Lecture 46 - Absorption of Nutrients Flashcards
Describe the nutrient makeup of a carrot
Even though it seems bland, it contains:
Lipid: cell membrane
Protein: structures in the cell
What are the basic food groups
Carbohydrates
Proteins
Fats
Vitamins and minerals
Describe what has to happen before nutrient absorption?
Describe how this is overcome
1/ get to the right location (Jejunum)
- peristalsis
2/ broken down into small enough molecules
- chemical
- mechanical
3/ physical contact with the mucosa
- villi for increased SA
Describe the overall process of digestion
Macronutrients –> molecules
How is food chemically digested?
Where is this happening?
Must first be broken up to increase surface area
Digestive enzymes coming into contact with the small food particles
Motor activity to mix the enzymes with the food
Most prominent in duodenum and upper Jejunum
Where do digestive enzymes come from?
Exocrine Pancreas
To a lesser extent: mucosal epithelial cells
Describe the function of digestive enzymes from the mucosal epithelial cells
Enzymes embedded in the membrane of the mucosa
Exposed to the food particles in the lumen for digestive action
Where does intestinal epithelium begin and end?
Inside of lip
Anus
Describe the epithelium in the mouth and oesophagus
Impervious
Doesn’t absorb anything
Doesn’t secrete anything
Describe the requirements of the epithelium of the stomach
Secretory
Protective (against acid)
What is the name for the cells in the small bowel?
Enterocytes
Describe the surface epithelium of the small bowel
Crypts/ pits and villi
Connected by tight junctions –> strong barrier
Describe the life of an enterocyte
Stem cells at base of the crypt Continually made Migrate up to tip of villous - change function as they move up Shed off
What is the structure of an enterocyte?
Apical surface
- brush border
- absorption
Basolateral surface
- diffusion of molecules out into Lamina propria
Different functions at each side
What is the function of enterocytes in the crypt and at the tip of the villous?
How does this change?
Crypt: secretory
Villous: absorption
Change in the protein expression in the cell as they migrate up
What is gluten?
Protein found in most grains
What is coeliac disease?
Autoimmune reaction in some people when gluten is eaten
Shedding of the epithelium from the villous in the small bowel
The symptoms resolve quickly after a few days
What is in saliva that starts to break down food?
Describe how this functions
Amylase
Breaks some of the bonds within sugars and starches (polysaccharides)
What is the role of secretin?
What causes its release?
Release: acid in the small intestine
Function:
- HCO3- secretion
- inhibits acid secretion
Which sugars can be absorbed by the intestinal epithelium?
Glucose
Fructose
Galactose
What is the role of CCK?
Brings about release of:
- bile (gall bladder contraction)
- pancreatic enzymes
Can disaccharides be absorbed?
No, only monosaccharides can be absorbed
What is happening in lactose intolerance?
No lactase gene
Can’t break down lactose (a disaccharide)
Diarrhoea
As we age, we have reduced tolerance of lactose
How is glucose transported across the apical surface of the enterocyte?
Na+/glucose linked transporter
SGLT
Across apical surface of enterocyte
How is galactose transported into the enterocyte?
Same as glucose
SGLT
How is fructose transported across the apical surface of the enterocyte?
GLUT 5
How is glucose transported across the basal surface of the enterocyte?
GLUT2
How is fructose transported across the basal surface of the enterocyte?
GLUT 2
How is galactose transported across the basal surface of the enterocyte?
GLUT 2
What is important about high fructose diets?
Skews expression towards GLUT 5 transporters
Associated with disease
How can protein be absorbed?
Must be first broken down into short peptides and amino acids
What are endopeptidases?
What are some examples?
Breaks peptide bonds within an amino acid chain
Trypsin
Pepsin
Cymotrypsin
Where is protein first broken down?
In the stomach
No pepsin in the mouth
What happens to lipids in the stomach?
Chyme is separated out
Lipids float to the top (fundus
Describe the release of pepsin
- Vagal activity and gastrin bring about release
- Released by chief cells as pepsinogen
- Activated the acid in the stomach
What is the function of exopeptidases?
List some examples
Cleave terminal amino acids on peptides
Carboxypeptidases
Aminopeptidases
Which enzymes can break a peptide down into single amino acids?
Exopeptidases
What secretes pepsin?
Chief cells
Describe the release of trypsin
- Released by pancreas under CCK signalling
- Initially inactive - PROTRYPSIN (so as to prevent digestion of pancreas)
- Endopeptidase activity
What is pancreatitis?
Blockage of pancreatic duct
Proteases activated whilst still in the pancreas
Start to break down the pancreas
Acute then extreme pain
Death
How are amino acids absorbed across the apical surface?
Na+ coupled cotransporters
How do di and tri peptides cross the apical surface?
H+ coupled transporters
How do amino acids cross the basal surface?
Na+ coupled co transporters
How can polypeptides get across the apical surface?
Endocytosis
Describe the general digestion of fat
Triglycerides:
- Lipases break TGs into:
- monoglyceride + free fatty acids
Phospholipids:
broken down by phospholipases
Cholesterol:
down not require breakdown
What is the structure of a triglyceride?
Glycerol + 3 fatty acids
What leads to the release of bile into the duodenum?
- Fat in the stomach
- Release of CCK from duodenum
- Contraction of gall bladder
- Release of bile from common bile duct
How does cholesterol get into cells?
Active transport
Describe the breakdown and subsequent uptake of fat
- Bile coats large fat droplets –> emulsification
- Pancreatic lipase & Colipase break down fats into monosaccharides –> micelles
3a. Monoglycerides, fatty acids diffuse into cells
3b. Cholesterol transported in
What is Colipase?
How is it activated?
Trypsin activates Colipase
Colipase activates lipase
What happens to free fatty acids, cholesterol and monoglycerides once inside the enterocyte?
Combine to form chylomicrons
Chylomicrons cross basolateral membrane via exocytosis
Chylomicrons enter lymphatic lacteals
What is a lacteal?
A lymphatic capillary that absorbs fats in the villi
What happens to water in the lumen of GIT?
Mostly reabsorbed:
- jejunum
- ileum
- colon
How much water is reabsorbed per day?
How much is excreted in the faeces?
8-9 L
200 ml
Which diseases occur when there is a failure to control water transport?
Cholera
Typhoid
Clostridium difficile
Rotavirus
How is cellulose digested?
Where?
Not digested
Fermented
Commensal bacteria carry this out
Caecum and proximal colon
What are the products of cellulose fermentation?
Short chain fatty acids (SCFAs)
Where does the energy required by the colonic mucosa come from?
The fermentation of cellulose
What else do the bacteria of our microbiome produce?
Neurotransmittes
Other bioactive substances
What are some of the pancreatic enzymes that CCK brings about the release of?
- protrypsin
- chymotrypsinogen
- procarboxypeptidase
How is the acid maintained as acidic?
Parietal cells of the stomach pump H+ into the lumen of the stomach
Describe the release of trypsin
- CCK stimulates pancreas
- Protrypsin released from pancrease
- Activated by a protein in the brush border
Describe the release of chymotrypsinogen and procarboxypeptidase
- CCK stimulates pancreas
- Release from pancreas into duodenum
- Activated by trypsin
Where is CCK released from?
Enteroendocrine cells of the duodenum
List the proteases
Pepsin Trypsin Chymotrypsin Carboxypeptidase Aminopeptidase
