What is absorption?
Absorption is the movement of electrolytes, water and nutrients from the gut lumen into the blood
Identify 3 carbohydrate monosaccharides
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
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