6.1 Digestion
6.1.1 Explain why digestion of large food molecules is essential.
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Large food molecules are usually polymers, such as polysaccharides, proteins and lipids,
which are too large to be absorbed from the digestive tract into the circulatory system for
transport because they are simply too large to move across the membranes of small
intestine epithelial cells.
After digestion, polysaccharides are broken down into monosaccharides, polypeptides
are broken down into amino acids, and lipids are broken down into glycerol and fatty
acids.
Monomers, such as monosaccharides, amino acids, glycerol, and fatty acids are small
enough to be absorbed by small intestine epithelial cells, moving these substances by
diffusion, facilitated diffusion, or active transport through membrane proteins.
6.1.2 Explain the need for enzymes in digestion. The need for increasing
the rate of digestion at body temperature should be emphasized.
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At body temperature (37°C in mammals), reaction rates are too slow to be efficient at
hydrolysis reactions of large food molecules
Hydrolytic reactions in the digestion of large food molecules, such as polysaccharides,
proteins and lipids into their monomers, are exothermic, but occur very slowly due to
considerable activation energy
Enzymes lower activation energy, catalyzing hydrolysis reactions of large food molecules
into their monomers
6.1.3 State the source, substrate, products and optimum pH conditions for
one amylase, one protease and one lipase. Any human enzymes can be
selected. Details of structure or mechanisms of action are not required.
Enzyme
Salivary amylase
Pepsin
Pancreatic lipase
Source
salivary glands
stomach
pancreas
Substrate
starch
proteins
Products
maltose
polypeptides
Optimum pH
7-8
1.5-2.5
Triglycerides (fats
and oils)
fatty acids and
glycerol
7
6.1.4 Draw and label a diagram of the digestive system. The diagram should
show the mouth, esophagus, stomach, small intestine, large intestine, anus,
liver, pancreas and gall bladder. The diagram should clearly show the
interconnections between these structures.
6.1.5 Outline the function of the stomach, small intestine and large intestine.
Stomach:
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A large, expandable, muscular and glandular organ
Stores and mixes food, aiding in both physical and chemical digestion
gastric pits secrete:
a) HCl, producing a stomach pH of about 2, facilitating pepsin activity, and killing foreign
pathogens, such as bacteria
b) pepsinogen, an inactive precursor which is converted to pepsin under acidic conditions
c) pepsin catalyzes the hydrolysis of large proteins and polypeptides into smaller polypeptides
d) mucus, which protects stomach cells from acidic conditions
e) chyme = product of stomach digestion, an acid fluid released from stomach into small intestine
via pyloric sphincter
Small intestine:
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Digestion:
a) pancreas releases bicarbonate = NaHCO3b) which neutralizes acidic chyme, producing a pH = 8, optimizing activities of intestinal enzymes
c) enzymes from pancreas, and small intestine epithelial cells hydrolyze large molecules into
smaller molecules
d) polypeptides digested into amino acids
e) polysaccharides & disaccharides digested into monosaccharides
f) triglycerides digested into fatty acids and glycerol
g) bile produced in liver, stored in gall bladder, released through pancreatic duct
h) emulsifying fat droplets into smaller particles on which pancreatic lipase can act more
efficiently
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motility by peristalsis: rhythmic contractions of circular and longitudinal smooth muscles
lining small intestine slowly force chyme down intestinal tract
absorption: lining of small intestine is folded, increasing surface area for absorption, and
each fold is folded again into villi, with each villus acting an absorptive unit
Large intestine:
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absorption of vitamin K produced by mutualistic bacteria
reabsorption of water, Na+, K+ from intestinal lumen to capillaries
motility by peristalsis: rhythmic contractions of circular and longitudinal smooth muscles
lining large intestine slowly force fecal matter down intestinal tract
6.1.6 Distinguish between absorption and assimilation.
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absorption: movement of chemical substances from the lumen of the digestive tract
across the membranes of cells lining the digestive tract by diffusion, facilitated diffusion,
or active transport, and then either into the circulatory or lymphatic systems for
distribution to all somatic cells
assimilation: following digestion and absorption, nutrients are taken into somatic cells and
converted to the biomass of the organism
6.1.7 Explain how the structure of the villus is related to its role in
absorption and transport of the products of digestion.
A. surface area:
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Folding: intestinal folding increases surface area by 3X;
Villi: within each fold, a second set of folds creates a series of villi, with each villus being
a finger-like projection, increasing intestinal surface area by an additional 10X;
Microvilli: along the lumen side of each small intestine epithelial cell a brush border of
microvilli additionally expands surface area by another 20X;
Thus, the total surface area increase = 3 x 10 x 20 = 600x
B. Membranes of epithelial cells:
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diffusion of fatty acids, monoglycerides, fat-soluble vitamins, some mineral ions through
membrane phospholipid bilayer
facilitated diffusion of some monosaccharides, some vitamins and mineral ions using
membrane proteins
active transport of amino acids, most monosaccharides, some mineral ions, using
membrane proteins & ATP produced by mitochondria in epithelial cells
C. Blood capillaries:
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oxygenated blood enters villus supplying oxygen for cellular respiration:
cell growth replacing lost/injured cells
ATP for active transport
deoxygenated blood leaves villus rich in absorbed nutrients: amino acids,
monosaccharides, mineral ions, vitamins
D. Lacteals = branches of lymphatic system:
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fatty acids and glycerol are reformed into triglycerides in epithelial cell smooth ER/Golgi
apparatus
triglycerides, with phospholipids and cholesterol, aggregate into chylomicrons which are
coated with proteins and then leave epithelial cells and enter lacteals
AHL