CH2 CHEM PT 3 - CARBOHYDRATES - Monosaccharides, Disaccharides, and Polysaccharides -

Introduction to Organic Compounds and Carbohydrates

• Organic compounds are defined by having a carbon backbone. Example given: a sugar with many carbon–carbon linkages, yielding a carbon-rich structure. Glucose is used as an example of a sugar (a simple carbohydrate).

• Do not panic about memorizing every structural detail of carbohydrates. The instructor shows both a long-form (open-chain) structure and a ring form of glucose to familiarize you with common textbook pictures; memorization of the exact ring/aromatic representation is not required for this course.

• Sugar glucose can be written in two forms:

  • Linear (open-chain) form

  • Ring form (cyclic) form

• The point is to recognize sugars when you see them, not to memorize every arrangement. The ring form simply reflects cyclization that occurs in solution.

Glucose: Forms and Notation

• Glucose is a simple sugar, also called a monosaccharide.

• It has the chemical formula C6H{12}O_6.

• In carbohydrates, carbon (C), hydrogen (H), and oxygen (O) are the key elements.

• Typical pattern for sugars: there are twice as many hydrogens as oxygens, i.e., H:O ≈ 2:1; glucose is a concrete example: C6H{12}O_6.

• In text form here, the ring form is drawn as a hexagon with oxygen at one vertex; other corners are carbon atoms. The ring form still corresponds to the same molecular formula and carbohydrate identity.

• The two forms of glucose illustrate a broader concept: many carbohydrates can be represented as either a linear chain or a ring, depending on the context and the portion of the molecule being emphasized.

Purpose and Functions of Carbohydrates

• Primary function: provide energy for the body.

  • When glucose (or other sugars) is consumed, the chemical bonds between carbon and hydrogen are broken in metabolic pathways.

  • Breaking bonds releases energy, which is captured in usable forms (e.g., ATP) to power cellular processes like muscle contraction, thinking, and other bodily functions.

• Carbohydrates serve as a major energy source in the diet and metabolism.

Carbohydrate Composition and Formula

• Elements present: carbon (C), hydrogen (H), oxygen (O).

• General observation (for this class): sugars typically have twice as many hydrogens as oxygens (H:O ≈ 2:1). A representative formula example is C6H{12}O_6 for glucose.

• For glucose specifically, there are:

  • Carbons: 6

  • Oxygens: 6

  • Hydrogens: 12

• In the open-chain representation, all atoms can be counted; in the ring form, hydrogens may not be drawn at every terminus, but they are present in the actual molecule.

• The emphasis here is on recognizing the chemical composition and the general color/typical formula rather than memorizing every dot-and-line detail.

Monosaccharides (Simple Sugars)

• Three monosaccharides highlighted for this course: glucose, fructose, galactose.

  • Glucose: a primary example of a simple sugar (monosaccharide).

  • Fructose: another common monosaccharide found in fruit, honey, etc.

  • Galactose: another monosaccharide that combines with glucose to form lactose.

• Monosaccharides are the simplest form of carbohydrates and cannot be broken down into smaller carbohydrate units by hydrolysis.

• They are polar and dissolve in water due to the electronegative oxygen atoms that pull electron density toward themselves, creating partial charges that interact with water.

• The instructor notes: in class, glucose is shown both in ring form and linear form, but you do not need to memorize or reproduce the exact ring structure.

Dimensionality of Carbohydrates: Monosaccharides → Disaccharides → Polysaccharides

• Monosaccharides can combine with each other to form disaccharides through a dehydration reaction (also called dehydration synthesis).

• Dehydration synthesis: two monosaccharides join by removing one water molecule (H_2O) to form a disaccharide.

  • The general principle: combining smaller units into larger ones involves the removal of water.

• When two monosaccharides combine, the resulting disaccharide contains two monosaccharide units bonded together.

• The three disaccharides introduced are:

  • Maltose: made from two glucose molecules.

  • Sucrose: made from glucose and fructose.

  • Lactose: made from glucose and galactose.

• All disaccharides end with the suffix -ose (e.g., maltose, sucrose, lactose).

• Dehydration is a synthesis process; it is anabolic (building up molecules).

• Reversibility: disaccharides can be broken back down into monosaccharides by hydrolysis (adding water) in a reversal process.

• Hydrolysis: the addition of water to break a bond, i.e., breaking a disaccharide into two monosaccharides with the consumption of water.

  • Etymology: hydro- = water, -lysis = to break.

  • In the hydrolysis direction, water is added to split the bond, effectively reversing dehydration synthesis.

Example Reactions (Dehydration Synthesis)

• Glucose + Glucose → Maltose + H_2O

  • Represented as: ext{Glucose} + ext{Glucose}
    ightarrow ext{Maltose} + ext{H}_2 ext{O}

• Glucose + Fructose → Sucrose + H_2O

  • Represented as: ext{Glucose} + ext{Fructose}
    ightarrow ext{Sucrose} + ext{H}_2 ext{O}

• Glucose + Galactose → Lactose + H_2O

  • Represented as: ext{Glucose} + ext{Galactose}
    ightarrow ext{Lactose} + ext{H}_2 ext{O}

• Note: these reactions illustrate the dehydration process by removing water to form larger sugar units.

• The term dehydration synthesis can be used to emphasize that synthesis (built molecules) is achieved by removing water.

Polysaccharides: Large, Branched Carbohydrate Structures

• Polysaccharides are long chains (or branched networks) of monosaccharide units connected by glycosidic bonds.

• They are formed by repeated dehydration synthesis, linking many monosaccharide units together.

• Three polysaccharides highlighted for this class:

  • Glycogen: polysaccharide used for storage of glucose in the body (animal storage form).

  • Starch: polysaccharide used for storage of glucose in plants and a major dietary carbohydrate (e.g., in potatoes, bread).

  • Cellulose: polysaccharide used for structural support in plants; contains glucose units linked in a way that humans cannot hydrolyze due to lack of appropriate enzymes.

• Cellulose is unique because humans cannot break the glycosidic bonds in cellulose with our digestive enzymes; we do not have the necessary enzymes to hydrolyze cellulose. As a result, cellulose passes through the digestive tract largely undigested and contributes to dietary fiber.

• Dietary fiber (cellulose) is important because it helps pull water through the digestive tract and promotes easier movement of stool.

• For this course, it is stated that all three polysaccharides listed (glycogen, starch, cellulose) are polar and dissolve in water; this is a general statement used for teaching purposes, though real-world solubility can vary by context and structure.

Digestion, Absorption, and Metabolism of Carbohydrates

• The digestive system breaks carbohydrates down to monosaccharides via hydrolysis, enabling absorption into the bloodstream.

• Digestive water is essential for hydrolysis because bond cleavage requires water.

• The human digestive system produces significant amounts of water-containing fluids to facilitate digestion:

  • Saliva: approximately 1 to 2 liters per day.

  • Stomach juice: approximately 1 to 2 liters per day.

  • Pancreatic juice: approximately 1 to 2 liters per day.

• When starch is ingested, the polysaccharide is broken down by hydrolysis into monosaccharides (the simple sugars).

• Absorbed monosaccharides enter the bloodstream and can be used for immediate energy or stored for later use (via dehydration synthesis) as glycogen.

• Storage form: glycogen is the storage polysaccharide in the body for glucose; starch is the storage form in plants for what we eat; cellulose remains as a structural component (and dietary fiber).

• The future unit will cover enzymes in more detail; enzymes are biological catalysts that enable specific hydrolysis or synthesis reactions (e.g., enabling starch breakdown or disaccharide formation).

Water, Solubility, and Polar Nature of Carbohydrates

• Carbohydrates are generally polar due to electronegative oxygen atoms, which promotes solubility in water. In this course, carbohydrates are described as polar and water-soluble in general terms.

• A simple conceptual model used in class is a string of beads (monosaccharide units) linked by dehydration synthesis to form polysaccharides; breaking them apart via hydrolysis corresponds to adding water; re-linking them via dehydration synthesis corresponds to storing energy.

Quick Reference: Key Terms and Concepts

• Monosaccharide: the simplest carbohydrate; cannot be hydrolyzed into smaller carbohydrate units.

• Disaccharide: formed when two monosaccharides join via dehydration synthesis; examples: maltose, sucrose, lactose.

• Polysaccharide: long chain or branched network of monosaccharide units; examples: glycogen, starch, cellulose.

• Dehydration synthesis (dehydration): reaction that links monosaccharides together by removing a water molecule; anabolic process.

• Hydrolysis: reaction that uses water to break bonds between monosaccharide units; catabolic process.

• Ring form vs linear form: glucose can be represented as a ring or a straight chain; both representations refer to the same molecule.

• Glycogen: storage polysaccharide in animals.

• Starch: storage polysaccharide in plants; major dietary carbohydrate.

• Cellulose: structural polysaccharide in plants; humans cannot enzymatically hydrolyze the glycosidic bonds in cellulose; provides dietary fiber.

• Endings of carbohydrate names: most disaccharides and many polysaccharides end with -ose (e.g., glucose, maltose, sucrose, lactose).

• Energy metabolism connection: bonds in carbohydrates are broken to release energy used for cellular work (muscle contraction, thinking, etc.).

• Relationship to digestion: saliva, gastric juice, and pancreatic juice collectively provide the aqueous environment needed for hydrolysis.

Connections to Broader Principles and Real-World Relevance

• Energy economics: carbohydrates are a primary energy source; understanding how they are broken down and stored helps explain metabolism, exercise physiology, and nutrition.

• Enzymes and digestion: the upcoming unit will discuss how enzymes specifically catalyze hydrolysis or synthesis reactions, dictating which carbohydrates can be metabolized efficiently (e.g., starch digestion vs. cellulose digestion).

• Fiber and health: cellulose-containing fiber aids in digestion and stool movement, illustrating the practical health implications of carbohydrate structure.

• Food science and labeling: common disaccharides like sucrose appear in food products; understanding dehydration synthesis helps explain how different sugars form and are digested.

Quick Practice Prompts (to test understanding)

• Identify which of the following are monosaccharides: glucose, fructose, lactose, maltose, glycogen.

• Write the dehydration synthesis equation for glucose + glucose forming maltose and water.

• Explain why humans cannot digest cellulose and how this relates to fiber in the diet.

• Distinguish between glycogen and starch in terms of storage location and biological role.

• Describe hydrolysis and dehydration synthesis in terms of energy flow (ex: is dehydration synthesis anabolic or catabolic?).

• Summary statement: Carbohydrates are organic molecules with a carbon backbone that play a central role in energy storage and supply, structural support (in some contexts), and dietary fiber. They range from simple monosaccharides like glucose to complex polysaccharides such as glycogen, starch, and cellulose, all linked and transformed primarily through dehydration synthesis and hydrolysis, processes intimately connected to water and enzyme activity that drive digestion and metabolism.