Digestion/Absorption/Metabolism Flashcards
Understand the function of each part of the digestive system
Mouth - chemical (amylase and lipase)
Mechanical
Oesophagus - no digestion/absorption
Stomach - Mechanical action (Oblique, circular and longitudinal break down food
Fundus is where undigested food is stored
Chyme (digested food) goes to pyloric antrum towards duodenum
SI - duodenum, jejunum, ileum
Duodenum - secrete bicarbonate rich mucus to help neutralise the acidity of the chyme. Digestion
Jejunum - majority of absorption
Ileum - little digestion
Large Intestine - most of the nutrients have been absorbed. Chyme is converted to faeces for excretion
What are the major cell types present in gastric pits and their major secretions?
D cells secrete somatostatin - decrease HCl produced by parietal cells.
G cells - secrete gastrin which can increase pH or decrease it depending on the levels
Endochromaffin cells - release histamine which act on parietal cells to increase HCl.
Parietal cells - release HCl in stomach
Chief cells - convert pepsinogen into pepsin for digestion of proteins
Mucosal cells secrete mucosa in the pit to prevent self digestion
Explain how gastric acid secretion is controlled
Gastric acid is secreted by parietal cells of the stomach in response to stimuli such as the presence of food in the stomach or intestine.
Activates histamine (H2) , acetylcholine (M3) and gastrin (CCK2) receptors in the basolateral membrane of the parietal cell
Signals activation of H+K+ ATPase – acid secretion
How does Omeprazole work?
Proton pump inhibitor
Reduce acid secretion
Describe how the macronutrient group proteins and complex carbohydrates are digested into di/tri-peptides and free amino acids, or mono- and di-saccharides, respectively.
Galactose to lactose via lactase enzyme
Starch to maltose, maltotriose, alpha dextrin via amylase. then broken down by brush border enzymes to glucose
Sucrose to fructose via sucrase enzyme
Polypeptides -> pepsin in stomach -> trypsin and chymotrypsin in duodenum to make oligopeptides and free amino acids. Oligopeptides further digested by endopeptidase, dipeptidase and amino peptidase to dipeptides and tripeptides.
What are the three main regions of the small intestine and their principle function(s)?
Duodenum - Digestion
Jejunum - Mainly Absorption and some digestion
Ileum - Absorption
What are the key transport proteins present at the luminal and contraluminal membranes in small intestinal enterocytes?
Carbohydrates -
Glucose and lactose enters the body via GLUT2 or mainly SGLT1 with the help of NA+/K+ ATPase.
Fructose enters via GLUT5
All leave by GLUT2
Proteins -
Amino acids are transported via sodium cotransporter
Di and Tripeptidese are absorbed via H+ dependent cotransporters and hydrolysed into amino acids
All leave via facilitated diffusion
Lipids -
Large TAG are emulsified to smaller TAG
Smaller TAG with help of bile acid and lipase enzyme make micelle
Micelle diffuse and two chains (long and medium)
Long - reesterify to make TAG and packaged into chylomicrons and enter via lacterals into lymphatic system.
Medium - diffuse into bloodstream
Explain how impaired activity of the Na+/K+ ATPase may contribute to the symptoms of IBD
IBD - inflammation of GI tract
Na+/K+ does not work then disrupts water balance
Causing diarrhoea due to excessive fluid accumulating in lumen
Compare and contrast the differences between carbohydrate handling by the liver and by skeletal muscle
Liver - store glucose in form of glycogen
Glucose can also be used to make ATP and enter Krebs cycle
GLUT2 transporter - glucose moves passively
Skeletal muscle - use the glucose for energy
GLUT4 transporter - stored in intracellular vesicles
Describe how insulin regulates the uptake / release of FFAs in white adipose tissue, listing the key enzymes involved
FFA from food enter circulation several hours after because of effect of insulin on adipose tissue is more complex.
Uptake
If body has high blood glucose, insulin causes the uptake of FFA into adipose tissue for storage as TAGS.
Insulin binds to adipocytes causing GLUT4 receptors activated and allow uptake of glucose into adipocytes. Metabolised to glycerol-3-Phosphate for TAG synthesis
Insulin activates enzyme lipoprotein lipase (LPL) located on endothelial cells around adipose tissue and this hydrolyses TAGS to FFAS from lipoprotein (chylomicrons) and VLDL.
The FFA enter the adipocytes through fatty acid transporter
FFA are esterified with glycerol to form TAGs, which are stored in lipid droplets within the cytoplasm.
Release
Fasting or need for energy, insulin levels decrease so hormone sensitive lipase (HSL) in adipocytes activate
Adrenaline, noradrenaline and glucagon levels increase
HSL responsible for hydrolysis of TAGS, releasing FFA and glycerol into the blood
Decreased insulin level lead to inhibition of LPL
Release of FFA are transported out of adipocytes into the blood stream by fatty acid transporters
In the blood, FFA can be taken up and utilised as energy substrates by other tissues, such as skeletal, muscle and liver through beta oxidation or converted into ketone bodies by liver during prolonged fasting.
Discuss the role of the insulin:glucagon ratio in the regulation of carbohydrate and fatty acid metabolism in liver and adipose tissue
Carbohydrates:
When blood glucose levels in the body are high, insulin levels increase and glucagon levels decrease. High insulin levels causes glucose to enter the liver via GLUT2 receptors.
Glucose to Glucose-6-phosphate via GK.
Insulin increases glycogen synthesis into glycogen and stimulates glycolysis breakdown of Glucose-6-phosphate into pyruvate to enter the krebs cycle to make ATP.
Insulin inhibits gluconeogenesis and glycogen breakdown and glucagon and adrenaline stimulate this.
Therefore, high insulin: glucagon ration causes more insulin for more glucose uptake and utilisation.
Fatty acids
High insulin levels promote fatty acid synthesis in adipose tissue
Insulin stimulates uptake of glucose by adipocytes via GLUT4 for FFA synthesis
Inside adipocytes, glucose is converted into acetyl-coA as FFA synthesis
Insulin also promotes conversion of excess glucose into glycerol-3-phosphate for TAG synthesis.
High insulin to glucagon ratio = storage of excess energy of TAGS in adipose tissue.
