AN SC 311 Lecture #3
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What is digestion?
- Process by which food stuffs are broken down in the GI tract into absorbable units
- during digestion, food and nutrients are considered to be outside the body since the GIT is a hollow tube outside the body
What is absorption?
the process of transport or transfer of digested products through the gut mucosal wall
What are the 4 types of digestive processes?
What are the ways the absorptive surface area of the GI tract is increased?
- kerckring folds increase the SA by 3 fold
- villi increase SA by 10 fold
- microvilli increase SA by 20-30 fold
- all together could lead to a 1000 fold increase
What are the 5 type of epithelial cells?
- columnar absorptive cells
- mucous (goblet) cells
- enteroendocrine (enterochormaffin) cells
- paneth cells
- undifferentiated columnar cells
What do columnar absorptive cells do?
what do goblet cells do?
what do enterochromaffin cells do?
release hormones into blood circulation
what do paneth cells do?
- remain in crypt base
- secrete antimicrobial molecules to provide host defence against gut microbes
what do undifferentiated columnar cells do?
- give rise to columnar absorptive cells, mucous, enterochromafffin, and paneth cells
- found up the lining of crypt and villi
What happens at the crypts of lieberkuhn?
generic cells are produced
How often do you see new cells in the SI lining cells?
- Every 4 days
- as you get older the time frame increases
Examples of monosaccharides?
Examples of disaccharides
examples of polysaccharides?
- starch - amylose, and amylopectin
what linkages does starch have? how about cellulose? How do you break each one down?
- starch have alpha 1-4 linkages between glucose. Enzymes can break them
- cellulose have beta 1-4 linkages between glucose. Animals need bacteria to break them down.
What does amylase break down?
Where do all viens converge in the GIT
portal vein which then leads to capillary beds in the liver
describe the 3 steps to digestion and absorption of carbohydrates
- 1. breakdown of starch by amylase (salivary and pancreatic)
- 2. breakdown of oligo- tri - or disaccharides to monsaccharides by oligogulosidase, maltase, lactase, sucrase from the duodenal and jejunal mucosa
- 3. only monosaccharides are absorbed - active transport using ATP via the Na- K - ATPase pump
- diffusion using concentration gradient
- fructose and galatose leads to glucose in the liver
How does lactase and sucrase change over time?
lactase is higher at birth and in early development but weans off later on. Sucrase doesnt come into activity till about day 18.
What are micelles composed of?
- bile salts
- fatty acids
List the 6 steps of digestion and absorption of fat?
- 1. Biles salts secreted by hepatocytes emulsify fat globule
- 2. pancreatic lipase converts emulsified fat to free fatty acids and monglycerides
- 3. Formation of micelle
- 4. Absorption of fatty acids and monoglycerides into mucosal cells by simple diffusion
- 5. Absorption of chylomicrons to lacteal
- 6. Absorption of bile salts in the ileum
Where is the majority of water reabsorbed?
Large intestine plus ceccum
4 steps of Absorption of Na and water?
- 1. Na flows into the cell and raises osmotic pressure in the intracellular space
- 2. delivery of net positive charge attracts negatively charged ions (Cl)
- 3. increased osmotic pressure allows water flow into the intercellular space
- 4. Na is expelled to extracellular space by the Na/K pump on basal lateral membrane and water follows.
What are the differences between microbial and enzymatic digestion?
- fiber can be digested
- end products are volatile fatty acids
- synthesis of microbial protein from non-protein such as urea
- synthesis of B vitamins
Who is able to perform microbial digestion?
ruminants are able to break stuff down before the stomach and horse are able to break down stuff after the stomach.
what are the requirements for microbial digestion?
- fluid environment
- neutralized environment
- continuous removal of the end products by absorption
- long retention time of digesta
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