Digestion and Absorption Flashcards
(44 cards)
1
Q
What is digestion?
A
The breakdown of nutrients into absorbable molecules.
2
Q
What is absorption?
A
Movement of nutrients, water and electrolytes from the gut lumen into the internal environment.
3
Q
Describe the structure of the intestinal mucosa.
A
- Surface of the small intestine is arranged in circular folds of keckring.
-
Villi project from the folds
- Surface of the villi are covered with epithelial cells (enterocytes) with mucus secreting cells (goblet cells).
- Apical surface of epithelial cells covered by microvilli - brush border.
4
Q
What is the main site of absorption in the intestine?
A
- Small intestinal epithelial cells.
5
Q
What are the fat-soluble and water-soluble vitamins which must be consumed in the diet?
What are the minerals (trace metals)?
A
-
Fat-soluble
- Vitamin A
- Vitamin D
- Vitamin E
- Vitamin K
-
Water-soluble
- B1, B2, B3, B5, B6, B12
- These are not produced by the body (or endogenous amounts are not sufficient).
-
Minerals
- Ca
- Fe
- Zn
- Mn
- Mg
- Phosphorus
6
Q
What are the reasons that we need to consume the vitamins in our diet that we cannot make?
A
- They are not produced by the body (or endogenous amounts are not sufficient).
- They may be required as cofactors, antioxidants, ‘hormone’ (Vitamin D - synthesised in the liver in response to sunlight exposure, but required in the diet if not exposed).
- Deficiencies result in pathologies:
- Rickets (D)
- Scurvy (C)
- Anaemia (B12)
- May be cytotoxic in increased concentration, therefore should stick to RDA (although water soluble should be okay as excess is excreted in the urine, not stored).
7
Q
Describe the digestion and absorption which happens in the mouth.
A
- Very little digestion - small amount of lipid, CHO.
- Almost no absorption (certain drugs).
8
Q
Describe the digestion and absorption which happens in the stomach.
A
- Some digestion - protein but not very important.
- Almost no absorption (certain drugs).
9
Q
Describe the digestion and absorption which happens in the small intestine.
A
- LOTS of absorption - CHO, lipids, proteins.
- Vital site of digestion and absorption (some regional differences between duodenum, ileum and jejunum).
10
Q
Describe the digestion and absorption which happens in the large intestine.
A
- Almost no digestion and absorption (apart from water), some “indigestible” substances used as fuel by gut flora.
11
Q
Which is the only type of carbohydrate which can be absorbed?
A
Monosaccharides
12
Q
Which carbohydrates are absorbed by Na+-dependent cotransport?
Where are they absorbed?
A
- Glucose
- Galactose
- Absorbed in the small intestine
13
Q
Which carbohydrates are absorbed by facilitated diffusion?
A
Fructose
14
Q
What are the factors which promote digestion of carbohydrates in the mouth, stomach and duodenum respectively?
A
-
Mouth
- Salivary α amylase
-
Stomach
- Amylase continues to act
-
Duodenum
- Pancreatic amylase
- Brush border enzymes (maltase, sucrase and lactase) - act on disaccharides, producing monosaccharides - fructose, glucose and galactose.
- Lactose intolerance = no enzyme so bacteria ferment sugar - gas and diarrhoea.
15
Q
What are the digestible and ingestible CHOs?
A
-
Starch (from plants) and glycogen (from animals):
- Linear chains of glucose molecules joined by α1-4 glycoside bonds initially digested by amylase.
-
Cellulose (from plants):
- Linear chains linked by β1-4 glycosidic bonds. No enzymes in humans to digest cellulose.
- But, there are ways to break through a cell wall that are no enzymic. Largely mechanical - chewing lettuce etc. We just cannot break down this bond chemically.
16
Q
What are amylases?
A
Free enzymes acting in the GI lumen and digest only internal α1-4 glycosidic bonds.
17
Q
Describe the different types of amylases.
A
- Salivary amylases - secreted from the mouth in response to the sight and smell of food - of lesser importance than…
-
Panceatic amylases - secreted from pancreas into the duodenum.
- Produces maltotriose, maltose and α-limit dextrins.
18
Q
Describe the products of amylase.
(Carbohydrate digestion of carbohydrates).
A
- Products of amylase - now digested by oligosaccharides.
- Attached to the enterocyte mucosal membrane of the brush border of epithelial cells.
- α-glucosidase - cleaves α1-4 glycosidic bonds to remove single glucose units from the non-reducing end of the polymer.
- Isomaltase - cleaves α1-6 glycosidic bonds in the α-limit dextrin oligosaccharides.
19
Q
Describe carbohydrate digestion by disaccharides.
A
- After carbohydrate digestion by carbohydrates:
- Then, products of amlyase and oligosaccharides are hydrolysed by the disaccharidases which are attached to the brush border membrane.
- Maltase - produces glucose.
- Sucrase - produces glucose and fructose.
- Lactase - produces galactose and glucose.
20
Q
What is the end product of disaccharide digestion of maltose?
A
- Glucose - Glucose
- α1-4 glycosidic bond digested by MALTASE
21
Q
What are the end products of disaccharide digestion of sucrose?
A
- Glucose - fructose
- α1-2 glycosidic bond digested by sucrase
22
Q
What are the end products of disaccharide digestion of lactose?
A
- Galactose-Glucose
- β1-4 glycosidic bond digested by lactase
23
Q
What are the final products of carbohydrate digestion?
A
- Glucose
- Galactose
- Fructose
24
Q
Describe the absorption of carbohydrates.
A
-
Secondary active transport
- SGLT1 (sodium-dependent glucose transporter 1) located on the apical membrane transports glucose and galactose.
-
Facilitated diffusion
- GLUT5 (glucose transporter 5) transports fructose across the apical membrane.
25
Summarise protein digestion.
* Begins in the stomach with **pepsin**.
* Completed in the small intestine with pancreatic and brush-border **proteases**.
* **Endopeptidases** hydrolyse the interior peptide bonds of proteins.
* **Exopeptidases** hydrolyse one amino acid at a time.
26
Describe the secretion of proteases.
Proteases are secreted as zymogens before being activated.
27
What are the peptidases (aka proteolytic enzymes) which are involved in protein digestion?
* **Endopeptidases** - cleave large polypeptides in the middle of the chain → oligopeptides (2-8AA long).
* **Pepsin** (stomach) - hydrolyses links with tyrosine, D alanine and leucine. Hydrolyses long polypeptide chains into shorter ones.
* **Trypsin** (SI) - hydrolyses links with arginine and lysine. (Trypsin inhibitor - protection of self - small protein, present in pancreatic juice to inhibit any trypsin found prematurely in the pancreatic cells or duct).
* **Chymotrypsin** (SI) - hydrolyses links with tyrosine, tryptophan, D alanine, methionine and leucine.
* **Elastase** (SI) - degrades elastin.
* **Exopeptidases -** cleave amino acids one at a time from either end of protein.
* Carboxypeptidases (C-terminal).
* Aminopeptidases (N-terminal).
28
What are the products of stomach and pancreatic luminal enzymes?
* Oligopeptides
* Amino acids
29
What are the brush border enzymes which continue protein digestion?
* Oligopeptidase
* Aminopeptidase
30
What are the products of protein digestion?
What are these products further broken down into and where does this happen?
* Amino acids
* Dipeptides
* Tripeptides
* In enterocyte they are further broken down into amino acids.
31
Describe the digestion and absorption of lipids.
* Dietary lipids are hydrophobic (insoluble in water) so causes problems with digestion and absorption.
* They must be solubilised before digestion and absorption can occur.
* Digestion begins in the stomach with the action of lingual and gastric lipases.
* Digestion is completed in the small intestine with the action of the pancreatic enzymes.
32
What are the 4 main types of lipids?
* Fats / oils (triacylglycerols (TAG) or triglycerides)
* Phospholipids
* Cholesterol and cholesterol esters
* Fatty acids
33
What are the 3 main types of enzymes involved in lipid digestion?
* Lipases
* Phospholipases
* Cholesterol esterases
* Bile salts also have an inportant role in achieving digestion / absorption, but are not enzymatic
34
Describe the lipid digestion of triacylglycerols (TAG) by lipases.
* In (mouth) and stomach:
* Salivary / gastric lipases (relatively unimportant ~10% of ingested lipids hydrolysed).
* Heat and movements in stomach mix food with lipases → lipid emulsion.
* Hydrolysis initially slow due to largely separate aqueous / lipid interface.
* As hydrolysis proceeds, rate increases due to fatty acids produced acting as surfactants breaking down lipid globules aiding emulsification.
* Emulsified fats ejected from the stomach to the duodenum.
* In duodenum:
* Pancreatic lipase (important - the main lipid digestive enzyme).
* Aided by bile salts from the gall bladder.
* HCO3- neutralises stomach acid → suitable pH.
35
Describe the digestion of lipids in the small intestine.
* Bile salts, lysolecithin and products of lipid digestion emulsify dietary lipids.
* Emulsification produces small droplets of lipids dispersed in an aqueous solution creating a large surface area for pancreatic enzyme digestion.
* Pancreatic enzymes (pancreatic lipase, cholesterol ester hydrolase and phospholipase A2) and the protein, colipase are secreted to complete digestion.
36
What is the role of bile salts in emulsions and micelles?
37
What are mixed micelles?
* Products of lipid digestion (cholesterol, monoglycerides, lysolecithin and free fatty acids) are solubilised in mixed micelles (diameter 5nm).
* Core contains the products of lipid digestion.
* Surface coating of bile salts which are amphipathic.
38
Describe chylomicrons.
* Chylomicrons (100nm diameter) have a core of triglycerides and cholesterol ester - phospholipids and apoproteins on the outside (80% / 20%).
* Chylomicrons are packaged into secretory vesicles on the golgi membrane and are exocytosed across the basolateral membrane.
* Chylomicrons are too big to enter vascular capillaries but enter the lymphatic capillaries (lacteals) by moving between the endothelial cells that line the lacteals.
* The lymphatic circulation carries the chylomicrons to the thoracic duct which empties into the blood stream.
39
Bile salts are essential for lipid digestion. But, there is a problem - describe this.
* There are not enough bile salts to deal with the average meal.
* ~3-4g in the body, but ~3-15g required per meal.
* After use, most are reabsorbed from the terminal ileum into the liver via enterohepatic circulation (via hepatic portal vein).
40
Describe the enterohepatic circulation of bile salts.
41
Describe the mechanism of absorption of fatty acids, monoglycerides and cholesterol.
* Absorbed in the small intestine.
* Mechanism of absorption:
* Bile salts form micelles.
* Diffusion of products into intestinal cells.
* Re-esterification within the cell to triglycerides and cholesterol.
* Chylomicrons form in the cell and transfer to lymph.
42
Describe the absorption of water in the small intestine.
* Absorption of H2O (and NaCl).
* Na+ actively absorbed in exchange for K+.
* K+ reabsorbed in exchange for H+.
* Cl- absorbed in exchange for HCO3-.
* H2O follows due to osmosis.
* Controlled by enteric nerve plexi and hormonal control.
* Aldosterone = increased water absorption.
43
Describe the bacterial actions in the large intestine.
* Movements through the large intestine are very slow, therefore bacteria thrive (1011 bacteria /g faeces).
* Formation of certain B vitamins (B12, thiamin, riboflavin).
* Formation of vitamin K
* B and K absorbed passively (probably - mechanisms remain uncertain).
* "Digestion"
* Certain bile constituents
* Other "ingestible" matter
44
Summarise the absorption of fluid in the small and large intestine.
* ~9L of fluid enters the GI tract each day.
* Approx. 2.3L from ingestion, the rest from secretion.
* Small intestine reabsorbs ~8L.
* Large intestine reabsorbs ~90% of the last litre by osmosis through cell walls into vascular capillaries inside villi.
* "Rough agents" in the large intestine cause problems - e.g. lactose → osmotic diarrhoea.
