S6) Completing digestion/absorption

Question 1

What is absorption?

Answer 1

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

Question 2

Identify 3 carbohydrate monosaccharides

Answer 2

• Fructose
• Galactose
• Glucose

Question 3

The goal of carbohydrate digestion is to get monosaccharides.

Why is this?

Answer 3

• End products of carbohydrate metabolism
• They can move out of the lumen

Question 4

Where does carbohydrate digestion conclude?

Answer 4

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

Question 5

Identify 3 common dietary carbohydrates

Answer 5

• Starch (polysaccharide)
• Lactose (disaccharide)
• Sucrose (disaccharide)

Question 6

Describe the structure of amylose

Answer 6

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

Question 7

Describe the structure of amylopectin

Answer 7

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

Question 8

Describe the structure of maltose

Answer 8

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

Question 9

Describe the structure of isomaltose

Answer 9

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

Question 10

Describe the structure of alpha dextrins

Answer 10

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

Question 11

Describe the completion of starch digestion by brush border enzymes

Answer 11

• Maltose (maltase) = glucose + glucose
• Alpha dextrins (isomaltase) = glucose
• Lactose (lactase) = glucose + galactose
• Sucrose (sucrase) = glucose + fructose

Question 12

Describe how glucose is absorbed into the intestinal epithelium

Answer 12

• Na⁺/K⁺ ATPase on basolateral membrane maintains low intracellular Na⁺
• SGLT-1 binds Na⁺ which allows glucose binding
• Na⁺ & glucose moves into cell

Question 13

Describe how fructose is absorbed into the intestinal lumen

Answer 13

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

Question 14

How is glucose transported into the blood?

Answer 14

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

Question 15

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

Answer 15

• Uptake of Na⁺ generates osmotic gradient (water follows)
• Glucose uptake stimulates Na⁺ uptake
• Hence, mixture of glucose and salt will stimulate maximum water uptake

Question 16

What kind of proteins are digested?

Answer 16

• Amino acids
• Dipeptides
• Tripeptides

Question 17

Describe protein digestion in the stomach

Answer 17

• 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

Question 18

Describe the role of the pancreas in protein digestion

Answer 18

• Pancreas releases proteases as zymogens
• Trypsinogen is converted to trypsin by enteropeptidase
• Trypsin then activates other proteases

Question 19

What do exopeptidases do?

Answer 19

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

Question 20

What do endopeptidases do?

Answer 20

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

Question 21

How are amino acids absorbed?

Answer 21

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

Question 22

Most protein products are ingested as dipeptides/tripeptides.

Describe how this occurs

Answer 22

• Dipeptides/tripeptides are transported by H⁺ co-transporter – peptide transporter 1
• Inside cell these are converted to amino acids by cytosolic peptidases

Question 23

Describe the transport of sodium in the small and large intestines

Answer 23

• 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

Question 24

Describe calcium absorption in the intestines when calcium intake is low

Answer 24

• Active transcellular absorption
• Ca²⁺ enters cell via facilitated diffusion
• This process requires Vitamin D (Calbindin), stimulated by PTH

Question 25

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

Answer 25

Passive paracellular absorption

Question 26

Describe iron absorption in the intestines

Answer 26

• Iron is absorbed across apical membrane (co-transport with H⁺)
• Gastric acid is important in the process

Question 27

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

Answer 27

• 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

Question 28

Describe iron storage

Answer 28

• Approx. half of iron is in haemoglobin
• Other half stored in ferritin complexes in bone marrow, liver and spleen

Question 29

How are water soluble vitamins absorbed?

Answer 29

Water soluble vitamins mainly absorbed by Na⁺ cotransport

Question 30

Describe how Vitamin B₁₂ is absorbed

Answer 30

• 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

Question 31

Identify 4 signs/symptoms of coeliac disease

Answer 31

• Diarrhoea
• Flatulence
• Weight loss
• Sensory loss in her hands

Question 32

What is Coeliac disease?

Answer 32

Coeliac disease is the intolerance of the gliadin fraction of gluten which is found in wheat, rye and barley and results in immune response

Question 33

The resulting immune response in coeliac disease damages the intestinal mucosa.

How does this manifest?

Answer 33

• Absence of intestinal villi
• Lengthening of intestinal crypts
• Lymphocytes infiltrate epithelium

⇒ Impaired digestion/malabsorption

Question 34

Identify 2 investigations for Coeliac disease

Answer 34

• Upper GI endoscopy
• Duodenal biopsies

Question 35

Identify 3 blood tests which can be used to diagnose Coeliac Disease

Answer 35

• Serology
• Electrolyte imbalances
• Anaemia

Question 36

What is the treatment for Coeliac disease?

Answer 36

Diet