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Simple Carbohydrates
- Monosaccharides: simple sugars
- primarily the 6-carbon sugar glucose
- Disaccharides: Two monosaccharide units joined by covalent bonds.
- Primarily Sucrose consisting of one glucose and one fructose residue.
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Complex Carbohydrates
- Oligosaccharides (3 to 10 saccharide units)
- Primarily trisaccharides
- Polysaccharides (more than 10 saccharide units)
- Primarily glycogen, starch, and cellulose.
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Monosaccharides of the ___ configuration are much more important nutritionaly because _____.
Monosaccharides of the D configuration are much more important nutritionally than their L isomers, because D isomers exist as such in dietary carbohydrate and are metabolized specifically in that form. Enzymes involved in carbohydrate digestion and metabolism are stereospecific for D sugars, meaning that they react only with D sugars and are inactive toward L forms.
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The three most common disaccharides in the diet are ___, ___, and ___. How are they made and where are they found?
Maltose: Two glucose units linked through an alpha 1-4 glycosidic bond. Found in malt beverages such as beer and malt liquors.
Lactose: Glactose linked by a beta 1-4 glycosidic bond to glucose. Found naturally only in milk and milk products.
Sucrose: Glucose and fructose - not a reducing sugar - linked via the anomeric hydroxyl of both residues. The most widely distributed of the disaccharides and is the most commonly used natural sweetener (cane sugar and beet sugar).
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Name three common oligosaccharides. Where are they found?
Raffinose (a trisaccharide), stachyoses (a tetrasacchride), and verbascose (a pentasaccharide) are made up of glucose, galactose, and fructose and are found in beans, peas, bran, and whole grains.
Human digestive enzymes do not hydrolyze them, but the bacteria within the intestine can digest them. This is the basis for flatulence that occurs after eating these foods.
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Define homopolysaccharide and heteropolysaccharide. Which one carries greater importance in nutrition?
If the structure is composed of a single type of monomeric unit it is called a homopolysaccharied.
If two or more different types of monosaccharides make up its structure, it is called a heteropolysaccharide.
Homopolysaccharides are of far greater importance in nutrition because of their abundance in many natural foods.
The polyglucoses starch (plants) and glycogen (animals), for example, are the major storage forms of carbohydrate in plant and animal tissues.
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The most common digestible polysaccharide in plants is _____. What is it made of and what foods is it found in?
Starch.
Starch can exist in two forms, amylose and amylopectin, both of which are polymers of D-glucose.
Amylose is unbranched with glucose attached through alpha 1-4 glycosidic bonds.
Amylopectin is a branched-chain polymer, with branch points occurring through alpha 1-6 bonds.
Both occur in cereal grains, potatoes, legumes, and other vegetables. Amylose contributes about 15 to 20% and amylopectin 80 to 85% of the total starch content.
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The major form of stored carbohydrate in animal tissues is _____. Describe it.
glycogen, which is localized primarily in liver and skeletal muscle. Like amylopectin, it is a highly branched polyglucose molecule--just far more branched than amylopectin.
The high degree of branching in glycogen and amylopectin offers a distinct metabolic advantage, because it presents a large number of nonreducing ends from which glucose residues can be cleaved.
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Glycogenolysis
The pathway by which gycogen is enzymatically broken down to glucose.
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What is cellulose? Explain it's function when digested?
Cellulose is the major component of cell walls in plants. It is a homopolysaccharide of glucose but its glycosidic bonds are beta 1-4, rendering the molecule resistant to the digestive enzyme alpha-amylase, which is stereospecific to favor alpha 1-4 linkages.
Because cellulose is not digestible by mammalian digestive enzymes, it is defined as a dietary fiber and is considered not to provide energy but rather a bulking agent and a potential energy source for some intestinal bacteria that are able to digest it.
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The most important dietary carbohydrates nutritionally are _____ and _____.
Polysaccharides and disaccharides. However, some free glucose and fructose are present in honey, certain fruits, and the carbohydrates added to processed foods such as high-fructose corn syrup.
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Glycosidases or, alternatively, carbohydrases.
The enzymes that hydrolyze polysaccharides and disaccharides to their constituent monosaccharide units for absorption from the GI tract into the bloodstream.
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The _____ is the major site of metabolism of galactose, fructose, and glucose.
Liver
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Glucose enters the cells by facilitated transport. In skeletal muscle and adipose tissue the process is insulin ___, whereas in the liver it is insulin ___.
dependent/independent
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When blood glucose levels are elevated, insulin is released by the ...
beta-cells of the pancreas
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Insulin stimulates the uptake of ___ by muscle and adipose and also inhibits the synthesis of ___ by the liver.
Insulin stimulates the uptake of glucose by muscle and adipose and also inhibits the synthesis of glucose (gluconeogenesis) by the liver.
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Glycogenesis
Making of glycogen
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Glycogenolysis
Breakdown of glycogen
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Glycolysis
Oxidation of glucose
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Glyconeogenesis
Production of glucose from noncarbohydrate intermediates
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Hexose monophosphate shunt
Production of 5-carbon monosaccharides and NADPH
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Tricarboxylic acid cycle (TCA)
Oxidation of pyruvate and acetyl CoA
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Glycemic index
The increase in blood glucose level over the baseline level during a 2-hour period following the consumption of a defined amount of carbohydrate (usually 50 g) compared with the same amount of carbohydrate in a reference food.
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Glycemic load (GL)
Glycemic load considers both the quantity and the quality of the carbohydrate in a meal.
The GL equals the glycemic index times the grams of carbohydrate in a serving of the food.
The higher the GL, the greater the expected elevation in blood glucose and in the insulinogenic effect of the food.
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