Download Fat digestion: 1. Bile salts and lecithin emulsify lipid particles

IPHY 3430 9-13-11
Exam location: Duane Physics G1B30
Sept. 22
Enzymatic Digestion in Small Intestine
1. Proteins degraded to small polypeptides
2. Carbohydrates degraded to disaccharides
3. Lipids must receive special treatment
before they can be attacked by enzymes.
Fat digestion:
1. Bile salts and lecithin emulsify
lipid particles (micelle)
2. Lipase breaks down to
monoglycerides and free fatty
acids.
1
Epithelial cell
Capillaries
Mucous cell
Villus
Central lacteal
Crypt of Lieberkühn
Arteriole
Venule
Lymphatic vessels
Microvilli
(brush border)
Epithelial
cell
Fig. 16-21, p. 614
2
Absorption of nutrients
1. Small peptides.
A. Peptides broken down to amino acids by
aminopeptidases in the epithelial wall or by
intracellular peptidases.
B. Most amino acids are absorbed into epithelial cell
by active transport (with expenditure of ATP and cotransport of Na+
C. Cl- and water follow Na+ into epithelial cell.
D. Amino acids exit cell via passive carrier molecules
and into blood. Na+ exits by active transport, Cl- and
water follow.
2. 
Absorption of Disaccharides
A. In epithelial cell, disaccharides broken down to
single sugars.
Lactose --> galactose and glucose by lactase
Maltose--> glucose and glucose by maltase
Sucrose--> glucose and fructose by sucrase
B. Glucose and galactose actively (with ATP) cotransported into epithelial cell with Na+. Cl- follows
with water.
Fructose moved by passive facilitated diffuion.
C. Glucose, galactose and fructose exit the cell by
passive facilitated diffusion, and enter blood by
diffusion
3
3. Absorption of Fats A. micelles containing monoglycerides and
free fatty acids deliver them to epithelial
lining of small intestine
B. monoglycerides and FFA passively diffuse through
cell membranes
C. monoglycerides and FFA resynthesized into triacylglycerides in cell
D. triacyglycerides surrounded with protein coat, forming a chylomicron. Fats, continued:
E. Chylomicron moved by exocytosis out
of epithelial cell into lymph circulation
F. Chylomicron transported by lymph to
circulatory system, and then to fat depot
where protein coat shed, and fat stored.
G. ATP used for bile and triacylglyceride
synthesis.
4
Fig. 16-16, p. 618
Absorption of other nutrients:
A. vitamins
1. Fat soluble (A,D,E, K)--absorbed passively
via micelle
2. Water soluble absorbed passively or with
facilitated diffusion
3. Vitamin B12 unique--needs gastric intrinsic
factor and receptor-mediated uptake
5
B. Ions
1. Active transport (Na+, K+, Ca++, Mg++, Mn++,
Fe++, PO4-, etc)
2. Cl-, HCO3- passive diffusion
C. Water
Moves by osmosis with all nutrients transported
[~10 L/day absorbed (most from digestive process);
less than 500 ml still in feces as it goes to large
intestine] Accomplishments in the small intestine
1. Complete breakdown of proteins, fats, and
carbohydrates and absorption of their
constituents
2. Almost complete absorption of ions, water,
vitamins.
3. Addition of bilirubin to feces for
elimination.
6
Large intestine
1. Store feces and move towards anus
2. Continue absorption of ions (mostly
Na + and Cl-) and water (less than 100
ml/day lost).
3. Bacterial breakdown of non-digestible
material and vitamin synthesis.
Bacterial action in large intestine
There are more bacteria in the large intestine than cells
in an adult body.
Bacteria use β-amylase to digest cellulose (some
glucose may be used by large intestinal cells)
Gases produced (CO2, methane, H2S) as waste
products
Vitamins K, B12, riboflavin, and thiamine produced in
small amounts
7
Elimination of feces
Feces contain bilirubin, non-digestible organic matter
from food (roughage), ferritin, any non-absorbed
nutrients (fats), bacterial breakdown products,
sloughed cells, water, and a few ions.
Control of anal sphincters:
Internal: smooth muscle controlled by PNS
External: skeletal muscle controlled by motor 8