Download The carbohydrates Nature of carbohydrates: • Energy production

The carbohydrates
Nature of carbohydrates:
• Energy production system:
- 3 steps to produce energy from a basic fuel supply:
(1) In human body, the body digests its basic fuel, carbohydrates, changing it to
glucose.
(2) The body then absorb & through blood circulation, carries this refined fuel to
cells that need glucose.
(3) Glucose is burned in these cells, & energy produced through the process of
metabolism.
• The carbohydrates are a group of naturally occurring carbonyl compounds
(aldehydes or ketones) that also contain several hydroxyl groups.
• A carbohydrates is composed of carbon (C), hydrogen (H), and oxygen (O).
• (CH2O)n when n = 5 then C5H10O5
• Glucose is the most important carbohydrate; most dietary carbohydrate is absorbed
into the bloodstream as glucose, and other sugars are converted into glucose in the
liver.
Classes of carbohydrates:
• It is taken up by the cells and either broken down to obtain energy (glycolysis) or
converted into other metabolites.
• Glucose is the major metabolic fuel of mammals and a universal fuel of the fetus. It
is the precursor for synthesis of all the other carbohydrates in the body, including
glycogen for storage; ribose and deoxyribose in nucleic acids; and galactose in
lactose of milk, in glycolipids, and in combination with protein in glycoproteins and
proteoglycans.
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• Diseases associated with carbohydrate metabolism include diabetes mellitus,
galactosemia, glycogen storage diseases, and lactose intolerance.
• CARBOHYDRATES ARE ALDEHYDE OR KETONE DERIVATIVES OF
POLYHYDRIC ALCOHOLS
1- Monosaccharide: Are those carbohydrates that cannot be hydrolyzed into
simpler carbohydrates: They may be classified as trioses, tetroses, pentoses,
hexoses, or heptoses, depending upon the number of carbon atoms; and as aldoses
or ketoses depending upon whether they have an aldehyde or ketone group.
Examples are listed in Table 1.
– They are quickly absorbed from the intestine into the blood stream and
carried to the liver.
– Glucose, fructose, lactose.
2- Disaccharide: Are condensation products of two monosaccharide units.
- Sucrose, Lactose, Maltose.
3- Oligosaccharides: Are condensation products of more than two to ten
monosaccharides; maltotriose is an example
4- Polysaccharide: Are condensation products of more than ten monosaccharide
units; examples are the starches and dextrins, which may be linear or branched
polymers.
- Starch, Glycogen, Dietary fiber.
Monosaccharide
• Glucose:
- Is the form of sugar circulating in the blood and is the primary fuel to the cells.
- The blood supply comes mainly from the digestion of starch, & sometimes
called dextrose.
- Is a moderately sweet sugar.
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• Fructose: is found mainly in fruits, consider the sweetest of the simple sugars.
• Galactose: comes mainly from the digestion of milk sugar, or lactose.
Disaccharides
• Sucrose:
- Is common table sugar, its two sugar units are glucose & fructose.
• Lactose:
- Is the milk sugar, formed in mammary glands
- Its two single sugar units are glucose & Galactose.
- Not found in plants.
- Aids in the absorption of calcium & phosphorus.
• Maltose:
- The two single sugar units that compose maltose are double glucose molecule,
occurs as a breakdown product of the starches contained in malt (“malt sugar” and
as an intermediate in the digestion of starch.
Polysaccharides
• Polysaccharides are sometimes classified as hexosans or pentosans, depending upon
the identity of the constituent monosaccharides.
• Starch and glycogen are storage polymers of glucose in plants and animals,
respectively.
• Starch is the major source of energy in the diet.
• 1) Starch:
- They are found in grains, legumes, and other vegetables, and some fruits.
- Breakdown more slowly and supply energy over a longer period of time.
- Most important dietary carbohydrate
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- The major food sources of starch include grains in the form of pasta, crackers,
bread, and other baked goods, legumes in the form of beans, & peas, potatoes, rice,
corn, & bulgur.
- Whole grains: is used for food products such as flours, breads, or cereals that are
produced from unrefined grain, which is grain that still retains its outer bran layers
and inner germ endosperm and their nutrients (i.e. dietary fiber, minerals, &
vitamins)
- Enriched grains: are refined grain products to which key nutrients, usually
minerals (i.e. iron), and vitamins (i.e. A, C, D, thiamin, riboflavin, & niacin), have
been added.
• 2) Glycogen: found in animal muscle tissue, and liver
• 3) Dietary fiber
Dietary fiber
• Dietary fiber: in humans there is a lack in the necessary enzymes to digest dietary
fiber, used to treat gastrointestinal problems.
- Cellulose: helps move the food mass along, stimulates normal muscle action in
the intestine, the main sources are the stems and leaves of vegetables and the
coverings of seeds and grains.
- Non-cellulose polysaccharides: they absorb water and swell to a larger bulk, thus
slowing the emptying of the food mass from the stomach, and preventing spastic
colon.
- Lignin: is a large compound that forms the woody part of certain plants, it also
combines with bile acids and cholesterol in the human intestine, preventing their
absorption
Complex carbohydrates
• Contain other sugar derivatives such as amino sugars, uronic acids, and sialic acids.
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• They include proteoglycans and glycosaminoglycans, associated with structural
elements of the tissues; and glycoproteins, proteins containing attached
oligosaccharide chains.
• They are found in many situations including the cell membrane.
Other sweeteners
• Used as sugar replacers
• 2 types:
1) Nutritive sweeteners:
- They are sweeteners such sugar alcohols that contribute to total calorie intake.
- Sugar alcohols (sorbitol, mannitol, & xylitol)
- 2-3 kcalories/gram.
- Sorbitol: sucrose substitute.
- They are absorbed slowly in the small intestine.
- Use for diabetic patients
- Excessive amount= diarrhea.
- lowered risk than sugar to cause dental caries.
2) Nonnutritive sweeteners:
- Alternative sweeteners.
- Are sugar substitutes that do not have any caloric value.
- Sweet taste without energy.
- Commonly used are aspartame & saccharin.
- Sweeter than table sugar.
Functions of carbohydrates
• 1) Primary energy function:
(a) Basic fuel supply:
- Primary fuel for the body.
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- Fuel factor for carbohydrates is 4 kcal/gram.
(b) Reserve of fuel supply:
- Glycogen reserves fuel supply
- Individual must eat carbohydrate foods regularly to meet energy demand (to
maintain blood glucose level & prevent a breakdown of fat and protein in tissue).
• 2) Special tissue functions:
- Liver: glycogen reserves in the liver & muscles, protect cells from depressed
metabolic function.
- Protein & fat: (1) Carbohydrates protects protein to be used in tissue growth &
maintenance (2) Prevent rapid breakdown of fat & produce ketones (strong acid),
this action called antiketogenic effect.
- Heart: fatty acids are the regular fuel for energy, in emergency situation
glycogen.
- Central nervous system: the brain need continues supply of glucose, otherwise
low sugar cause brain damage.
Table 1. Classification of important sugars
Aldoses Ketoses
Trioses (C3H6O3) Glycerose Dihydroxyacetone
Tetroses (C4H8O4) Erythrose Erythrulose
Pentoses (C5H10O5) Ribose Ribulose
Hexoses (C6H12O6) Glucose Fructose
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