Metabolism part 1: Energy Sources Flashcards
What are the three enzymes responsible for the digestion of protein? And where are they located?
Pepsin, Trypsin, Chymotrypsin.
Pepsin comes from the stomach, while trypsin and chymotrypsin come from the pancreas and act on the small intestine.
What are zymogen proteins? And what is their purpose?
Inactivated proteins, - Proteolytic enzymes are not secreted in their active form as they would start attacking the human body. To get around this, they are secreted in zymogen form and need to be activated by a certain stimulus.
How is pepsin activated, and what is its purpose
- Pepsinogen is secreted in the stomach by chief cells.
- Pepsi is activated by the presence of H+ in the stomach lumen
- Activated pepsin can go on and activate other pepsin molecules as well as break proteins into polypeptides
Where are trypsin and chymotrypsin secreted?
- Are secreted by the pancreas into the small intestine in
inactivated form.
What happens when trypsinogen is activated? And where does it occur?
- Trypsinogen is activated into trypsin by a substance on the
small intestine wall called enterokinase. - Activated trypsin then goes on to activate all other
proteases from their inactivated form
What hormones are responsible for controlling the secretion of pepsin, trypsin, and chymotrypsin?
Gastrin, Secretin, CCK.
What is the purpose of Gastrin.
Occurs with high amino acids in the stomach. Stimulates acid and pepsinogen release
What is the purpose of secretin?
occurs with high HCl in the duodenum, causes release of bicarbonate from the pancreas
What is the purpose of CCK?
Occurs with amino acids or fats in the duodenum. Causes the pancreas to secrete all enzymes.
What is direct calorimetry?
Direct calorimetry is a measure of the heat produced by oxidative processes.
What are the strengths of direct calorimetry?
- Very accurate
- Able to measure a basal metabolic rate.
What are the weaknesses of direct calorimetry?
- Unable to measure complicated activities.
- Expensive and time consuming.
What is indirect calorimetry?
Indirect Calorimetry calculates heat production by measuring gas exchange. During resting conditions food sources are broken down to produce energy. The oxygen consumed and the carbon dioxide produced are measured to provide a measurement of energy expenditure.
Carbohydrates
- Carbohydrates are long chains of carbon rings.
- They are attached together via 1,4 alpha glycosidic bonds.
- Breaking these bonds is how carbohydrates are digested.
How are carbohydrates broken down?
- The enzyme for carbohydrate digestion is called amylase. - Amylase acts on the 1,4 alpha glycosidic bonds and breaks them forming disaccharides.
- Amylase is secreted in the mouth and from the pancreas.
- The secretions from the pancreas act on the carbohydrates in the small intestine.
How are carbohydrates broken down from disaccharides to monosaccharides?
Enzymes on the microvilli of the small intestine.
These are Sucrase, maltase, and lactase.
What does Surcase, maltase, and lactase break down?
- Sucrase breaks up sucrose into fructose and glucose.
- Maltase breaks up maltase into two glucoses.
- And lactase breaks up lactose into galactose and glucose.
Absorption of carbohydrates
- Carbohydrate absorption occurs once the molecule is made into single sugars.
There are three main pathways of carbohydrate absorption:
Paracellular, Co-transportation, and transportation alone.
Paracellular transportation of carbohydrates
Monosaccharides diffuse through the tight junctions between cells.
Lone transport of carbohydrates
Transporters that move monosaccharides along by themselves (GLUT5).
Co-transportation of carbohydrates
Co-transported with sodium via SGLT1 transporter
Protein absorption
Same as carbohydrates, but with amino acids.
Digestion and absorption of nucleic acids
- Nucleic acids are broken down by acid in the stomach into nucleotides.
- The nucleotides are further broken down into nucleosides by pancreatic nucleases.
- The nucleosides of the DNA are finally absorbed by the small intestine via sodium dependent channels.
Digestion of lipids
Lipids are non-polar, which means that they can cross cell membranes without transporters. However, they cannot mix with the contents of the gut lumen.
Make up of Triacyclglycerols
A glycerol molecule and three fatty acids.
Purpose of chemical digestion breaks up triacyclglycerol into….
two fatty acids and a monoacylglycerol.
To break up the triacyclglycerol into monoacylglycerol and two fatty acids…
an enzyme in the pancreas called pancreatic lipase breaks the ester bonds holding them together. Pancreatic lipase in the gut lumen and lipids to do not mix with the fluid on their own, therefore bile salts are needed to act as a bridging molecule.
What are bile salts
formed in the liver and stored in the bile duct, bile salts are amphipathic, meaning they have a polar end and a non-polar end. This allows them to form emulsion droplets (emulsified fat).
Steps of lipid digestion and absorption
- Mixing and emulsion droplet formation.
- Breakdown via pancreatic lipase.
- Formation and movement of micelles.
- Absorption into the enterocytes.
Mixing and emulsion of droplet formation
The stomach and small intestine undergo segmentation to mix the fat and the gut fluid very briefly.
Before they separate, bile salts bind their non-polar end to the fats and polar end to the gut fluid.
This is the formation of emulsion droplets.
Breakdown via pancreatic lipase
Pancreatic lipase binds to the emulsion droplet and starts breaking down TAG into MAG and 2 fatty acids.
The structure holding the now digested material is now called a micelle. These micelles migrate towards the enterocyte walls.
- Formation and movement of micelles
Newly formed micelles move towards the enterocyte wall. They release the monoacylglycerols and fatty acids which diffuse into the cell.
The bile salts do not cross the enterocyte wall at this point. Only the contents of the micelle.
- Absorption into the enterocytes
Because they are non-polar the monoacylglycerols and fatty acids cross easily into the enterocyte
What happens after absorption into the enterocytes?
Once they are absorbed, while still in the enterocyte, the monoacylglycerol and fatty acids are turned back into triacyclglycerol. They are then packaged into chylomicrons, which enter the lacteal and are transported away from the intestine.
Chylomicrons
Consist of triglycerides, phospholipids, cholesterol, and proteins.
Transport lipids from the intestines to tissues in the body.
VLDL’s
transport lipids from the liver to the peripheral tissues.
LDL’s
Transports cholesterol from the liver to the peripheral tissues and is called LDL.
LDL is made in the liver from VLDL remnants.
HDL’s
- Take cholesterol from the peripheral tissues back to the liver for processing.
What happens to the chylomicrons when they leave the enterocytes
Fat exits the enterocytes as triacyclglycerols (in chylomicrons). These pass into the lacteals then into the blood. From the blood they go to the tissues. At these tissues, lipoproteins break down the TAG’s into monoacylglycerol and two fatty acids and absorbs them into the cell.
When this happens, the chylomicron remnants (like phospholipids and cholesterol) need to be reabsorbed. These go back in the blood, and into the liver to be reabsorbed
Composition of chylomicrons
98% triglycerides, 1% cholesterol, 1% phospholipids
Composition of VLDL’s
60% triglycerides, 20% cholesterol, 20% phospholipids
Composition of LDL’s
15% triglycerides, 60% cholesterol, 25% phospholipids.
Composition of HDL’s
10% triglycerides, 30% cholesterol, 60% phospholipids.
What role does vitamin A play in the body
Converted to retinal to allow vision
What role does vitamin D play in the body
Required for calcium absorption
What role does vitamin E play in the body
Maintains cell membrane integrity
What role does vitamin K play in the body
Required for gamma carboxylation
What role does vitamin B1 play in the body
Part of TPP enzyme
What role does vitamin B2 play in the body
Part of FAD enzyme
What role does vitamin B3 play in the body
Part of NAD enzyme
What role does vitamin B5 play in the body
Part of synthesis of CoA
What role does vitamin B6 play in the body
Part of pyridoxal phosphate enzyme
What role does vitamin B7 play in the body
Coenzyme of carboxylase.
What role does vitamin B9 play in the body
Formation of the neural tube
Synthesis of pyridines.
What role does vitamin B12 play in the body
General nutrient catabolism.
What role does vitamin C play in the body
Antioxidant, Collagen synthesis
What role does Selenium play in the body
Breakdown of free radicals to stop them attacking and mutating membrane.
What role does iodine play in the body
Binds and stabilises thyroid hormone allowing It to be converted to active form and complete its role all over the body.
What role does Iron play in the body
Essential component of myoglobin and haemoglobin which are for oxygen transport and storage.
