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Essentials of Anatomy and Physiology
Fifth edition
Seeley, Stephens and Tate
Chapter 17: Cellular
Metabolism
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 2.1
Processes of the Digestive System
Figure 14.11
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Slide 14.46
Control of Digestive Activity
• Mostly by reflexes via the
parasympathetic division
• Chemical and mechanical receptors
trigger reflexes
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Slide 14.47a
Control of Digestive Activity
• Stimuli include:
• Stretch of the organ
• pH of the contents
• Presence of breakdown products
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Slide 14.47b
Control of Digestive Activity
• Reflexes include:
• Activation or inhibition of glandular
secretions
• Smooth muscle activity
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Slide 14.47b
Digestion and Absorption in the
Stomach
• Proteases act on:
• Pepsin –protein digestion
• Rennin –milk protein digestion
• Absorption of:
•Water, alcohol and aspirin
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Slide 14.55
Digestion in the Small Intestine
• Pancreatic enzymes provide…
• Complete digestion of starch
• Amylase
• Other carbohydrases
• About half protein digestion (trypsin, etc.)
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Slide 14.57a
Digestion in the Small Intestine
• Pancreatic enzymes, cont…
• Fat digestion (lipase)
• Nucleic acid digestion (nucleases)
• Alkaline content neutralizes acidic chyme
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Slide 14.57b
Stimulation of the Release of Pancreatic
Juice
• Vagus nerve
• Local hormones
• Secretin
• Cholecystokinin
Figure 14.15
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Slide 14.58
Absorption in the Small Intestine
• Water
• Products of digestion
• Most molecules absorbed by active
transport
• Lipids absorbed by diffusion
• Nutrients transported to the liver
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 14.59
Nutrition
• Nutrient – substance used by the body
for growth, maintenance, and repair
• Categories of nutrients
• Carbohydrates
• Lipids
• Proteins
• Vitamins
• Mineral
• Water
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Slide 14.63
Cellular Metabolism
• “All the chemical reactions necessary to
maintain life”
• Anabolism: a constructive process during
which larger molecules are built from
smaller ones
• Usually involves condensation
•AKA dehydration synthesis
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Slide 14.67
Cellular Metabolism, con’t…
•Carbohydrates
•Monosaccharides = simple sugars
•Glucose, fructose
•Disaccharides = Combinations of
monosaccharides, removal of water
•Sucrose, lactose, maltose
• Polysaccharides: usually polymers of
glucose
•Starch, cellulose, chitin
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Slide 14.67
Cellular Metabolism, con’t…
•Lipids
•1 glycerol + 3 fatty acids
neutral fat + 3 H2O
•These are triglycerides
•Further modifications produce:
•Phospholipids (cell membrane)
•glycolipids (cell membrane)
•Lipoproteins (cell membrane,
blood)
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Slide 14.67
Cellular Metabolism, con’t…
•Proteins
•Two amino acids
a dipeptide + H2O
•Covalent bond formed is a peptide
bond
•Unique to proteins
•Polypeptides: 2-100 amino acids
•Protein: >100 amino acids
•Require additional modification to
become functional
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Slide 14.67
Cellular Metabolism, con’t…
•Proteins
•Modification occurs on four levels
•Primary: string of amino acids
•Secondary: helix or “pleat” structures
•Tertiary: 3-D folding
•Quarternary: two or more 3-D
proteins that act as a functional unit
•i.e., hemoglobin, collagen
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Slide 14.67
Cellular Metabolism, con’t…
•Proteins
•Recall from Chemistry:

•Proteins each have a unique 3-D
shape
•Shape determines function
•Loss of shape leads to loss of
function
•“denaturing” proteins with heat, pH
changes
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Slide 14.67
Cellular Metabolism, con’t…
•Proteins
•May be structural or functional
•Structural:
•Play a role in cellular architecture
•Collagen, fibrin, actin, myosin, etc.
•Functional:
•Play a role in cell metabolism
•Enzymes, neurotransmitters,
antibodies, etc.
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Slide 14.67
Cellular Metabolism, con’t…
•Enzymes:
•Biological catalysts
•Highly specific for a substrate
•Substrate: substance upon which an
enzyme acts
• i.e., peptidases act only on
peptide bonds in small polypeptides
•Produced only in presence of substrate
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Slide 14.67
Cellular Metabolism, con’t…
•Enzymes:
•Huge protein molecules
•Alter shape to conform to shape of
substrate (“wrap around” effect)
•Average 1500/cell (>5000 in liver cells)
•Most require co-enzymes
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Slide 14.67
Cellular Metabolism, con’t…
•Enzymes:
•Recognize substrate by shape of
binding site
•Serve to lower energy required for
reaction to occur (activation energy)
•therefore speed up reactions
•Not changed or used up during reaction
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 14.67
Cellular Metabolism, con’t…
•Co-Enzymes:
•Required to activate enzymes
•Facilitate enzymatic reactions
•May be a metal ion (Zn++, Cu++, Fe++)
•May be a vitamin
•Vitamins are co-enzymes
•Only function if “their” enzyme is
available
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 14.67
Cellular Metabolism
• Catabolism: substances are broken down into
molecules
• “destructive” process
• Large molecules broken down into smaller
molecules
• Usually by hydrolysis
• “splitting with water”
• Adds H2O back into molecule
• Breaks covalent bonds
Cellular Metabolism
• Catabolism
• Energy is released when bonds break
• Reverse of dehydration synthesis
(condensation)
• Hydrolysis = chemical digestion
• Occurs simultaneously (and
continuously) with anabolism
• Processes controlled by enzymes
Cellular Energy
• Cellular energy is chemical energy
• Derived from breaking chemical
bonds
• ~ ½ Energy is stored as ATP
• ~ ½ Energy is released as heat
• Helps maintain body temperature
• Enzymes control in the process
Cellular Energy
• All nutrient molecules are ultimately
degraded or converted to glucose
• Only glucose can be used to make
ATP
• Oxidation: cellular process of
chemically breaking apart a glucose
molecule to release energy
Cellular Energy
• Glucose oxidation occurs in 2 phases
• Anerobic metabolism
• Occurs in cytoplasm
• Without oxygen
• AKA glycolysis
• Splits glucose into two 3-Carbon
molecules: pyruvate
Cellular Energy
• Glycolysis
• Process also produces 2 ATPs
• In yeast, plant cells:
• Pyruvate can undergo alcoholic
fermentation
• In bacteria, animal cells:
• Pyruvate can produce lactic acid
Cellular Energy
• Aerobic metabolism
• Uses oxygen
• AKA Kreb’s Cycle or Citric Acid
cycle or Tricarboxylic Acid (TCA)
Cycle
• Occurs in mitochondria
• Makes more ATP than anerobic
processes
Cellular Energy
• Aerobic metabolism
• CO2 and H2O are waste products
• CO2:
• Diffuses out of cells
• Dissolves in plasma
• Produces HCO3- in blood
• Exhaled from lungs
Cellular Energy
• Aerobic metabolism
• H2O:
• “metabolic” water
• Exhaled from lungs
• Final products of glucose oxidation:
• CO2, H2O, ATP
Cellular Energy
• Aerobic metabolism
• For each molecule of glucose:
• 2 ATP formed in glycolysis
• 36 ATP formed in TCA cycle
• Energy stored in phosphate
bonds
• A reversible reaction
Metabolic Pathways
• “A particular sequence of enzymatic
reactions”
• Such as glycolysis, TCA cycle
• Carbohydrate pathways
• Carbos should comprise most of
our diet (~ 50% complex carbs)
• Used as a primary energy source
• Produce 4kcal/gm
Metabolic Pathways
• Carbohydrate pathways
• Excess carbs converted to energy
storage forms
• Glycogen (muscle, liver)
• Adipose tissue (hips)
• Process is anabolism
Metabolic Pathways
• Lipid pathways
• Metabolism controlled by liver
• Should comprise <30% of calories
in diet
• Get 9 kcal/gm (more ATP!)
• Must be degraded into glycerol,
fatty acids, then pyruvate
• A reversible catabolic process
Metabolic Pathways
• Protein pathways
• Proteins should comprise ~30% of
diet
• Get 4 kcal/gm
• Catabolism is more complex
• Proteins contain nitrogen
Metabolic Pathways
• Protein pathways
• Deamination: removal of nitrogen
from amino acids
• Occurs in liver
• Nitrogen is converted to urea
» A nitrogenous waste product
• Sent to kidneys for excretion
Metabolic Pathways
• Protein pathways
• After deamination:
• amino acid “skeleton” is
processed
• in TCA cycle
• May produce CO2, H2O, ATP
• May form glucose or fat
Metabolic Pathways
• Protein pathways
• Glucose formed from amino acid
skeletons may be re-converted to
amino acids
• “Essential” amino acids:
• Body cannot make these
• Must obtain in the diet
Regulation of Metabolic
Pathways
• Enzyme “saturation”
• Too much substrate for number of
enzyme molecules
• Reaction rate cannot increase
• A single enzyme can control an
entire metabolic pathway
• “rate limiting” enzyme
Digestive System: Disorders
• Ulcers: bacterial infection with H. pylori
• Vomiting: controlled by center in medulla
oblongata
• Activity of tract slows in old age
• Fewer digestive juices
• Peristalsis slows
• Diverticulosis and cancer more common
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Slide 14.92b
Digestive System: Disorders
• IBS: irritable bowel syndrome
• Crohn’s disease (autoimmune)
• Constipation
• Diarrhea
• Colitis
• Colon polyps/cancers
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 14.92b