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DIGESTIVE SYSTEM
By Arvina D W
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Food and Nutrition
A. Food and Energy
B. Nutrients
1. Water
2. Carbohydrates3. Fats
4. Proteins
5. Vitamins
6. Minerals
C. Nutrition and a Balanced Diet
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NUTRIENTS
include
include
such as such as
are made of include includeare made using
Nutrients
Carbohydrates Fats MineralsProteins Vitamins
Simple Complex
Amino
acids Calcium Iron
Fatty Acids Glycerol
Sugars Starches Fat-solubleWater-
soluble
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Body mass index
Body mass index (kg/m2) = Weight (kg) / Height (m
2)
Female (kg/m2) Male (kg/m2)
Thin < 17 (kg/m2
) < 18 (kg/m2
)
Normal 17 23 (kg/m2) 18 25 (kg/m2)
Fat 23 27 (kg/m2) 25 27 (kg/m2)
Obesity > 27 (kg/m2) > 27 (kg/m2)
Source : Health Department 2003 See food intake and body weight
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Carbohydrates
Carbohydrates
The diversity of dietary carbohydrates necessitates discussion
of several classes of these molecules, ranging from simple
sugars to huge, branched polymers
Monosaccharides or simple sugars are either hexoses (6-carbon) like glucose,
galactose and fructose, or pentoses (5-carbon) like ribose. These are the
breakdown products of more complex carbohydrates and can be efficiently absorbedacross the wall of the digestive tube and transported into blood.
Disaccharides are simply twomonosaccharides linked together by a glycosidic bond. The
disaccharides most important in nutrition and digestion are:
lactose or "milk sugar": glucose + galactose
sucrose or "table sugar": glucose + fructose
maltose: glucose + glucose
Oligosaccharides, which include disaccharides, are relatively short chains of monosaccharides
which typically are intermediates in the breakdown of polysaccharides to monosaccharides.
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Polysaccharides
You should be familiar with three important polysaccharides, each of which is a large polymer of
glucose:
Starch is a major plant storage form of glucose.Cellulose is the other major plant carbohydrate. It is the major constituent of plant cell walls,
and more than half of the organic carbon on earth is found in cellulose. Herbivores subsist
largely on cellulose, not because they can digest it themselves, but because their digestive tracts
teem with microbes that produce cellulases that hydrolyze cellulose.
Glycogen is the third large polymer of glucose and is the major animal storage carbohydrate.
Like starch, the glucose molecules in glycogen are linked together by alpha(1-4) glycosidic
bonds.
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Fatty acids are present in only small amounts in animal and plant tissues, but are the building
blocks of many important complex lipids. True fatty acids possess a long hydrocarbon chain
terminating in a carboxyl group. Nearly all fatty acids have an even number of carbons and havechains between 14 and 22 carbons in length. The principle differences among the many fatty acids
are the length of the chain (usually 16 or 18 carbons) and the positions of unsaturated or double
bonds. For example, stearic acid (pictured below) has 18 carbons and is saturated.
The so-called "short-chain" or volatile fatty acids are 2 to 4-carbon molecules of great importance
in intermediary metabolism and as the mainstay of ruminant nutrition. They are represented by acetic,
butyric and proprionic acids.
The most abundant storage form of fat in animals and plants, and hence the most important dietary
lipid, is neutral fat or triglyceride. A molecule of triglyceride is composed of a molecule of glycerol in
which each of the three carbons is linked through an ester bond to a fatty acid. Triglycerides cannot
be efficiently absorbed, and are enzymatically digested by pancreatic lipase into a 2-monoglyceride
and two free fatty acids, all of which can be absorbed. Other lipases hydrolyse a triglyceride into
glycerol and three fatty acids.
A triglyceride
(triacylglycerol):
tristearin
FATS
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PROTEIN
ProteinsProteins are polymers of amino acids linked together by peptide bonds. Very short proteins,
typically 3 to 10 amino acids in length, are called peptides
Although very small peptides can be absorbed to a limited degree, for all intents and purposes,
proteins must be reduced to single amino acids before they can be absorbed. Enzymes that
hydrolyze peptide bonds and reduce proteins or peptides to amino acids are calledproteases orpeptidases.
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VITAMINS
Vitamin
A (retinol)
D (calciferol)
E (tocopherol)
K
B1 (thiamine)
B2 (riboflavin)
Sources
Yellow, orange, and dark green
vegetables; dairy products
Fish oils, eggs; made by skin
when exposed to sunlight;added to dairy products
Green leafy vegetables, seeds,
vegetable oils
Green leafy vegetables; made
by bacteria that live in human
intestine
Whole grains, pork, legumes,
milk
Dairy products, meats,
vegetables, whole-grain cereal
Function
Important for growth of skin
cells; important for night vision
Promotes bone growth;
increases calcium andphosphorus absorption
Antioxidant; prevents cellular
damage
Needed for normal blood
clotting
Normal metabolism of
carbohydrates
Normal growth; part of electron
transport chain; energy
metabolism
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VITAMINS
Vitamin
Niacin
B6 (pyridoxine)
Pantothenic acid
Folic acid
B12(cyanocobalamin)
Sources
Liver, milk, whole grains, nuts,
meats, legumes
Whole grains, meats,
vegetables
Meats, dairy, whole grains
Legumes, nuts, green leafy
vegetables, oranges, broccoli,
peas, fortified bread and cereal
Meats, eggs, dairy products,enriched cereals
Function
Important in energy metabolism
Important for amino acid
metabolism
Needed for energy metabolism
Coenzyme involved in nucleic
acid metabolism; prevents
neural-tube defects in
developing fetuses
Coenzyme in nucleic acidmetabolism; maturation of red
blood cells
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MINERALS
Mineral
Calcium
Phosphorus
Potassium
Chlorine
Sodium
Magnesium
Iron
Fluorine
Iodine
Zinc
Sources
Diary products; salmon; sardines;
kale; tofu; collard greens; legumes
Dairy products; meats; poultry; grains
Meats; dairy products; many
fruits and vegetables; grains
Table salt; processed foods
Table salt; processed foods
Whole grains; green leafy vegetables
Meats; eggs; legumes; whole grains;
green leafy vegetables; dried fruitFluoridated drinking water; tea;
seafood
Seafood; dairy products; iodized salt
Meats; seafood; grains
Function
Bone and tooth formation; blood clotting;
nerve and muscle function
Bone and tooth formation; acid-base balance
Acid-base balance; body water balance;
nerve function
Acid-base balance; formation of gastric juice
Acid-base balance; body water balance;
nerve function
Activation of enzymes in protein synthesis
Component of hemoglobin and of electron
carriers used in energy metabolismMaintenance of tooth structure; maintenance
of bone structure
Component of thyroid hormones
Component of certain digestive enzymes
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FOOD GUIDE PYRAMID
Fats
Sugars
Fats, Oils, and Sweets (use sparingly)Soft drinks, candy, ice cream, mayonnaise, and
other foods in this group have relatively few
valuable nutrients.
Milk, Yogurt, and Cheese Group
(2-3 Servings)Milk and other dairy products are rich in
proteins, carbohydrates, vitamins, and
minerals.
Meat, Poultry, Fish, Dry Beans, Eggs,and Nut Group
(2-3 servings)
These foods are high in protein.
They also supply vitamins and minerals.
Vegetable Group
(3-5 servings)
Vegetables are a low-fat
source of carbohydrates,
fiber, vitamins, and minerals.
Fruit Group
(2-4 servings)
Fruits are good sources of
carbohydrates, fiber, vitaminsand water.
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The Process of Digestion
A. The Mouth
1. Teeth (Mechanical Digestion )
2. Saliva (Chemical Digestion)
B. The Esophagus
C. The Stomach
1. Chemical Digestion2. Mechanical Digestion
D. The Small Intestine
E. Absorption in the Small Intestine
F. The Large Intestine
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THE ORGANS OF DIGESTIVE SYSTEM
Pharynx
Mouth
Salivary glands
Esophagus
Liver
Gallbladder(behind liver)
Rectum
Stomach
Pancreas(behind stomach)
Large intestine
Small intestine
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PREGASTRIC DIGESTION
There are 3 stages to provide food for the gut to work on in the mouth:
1.Prehension the process of seizing or grasping or otherwise getting food into the
mouth.
2.Mastication the first step in the breakdown of complex foodstuffs and serves several
functions, including:
a. breaking large pieces into small pieces, resulting in a massive increase in
surface area which is where digestive enzymes work
b. softening of food and transformation into a size conducive to swallowing
c. lubrication of food by impregnating it with saliva
3.Swallowing the bolus is swallowed and passed on to the next part of the gut.
see the human teeth
Digestion is the process that converts the ingested foods into a form that can be
absorbed and transported.
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Dental Formulae
Deciduous2 1 0 2
2 1 0 2= 10 Permanent
2 1 2 3
2 1 2 3= 16
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SALIVA
Saliva is produced in and secreted from salivary glands. The basic secretory units of salivary
glands are clusters of cells called an acini.
The three major pairs of salivary glands are:
parotid glands produce a serous, watery secretion
submaxillary (mandibular) glands produce a mixed serous and mucous secretion
sublingual glands secrete a saliva that is predominantly mucous in character
The basis for different glands secreting saliva of differing composition can be seen by examining
salivary glands histologically. Two basic types of acinar epithelial cells exist:
serous cells, which secrete a watery fluid, essentially devoid of mucus
mucous cells, which produce a very mucus-rich secretion
Saliva contains of mucus to lubricate the food and hydrogencabonate to provide the ideal condition
of pH for amylase to work.
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ESOPHAGUS
The bolus can be ensured to travel down the esophagus by epiglottis.
The bolus is swallowed and enters the esophagus and it will be pushed down towards the
stomach by peristalsis (wave of esophagus muscular contraction).
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STOMACH
The stomach contains digestive glands that produce 3 important secretion :
mucus protect the walls of the stomach from attack by gastric juices
pepsinProteases: Pepsinogen, an inactive zymogen, is secreted into gastric juice from
both mucous cells and chief cells. Once secreted, pepsinogen is activated by stomach acid
into the active protease pepsin, which is largely responsible for the stomach's ability to initiate
digestion of proteins. Chief cells also secrete chymosin (rennin), a protease that coagulates
milk protein.
hydrochloric acid provides the acidic conditions neede for the action of pepsin.
Hormones: The principal hormone secreted from the gastric epithelium is gastrin, a peptide
that is important in control of acid secretion and gastric motility
Inside the stomach the food is chuened up with the gastric juices into a creamy liquid called
chyme.
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SMALL INTESTINE
The digestion of molecules is completed in the small intestine, using digestive juices that
contain:
enzymes from the pancreas amylase, protease, and lipase (see table 2)
enzymes from the intestine wall maltase, sucrase (see table 2)
bile from the liver.
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LARGE INTESTINE
Some of food d that enter the small intetine cant be digested and absorbed into the
bloodstream.
These foods are :
indigestible fiber
undigested food some digestible food that may not be completely digested in
the small intestine
Those foods make up the bodys solid and pass into the large intestine.
In the large intestine wateris absorbed
waste materials form fecal material (faeces)
See Gastrointestinal Transit: How Long Does It Take?
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THE DIGESTIVE ENZYMES
Site
Mouth
Stomach
Small intestine
(from pancreas)
Small intestine
Enzyme Role in Digestion
Salivary amylase
Pepsin
Amylase
Trypsin
Lipase
Maltase, sucrase, lactase
Peptidase
Breaks down starches into
disaccharides
Breaks down proteins into large
peptidesContinues the breakdown of
starch
Continues the breakdown of
protein
Breaks down fat
Breaks down remainingdisaccharides into
monosaccharides
Breaks down dipeptides into
amino acids.
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LIVER & PANCREAS
Liver
Gallbladder
Duodenum
Bile duct
Pancreas
Pancreatic duct
To rest of small intestineCholecystokinin is the principle stimulus for delivery of pancreaticenzymes and bile into the small intestine.
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SMALL INTESTINE
Small Intestine Villus
Circular folds
Epithelial cells
Capillaries
Lacteal
Vein
Artery
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EXERCISE
1. What happens to solids that are indigestible - for example, a rock or a penny? Will it remain
forever in the stomach?"
If the indigestible solids are large enough, they indeed cannot pass into the small intestine, and
will either remain in the stomach for long periods, induce a gastric obstruction or, as every catowner knows, be evacuated by vomition However, many of the indigestible solids that fail to pass
through the pylorus shortly after a meal do pass into the small intestine during periods between
meals.