Carbohydrates: An Introduction

Carbohydrates: Introduction

• Carbohydrates are a type of macronutrient that provides energy to the body.
• They are composed of polyhydroxy aldehydes or ketones, meaning they can be broken down into simple sugars.
• They are present in foods like breads, fruits, vegetables, and milk products.

Monosaccharides: The Building Blocks of Carbohydrates

• Monosaccharides are the simplest sugars and have 3 to 6 carbon atoms with an aldehyde or ketone group and multiple hydroxyl (-OH) groups.
• They are classified based on the number of carbon atoms: triose (3C), tetrose (4C), pentose (5C), and hexose (6C).
• Aldoses are monosaccharides with an aldehyde group, while ketoses have a ketone group.
• Glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose) are constitutional isomers with the formula C3H6O3.

Fischer Projection Formulas

• Carbohydrates have one or more chirality centers, except for dihydroxyacetone.
• Glyceraldehyde has one chirality center, leading to two possible enantiomers.
• The prefix "D" is used when the -OH group is on the right side of the carbon chain in a Fischer projection formula, while "L" indicates that the -OH group is on the left.
• Most naturally occurring sugars are D sugars.

Common Monosaccharides

• Glucose (dextrose) is blood sugar and is found in the bloodstream at normal levels of 70-110 mg/dL.
• Excess glucose is stored as glycogen (a polysaccharide) or as fat.
• Insulin regulates blood glucose levels by promoting glucose uptake into tissues and glycogen formation.
• People with diabetes have insufficient insulin production.
• Galactose is a component of the disaccharide lactose.
• Galactosemia is a genetic disorder where individuals lack an enzyme needed to metabolize galactose. This leads to the buildup of galactose, causing issues like cataracts and cirrhosis.
• Fructose, a ketohexose, is found in honey and is twice as sweet as table sugar with the same calorie content per gram.

Cyclic Forms of Monosaccharides

• Monosaccharides can form cyclic structures through reactions between the aldehyde (or ketone) group and an alcohol group within the molecule.
• The cyclic form of a monosaccharide is called a hemiacetal, and the carbon involved in the ring formation is called the anomeric carbon.
• D-glucose exists in two cyclic forms: the α-anomer and the β-anomer.
• These forms are represented using Haworth projections.
• Mutarotation refers to the equilibrium between these two forms in solution.

Chemical Reactions of Monosaccharides

• Reduction of the Aldehyde Carbonyl Group: Aldoses can be reduced to alditols using H2 and a catalyst (Pd).
• Oxidation of the Aldehyde Carbonyl Group: Aldoses are readily oxidized to carboxylic acids using Benedict's reagent. Ketoses, however, need to be rearranged in basic conditions to form an aldose before being oxidized.

Disaccharides

• These carbohydrates are formed by the linkage of two monosaccharides.
• The bond between the monosaccharides is called a glycosidic linkage.
• Glycosidic linkages can be α or β, depending on the orientation of the hydroxyl group on the anomeric carbon.
• Lactose, made of glucose and galactose, is found in milk and is broken down by the enzyme lactase. Lactose intolerance occurs when individuals lack this enzyme.
• Sucrose (table sugar) is composed of glucose and fructose. It is a sweet carbohydrate with a high calorie content.
• Artificial sweeteners like aspartame (Equal), saccharine (Sweet’n Low), and sucralose (Splenda) have been developed as sugar substitutes to reduce caloric intake.

Polysaccharides: Complex Chains of Monosaccharides

• Polysaccharides are composed of three or more monosaccharides linked together.
• The three main polysaccharides found in nature are cellulose, starch, and glycogen.
• All polysaccharides are composed of repeating glucose units connected by glycosidic bonds.

Types of Polysaccharides

• Cellulose is an unbranched polymer of glucose units linked by β-1,4-glycosidic bonds.
• It is found in plant cell walls, providing structural support.
• Humans lack the enzyme needed to digest cellulose, so it is a source of dietary fiber.
• Starch is a polymer of glucose units linked primarily by α-1,4-glycosidic bonds.
• Starch is found in corn, rice, wheat, and potatoes.
• Amylose is a type of starch with unbranched chains, while amylopectin is branched.
• Humans can digest both amylose and amylopectin using the enzyme amylase.
• Glycogen is the main storage form of carbohydrates in animals, primarily found in the liver and muscle cells.
• It is highly branched, similar in structure to amylopectin, with α-1,4- and α-1,6-glycosidic bonds.

Glycosaminoglycans (GAGs)

• These are unbranched carbohydrates formed by repeating units of amino sugars and glucuronate.
• Examples include hyaluronate (lubricates joints) and chondroitin (found in cartilage and tendons).
• Heparin, a GAG stored in liver mast cells, helps prevent blood clotting.

Chitin

• A polysaccharide composed of N-acetyl-D-glucosamine units linked together by β-1,4-glycosidic bonds.
• This is a major component of the exoskeletons of insects and crustaceans.

Blood Type

• Human blood types (A, B, AB, and O) are determined by the presence of specific monosaccharides attached to a membrane protein on red blood cells.
• Blood type A has an additional monosaccharide compared to blood type O.
• Blood type B has an extra D-galactose unit compared to type O.
• Blood type AB has both A and B carbohydrates.
• Antibodies in individuals' blood can react with other blood types, which explains why blood type O is considered a universal donor and blood type AB is a universal recipient.

Lipids

• Defined by their physical properties, not by functional groups
• Primarily nonpolar due to many C-C and C-H bonds, resulting in water insolubility
• Soluble in organic solvents
• Categorized as hydrolyzable and nonhydrolyzable lipids
  • Hydrolyzable lipids:
    • Can be broken down into smaller molecules using water (hydrolysis)
    • Include waxes, triacylglycerols, and phospholipids
  • Nonhydrolyzable lipids:
    • Cannot be broken down into smaller molecules using water
    • Include steroids, fat-soluble vitamins, and eicosanoids (prostaglandins)
• Fatty acid is a component of most hydrolyzable lipids
  • Carboxylic acid with a long carbon chain (12-20 carbon atoms)
  • Most naturally occurring fatty acids have an even number of carbon atoms
  • Saturated fatty acids: No double bonds in their hydrocarbon chains
  • Unsaturated fatty acids: One or more double bonds (usually cis)
    • Higher number of double bonds in fatty acids decreases melting point (MP)

Waxes

• Esters formed from a fatty acid and a long-chain alcohol
• Hydrophobic in nature
• Form protective coatings on bird feathers, sheep wool, and beeswax
• Can be hydrolyzed with water in the presence of acid or base to reform the original carboxylic acid and alcohol

Triacylglycerols (Triglycerides)

• Three esters formed from glycerol and three fatty acid molecules
• Stored in adipose tissue
  • Number of adipose cells remains constant, but their size changes with weight gain or loss
  • To metabolize triacylglycerols for energy, lipases hydrolyze the esters
  • Complete metabolism of triacylglycerols results in CO2, H2O, and a large amount of energy
• Simple triacylglycerols: Have three identical fatty acid side chains
• Mixed triacylglycerols: Have two or three different fatty acids
• Saturated triacylglycerols: Contain only saturated fatty acids
  • Make up most animal fat
  • Solid at room temperature
• Unsaturated triacylglycerols: Contain one or more double bonds
  • Vegetable oils
  • Liquid at room temperature
  • Monounsaturated: One C=C bond
  • Polyunsaturated: Many C=C bonds
• More double bonds in triacylglycerols decrease melting point
• Fats: Higher MP, solid at room temperature
  • Derived from fatty acids with few double bonds
  • Used for cell membranes, insulation, and energy storage
  • 20-35% of a person's caloric intake should come from fats
  • High intake of saturated triacylglycerols can lead to heart disease
  • Saturated fats can stimulate cholesterol synthesis, leading to cholesterol plaques in arteries
• Oils: Lower MP, liquid at room temperature
  • Derived from fatty acids with a larger number of double bonds

Phospholipids

• Contain a phosphate atom
• Main types:
  • Phosphoacylglycerols: Major component of most cell membranes
  • Sphingomyelins: Contain a sphingosine backbone instead of glycerol
• Phosphoacylglycerols:
  • Resemble triacylglycerols, but with a phosphodiester bonded to an alcohol replacing the third fatty acid
  • Types: Cephalin (phosphatidylethanolamine) and Lecithin (phosphatidylcholine)
• Sphingomyelins:
  • Do not contain a glycerol backbone, but a sphingosine backbone
  • Contain a single fatty acid bonded to the backbone by an amide bond
  • Components: Sphingosine, amide, choline
• Phospholipids:
  • Two fatty acid side chains form nonpolar "tails" that lie parallel to each other
  • Phosphodiester end forms a charged or polar "head"

Other Relevant Terms

• Linoleic Acid: Omega-6 acid
• Linolenic acid: Omega-3 acid
• Omega-n fatty acids: Essential fatty acids, cannot be synthesized by the body, must be obtained through diet
• Esterification: Formation of an ester bond
• Saponification: Basic hydrolysis of a triacylglycerol
• Spermaceti wax: A type of wax with long nonpolar carbon chains
• Adipose tissue: Where triacylglycerols are stored
• Lipases: Enzymes that hydrolyze esters in triacylglycerols
• Trans fat: Acts like saturated fats, increases cholesterol levels in the blood
• Cholesterol: A lipid that can form plaques in arteries, leading to heart disease

This quiz covers the basic concepts of carbohydrates, including their structure, function, and classification. It delves into monosaccharides, their importance as building blocks, and introduces Fischer projection formulas. Perfect for students looking to enhance their understanding of macronutrients.