Digestion & Absorption of Carbs and Proteins Flashcards
What are enterocytes?
Enterocytes are specialized cells lining the inner surface of the small intestine, where they play a crucial role in the absorption of nutrients and water from digested food.
These cells are a type of epithelial cell with unique structural features that facilitate their functions in nutrient absorption and barrier maintenance.
What types of gland is the pancreas?
Both exo- and endocrine
Exocrine = enzymes, electrolytes & mucin
Endocrine = insulin, glucagon, pancreatic polypeptides & somatostatin
What are acinar cells?
Specialized cells found in exocrine glands throughout the body, including the pancreas, salivary glands, lacrimal glands, and sweat glands. These cells are responsible for the production and secretion of various substances, including enzymes, hormones, and fluids, into ducts or directly into the external environment.
What do pancreatic acinar cells do?
Secrete peptidases, lipases, alpha-amylases & nucleases
What happens if acinar cells don’t work?
Malabsorption sydromes occur
What are ductal cells?
These cells line the ducts of exocrine glands and are responsible for modifying and transporting the secretions produced by the glandular cells (such as acinar cells) to their final destination.
What do ductal cells secrete?
Pancreatic juice with high concentration of HCO3-
What is the role of HCO3- in pancreatic juice?
Regulate the pH of upper intestine
What happens if there is a failure in pancreatic juice secretion?
Duodenal ulcers
Acinar cells vs duct cells
Acinar cells = CCK, ACh causing enzyme and Cl- secretion
Duct cells = secretin = HCO3- secretion
Why does the concentration of HCO3- increase as the rate of pancreatic juice secretion increase?
Primarily due to the action of ductal cells in the pancreas, which actively secrete bicarbonate ions into the pancreatic ducts.
This process is regulated by various signalling pathways and mechanisms to maintain the optimal pH environment for pancreatic enzyme activity and digestion in the small intestine.
Where does the HCO3- secreted into pancreatic juice come from?
Plasma
When secretion of HCO3- in pancreatic juice is high = acid tide in the plasma
What enzymes are found in pancreatic juice?
Pancreatic alpha-amylase, lipases & proteases
Role of pancreatic alpha-amylase
Hydrolyzes, glycogen, starch, complex CH2O
NOT cellulose
Describe pancreatic lipases
All secrete in their active form
Water-insoluble esters require bile salts to work
Name water-soluble esters and their properties
Short chain fatty acids
Do not require bile salts to be digested
Name some pancreatic proteases
Trypsin
Chymotrypsin
Carboxypeptidase A & B
Secreted in inactive zymogen form
What generally stimulates pancreatic secretion?
Parasympathetic system
What generally inhibits pancreatic secretion?
Sympathetic system
Partially mediated by vasoconstriction
What are the phases of pancreatic secretion?
Cephalic phase
Gastric phase
Intestinal phase
What is the most important phase of pancreatic secretion?
Intestinal phase
How is the cephalic phase of pancreatic secretion activated?
This phase is primarily under neural control and is initiated by the central nervous system in anticipation of food ingestion
Activated through sight, smell or taste of food
Enhanced by vagal stimulation
Secrete substantial amounts of enzymes & HCO3-
What is the gastric phase of pancreatic secretion?
The stage of pancreatic enzyme and bicarbonate secretion that occurs in response to the presence of food in the stomach.
This phase is initiated by distension of the stomach wall and the presence of partially digested food (chyme), which triggers neural and hormonal signals that stimulate pancreatic secretion
Release of gastrin to produce low-volume high-enzyme pancreatic secretion
Explain why intestinal phase of pancreatic secretion is important?
70% of total secretion
What stimulates the intestinal phase of pancreatic secretion?
CCK & secretin = released from endocrine cells in duodenum ad upper jejunum
What is the function of CCK?
Potent stimulant of pancreatic enzyme secretion
What does potentiated mean?
Potentiated typically refers to the enhancement or amplification of a physiological response or effect.
When a substance or stimulus is described as potentiated, it means that its potency or effectiveness has been increased, often through interaction with other factors or mechanisms.
What is CCK potentiated by?
Secretin
By itself secretin has no effect on CCK
What causes secretin and CCK secretion and what are their functions?
Chyme in the intestine
Secretion of pancreatic juice rich in bicarbonate = secretin
Secretion of pancreatic juice rich in enzymes = CCK
What potentiates the effect of both CCK & secretin?
ACh
What nerve stimulates pancreatic secretion?
Vagus nerve
What is a major stimulus for CCK secretion?
Amino acids = mainly phenylalanine & tryptophane
Fatty acids
Monoglycerides
What is a potent stimulus for the release of secretin?
Low pH = less than 4.5
What must happen to carbohydrates before being absorbed?
They must be digested into monosaccharides before being absorbed
What enzymes digest carbohydrates and where are they found?
Luminal = alpha-amylase
Mucosal = disaccharidases
Where is pancreatic alpha-amylase most concentrated?
Duodenum
Where does starch digestion begin?
Mouth by salivary alpha-amylase
What is the rate limiting step of mucosal digestion?
Rate for oligosaccharides to be digested into monosaccharides by brush border dissacharidases
Where is mucosal enzyme activity greatest?
Brush border of jejunum
What are the end products of starch and glycogen breakdown?
Maltase = glucose
Sucrase = fructose
Lactase = galatose
Where has highest capacity to absorb sugars?
Duodenum & upper jejunum
What mechanism are glucose, galatose & xylose absorbed by?
Na+ dependent transport system
What mechanism is fructose absorbed by?
Facilitated transport
What happens to fructose in the epithelial cells?
Rapidly converted into glucose & lactic acid = maintaining concentration gradient for diffusion
Is monosaccharide absorption regulated?
No so can absorb over 5kg sucrose a day
What happens in the event of failure to absorb carbohydrates?
Results in diarrhoea and intestinal gas
Intestinal gas from bacterial fermentation
What is solvent drag?
The movement of solvent molecules (usually water) carries solute particles along with it as it flows through a semipermeable membrane or across an epithelial barrier. This process occurs due to the movement of solvent molecules driven by osmotic or hydrostatic pressure gradients, which can drag solute particles along with them
Solvent drag plays a role in carbohydrate absorption by facilitating the movement of water across the intestinal epithelium. As water moves from the intestinal lumen into the epithelial cells and then into the bloodstream, it creates a flow that can carry dissolved carbohydrate molecules with it. This helps to enhance the absorption of carbohydrates from the intestinal lumen into the bloodstream, particularly in regions where carbohydrate concentration is high and water movement is facilitated by osmotic gradients.
What happens if you consume lactose when you have lactase deficiency?
Accumulation of lactose in bowel lumen
Lactic acid production by bacteria
Increased luminal osmolality
Fluid accumulation in lumen
Luminal distension & enhanced peristalsis
Watery diarrhoea
What can be a cause of lactose intolerance?
Age = lactase decreases
Congenital = genetic mutation that impairs the production of lactase
Infection = lactase is sensitive to infectious &
inflammatory diseases that affect
intestine
Name the enzymes that digest proteins
Gastric pepsin
Pancreatic proteases
Enterocytes peptidases
Where is most protein digested and absorbed?
50% Duodenum & jejunum
20-50% reaches ileum
10% reaches colon
What happens to proteins in the colon?
Digested by micro-organisms
Protein in stool from bacterial and cellular debris
What are proteins digsted into?
Small polypeptides & amino acids before being absorbed
Di, tri & tetrapeptides are 3-4x more concentrated than single aa in enterocytes
How are digested proteins transported?
Na+ dependent transport systems for tri, dipeptides & L-amino acids
What form of digested peptides are most absorbed?
Tri- & dipeptides are absorbed in greater quantities than amino acids
