3. Slide 14.47a
Control of Digestive ActivityControl of Digestive Activity
• Mostly by reflexes via the
parasympathetic division
• Chemical and mechanical receptors
trigger reflexes
4. Slide 14.47b
Control of Digestive ActivityControl of Digestive Activity
• Stimuli include:
• Stretch of the organ
• pH of the contents
• Presence of breakdown products
5. Slide 14.47b
Control of Digestive ActivityControl of Digestive Activity
• Reflexes include:
• Activation or inhibition of glandular
secretions
• Smooth muscle activity
6. Slide 14.55
Digestion and Absorption in theDigestion and Absorption in the
StomachStomach
• Proteases act on:
• Pepsin –protein digestion
• Rennin –milk protein digestion
• Absorption of:
• Water, alcohol and aspirin
7. Slide 14.57a
Digestion in the Small IntestineDigestion in the Small Intestine
• Pancreatic enzymes provide…
• Complete digestion of starch
• Amylase
• Other carbohydrases
• About half protein digestion (trypsin, etc.)
8. Slide 14.57b
Digestion in the Small IntestineDigestion in the Small Intestine
• Pancreatic enzymes, cont…
• Fat digestion (lipase)
• Nucleic acid digestion (nucleases)
• Alkaline content neutralizes acidic chyme
9. Slide 14.58
Stimulation of the Release of PancreaticStimulation of the Release of Pancreatic
JuiceJuice
• Vagus nerve
• Local hormones
• Secretin
• Cholecystokinin
Figure 14.15
10. Slide 14.59
Absorption in the Small IntestineAbsorption in the Small Intestine
• Water
• Products of digestion
• Most molecules absorbed by active
transport
• Lipids absorbed by diffusion
• Nutrients transported to the liver
11. Slide 14.63
NutritionNutrition
• Nutrient – substance used by the body
for growth, maintenance, and repair
• Categories of nutrients
• Carbohydrates
• Lipids
• Proteins
• Vitamins
• Mineral
• Water
12. Slide 14.67
Cellular MetabolismCellular 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
13. Slide 14.67
Cellular Metabolism, con’t…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
15. Slide 14.67
Cellular Metabolism, con’t…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
16. Slide 14.67
Cellular Metabolism, con’t…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
17. Slide 14.67
Cellular Metabolism, con’t…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
18. Slide 14.67
Cellular Metabolism, con’t…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.
19. Slide 14.67
Cellular Metabolism, con’t…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
20. Slide 14.67
Cellular Metabolism, con’t…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
21. Slide 14.67
Cellular Metabolism, con’t…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
22. Slide 14.67
Cellular Metabolism, con’t…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
23. 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
24. 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
25. 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
26. 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
27. 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
28. 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
29. 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
30. 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
32. 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
33. 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
35. 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
39. 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
40. 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
41. Slide 14.92b
Digestive System: DisordersDigestive 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
