S6) Completing digestion/absorption Flashcards Preview

Y2 SEM 3: Gastro-intestinal System > S6) Completing digestion/absorption > Flashcards

Flashcards in S6) Completing digestion/absorption Deck (36)
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What is absorption?

Absorption is the movement of electrolytes, water and nutrients from the gut lumen into the blood 


Identify 3 carbohydrate monosaccharides 

- Fructose

- Galactose

- Glucose 


The goal of carbohydrate digestion is to get monosaccharides.

Why is this?

- End products of carbohydrate metabolism

- They can move out of the lumen


Where does carbohydrate digestion conclude?

Final enzyme digestion takes place in the brush border by ‘brush border hydrolases’


Identify 3 common dietary carbohydrates

- Starch (polysaccharide)

- Lactose (disaccharide)

- Sucrose (disaccharide) 


Describe the structure of amylose

Amylose is a polysaccharide made of α-D-glucose units, bonded to each other through α(1→4) glycosidic bonds


Describe the structure of amylopectin

Amylopectin is a water soluble polysaccharide and highly branched polymer of glucose found in plants containing both α(1→4) and α(1→6) glycosidic bonds


Describe the structure of maltose

Maltose is a disaccharide formed from two units of glucose joined with an α(1→4) bond


Describe the structure of isomaltose

Isomaltose is an isomer of maltose, formed from two units of glucose joined with an α(1→6) bond


Describe the structure of alpha dextrins

Alpha limit dextrin is a short chained branched amylopectin remnant, produced by hydrolysis of amylopectin with alpha amylase


Describe the completion of starch digestion by brush border enzymes

- Maltose (maltase) = glucose + glucose

- Alpha dextrins (isomaltase) = glucose

- Lactose (lactase) = glucose + galactose

- Sucrose (sucrase) = glucose + fructose 


Describe how glucose is absorbed into the intestinal epithelium

- Na+/K+ ATPase on basolateral membrane maintains low intracellular Na+

- SGLT-1 binds Na+ which allows glucose binding

- Na+ & glucose moves into cell 


Describe how fructose is absorbed into the intestinal lumen

Fructose is transported by facilitated diffusion using the GLUT5 transporter to enter enterocyte


How is glucose transported into the blood?

GLUT2 transports glucose out of enterocyte which then diffuses down gradient into capillary blood 


Explain the principle of oral rehydration in terms of sodium and glucose absorption

- Uptake of Na+ generates osmotic gradient (water follows)

- Glucose uptake stimulates Na+ uptake

- Hence, mixture of glucose and salt will stimulate maximum water uptake


What kind of proteins are digested?

- Amino acids

- Dipeptides

- Tripeptides


Describe protein digestion in the stomach

- Pepsinogen released from chief cell is converted to pepsin by HCl

- Pepsin acts on protein to form oligopeptides/amino acids which move into the small intestine


Describe the role of the pancreas in protein digestion

- Pancreas releases proteases as zymogens

- Trypsinogen is converted to trypsin by enteropeptidase

Trypsin then activates other proteases 


What do exopeptidases do?

Exopeptidases break bonds at the end of the polypeptide to produce dipeptides or amino acids e.g. carboxypeptidases (A&B)


What do endopeptidases do?

Endopeptidases break bonds in the middle of the polypeptide to produce smaller polypeptide chains e.g. trypsin, chymotrypsin, elastase 


How are amino acids absorbed?

Amino acids are transported into cell using a Na+-amino acid co-transporter


Most protein products are ingested as dipeptides/tripeptides.

Describe how this occurs

- Dipeptides/tripeptides are transported by H+ co-transporter – peptide transporter 1

- Inside cell these are converted to amino acids by cytosolic peptidases 


Describe the transport of sodium in the small and large intestines

- Both intestines have Na+-K+ ATPase on basolateral membrane

- On the apical membrane:

I. Small intestine – Na+ is co-transported

II. Large intestine – Na+ channels 


Describe calcium absorption in the intestines when calcium intake is low

- Active transcellular absorption

- Ca2+ enters cell via facilitated diffusion

- This process requires Vitamin D (Calbindin), stimulated by PTH


Describe calcium absorption in the intestines when calcium intake is normal/high

Passive paracellular absorption 


Describe iron absorption in the intestines

- Iron is absorbed across apical membrane (co-transport with H+

- Gastric acid is important in the process


Compare and contrast iron uptake when iron levels are high/low

When Iron levels are low – iron binds to transferrin (transported to stores)

- When Iron levels are high – iron contained in ferritin complexes (trapped in cell) and lost when enterocyte is replaced 


Describe iron storage

- Approx. half of iron is in haemoglobin

- Other half stored in ferritin complexes in bone marrow, liver and spleen 


How are water soluble vitamins absorbed?

Water soluble vitamins mainly absorbed by Na+ cotransport 


Describe how Vitamin B12 is absorbed

Vitamin B12 absorbed in terminal ileum bound to intrinsic factor

- Intrinsic factor is secreted by gastric parietal cells, hence, B12 deficiency caused by gastritis/terminal ileal removal