Digestion and Absorption Flashcards
Name the 2 functional periods the body undergoes in providing energy for cellular activities.
Absorptive
Postabsorptive
What occurs during the absorptive state?
Ingested nutrients enter the blood from the GI tract. Some nutrients provide energy for the body and others are added to energy stores.
What occurs during the postabsorptive state?
The GI tract is empty of nutrients and the body’s own stores must supply energy.
Name 3 important fatty acids we absorb.
Palmitic, stearic, oleic acid
Where does triglyceride digestion predominantly occur?
Small intestine
Where is lipase synthesised?
Pancreas.
Give the equation for the splitting of a triglyceride by lipase.
Triglyceride —> Monoglyceride + 2 Fatty acids
Name the process which converts lipids in ingested food to very small droplets.
Emulsification
Give 2 things that emulsification requires.
- Mechanical disruption of large droplets into smaller droplets. Provided by motility of GI tract
- Emulsifying agent: provided by phospholipids and bile salts. They prevent the aggregation of fat into larger droplets by repelling.
The coating of lipid with emulsifiers impairs the accessibility of lipase. How is this overcome?
Pancreas secretes colipase which binds onto the droplet as well as binding to lipase, holding it in place.
What are micelles?
Holding stations for lipids
In the small intestine, the fatty acids and monoglycerides are re-synthesised into triglycerides. In which organelles does this take place?
Smooth ER. The fat droplets are then pinched off in vesicles to the Golgi apparatus where they are modified to chylomicrons. The chylomicrons fuse with the plasma membrane and enter the interstitial fluid via exocytosis.
What do chylomicrons contain?
triglycerides, phospholipids, cholesterol and fat soluble vitamins
Give the sources of fatty acids for triglyceride synthesis.
- Glucose that enters adipose tissue is broken down to provide building blocks for the synthesis of fatty acids
- Glucose that is used in the liver to form VLDL triglycerides
- Ingested triglycerides, transported in blood in chylomicrons and taken up by adipocytes
Where are vitamins A,D,E + K (fat soluble vitamins) absorbed?
Ileum
Where are vitamins C+B (water soluble vitamins) absorbed?
Jejunum
One exception of absorption is vitamin B-12. How is this absorbed?
Vitamin B-12 must first bind to intrinsic factor (secreted by parietal cells). Intrinsic factor bound to B-12 binds to epithelial cells in the lower ileum so it can be absorbed via endocytosis.
What is vitamin B-12 used for? What can a deficiency lead to?
Vitamin B-12 is needed for erythrocyte formation. Deficiency can lead to pernicious anaemia - usually caused by lack of intrinsic factor.
Where is protein mainly digested and absorbed?
Duodenum
How many amino acids can we not manufacture?
8 - these are essential.
Amino acids exist as optical isomers. Which form is found in the proteins we use?
L-form
Give the peptide bond.
CONH (HN-C=O)
Describe the digestion of protein.
- Begins in stomach. Pepsin cleaves peptide linkage to form fragments
- In small intestine, fragments are further broken down by endopeptidases e.g. trypsin or exopeptidases e.g. carboxyl dipeptidases
Where is pepsinogen I found?
HCL secreting region (body) of stomach
Where is pepsinogen II found?
Pyloric region of stomach
What is the optimal PH for pepsin?
1.6-3.2, so its action is terminated on exit from stomach
Where does digestion and absorption of carbohydrates occur?
In duodenum
Name some important monosaccharides.
Glucose, fructose, galactose
Name some important disaccharides.
Lactose - beta linkage (those who are lactose intolerant do not have sufficient beta enzymes to break down lactose)
Sucrose - alpha linkage
Name some important polysaccharides.
Starch, glycogen
Which isomer of sugar molecules are utilised in metabolism?
D-isomers
Describe the linkage in glycogen.
Alpha 1,4 glycosidic bonds, alpha 1,6 glycosidic bonds
Describe the linkage in starch.
Majority alpha 1,4 glycosidic bonds, less alpha 1,6 bonds than glycogen
Describe how starch is digested.
- Starch is degraded by ptyalin (alpha amylase) in the mouth. Ph=6.7
- Small intestine is responsible for 95% of starch digestion. Here, pancreatic alpha amylase catalyses alpha1,4 bonds to form: maltose, maltotriose, glucose
- These products are further digested by oligosaccharidases in the brush border of microvilli to form: maltase, lactase, sucrase
- These products are further broken down to monosaccharides glucose, galactose and fructose.
Describe the absorption of carbohydrates and the products they are converted to.
- Hexoses and pentoses are rapidly absorbed across the intestinal mucosa and drain into the hepatic portal vein
- The carbohydrates enter the liver and they are converted to glucose
- Liver also converts glucose to glycogen for storage
- Liver converts some glucose into alpha-glycerol phosphate and fatty acids which are later used to synthesise triglycerides (occurs in hepatocytes as well as adipocytes).
What does VLDL stand for?
Very low density lipoproteins - contain more fat than protein an fat is less dense than protein
Where are bile salts reabsorbed in the GI tract?
Terminal ileum
