Biological Macromolecules and Their Functions
Learning Objectives
By the end of this section, you will be able to:
Understand macromolecule synthesis
Explain dehydration (or condensation) and hydrolysis reactions
Introduction to Biological Macromolecules
Biological macromolecules are large molecules essential for life, built from smaller organic molecules.
There are four major classes of biological macromolecules:
Carbohydrates
Lipids
Proteins
Nucleic acids
These macromolecules are important components of cells and perform a wide range of functions.
Together, they constitute the majority of a cell's dry mass (with water making up the majority of complete mass).
Biological macromolecules are organic and contain carbon, often bound to hydrogen, and may also include oxygen, nitrogen, and other minor elements.
Dehydration Synthesis
Dehydration Synthesis:
This process combines monomers (single subunits) into polymers (larger molecules) with the release of water.
The term literally means "put together while losing water."
The mechanism involves:
The hydrogen atom of one monomer combining with the hydroxyl group of another monomer, resulting in the formation of a water molecule.
The sharing of electrons between monomers to form covalent bonds.
Example:
Two glucose molecules link to form maltose, a disaccharide, releasing one water molecule in the process.
Monomers can join together in various configurations, resulting in a diverse array of polymers.
Example: Glucose monomers form starch, glycogen, and cellulose.
Hydrolysis
Hydrolysis:
This is the process by which polymers break down into monomers by the addition of water.
A chemical reaction occurs where water is inserted across the covalent bond, resulting in the breaking apart of the compound.
The breakdown process involves:
One part of the polymer gaining a hydrogen atom (H+) and the other part gaining a hydroxyl group (OH–) from the split water molecule.
Reverse of Dehydration Synthesis.
Example:
Maltose can be hydrolyzed to yield two glucose monomers by adding a water molecule.
Enzyme Role:
Both dehydration synthesis and hydrolysis reactions are catalyzed by specific enzymes.
Dehydration reactions require energy as they form new bonds, while hydrolysis reactions break bonds and release energy.
Specific enzymes are required for breaking down each type of macromolecule:
Amylase, sucrase, lactase, maltase: breakdown carbohydrates.
Proteases (pepsin, peptidase): breakdown proteins.
Lipases: breakdown lipids.
Carbohydrates: Role and Classification
Learning Objectives:
Discuss the role of carbohydrates in cells (animals and plants).
Explain carbohydrate classifications.
List common monosaccharides, disaccharides, and polysaccharides.
Carbohydrates are a primary source of energy in our diets (e.g., grains, fruits, vegetables).
They provide energy primarily through glucose, a simple sugar.
Molecular Structure:
Formula: CnH{2n}O_n (where n = number of carbons).
The ratio of carbon to hydrogen to oxygen is 1:2:1.
This ratio is the origin of the term "carbohydrate" (carbon and water).
Carbohydrate Subtypes:
Monosaccharides (simple sugars)
Disaccharides (two sugars)
Polysaccharides (many sugars)
Monosaccharides
Definition: Simple sugars.
Common examples include glucose, galactose, and fructose.
Molecular characteristics:
Typically have between 3-7 carbon atoms.
Names usually end with -ose.
Functional Groups:
Aldose (having an aldehyde group) and Ketose (having a ketone group).
Types based on carbon number:
Trioses: 3 carbons
Pentoses: 5 carbons
Hexoses: 6 carbons
Structural Isomers:
Glucose, galactose, and fructose are structural isomers.
They have the same formula C6H{12}O_6 but different arrangements leading to distinct characteristics.
Ring Form Behavior
Monosaccharides can exist as linear chains or as ring shapes (usually in aqueous solutions).
When forming rings:
The anomeric carbon (carbon 1) can be in two configurations: Alpha (α) if the hydroxyl group is below and Beta (β) if above.
Disaccharides
Definition: Formed by joining two monosaccharides via dehydration synthesis, releasing a water molecule.
Common examples:
Sucrose: Glucose + Fructose
Lactose: Glucose + Galactose
Maltose: Glucose + Glucose
Glycosidic Bond:
The covalent bond formed between monosaccharides is known as a glycosidic bond (or glycosidic linkage).
Can be alpha or beta based on the orientation of the OH group on the first carbon of the first monomer.
Polysaccharides
Definition: Long chains of monosaccharides linked through glycosidic bonds.
Examples:
Starch: Plant storage form of glucose (comprised of amylose and amylopectin).
Glycogen: Animal storage form of glucose, stored in liver and muscle cells.
Cellulose: Comprises plant cell walls, connected by β-1,4 glycosidic bonds, providing structural support.
Chitin: A nitrogen-containing polysaccharide forming exoskeletons of arthropods.
Storage and Energy Release:
Glycogen breaks down to glucose through glycogenolysis when blood glucose levels drop.
Enzymes (like amylase and lipases) facilitate energy extraction from carbohydrates during digestion.
Benefits of Carbohydrates
Carbohydrates are essential to a balanced diet and provide numerous health benefits:
Serve as an immediate energy source (4.3 Kcal/g).
Provide dietary fiber (mostly cellulose) to regulate digestion, support bowel movements, and lower cholesterol levels.
Linked to a reduced risk of chronic diseases, such as colon cancer.
Essential for energy metabolism, cellular respiration, and overall metabolic processes.
Lipids: Overview
Learning Objectives:
Describe the major types of lipids.
Explain the role of fats in energy storage.
Differentiate between saturated and unsaturated fatty acids.
Describe phospholipids and their role in cells.
Define the basic structure of steroids and their functions.
Explain how cholesterol maintains the fluidity of the plasma membrane.
Lipids are primarily hydrophobic molecules and perform various functions, including energy storage, insulation, and forming cellular membranes.
Types of Lipids: Fats, oils, waxes, phospholipids, and steroids.
Fats and Oils
Fats consist of glycerol and fatty acids.
Glycerol: An alcohol with 3 carbons, 5 hydrogens, and 3 hydroxyl groups.
Fatty Acids: Long hydrocarbon chains bonded to a carboxyl group (C with a double-bonded O), can vary in carbon number (4-36, typically 12-18).
Fats are often referred to as triacylglycerols or triglycerides, created via ester bonds between fatty acids and glycerol, releasing three water molecules.
Saturated vs Unsaturated Fatty Acids
Saturated Fatty Acids:
Contain only single bonds in their hydrocarbon chains; examples include stearic acid.
Are solid at room temperature (e.g., animal fats).
Unsaturated Fatty Acids:
Contain one or more double bonds (e.g., oleic acid).
Are liquid at room temperature (e.g., oils).
Types:
Monounsaturated: One double bond
Polyunsaturated: Multiple double bonds.
Trans Fats: Artificially hydrogenated oils, which transform cis double bonds into trans configurations, potentially increasing heart disease risk.
Omega Fatty Acids
Essential Fatty Acids: Omega-3 and Omega-6, which the body cannot synthesize and must be ingested.
Omega-3 fatty acids, such as alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), are particularly beneficial for heart health and inflammation reduction.
Phospholipids
Phospholipids make up the plasma membrane structure with a glycerol backbone, two fatty acids, and a phosphate group.
They are amphipathic: hydrophobic tails and a hydrophilic head, forming bilayers in aqueous environments.
Essential for membrane dynamics and cellular functions.
Steroids
Steroids have a unique fused ring structure with four linked carbon rings and are hydrophobic.
Cholesterol: A key steroid produced in the liver, precursor to steroid hormones and vitamin D; necessary for maintaining membrane fluidity.
Proteins: Overview
Learning Objectives:
Describe functions of proteins in cells and tissues.
Explain the relationship between amino acids and proteins.
Discuss four levels of protein organization.
Highlight the link between protein shape and function.
Proteins are composed of amino acid polymers arranged in specific sequences, influencing their shape and functionality.
Types and Functions of Proteins
Proteins have numerous roles: structural, regulatory, contractile, protective, transport, storage, and enzymatic.
Enzymes: Biological catalysts that speed up reactions (e.g., salivary amylase).
Hormones: Chemical signaling molecules regulating physiological processes (e.g., insulin).
Table of Protein Types and Functions:
Type
Examples
Functions
Digestive Enzymes
Amylase, lipase, pepsin
Catabolize nutrients
Transport
Hemoglobin, albumin
Carry substances
Structural
Actin, tubulin, keratin
Construct structures like cytoskeleton
Hormones
Insulin, thyroxine
Coordinate body system activity
Defense
Immunoglobulins
Protect against pathogens
Contractile
Actin, myosin
Muscle contraction
Storage
Legume proteins, egg white
Provide nourishment
Amino Acids as Building Blocks
Proteins are made from 20 common amino acids, each with:
A central carbon (alpha carbon)
An amino group (–NH2)
A carboxyl group (–COOH)
A unique R group (side chain)
Amino acids can be categorized:
Based on polarity (polar vs nonpolar), charge (positive/negative), etc.
Essential amino acids cannot be synthesized by the body and must be acquired through diet. Examples include isoleucine, leucine, and cysteine.
Peptide Bond Formation: A dehydration reaction connects amino acids, forming polypeptides:
Bonds between the carboxyl group of one amino acid and the amino group of another release a water molecule.
Levels of Protein Structure
Primary Structure:
Unique sequence of amino acids in a polypeptide chain.
Illustrated by insulin's two polypeptide chains linked by disulfide bonds.
Secondary Structure:
Local folding into structures like alpha-helix and beta-pleated sheets, stabilized by hydrogen bonds.
Tertiary Structure:
Overall three-dimensional shape due to R group interactions and bonding.
Hydrophobic interactions, ionic bonds, and disulfide linkages contribute to this structure.
Quaternary Structure:
Formed from multiple polypeptide chains interacting, e.g., insulin.
Denaturation
The disruption of protein structure due to heat, pH changes, or chemicals may lead to loss of functionality.
Denaturation may be reversible or irreversible.
Example: Cooking an egg denatures albumin.
Nucleic Acids: Overview
Learning Objectives:
Describe the structure of nucleic acids and their types.
Explain the roles of DNA and RNA.
Nucleic acids are vital for genetic information storage and transfer.
Two primary types: DNA and RNA.
Structure and Function of DNA
DNA: Comprised of nucleotides containing a nitrogenous base, ribose sugar, and a phosphate group, forming a double helix.
Functions as genetic material in all living organisms, controlling cellular activities by regulating gene expression.
Features:
Antiparallel strands, with complementary base pairing (A-T, G-C).
Structure and Function of RNA
RNA: Involved in protein synthesis and contains ribose sugar; is single-stranded and uses uracil (U) instead of thymine (T).
Major types:
mRNA: Carries genetic information from DNA for protein synthesis.
rRNA: Integral component of ribosomes.
tRNA: Transfers amino acids to the growing polypeptide chain during synthesis.
miRNA: Regulates gene expression.
Central Dogma of Molecular Biology
Describes the flow of genetic information:
DNA → RNA → Protein.
Transcription: DNA is transcribed to RNA in the nucleus.
Translation: RNA is translated into protein in the cytoplasm.
Conclusion
Biological macromolecules are diverse and crucial for life processes.
Understanding their structures, functions, and interactions is key to many biological sciences, including biochemistry, molecular biology, and genetics.