismal survival.

Carbohydrates

Overview of Carbohydrates

Carbohydrates are organic molecules that serve as energy sources and structural components in living organisms. They include simple sugars and polymers of sugars.

• General Structure: Contain carbon (C), hydrogen (H), and oxygen (O), typically in the ratio (CH2O)n.

• Functional Groups: Contain a carbonyl group (C=O) and multiple hydroxyl groups (-OH).

• Monosaccharides: Simple sugars such as glucose (C6H12O6), which are nutrients and fuel for cells and are used in cellular respiration.

• Functions: Serve as building blocks for amino acids and as monomers for di- and polysaccharides.

Types of Carbohydrates

• Monosaccharides: The simplest form of carbohydrates (e.g., glucose, fructose). They have molecular formulas that are multiples of CH2O.

• Disaccharides: Formed by joining two monosaccharides via covalent bonds (glycosidic linkage). The most common disaccharide is sucrose (glucose + fructose). Plants transport carbohydrates as sucrose.

• Polysaccharides: Polymers with many monosaccharide units joined by dehydration reactions.

Storage Polysaccharides

• Starch: Polymer of glucose monomers; storage form in plants.

• Glycogen: Polymer of glucose; storage form in animals, found in liver and muscle cells.

Structural Polysaccharides

• Cellulose: Major component of plant cell walls; provides structural support.

• Chitin: Forms the exoskeleton of arthropods.

Example: Monosaccharide Identification

• Practice Problem: A monosaccharide with 4 carbons has the formula C4H8O4 (threose).

Proteins

Overview of Proteins

Proteins are complex molecules made of one or more polypeptides, which are polymers of amino acids. The three-dimensional shape of a protein determines its function.

• Elements: Composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S).

• Structure: The sequence and arrangement of amino acids determine the protein's shape and function.

Amino Acids

• Definition: Organic molecules with an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group) attached to a central carbon.

• Types of Side Chains:

  • Nonpolar (hydrophobic)

  • Polar (hydrophilic)

  • Charged/ionic (hydrophilic)

• Properties: The side chain determines the unique characteristics and reactivity of each amino acid.

Peptide Bonds and Polypeptides

• Peptide Bond: Formed by a dehydration reaction between the carboxyl group of one amino acid and the amino group of another.

• Polypeptide: A chain of amino acids linked by peptide bonds. Each polypeptide has a unique sequence and directionality (N-terminus: free amino group; C-terminus: free carboxyl group).

Levels of Protein Structure

• Primary Structure: Linear sequence of amino acids determined by genetic information.

• Secondary Structure: Coils and folds due to hydrogen bonding within the polypeptide backbone (e.g., α-helix, β-pleated sheet).

• Tertiary Structure: Overall 3D shape due to interactions between side chains (hydrophobic interactions, disulfide bridges).

• Quaternary Structure: Association of two or more polypeptides (only in some proteins).

Functions of Proteins

• Antibodies: Protect the body from disease.

• Enzymes: Catalyze chemical reactions.

• Messengers: Transmit signals (e.g., hormones).

• Structural: Provide support (e.g., collagen).

• Transport/Storage: Bind and carry molecules (e.g., hemoglobin).

Example: Cystic Fibrosis

• Practice Problem: Cystic fibrosis results from a deletion of three nucleotides in the CFTR gene, affecting the protein's structure and function, leading to disease.

Nucleic Acids

Overview of Nucleic Acids

Nucleic acids are polymers made of nucleotide monomers. They store, transmit, and express hereditary information. The two main types are DNA and RNA.

• DNA (Deoxyribonucleic Acid): Stores genetic information.

• RNA (Ribonucleic Acid): Involved in gene expression and protein synthesis.

Structure of Nucleotides

• Components: Each nucleotide consists of a nitrogenous base, a five-carbon sugar (pentose), and one or more phosphate groups.

• Nitrogenous Bases:

  • Pyrimidines: Single six-membered ring (cytosine, thymine [DNA], uracil [RNA]).

  • Purines: Fused double ring (adenine, guanine).

• Sugars: Deoxyribose in DNA, ribose in RNA.

• Phosphate Group: Attached to the 5' carbon of the sugar.

Polynucleotides and Directionality

• Phosphodiester Linkage: Connects adjacent nucleotides, forming the sugar-phosphate backbone.

• Directionality: 5' to 3' end; sequence of bases encodes genetic information.

DNA Structure

• Consists of two antiparallel polynucleotide strands forming a double helix.

• Strands held together by hydrogen bonds between complementary bases:

  • Adenine (A) pairs with Thymine (T)

  • Guanine (G) pairs with Cytosine (C)

RNA Structure

• Single-stranded polynucleotide, variable in shape due to internal base pairing.

• Adenine pairs with Uracil (U); Cytosine pairs with Guanine.

Example: DNA Complementarity

• Practice Problem: Given 5'-CATGTCAAC-3', the complementary strand is 3'-GTACAGTTG-5'.

Lipids

Overview of Lipids

Lipids are a diverse group of hydrophobic molecules that are not true polymers. They are generally small and nonpolar, making them insoluble in water.

• Main Types: Fats, phospholipids, steroids.

Fats

• Composed of glycerol (an alcohol) and fatty acids (long hydrocarbon chains with a carboxyl group).

• Fatty acids join to glycerol via ester linkages (bond between hydroxyl and carboxyl groups).

• Saturated Fatty Acids: No double bonds between carbons; saturated with hydrogen.

• Unsaturated Fatty Acids: One or more double bonds; causes kinks in the chain.

Phospholipids

• Major component of cell membranes.

• Structure: Two fatty acids and a phosphate group attached to glycerol.

• Form bilayers in water: hydrophobic tails face inward, hydrophilic heads face outward.

Steroids

• Characterized by four fused carbon rings with various functional groups attached.

• Examples: Cholesterol, testosterone.

Comparison Table: Macromolecules

Macromolecule	Elements	Monomer	Polymer
Carbohydrates	Carbon, hydrogen, oxygen	Monosaccharide	Polysaccharide
Proteins	Carbon, hydrogen, oxygen, nitrogen, sulfur	Amino acid	Polypeptide
Lipids	Carbon, hydrogen, oxygen (sometimes phosphorus for phospholipids)	Glycerol and fatty acids	Does not form true polymers
Nucleic Acids	Carbon, hydrogen, oxygen, nitrogen, phosphorus	Nucleotide	DNA, RNA (polynucleotide)