Problem 2.02: Study Guide 2 - "Absorption in the Small Intestine"

Describe Protein digestion in the brush border of the small intestine?

- Protein digestion is complete by two peptidases in brush border - Aminopeptidase: cleaves off amino acid at the amino end of peptide - Dipeptidase: splits dipeptides (two amino acids joined) into single amino acids

SAQ: Describe the absorption of proteins. Where do the proteins absorbed come from? Where does protein absorption occur?

- Protein is absorbed mainly as amino acids in the duodenum and jejunum. - Small chain peptides and native proteins may also be absorbed in the gut - Amino acids are absorbed by active transport and by secondary active transport with Na, dipeptides and tripeptides are absorbed by secondary active transport with H+ - Native proteins may be absorbed by endocytosis

In the stomach pepsin coverts _____ into ________

- Proteins - to peptides (small fragments of proteins)

SAQ: What are short chain fatty acids and how are they absorbed?

- Short chain fatty acids, have less than 10-12 carbon atoms and are absorbed by simple diffusion in the intestinal mucosal cells

How much fluid does the small intestine absorbed compared to large intestine?

- Small intestine absorbed 8.3 L the remainder passes into large intestine, where most of the rest is absorbed 0.9 L - Only 0.1 L water is excreted in feces each day

How are electrolytes absorbed?

- Sodium ions are absorbed by diffusion and secondary active transport and move out via Na/k pump - Negatively charged ions can move passively following Na or actively be transported - Calcium ions are absorbed actively in process stimulated by calcitrol - Iron, potassium, Magnesium and phosphate ions are also absorbed via active transport mechanisms.

Further describe forms of transport in which proteins are absorbed in the small intestine?

- Some amino acids enter absorptive cells of the villi via Na-dependent secondary active transport processes similar to glucose transporter - Other amino acids are actively transported by themselves - Symporter brings dipeptides and tripeptides together with H which are then hydrolyzed to single amino acids inside the absorptive cells

In the stomach lingual and gastric lipases converts ______ into ______

- Some triglycerides into - fatty acids, diglycerides and monoglycerides

What ends the activity of salivary amylase?

- The acidic pH of the Stomach

How are ingested molecules of sucrose, lactose and maltose are acted on?

- These three disaccharides are not acted on until they reach the small intestine - 3 brush border enzymes digest the disaccharides into monosaccharides - Sucrose breaks into molecule of glucose and fructose - Lactase digests into glucose and galactose - Maltase splits maltose and maltotriose into 2 or 3 molecules of glucose.

How do chylomicrons leave the absorptive cells? Where do they go?

- They leave via exocytosis - Because they are so bulky, chylomicrons cannot enter the blood capillaries, the pores are too small; instead enter lacteals which have large pores - From lacteals, transpoted by lymphatic vessel to thoracic duct and enter the blood junction of left internal jugular vein and left subclavian veins

SAQ: Describe Exocytosis and give example

- Transport of large molecules in a membrane bound vesicle through the membrane form inside to the outside of the cell - e.g. extrusion of proteins from cells after synthesis

SAQ: Describe Endocytosis and give an example

- Transport of large molecules in a membrane bound vesicle through the membrane from the outside to the inside of a cell - e.g. absorption of native proteins from the colostrum in a neonate.

What enzymes in pancreatic juice deal with protein digestion?

- Trypsin - Chymotrypsin - Carboxypeptidase - Elastase

SAQ: Describe Secondary active transport and give an example:

- Use of the concentration gradient of Na to energise a carrier protein to transport a substance against concentration gradient. - E.g. glucose absorption into intestinal mucosal cells

By what mechanism is water absorbed in the GI tract

- Via osmosis from the lumen of the intestines through the absorptive cells and into blood capillaries - Because water can move across the intestinal mucosa in both directions, the absorption of water form small intestine depends on absorption of electrolytes and nutrients to maintain osmotic balance with blood - The absorbed electrolytes and nutrients establish a concentration gradient for water that promotes water absorption by osmosis

SAQ: Describe the process of water absorption form the GI tract. Where does it occur?

- Water absorption is by osmosis and absorption can occur in any part of the GIT

Chemically what does feces consist of:

- Water, inorganic salts, sloughed off epithelial cells from the mucosa, bacteria, products of bacterial decomposition, unabsorbed digested material and indigestible.

Why are large short-chain fatty acids, long chain fatty acids and monoglycerides absorbed differently?

- because they are larger and hydrophobic - since they are not water soluble, they have difficulty being suspended in watery environment of the intestinal chime

Explain the role of bile in the digestion of lipids

- before the large lipid globule containing triglycerides can be digested, it must be emulsified into several small lipid globules providing a larger surface area for pancreatic lipase to function - Bile salts (sodium salts and potassium salts) are amphipathic - Amphipathic nature allows them to emulsify: the hydrophobic region interact with large lipid globule while the hydrophilic regions interact with intestinal chime - Small lipid globule that result measures about 1mm in diameter

In what form is proteins absorbed?

- most proteins are absorbed as amino acids via active transport processes that occur mainly in the duodenum and jejunum.

Bacteria products that are absorbed in the colon include:

- several vitamins needed for normal metabolism - among them some B vitamins and vitamin K

SAQ: Several years following abdominal surgery involving resection of part of his intestine, Henry is found to have anaemia. Further investigation reveals that his Erythrocyte MCV is abnormally high. Explain most likely cause? What part of his intestine is most likely resected, and what relation does this have to his anemia? Why did it take several years for anaemia to develop?

-Henry has a macrocytic anaemia i.e. an anemia characterised by large-sized red cells) hence the high MCV). This is caused by folate deficiency or vitamin B12 deficiency. - Vitamin B12 in the diet complexes with intrinsic factor (secreted by parietal cells of the stomach along with HCL) and is absorbed in the distal ileum. - Resection of the distal ileum, prevent vitamin B12 absorption and leaves eventually to macrocytic anaemia - 3-6 years worth of Vitamin B12 is stored in liver, hence the long delay between the loss of ability to absorb vitamin B12 and development of anemia

Enzymes that split triglycerides and phospholipids are called

Lipases

Describe Micelles

- Each of which is 2-10nm in diameter and includes 20-50 bile salt molecules - Formed due to amphipathic nature: hydrophobic regions of bile salts interact with fatty acids or monoglycerides and hydrophilic regions interact with the watery intestinal chyme - Also solubilize other large hydrophobic molecules such as fat-soluble vitamins and cholesterol

Describe digestion in the large intestine (colon):

- The final stage of digestion occur in the colon through the activity of bacteria that inhabit the lumen - mucus is secreted by the glands of large intestine, but no enzymes are secreted - bacteria ferment any remaining carbs and release hydrogen, CO2 and methane gas - these gases contribute to flatus (gas) in the colon

SAQ: Describe Facilitated diffusion and give an example:

- The movement of a substance down its concentration gradient at a faster rate than expected, due to the use of special transporters present in the cell membrane. - e.g. movement of glucose into the cell by action of insulin

SAQ: A monoglyceride may be large than an amino acid. Why can monoglycerides be absorbed by simple diffusion whereas amino acids cannot

- The plasma membrane is a phospholipid bilayer and so the monoglyceride, being a lipid (nonpolar) as well, can diffuse across easily, unlike the amino acids which are polar.

SAQ: What is the entero-hepatic circulation?

- The process whereby a substance which is produced and excreted by the liver cells - sent to the intestine via the bile duct (e.g. bile salts), is reabsorbed from the intestine and carried in the blood to the liver to be re-excreted.

Describe the absorption of Small short-chain fatty acids

- They are hydrophobic, contain less then 10-12 carbon atoms, and are more water soluble - Thus they can dissoclve in the watery intestinal chime, pass through the absorptive cells via simple diffusion - following same route taken by monosacchardies and amino acids into blood capillaries of villus

What is the most abundant lipids in the diet?

- Triglycerides, which consist of a molecules of glycerol bonded to 3 fatty acid molecules.

Actions of each protein digestive enzymes in pancreatic juice?

- Trypsin, chymotrypsin and elastase all cleave the peptide bone between specific amino acid and its neighbour - Carboxypeptidase splits off the amino acid at the carboxyl end of the peptide

SAQ: What is triglycerides?

-Most lipid in diets is in the form of the triglycerides which form globules of fat, which must be emulsified by bile salts in order for digestion to occur - Composed of three fatty acids attached to glycerol molecule

Where exactly is Vitamin B12 absorbed?

Ileum via an active transport along with intrinsic factor produced by stomach

SAQ: What 2 organs of the digestive system secrete the most fluid?

Stomach and pancreas.

Chyme entering the small intestine therefore contain what type of carbohydrates, proteins and lipids:

- Partially digested

In what form is carbohydrates absorbed?

- Monosaccharides

Capacity of small intestine to absorb monosaccharides:

120gram per hour

Total volume of fluid that enters the small intestine each day;

- 9.3 L - includes ingestion of liquids and from various gastrointestinal secretions

After amylase has spilt starch into smaller fragments, what brush border enzyme acts on the resulting -dextrins?

- A bursh border enzyme called -dextrinase act on the resulting dextrins, clipping off one glucose unit at time

SAQ: What is the difference between passive and active transport processes?

- Active transport processes consume energy to move substances against concentration gradients - Passive, substance move along these gradients and thus do not need further driving energy

Describe Enterohepatic Circulation:

- After participating in emulsification and absorption of lipids, most bile salts are reabsorbed by active transport in the final segment of the small intestine (ileum) and returned to liver through hepatic portal system for recycling - This cycle of bile salt secretion, reabsorption by ileum and resecretion into bile is this circulation.

Action of Pancreatic Amylase?

- An enzyme in pancreatic juice that acts in the small intestine - Although acts on both glycogen and starches, it has no effect on another polysaccharide called cellulose, an indigestible plant fibre that is commonly referred to as roughage

SAQ: People who have a certain type of gastritis develop a similar type of anaemia what kind of gastritis? How does it lead to anaemia?

- Atrophic gastritis (Achlorhydria), characterised by lack of parietal cells - Hence lack of intrinsic factor also resulting in inability to absorb vitamin B12, eventually, leading to anaemia.

Action of bacteria in remaining proteins in the colon:

- Bacteria also convert any remaining protein to amino aicds and break down amino acids into simpler substances: indole, skatole, hydrogen sulphide and fatty acids - Indole and skatole some is eliminated in feces and contributes to smell the rest is absorbed and transported into liver where these compounds are converted to less toxic substances and excreted in urine

Protein digestion starts where?

- Begins in the stomach where proteins are fragmented into peptides by the action of pepsin

Describe how large short-chain fatty acids, long chain fatty acids and monoglycerides made more soluble?

- Besides their role in emulsification, bile salts also help make these more soluble - The bile salts in intestinal chyme surround them, forming tiny spheres called micelles

Action of Pancreatic Lipase?

- Break down triglycerides into fatty acids and monoglycerides - Fatty acids can either be short chain fatty acids or long chain fatty acids

In the mouth salivary amylase converts _____ into _______

- Converts starch - to maltose (disaccharide), maltrioise (trisaccharide) and -dextrins (short chain banched fragments of starch)

Action of Bacteria in the colon on Bilirubin:

- Decomposes bilirubin to simpler pigments -> stercobilin -> feces brown color

SAQ: Describe primary active transport and give an example:

- Direct use of ATP to produce a high affinity binding site on carrier protein transporting substances against concentration and electrochemical gradient. - E.g. Na/K pump transporting Na out of and K into the cell

How absorbed monosaccharides move into the capillaries of villi

- Facilitated diffusion on basolateral surfaces

Benefits of including healthy fats in diet:

- Fats delay gastric emptying, which helps person feel full - fats enhance feeling of fullness triggering release of CCK - Necessary for absorption of fat-soluble vitamins

Where do many of the electrolytes absorbed by the small intestine come from?

- Gastrointestinal secretions - some are part of the ingested foods and liquids

SAQ: Describe the absorption of glucose and galactose in the small intestine. How does glucose absorption influence the absorption of water?

- Glucose and galactose are monosaccharides resulting from the digestion by brush border disaccharidases of disaccharides in the diet. - These monosaccharides are then absorbed by secondary active transport, and are coupled with Na absorption, into intestinal mucosal cells - From where they are released to be further transported into blood by facilitated diffusion or simple diffusion. - What follows glucose by osmotic process

Where does lipid digestion occur? Which location does it predominate?

- In the stomach by Lingual and gastric lipase - Predominates in the small intestine by action of pancreatic lipase

Explain the fate of large short chain fatty acids, long chain fatty acids and monoglycerides inside the absorptive cells:

- Inside they recombine to form triglycerides, which aggregate into globules along with phospholipids and cholesterol and become coated with proteins - Creating large masses (80nm) called CHYLOMICRONS

What other substances also in absorbed by the large intestine besides water?

- Ions including sodium and chloride - some vitamins

SAQ: What are chylomicrons and how are they involved in lipid absorption?

- Large lipoprotein complexes formed by aggregation of triglycerides, phospholipids and cholesterol coated with proteins. Chylomicrons are thus like packaging structures through which lipids are absorbed.

SAQ: Describe the absorption of long chain fatty acids and role of bile salts in this process.

- Long chain fatty acids form micelles with bile salts to be transported across the unstirred water layer of the intestinal mucosa and from there - diffuse into the cytosol of cell from where there is reprocessing into fats to be transported into lacteals

As a result of their emulsification and digestion, triglycerides are mainly broken down into:

- Monoglycerides and fatty acids - fatty acids can be either short-chain fatty acids or long-chain fatty acids

Explain the form of transportation of Monosaccharides into small intestine cells?

- Monosacchardies pass from the lumen through apical membranes via facilitated diffusion or active transport - Fructose, a monosaccharide found in fruit is transported via facilitated diffusion - glucose and galactose are transported into absorptive cells of the villi via secondary active transport that is coupled to active transport of Na + - Transporter has binding sites for one glucose molecules and two sodium ions

SAQ: Describe Lipid digestion, where it occurs and how it occurs?

- Most of lipid digestion occurs in small intestine (despite that fact that lipid digestion begin with lingual lipase and gastric lipase in the mouth and stomach).

SAQ. Describe Bulk flow and give an example

- Movement of several particles in a common median from an area of higher pressure to an area of lower pressure e.g. air during inspiration.

SAQ: Describe simple diffusion and give example:

- Movement of solute from an area of higher concentration to an area of lower concentration e.g. O2 in the lungs diffuses into pulmonary capillaries

SAQ: Describe Osmosis and give example:

- Movement of solvent across a semi-permeable membrane from an area of lower to an area of higher solute concentration - e.g. water movement from intracellular compartment to the extracellular compartment in the event of dehydration, when osmolarity of the of circulation is greater than intracellular osmolarity

SAQ: Describe the absorption of sodium?

- Na moves into the epithelial cells of the intestine by diffusion and by secondary active transport. - Sodium is then actively transport by Na,K- pump out of the epithelial cells into the blood.

Explain how large short-chain fatty acids, long chain fatty aicds and monoglycerides absorbed?

- Once micelles are formed, they move from the interior of the small intestinal lumen to the brush border of absorptive cells - from here the fatty acids and monoglycerides diffuse out of the micelles into absorptive cells leaving the micelles behind - Micelles continually repeat their ferrying function

Describe mechanical digestion in the Large intestine:

- One movement characteristic of the large intestine is haustral churning - Haustra remain relaxed and become distended while they fill up - When distension reach certain point, the wall contracts and squeeze content into the next haustrum - Peristalsis also occurs although at a slower rate than in more proximal portions. Final type of movement is mass peristalsis, a strong peristaltic wave that begins at middle of transverse colon driving contents of the colon into rectum

Completion of the digestion of carbohydrates, proteins, lipids is collective effort of:

- Pancreatic juice, bile and intestinal juice in small intestine

All dietary lipids are absorbed by what type of transport?

Simple diffusion

True or False: Glucose and Galactose compete to ride the same transporter

True

How do Amino acids move out of absorptive cells?

Via diffusion to capillaries of the villus