BackCarbohydrates: Structure, Classification, and Dietary Importance
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Carbohydrates
Definition and General Properties
Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen. They are classified as simple or complex sugars and play essential roles in energy storage, structural integrity, and cellular recognition.
Key Point 1: Carbohydrates include sugars, starches, and fibers.
Key Point 2: The general formula for carbohydrates is , where n is a whole number (typically 3 or higher).
Example: Glucose () is a common carbohydrate.
Classes of Carbohydrates
Monosaccharides
Monosaccharides are the simplest carbohydrates, consisting of a single sugar unit. They are sweet-tasting and cannot be broken down into smaller carbohydrates.
Key Point 1: Monosaccharides contain carbon, hydrogen, and oxygen.
Key Point 2: The general formula is .
Example: Glucose () is a monosaccharide.
Disaccharides
Disaccharides are carbohydrates composed of two monosaccharide units joined together. They can be split into two monosaccharides by hydrolysis.
Key Point 1: Disaccharides are formed by the condensation of two monosaccharides.
Key Point 2: Ordinary table sugar, sucrose (), is a disaccharide that can be hydrolyzed into glucose and fructose.
Oligosaccharides
Oligosaccharides are carbohydrates containing three to nine monosaccharide units. They play important roles in biological recognition, such as blood typing.
Key Point 1: Oligosaccharides consist of 3–9 monosaccharide units.
Key Point 2: The ABO blood-typing groups are examples of oligosaccharides.
Polysaccharides
Polysaccharides are carbohydrates composed of ten or more monosaccharide units. They can be linear or branched and serve as energy storage or structural materials.
Key Point 1: Poly means "many"; polysaccharides have long chains of sugar units.
Key Point 2: Starch is a plant polysaccharide with branched chains of glucose, which can be broken down for energy.
Example: Amylose and amylopectin are two forms of starch.
Dietary Fiber
Role of Fiber in Nutrition
Dietary fibers are carbohydrates that cannot be digested by human enzymes. They are classified as soluble or insoluble, each with distinct physiological effects.
Key Point 1: Soluble fiber mixes with water to form a gel-like substance, promoting satiety and slowing sugar and cholesterol absorption.
Key Point 2: Insoluble fiber does not mix with water but adds bulk to the diet, aiding in digestion and preventing constipation.
Example: Oatmeal, legumes, apples, and carrots are high in soluble fiber; whole grains, seeds, brown rice, cabbage, and vegetable skins are sources of insoluble fiber.
Comparison of Soluble and Insoluble Fiber
Type of Fiber | Mixes with Water? | Main Effects | Sources |
|---|---|---|---|
Soluble | Yes | Forms gel, slows absorption, increases satiety | Oatmeal, legumes, apples, psyllium, carrots |
Insoluble | No | Adds bulk, prevents constipation | Whole grains, seeds, brown rice, cabbage, vegetable skins |
Additional info: Fruit pectins used in making jellies contain soluble fiber; cellulose is an insoluble fiber.
