Transcript Digestion, Absorption, and Metabolism

Williams' Basic Nutrition & Diet
Therapy
14th Edition
Chapter 5
Digestion, Absorption, Transport
and Metabolism
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1
Lesson 5.1: Digestion and the
Digestive Organs
1. Through a balanced system of mechanical and
chemical digestion, food is broken down into smaller
substances and the nutrients are released for
biological use.
2. Special organ structures and functions accomplish
these tasks through the successive parts of the
overall system.
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2
Digestion (p. 64)

Basic principles

Principle of change
• The body cannot use food as it is eaten.
• Food must be changed into simpler substances to be
absorbed and then used by cells to sustain life.

Principle of wholeness
• The parts of the digestive process comprise a continuous
whole.
• Food components travel through the gastrointestinal (GI)
system until they are delivered to cells or excreted.
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3
Digestion (cont’d) (p. 65)
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4
Mechanical and Chemical
Digestion (p. 65)



Mechanical and chemical actions make up the
digestive process.
Food must undergo these changes to be delivered to
cells.
Specific actions occurring during digestion of
carbohydrates, proteins, and fats are discussed in
other chapters.
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5
Mechanical Digestion (p. 66)

Mechanical digestion: gastrointestinal motility

Muscles in GI wall produce:
• Tonic contractions, which produce continuous movement
• Periodic muscle contraction and relaxation, which mix
food mass and move it forward
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6
Mechanical Digestion (cont’d)
(p. 66)

Mechanical digestion: gastrointestinal motility
(cont’d)

Nerves from esophagus to anus:
• Control muscle tone in wall
• Regulate rate and intensity of contractions
• Coordinate various movements
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7
Chemical Digestion (p. 66)

Chemical digestion: gastrointestinal
secretions





Hydrochloric acid and buffer ions: produce the correct pH
necessary for enzyme activity
Enzymes: specific digestive proteins for breaking down
nutrients
Mucus: lubricates and protects the GI tract tissues and helps
mix the food mass
Water and electrolytes: carry and circulate the products of
digestion through the tract and into the tissues
Bile: divides fat into smaller pieces to assist fat enzymes
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8
Case Study

Tim eats a meal consisting of the following
foods:

Grilled Chicken with lemon seasoning
 Baked potato with broccoli and cheese
 Mixed greens with lemon juice and olive oil and
parmesan cheese
 1 cup of unsweetened ice tea
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9
Digestion in the Mouth and
Esophagus (p. 66)

Mechanical digestion




Mastication breaks down food
Food is swallowed and passes down esophagus
Muscles at tongue base facilitate process
Gastroesophageal sphincter at stomach entrance
relaxes, allowing food to enter, then constricts to
retain food
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10
Digestion in the Mouth and
Esophagus (cont’d) (p. 66)

Chemical digestion




Salivary glands secrete material containing
salivary amylase or ptyalin
Ebner’s glands at the back of the tongue secrete a
lingual lipase
Salivary glands also secrete a mucous material to
lubricate and bind food particles, facilitating the
swallowing of the food bolus
Secretions from the mucous glands in the
esophagus help move food toward the stomach
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11
Case Study (cont’d)

Explain what is happening in the mouth and
esophagus in regard to Tim’s meal
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12
Digestion in the Stomach (p. 67)
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13
Digestion in the Stomach (cont’d)
(p. 67)

Mechanical digestion




Under sphincter control, the food enters upper
portion of the stomach (fundus)
Stomach muscles knead, store, mix, and propel
food mass forward
By the time food mass reaches the lower portion
of the stomach (antrum), it is a semiliquid
acid/food mix called chyme
Pyloric valve slowly releases chyme into the first
section of the small intestine (duodenum)
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14
Digestion in the Stomach (cont’d)
(p. 67)

Chemical digestion: three types of acid
secretions



Hydrochloric acid: parietal cells in the stomach
lining secrete acid to promote gastric enzyme
activity
Mucus: secretions protect the stomach lining from
the erosive effect of the acid and also bind and
mix the food mass and help move it along
Enzymes: pepsinogen is secreted by stomach
cells and is activated by acid to become pepsin, a
protein-splitting enzyme
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15
Case Study (cont’d)

Explain what is happening to Tim’s meal in the
stomach at this time.
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16
Digestion in the Small Intestine
(p. 67)

Mechanical digestion

Peristaltic waves push food forward
 Pendular movements stir chyme
 Segmentation rings chop food mass into lumps
 Longitudinal rotation rolls food in spiral motion,
mixing it
 Surface villi motions stir and mix chyme
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17
Digestion in the Small Intestine
(cont’d) (p. 68)

Chemical digestion: pancreatic enzymes



Carbohydrate: pancreatic amylase converts starch
to maltose and sucrose
Protein: trypsin and chymotrypsin split large
protein molecules into small peptide fragments
and eventually into single amino acids;
carboxypeptidase removes end amino acids from
peptide chains
Fat: pancreatic lipase converts fat to glycerides
and fatty acids
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18
Digestion in the Small Intestine
(cont’d) (p. 68)

Chemical digestion: intestinal enzymes



Carbohydrate: Disaccharidases convert
disaccharides into monosaccharides.
Protein: Enterokinase activates trypsinogen from
the pancreas to become trypsin; amino peptidase
removes end amino acids from polypeptides;
dipeptidase splits dipeptides into amino acids.
Fat: Intestinal lipase splits fat into glycerides and
fatty acids.
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19
Case Study (cont’d)

Explain how Tim’s meal is progressing through the
intestine with each enzyme activity.
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20
Digestion in the Small Intestine
(cont’d) (p. 68)

Chemical digestion



Mucus: protects mucosal lining
Bile: emulsifying agent, aids fat digestion and
absorption
Hormones: secretin for alkaline environment,
cholecystokinin triggers release of bile
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21
Digestion in the Small Intestine
(cont’d) (p. 69)
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22
Case Study (cont’d)

What role does bile play in the digestive process of
Tim’s meal?
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Lesson 5.2: Absorption and
Metabolism
3. Absorption, transport, and metabolism allows for the
distribution, use, and storage of these nutrients
throughout the body.
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24
Absorption and Transport (p. 69)

At this point in the process:




Carbohydrates: reduced to simple sugars
(glucose, fructose, galactose)
Fats: changed into fatty acids and glycerides
Proteins: changed into single amino acids
Vitamins and minerals: liberated from food
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25
Case Study (cont’d)

Discuss the final products formed from digestion of
Tim’s meal that can be readily used by the cells
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26
Absorption and Transport (cont’d)
(p. 69)

Amount of nutrients consumed depends on
bioavailability:



Amount of nutrient present
Competition between nutrients for absorption sites
Form in which nutrient is present
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27
Small Intestine (p. 71)

Small intestine: three absorbing structures




Mucosal folds: surface of small intestine piles into
many folds
Villi: small, fingerlike projections
Microvilli: cover each villus
Make inner surface 600 times greater than
outer surface of intestinal wall
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28
Small Intestine (cont’d) (p. 72)
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29
Absorption Processes (p. 71)




Simple diffusion: particles move outward in all
directions toward areas of lesser concentration
Facilitated diffusion: uses a protein channel for
carrier-assisted movement of larger particles
Active transport: carrier partner (e.g., sodium) moves
particles across a membrane
Pinocytosis: larger materials are engulfed by a cell
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30
Absorption Processes (cont’d)
(p. 72)
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31
Absorption in the Large Intestine
(p. 73)



Water: main absorptive task of large intestine is to
absorb water; small amount remains for feces
Dietary fiber: contributes bulk to help form feces
Macronutrients and micronutrients: absorbed to
lymph or blood
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32
Transport (p. 73)

Vascular system: veins and arteries




Carry water-soluble nutrients, oxygen, other vital
substances
Transport wastes for removal
Portal circulation first carries nutrients to liver for
cell enzyme work
Lymphatic system

Carries non–water-soluble fatty materials
 Lymph vessels in villi absorb materials
 Route to larger lymph vessels
 Eventually to blood stream through thoracic duct
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33
Metabolism (p. 73)

At this point, individual macronutrients in
food:



Have been broken down
Absorbed into bloodstream or lymphatic system
Next, nutrients can be converted into energy or
stored
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34
Energy for Fuel (p. 73)


Mitochondria of cells is where metabolism
takes place
Two forms of metabolism


Catabolism: breaking down of larger substances
into smaller units
Anabolism: building up of larger substances from
smaller units
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35
Energy for Fuel (cont’d) (p. 73)



Metabolic processes ensure that the body has energy
in the form of adenosine triphosphate (ATP)
Metabolism of glucose from carbohydrates yields less
energy than metabolism of fat, but glucose is the
body’s primary source of energy
Protein can be an energy source, but it is relatively
inefficient
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36
Stored Energy (p. 74)




Energy beyond that needed at present is stored for
future
Glucose is converted to glycogen via glycogenesis,
stored in liver or muscles
When glycogen reserves are full, excess is stored as
fat via lipogenesis
Excess protein/amino acids converted to glucose via
gluconeogenesis
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37
Stored Energy (cont’d) (p. 74)
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38
Errors in Digestion and
Metabolism (p. 75)

Genetic defects: cell is missing enzyme
controlling metabolism of a specific nutrient

Phenylketonuria (PKU)
• Enzyme responsible for metabolizing essential amino
acid phenylalanine is missing
• Untreated, causes permanent mental retardation and
CNS damage
• With proper treatment, affected children may have
normal and healthy lives
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39
Errors in Digestion and
Metabolism (cont’d) (p. 75)

Genetic defects (cont’d)

Galactosemia
• Enzyme responsible for metabolizing galactose to
glucose is missing
• All sources of lactose must be eliminated from diet
• Untreated, can cause brain and liver damage
• Screening and treatment can enable normal life
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40
Errors in Digestion and
Metabolism (cont’d) (p. 77)

Genetic defects (cont’d)

Glycogen storage diseases (GSD)
• Group of rare genetic defects
• Absence of enzymes required for synthesis or
breakdown of glycogen
• Form of disease depends on enzyme missing
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41
Other Intolerances or Allergies
(p. 78)


Not genetic inborn errors of metabolism
Rather, caused by food intolerances or
allergies


Lactose intolerance: deficiency of any of the
disaccharidases in small intestine
• Insufficient lactase to break down milk
• Abdominal cramping and diarrhea
Produces containing lactose must be avoided
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42
Other Intolerances or Allergies
(cont’d) (p. 78)

Allergies



Inappropriate immune responses
Not necessarily a problem with digestion or
metabolism
Covered in Chapter 18
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