Bio 100 LEC Chapter 5 Part 1 UPDATED

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Bio 100 LEC Chapter 5 Part 1

Chapter 5: The Structure and Function of Large Biological Molecules

Introduction

This chapter explores the categories, properties, and functions of large biological molecules, focusing on their structural diversity and biological roles. Understanding these molecules is fundamental to cell biology, metabolism, and genetics.

Macromolecules and Polymers

Concept 5.1: Macromolecules are Polymers, Built from Monomers

Macromolecules are large biological molecules capable of forming polymers, which are long chains made from repeating subunits called monomers. The diversity in arrangement and organization of monomers leads to a wide variety of polymers with distinct properties.

Macromolecules: Carbohydrates, proteins, and nucleic acids are true macromolecules because they form polymers.
Monomers: The simplest subunit; the nature of the monomer dictates the properties of the resulting polymer.
Polymerization: The process of joining monomers to form polymers.

Colored blocks representing monomers and polymers

Synthesis and Breakdown of Polymers

Polymers are synthesized and broken down by specific chemical reactions:

Dehydration Reaction (Condensation): Joins monomers by removing a water molecule, forming a covalent bond.
Hydrolysis: Breaks polymers into monomers by adding a water molecule.
Enzymes: Biological catalysts that facilitate these reactions.

Equation for Dehydration Reaction:

Equation for Hydrolysis:

Diagram of dehydration and hydrolysis reactions

Diversity of Polymers

Variation Among Biological Molecules

The abundance and types of large biological molecules vary between cells and species. Polymers may serve structural or energy storage functions, and their stability and breakdown are essential for cellular processes.

Structural Polymers: Provide integrity and support (e.g., cellulose).
Energy Storage Polymers: Built to be broken down for energy (e.g., starch, glycogen).

Various foods representing diversity of biological polymers

Carbohydrates

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

Carbohydrates are macromolecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. They include sugars and their polymers, serving as energy sources and structural materials.

Monosaccharides: Simple sugars (e.g., glucose, fructose) with formulas usually multiples of CH2O.
Classification: Based on carbonyl group location (aldose or ketose) and number of carbons (triose, pentose, hexose).
Energy Storage: Abundant C-H bonds make carbohydrates ideal for energy storage, released during oxidation.

Table of monosaccharides: aldoses and ketoses

Linear and Ring Forms of Glucose

Glucose can exist in both linear and ring forms, especially in aqueous environments. The ring form is predominant in cells and can be either alpha or beta, depending on the orientation of the hydroxyl group at carbon 1.

Alpha Glucose: Hydroxyl group below the plane of the ring.
Beta Glucose: Hydroxyl group above the plane of the ring.

Linear and ring forms of glucose

Formation of Disaccharides

Monosaccharides can be joined to form disaccharides (two sugars) or oligosaccharides (a few sugars). The joining occurs via dehydration reactions, resulting in glycosidic linkages.

Maltose: Formed from two glucose molecules via a 1-4 glycosidic linkage.
Sucrose: Formed from glucose and fructose via a 1-2 glycosidic linkage.

Formation of maltose and sucrose disaccharides

Polysaccharides: Starch, Glycogen, and Cellulose

Polysaccharides are polymers of monosaccharides linked by glycosidic bonds. Their structure and function depend on the type of monomer and linkage.

Starch: Energy storage in plants; composed of alpha glucose monomers. Includes amylose (unbranched) and amylopectin (branched).
Glycogen: Energy storage in animals; highly branched polymer of glucose, stored in liver and muscle.
Cellulose: Structural component of plant cell walls; composed of beta glucose monomers, forming rigid, linear rods aggregated into microfibrils.

Starch structure: amylose and amylopectin

Glycogen structure: highly branched

Cellulose structure: microfibrils in plant cell wall

Alpha and Beta Linkages in Polysaccharides

The orientation of glucose monomers (alpha or beta) determines the type of glycosidic linkage and the properties of the polysaccharide. Alpha linkages are digestible by human enzymes, while beta linkages are not.

Starch: 1-4 alpha linkages.
Cellulose: 1-4 beta linkages.

Comparison of alpha and beta glucose linkages

Digestive Differences: Humans vs. Herbivores

Humans can digest starch (alpha linkages) but not cellulose (beta linkages), which passes as insoluble fiber. Herbivores, such as cows, rely on symbiotic microbes to hydrolyze cellulose and access its nutrients.

Mutualism: Microbes benefit from the environment, herbivores gain nutrients.
Nutritional Implications: Cellulose aids in digestive tract health for humans, but is not a nutrient source.

Human eating salad and cow eating grass

Lipids

Concept 5.3: Lipids are a Diverse Group of Hydrophobic Molecules

Lipids are large biological molecules that are not true polymers. They are defined by their insolubility in water and consist mainly of hydrocarbon regions. Lipids include fats, phospholipids, and steroids, each with distinct roles.

Fats and Oils: Energy storage and thermal insulation.
Phospholipids: Structural role in cell membranes.
Steroids: Regulation, hormones, and vitamins.

Slide introducing lipids as hydrophobic molecules

Fats: Structure and Function

Fats (triacylglycerols) are formed from glycerol and fatty acids via dehydration synthesis, resulting in ester linkages. Fats are used for long-term energy storage, cushioning, and insulation.

Glycerol: Three-carbon alcohol with hydroxyl groups.
Fatty Acid: Carboxyl group attached to a hydrocarbon chain.
Triglyceride: Glycerol bound to three fatty acids.

Formation of fat molecule from glycerol and fatty acids

Saturated and Unsaturated Fats

The properties of fats depend on the presence or absence of double bonds in fatty acids:

Saturated Fats: No double bonds; tightly packed; solid at room temperature (e.g., animal fats).
Unsaturated Fats: One or more cis double bonds; kinks prevent tight packing; liquid at room temperature (e.g., oils).

Comparison of saturated and unsaturated fats

Trans Fats

Trans fats are artificially created by manipulating unsaturated fats to have trans double bonds, resulting in a more compact structure. Trans fats are linked to increased risk of cardiovascular disease.

Cis vs. Trans: Cis double bonds cause kinks; trans double bonds allow tighter packing.
Health Implications: Trans fats are more harmful than saturated fats.

Foods rich in trans fats

Phospholipids

Phospholipids consist of a glycerol backbone, two fatty acid tails, and a phosphate group attached to a polar molecule. They are amphipathic, with hydrophilic heads and hydrophobic tails, and spontaneously form bilayers in aqueous environments, creating cell membranes.

Hydrophilic Head: Glycerol, phosphate, and polar group.
Hydrophobic Tails: Hydrocarbon chains.
Bilayer Formation: Hydrophobic tails face inward, hydrophilic heads face outward.

Phospholipid structure and bilayer formation

Steroids

Steroids are lipids with a four-ring hydrocarbon skeleton. Sterols are steroids with hydroxyl groups. Cholesterol is a key steroid in animal cell membranes and a precursor for other steroids like testosterone and estrogen.

Structure: Four fused rings with attached functional groups.
Function: Membrane component, hormone precursor.
Health Implications: Cholesterol levels are linked to cardiovascular health.

Steroid structure: sterol

Additional info: Enzyme specificity, functional group chemistry, and the role of polymers in cellular metabolism will be discussed in later chapters. This guide provides foundational knowledge for understanding biological macromolecules and their functions.