CELLULAR BIOLOGY - Non-ruminant Digestion and Metabolism Flashcards
What are the main nutritional energy sources used by most mammals?
Carbohydrates
Lipids
Proteins
Which enzyme cleaves alpha 1-4 glycosidic bonds?
Amylase
Where does carbohydrate digestion begin in pigs?
Carbohydrate digestion begins at the mouth by the actions of lingual amylase
Where in the digestive tract does the majority of carbohydrate digestion occur?
Carbohydrate digestion occurs predominately within the small intestine by the actions of pancreatic amylase
What are the four main steps of carbohydrate digestion?
- Enzymatic digestion of polysaccharides into disaccharides
- Enzymatic digestion of disaccharides into monosaccharides
- Absorption of monosaccharides into the small intestinal wall and into the bloodstream
- Transport of monosaccharides to target cells
Describe the process of enzymatic digestion of polysaccharides into disaccharides
Amylase cleaves the alpha 1-4 glycosidic bonds to break down polysaccharides into disaccharides
Describe the process of enzymatic digestion of disaccharides into monosaccharides
Cell surface enzymes on the enterocyte microvilli cleave disaccharides into monosaccharides (glucose)
Describe how glucose is absorbed into the small intestinal wall and into the bloodstream
Glucose in the small intestinal wall is absorbed into the enterocytes and into the bloodstream via active transport or facilitated diffusion
Describe the process of glucose transport to target cells
Glucose is transported around the body dissolved in the blood and taken up into target cells via GLUT proteins
What are the four metabolic processes that glucose is involved in?
Glycolysis
Glycogenesis
Glycogenolysis
Fatty acid synthesis
What is glycolysis?
Glycolysis is the breakdown of glucose into pyruvate
What is glycogenesis?
Glycogenesis is glycogen synthesis from glucose
What is the enzyme which converts glucose into glycogen during glycogenesis?
Glycogen synthase
What physiological condition stimulates glycogenesis?
When blood glucose levels are high, insulin secretion activates glycogen synthase which converts glucose into glycogen
Where in the body is glucose stored as glycogen?
Hepatocytes
Skeletal muscle cells
What is glycogenolysis?
Glycogenolysis is the breakdown of glycogen into glucose
What is the enzyme which converts glycogen back into glucose during glucogenolysis?
Glucose-6-phosphatase
What physiological condition stimulates glyocgenolysis?
When blood glucose levels are low, glucagon secretion activates glucose-6-phosphatase which converts glycogen back into glucose
Why do only the glycogen stores in the liver undergo glycogenolysis and contribute to blood glucose levels?
Liver cells have the enzyme glucose-6-phosphatase whereas skeletal muscle cells do not and thus the glycogen stores within these muscle cells can only be used as an energy source for the muscle itself and do not contribute to the blood glucose levels
What physiological condition stimulates fatty acid synthesis from glucose?
When blood glucose levels are high, insulin secretion stimulates excess glucose to be converted into fatty acids and stored as triacylglycerols (TAGs)
Where in the body are fatty acids stored as triacylglycerols (TAGs)?
Hepatocytes
Adipose cells
Where within the cell does fatty acid synthesis occur?
Fatty acid synthesis occurs within the cytoplasm of the cell
Describe the process of the fatty acid synthesis pathway
- Within the cytoplasm, glycolysis converts glucose into pyruvate which passes into the mitochondria
- Within the mitochondria, pyruvate is converted into acetyl CoA
- Acetyl CoA undergoes a condensation reaction with oxaloacetate to produce citrate which enters the citric acid cycle
- Citrate can cross the mitochondrial membrane back into the cytoplasm where it is broken down into acetyl CoA and oxalocaetate
- Acetyl CoA undergoes fatty acid synthesis in the cytoplasm and is stored as triacylglycerols (TAGs) in liver and adipose cells
Where in the digestive tract does the majority of lipid digestion occur?
Lipid digestion occurs predominately within the small intestine by the actions of pancreatic lipase
Other than pancreatic lipase, what are the other enzymes involved in lipid digestion?
Lingual lipase
Gastric lipase
Lipoprotein lipase
What is the role of bile salts in lipid digestion?
Bile salts mediate emulsification of lipids
Describe the process of lipid emulsification and why it is so important for lipid digestion
Lipids are hydrophobic molecules and thus clump together in the small intestine to form fat droplets. Small intestinal motility breaks down these fat droplets and mixes them with bile, coating them in bile salts, forming lipoprotein complexes, prevent the lipids from re-clumping and to increase the surface area of the lipids for enzymatic digestion
What is the structure of lipoprotein complexes?
Lipoprotein complexes have a central hydrophobic core containing triacylglycerols (TAGs) and cholesterol. This hydrophobic core is surrounded by hydrophilic membrane consisting of phospholipids and free cholesterol
What are the specific lipoprotein complexes formed within the small intestine?
Chylomicrons
What are the four steps of triacyglycerol (TAG) digestion?
- Triacyglycerol (TAG) partial digestion in the small intestinal lumen
- Triacyglycerol (TAG) resynthesis in the enterocytes
- Triacyglycerol (TAG) transport to target cells
- Triacyglycerol (TAG) hydrolysis
Describe the process of triacyglycerol (TAG) partial digestion in the small intestinal lumen
Colipase enzyme anchors pancreatic lipase to the surface of the chylomicrons, allowing pancreatic lipase to partially digest the triacyglycerols (TAGs) through gaps in the chylomicrons. This partial digestion breaks down the triacyglycerols (TAGs) in fatty acids and a monoacyl glycerol which diffuse into the enterocytes
Describe the process of triacyglycerol (TAG) resynthesis in the enterocytes
Fatty acids and monoacyl glycerol are resynthesised into triacyglycerols (TAGs) within the enterocytes and repackaged into chylomicrons
Describe the process of triacyglycerol (TAG) transport to target cells
Chylomicrons in the enterocytes undergo exocytosis into the lymphatic system and then into the bloodstream to target cells
Where in the body are lipoprotein lipases found?
Luminal surface of capillaries
Cardiac muscle cells
Skeletal muscle cells
Adipose cells
Describe the process of triacyglycerol (TAG) hydrolysis
Lipoprotein lipases hydrolyse the triacyglycerols (TAGs) within the chylomicrons into fatty acids and glycerol which can diffuse into target cells
What happens to the chylomicron remnant after triacyglycerol (TAG) hydrolysis?
The chylomicron remnant still contains cholesterol so is transported to and taken up by the liver where the cholesterol is released
Which physiological condition stimulates fatty acid metabolism?
When blood glucose levels are low, glucagon secretion stimulates fatty acid metabolism
Where in the cell does fatty acid metabolism occur?
Fatty acid metabolism occurs within the matrix of the mitochondria
What are the three steps of fatty acid metabolism?
- Entry into mitochondria
- Beta oxidation
- Acetyl CoA oxidation
How do short chain fatty acids enter the mitochondria for fatty acid metabolism?
Short chain fatty acids diffuse into the mitochondria for fatty acid metabolism
How do long chain fatty acids enter the mitochondria for fatty acid metabolism?
Long chain fatty acids require a carnitine shuttle to enter the mitochondria for fatty acid metabolism
Describe how the Carnitine shuttle transfers long chain fatty acids into the mitochondria
- Acyl CoA synthase activates fatty acids in the cytoplasm, causing them to react with coenzyme A to form fatty acid CoA
- The CoA of the fatty acid CoA is exchanged for carnitine, forming fatty acid carnitine
- Fatty acid carnitine can pass through the mitochondrial membrane via carnitine palmitoyl transferase I (CPT I)
- In the mitochondria, fatty acid carnitine is reversed to fatty acid CoA by carnitine palmitoyl transferase II (CPT II)
Describe the beta oxidation step of fatty acid metabolism
Within the mitochondria, each beta oxidation cycle releases two carbons from the acetyl CoA until the whole fatty acid is released as acetyl CoA
Describe the Acetyl CoA oxidation step of fatty acid metabolism
The Acetyl CoA released from the fatty acid chain enters the citric acid cycle and undergoes oxidative phosphorylation to produce ATP
What is the theoretical yield of ATP from one round of beta oxidation in fatty acid metabolism?
The theoretical yield is 17 ATP
In which cell types are fatty acids resynthesised for storage as triacyglyerols (TAGs)?
Fatty acids are resynthesised for storage as triacyglyerols (TAGs) in adipose cells
Where in the body is hormone sensitive lipase found?
Hormone sensitive lipase is found within adipose cells
What is the function of hormone sensitive lipase?
When blood glucose levels are high, insulin secretion stimulates hormone sensitive lipase to cleave triacyglyerols stored in adipose cells into fatty acids and glycerol so they can be used as a source of energy
What is ketogenesis?
Ketogenesis is the production of ketone bodies when fatty acids are metabolised to excess
Where in the body does ketogenesis occur?
Ketogenesis occurs in the liver
What are the three ketone bodies?
Hydroxybutyrate
Acetoacetate
Acetone
What are the three physiological conditions that stimulate ketogenesis?
Fasting
Starvation
Carbohydrate restricted diet
Why does insufficient dietary carbohydrates lead to ketone body formation?
If there are insufficient carbohydrates, this will stimulate intense gluconeogenesis to increase glucose production. Gluconeogenesis removes oxaloacetate from the citric acid cycle, preventing acetyl CoA from entering the citric acid cycle and becoming ATP so instead is converted into ketone bodies in the liver
Which enzyme mediates the conversion of ketone bodies back into Acetyl CoA for ATP production?
CoA transferase
Which two cell types cannot utilise ketone bodies as an energy source?
Erythrocytes: no mitochondria
Liver cells: no CoA transferase enzyme
What is ketosis?
Ketosis is the excess accumulation of ketone bodies
What is ketoacidosis?
Ketoacidosis is the pathological state of uncontrolled ketone body production leading to metabolic acidosis
What is the main cause of ketoacidosis?
Insulin deficiency (type one diabetes)
Where in the digestive tract does the majority of protein digestion occur?
Protein digestion occurs predominately in the stomach due to the actions of pepsin
What are the three steps of protein digestion?
- Protein digestion in the stomach
- Protein digestion in the small intestine
- Amino acid absorption into the small intestinal wall and into the bloodstream
Describe the process of protein digestion within the stomach
Distention of the stomach due to bolus entry stimulates G-cells in the gastric mucosa to secrete gastrin. Gastrin stimulates chief cells to secrete pepsinogen and parietal cells to secrete HCL. HCL activates pepsinogen to form pepsin which cleaves the peptide bonds between the amino acids making up proteins
Describe the process of protein digestion in the small intestine
The pancreas secretes proenzymes into the small intestine which when activated, will cleave the peptide bonds between the amino acids making up proteins
What are the two main proenzymes secreted by the pancreas into the small intestine to digest proteins?
Trypsinogen
Chymotripsinogen
What activates trypsinogen?
Trypsinogen is activated by enteropepsinogen to form trypsin
What activates chymotrypsinogen?
Chymotrypsinogen is activated by trypsin to form trypsinogen
Describe the absorption of amino acids into the small intestinal wall and the bloodstream
Amino acids are absorbed into the enterocytes and into the bloodstream via diffusion or transport proteins and travel to the liver and the kidneys
Where does catabolism of amino acids occur?
Catabolism of amino acids occurs in the liver
What are the two steps that amino acids have to undergo in amino acid catabolism?
Removal of the amino group
Breakdown of the carbon skeleton
What are the two reactions that occur to remove the amino group from the amino acid?
Transamination reaction
Deamination reaction
Describe the transamination reaction involved in amino acid catabolism
Transamination reaction is the transfer of an amino group from the amino acid to an acceptor molecule
What is the most common acceptor molecule seen in transamination reactions?
Ketoglutarate
Describe the deamination reaction involved in amino acid catabolism
Deamination reaction is the removal of the amino group from the acceptor molecule in the form of ammonium by the glutamate dehydrogenase enzyme. Ammonium is then directed to the urea cycle
What is the difference between glucogenic amino acids and ketogenic amino acids?
Glucogenic amino acids: carbon skeletons enter gluconeogenesis to produce glucose
Ketogenic amino acids: carbon skeletons are converted into ketone bodies
What are the only two exclusively ketogenic amino acids?
Leucine
Lysine
