Exam 3 (Lecture 11) - Digestion and Absorption of Nutrients 1 Flashcards
Define digestion. Define absorption. What are the types of nutrient assimilation disturbances?
Digestion:
- Process of breaking down complex nutrients (CHOs, proteins, and fats) into simple molecules for
absorption.
Absorption:
- Process of transporting simple molecules across the intestinal epithelium.
Nutrient assimilation disturbances:
- Maldigestive diseases (Exocrine Pancreatic Insufficiency = EPI)
- Malabsorptive diseases (Parvovirus)
Describe the three levels of surface convolutions that serve to expand the surface area of the small intestinal mucosa.
1) Plica circulares
- Large folds of mucosa (not present in all species)
2) Villi
- Finger-like projections
- Present in all species and increase intestinal surface area by 10-14X compared with a flat surface of equal
size
3) Brush border of villi
- Composed of submicroscopic microvilli
Describe the growth and development of the intestinal epithelium.
1) The rate of cell replication in the crypts is stimulated by several GI hormones.
2) When appetite and feed intake increase, there is an overall increase in the secretion of GI hormones.
3) Division and replication of enterocytes occur in the crypts only.
4) As crypt cells multiply, they migrate up villi epithelial cells, pushing other epithelial cells ahead.
- Continuous progression of cells migrating up the villi
5) During villi migration, cells mature, change from relatively undifferentiated cells in the crypts to highly specialized absorptive cells.
6) Mature villus epithelial cells are sloughed off due to age and constant exposure to gut contents.
*Villi length is determined by:
- Rate at which villus epithelial cells are lost at the villi tips
- Rate at which crypt epithelial cells replace villi tips
What is found at the base of the villi? Name the outermost layer of villi and crypt mucosa. Each cell in this layer is called an ___________.
1) Crypt cells
2) Epithelium
3) Enterocyte
Describe the structure and function of the following: apical cell membrane, basolateral cell membrane, tight junction, lateral space, goblet cells, and brush border surface.
Apical membrane:
- Covers the surface of the cell facing the lumen (apex)
- Contains microvilli
Basolateral membrane:
- Base and sides of the cell (not facing the gut lumen)
- Serves an important role in intestinal absorption
- nutrients absorbed into the enterocytes through the apical membrane must exit through the basolateral
membrane before gaining access to the bloodstream
Tight Junction:
- Attachments between adjacent enterocytes
- Connections serve a special function in the process of digestion and absorption
- Form a narrow band of attachment between adjacent enterocytes
- Band is near the apical end of the cells and divides the apical membrane from the basolateral membrane
Lateral space:
- Potential space formed between enterocytes formed when the tight junctions leave the majority of the
basolateral membrane unattached to its neighboring membrane on and adjacent enterocyte
Goblet cells:
- Interspersed among the enterocytes
- Secrete a rich layer of mucus that covers the mucosa
Brush border surface:
- Mucus blends into the glycocalyx, with the two layers forming a viscous coating that traps molecules near
the apical membrane
- Unstirred water layer
- present at intestinal epithelium; is quiet and flows at a much slower rate than water in the central part of
the lumen
- Unstirred water layer, mucus, and glycocalyx form an important diffusion barrier through which nutrients
must pass before entering the enterocytes
What is the pathway for electrolyte absorption into the blood?
Lumen of SI > tight junctions > lateral space > blood capillaries within lamina proprietary
How is chemical digestion accomplished?
Hydrolysis
What are the two general classes of digestive enzymes? Where are the enzymes located and how do the enzymes work?
1) Enzymes that act within the lumen of the gut
- Originate from the major GI glands, including salivary glands, gastric glands, and pancreas
- Incomplete hydrolysis of nutrients, resulting in the formation of short-chain polymers from the
original macromolecules
2) Enzymes that act at the membrane surface of the epithelium
- Secretions of these glands become thoroughly mixed with ingest
- Enzymes that are chemically bound to the surface epithelium of the small intestine complete the hydrolytic
process
- break the short-chain polymers resulting from luminal phase digestion into monomers that can be
absorbed across the epithelium
Describe the three general types of plant carbohydrates and what occurs with each during the luminal phase of CHO digestion.
3 General Types of Plant CHOs:
- Fiber
- Sugar
- Starch
Fiber:
- Plant fibers are not subject to hydrolytic digestion by mammalian enzymes and therefore cannot be
digested directly
Sugar:
- Simple (monosaccharides) = glucose, galactose, fructose are most important in animal diets
- Complex (polysaccharides; disaccharides, trisaccharides, oligosaccharides) = lactose and sucrose
- Named based on number of repeating simple sugar subunits (Oligosaccharides contain 3-10 monomers)
- Monosaccharides are present, preformed in small quantities in normal diets; however, most
monosaccharides absorbed from the gut arise from the enzymatic hydrolysis of more complex sugars
Starch:
- Two chemical forms: amylose (alpha-1,4 linkages) and amylopectin (alpha-1,6 linkages)
- Luminal starch digestion enzyme = alpha-amylase (found in pancreas, present in all species)
- Starch chains are broken at their midsections
- Chains continue to be cleaved until disaccharide (maltose) and trisaccharide (maltotriose) units formed
What are the final products of luminal phase CHO digestion?
Creation of many disaccharides, trisaccharides, and oligosaccharides
- Complex sugars are not hydrolyzed any further in the luminal phase of digestion
Describe the membranous phase of CHO digestion.
Involves lactase, maltase, isomaltase, and sucrase
Substrates for the enzymes must diffuse into the glycocalyx before hydrolysis can occur
Unstirred water layer, mucus, and glycocalyx form a diffuse zone separating the mucosal surface from the lumen of the intestine
Membranous phase digestive enzymes project from the apical membrane into this surface layer
Quiet surface layer forms a microenvironment in which membranous phase digestion occurs
Peptides and polysaccharides in the intestinal lumen must diffuse into the surface layer before membranous phase of digestion can take place
