Digestion and Absorption

Question 1

1. what are:
   a) sugars
   b) polysaccharides
   c) oligosaccharides
2. what is the difference between intrinsic and extrinsic sugars?
3. how is starch digested?
4. how are disaccharides digested?
5. describe how glucose is absorbed

Answer 1

1a) mono and disaccharides
1b) starch and non-starch polysaccharides
c) sigars consisting of 3-4 monosaccharide units. Most are not digested but fermented by intestinal bacteria
2. intrinsic sugars are those found in plant cell walls. Extrinsic sugars are those found in free solution in food

3. amylase, secreted by pancreas and salivary glands catalyses the hydrolysis of starch
4. disaccharides are digested by brush border enzymes such as galactase, maltase, sucrase and lactase
5. SODIUM DEPENDENT TRANSPORT
   - Na/K ATPase, as well as sodium secreted by pancreas create a sodium gradient favouring sodium absorption
   - glucose is symported across the apical membrane with sodium via SGLT
   - glucose is transported by facilitated diffusion across the basolateral membrane via GLUT1.

Question 2

1. why are most proteins resistant to digestive enzymes?
   What enables enzymatic digestion?
2. Name the 2 main types of protein digestive enzymes, what they do and where they act
3. What are proteases secreted as and why?
4. Name the 6 proteases and where they act
5. How are proteins absorbed?

Answer 2

1. because of their secondary and tertiary structures, their peptide bonds are inaccessible by proteases
   denaturation is achieved by cooking and gastric acid
2. endopeptidases - cleave peptide bonds that exist between specific amino acids
   exopeptidases - remove terminal amino acids one at a time
   exopeptidases act first in the stomach and small intestine
   endopeptidases are secreted in pancreatic juice or by intestinal mucosal cells
3. precursors. This prevents them from digesting themselves or tissue proteins before being secreted
4. pepsin (pepsinogen) - stomach
   trypsin (trypsinogen) - small intestine
   chymotrypsin (chymotrypsinogen) - small intestine
   elastase (pro-elastase) - small intestine
   carboxypeptidase (pro-carboxypeptidase) - secreted in pancreatic juice
   aminopeptidase (pro-aminopeptidase) - secreted by intestinal cells
5. free amino acids are transported by sodium dependent transport in a similar way to glucose
   large peptides can be absorbed intact by tanscellular or paracellular transport

Question 3

1. the cephalic, gastric and intestinal phases refer to what?
2. what occurs during the cephalic phase?
3. describe the neural mechanisms that govern the gastric phase
4. describe the hormonal regulation of the gastric phase
5. what is the purpose of the intestinal phase
6. describe the neural regulation of the intestinal phase
7. describe the hormonal regulation of the intestinal phase

Answer 3

1. controlled secretion of digestive enzymes
2. smell, thought and initial taste of food activates CNS centres. Facial and glossopharyngeal nerves are stimulated to stimulate salivary production; vagal stimulation promotes gastric acid secretion.
3. food distends stomach and stimulates stretch receptors. Chemoreceptors monitor pH of chyme
   stomach distension or increase in pH stimulates parasymp and enteric neurons in the submucosal plexus
   causes peristaltic waves, gastric juice secretion and promotes gastric emptying
4. gastrin is produced by G cells in response to stomach distension, high pH and parasympathetic stimulation. It is secreted into blood stream and acts on gastric glands to produce gastric juice, and increases motility of stomach. Gastrin secretion is inhibited when pH drops below 2
5. act to slow the exit of chyme from the stomach
6. distension of duodenum activates stretch receptors. medulla inhibits parasympathetic stimulation to stomach. this inhibits gastric motility and an increases contraction of pyloric sphincter
7. CCK is secreted by intestinal cells in response to chyme. stimulates secretion of pancreatic juice, contraction of gall bladder and pyloric sphincter, and relaxes sphincter of Oddi. Also acts on hypothalamus to produce feeling of satiety

Question 4

1. what are non-starch polysaccharides?

2. Name 3 advantages of non-starch polysaccharides

Answer 4

1. polysaccharides that are not digested by human enzymes
2. increase viscosity of intestinal contents thus slows absorption and lowers the spike in blood sugar
   adsorb bile salts (which aids cholesterol clearance) and carcinogens
   substrates for bacterial fermentation. This yields short chain fatty acids that provide fuel for intestinal cells and may have anti-proliferative and anoretic effects

Question 5

1. what is the glycemic index?
2. describe the difference in absorption between fibre rich and low fibre diets
3. what is the microbiome?

Answer 5

1. a measure of the extend and speed at which a carbohydrate is hydrolysed and the resultant monosaccharides absorbed
   relative ability of food to increase blood glucose
2. fibre rich foods are absorbed slowly through the small intestine. low fibre foods are absorbed quickly in the first part of the small intestine.
3. gut bacteria.

Question 6

1. what are the 4 fat soluble vitamins?
2. what are the essential fatty acids and why do they need to be taken in via the diet?
3. what is the common basic structure of a triglyceride?
4. what are unsaturated fatty acids?

Answer 6

1. A, D, E and K
2. omega 3 and omega 6. They can’t be synthesised in the body
3. 3 fatty acid chains that are ester bonded to a glycerol
4. fatty acid chains that contain at least one double bond.

Question 7

1. Name the 2 problems associated with fat absorption and digestion
2. How are these problems overcome?

Answer 7

1. Fats are insoluble but need to be digested in an aqueous environment
   fats need to be transported around the body in an aqueous environment
2. emulsification into small droplets and the stabilisation by bile salts and amphiphilic lipids for enzyme action
   TAG rich particles are coated with protective proteins and amphipathic lipids to be transported around the body

Question 8

Describe the basic stages of fat digestion

Answer 8

1. emulsification in the mouth and stomach into small droplets, which increases the surface area for enzyme attack
2. bile salts and amphipathic lipids coat emulsion particles to prevent them from re-aggregating into bulk TAG
3. lipases degrade TAG
4. bile salts stabilise products of lipase into micelles to facilitate absorption

Question 9

1. what is emulsification?
2. name 3 structures where lipase is secreted
3. what does lipase produce? (products of TAG digestion)
4. what is special about the products of TAG digestion?
5. Why is the digestion of TAG incomplete?
6. what is colipase?

Answer 9

1. breakdown of fat globules into tiny droplets which provides a larger surface area for lipase to act on
2. salivary glands, stomach, and pancreas
3. monoaglycerol and free fatty acids
4. they are amphiphillic, thus emulsify the lipid into increasingly small droplets/micelles
5. most TAG is only degraded to Monoaglycerol, because the digestion of monoaglycerol is slow
6. a amphiphilic protein that binds and anchors pancreatic lipase at the surface of the emulsification droplet

Question 10

Describe the stages of fat absorption

Answer 10

1. micelles travel to the apical brush border and release products
2. absorbed across intestinal wall via transport proteins
3. inside enterocytes, they are re-esterified
4. packaged with protiens into chylomicrons
5. cylomicrons are secreted into the lacteal of the enterocyte.
6. chylomicrons enter the general circulation at the thoracic duct.

Question 11

1. what is lipoprotein lipase?

2. are bile salts absorbed?

Answer 11

1. a protein found in adipose tissue and muscle cells that catalyses the hydrolysis of chylomicrons so that fatty acids can be stored.
2. no. they are transported to the ileum for reabsorption and recycling

Question 12

1. name 5 functions of omega 3
2. name 5 functions of omega 6
3. what is the general role of white adipose tissue?
4. what is the general role of brown adipose tissue?

Answer 12

1. controls blood clotting, builds cell membranes in brain. enhances brain and joint function. reduces risk of cancer, heart disease and diabetes. reduces inflammation
2. regulates brain functioning, growth and development, and metabolism. Stimulates hair and skin growth. Maintains healthy bones and reproductive system
3. energy storage
4. heat generation