Digestion & Absorption Flashcards
Principle dietary nutrients
- Carbs
- Protein
- Fat
- Vitamins
- Mineral
- Water (absorbed through skin)
Monosaccharides
- Glucose
- Galactose
- Frutose
(all are 6C sugars)
Lactose is composed of
Glucose + Galactose (lactase)
Sucrose is composed of
Glucose + Fructose (sucrase)
Maltose is composed of
Glucose + Glucose (maltase)
Disaccharides
- 2 monosaccharides linked together by glycosidic bond
- Broken down to constituent monomers by brush border enzymes in small intestine
Polysaccharides
- Starch
- Cellulose
- Glycogen
Plant carbs
Starch & Cellulose
Animal carbs
Glycogen
Starch
- Plant storage form of glucose
- α-amylose: glucose linked in straight chains
- Amylopectin: glucose chains highly branched
- Glucose monomers linked by α-1,4 glycosidic bonds hydrolysed by amylases (saliva, pancreas)
Cellulose
- Constituent of plant cell walls
- Unbranched, linear chains of glucose monomers linked by β-1,4 glycosidic bonds
- Dietary fibre (no enzymatic digestion in vertebrates - require bacteria (cellulase))
Glycogen
- Animal storage form of glucose
- Glucose monomers linked by α-1,4 glycosidic bonds
Animals can only digest
α-1,4 glycosidic bonds (starch, glycogen) using amylase
Paracellular
Absorbed in tight junctions (water)
Transcellular
Goes through the cell
Vectorial transport
Through transport proteins
SGLT1 & GLUT2 transport:
glucose & galactose
GLUT1 found in
RBC
SGLT1 found in
Kidney
Describe the primary active transport system
Na pumped out of cell, into blood and creates a gradient.
Na will then move into cell from lumen and bring glucose with it using sodium glucose transporter
Glucose transported into blood by GLUT2 transporter
Water goes into blood through osmotic gradient through tight junction
Secondary active transport system
Fructose transport system
GLUT5 and then GLUT2 transports fructose
Does not require Na, so no water uptake due to no Na uptake
What are peptides
Small proteins
3-10 amino acids in length
Proteins undergo
Post-translation modification
e.g addition of CHO = glycoprotein; Lipid = lipoprotein
What are proteases/peptidases
Enzymes which hydrolyse peptide bonds and reduce proteins/peptides to amino acids
What do Endopeptidases do?
Breaks peptide bonds in central region
What do Exopeptidases do?
Break peptide bonds at terminal ends of protein (amino peptidases (amino end) & carboxypeptides (carboxyl end))
Describe the secondary active transport system
Na pumped into blood, Na gradient made and amino acid moves in with Na into cell by sodium amino acid transporter (SAAT1)
Amino acid moved into blood through its own transporter
Water moved into blood through tight junction
Golgi apparatus processed the amino acids in the cell
Dipeptides and tripeptides transport
Dipeptides and tripeptides can be absorbed through PepT1 transporter that moves in peptide with hydrogen molecule
Hydrogen then pumped out with sodium hydrogen exchanger (NHE3), Na moved into cell from lumen
Na moved out of cell through sodium potassium pump
Peptide moves into blood through an unknown transporter
All ingested fat is in the form of
Triaglycerol
Fat digestion in small intestine is by
Pancreatic lipase
Is lipase a water-soluble enzyme?
Yes
Emulsification:
Dividing large lipid droplets into smaller droplets (1mm diameter) - increased surface area & accessibility to lipase action
What does emulsification require?
- Mechanical disruption of large lipid droplets into small droplets
- Emulsifying agent - prevents small droplets reforming into large droplets
Bile secretes:
bile salts + phospholipids
Absorption of lipase digestion products is enhanced by formation of
Micelles
Micelles are
similar to emulsion droplets but much smaller (4-7µm diameter)
Micelles formed from:
bile salt + monoglycerides + fatty acids + phospholipids
Polar portions of molecules form at
micelle surface
Non-polar portions form
micelle core
Micelle breakdown process
- Release of small amounts of free fatty acids & monoglycerides into solution
- There diffuse across plasma membrane to be absorbed
- Micelles are not absorbed
After entering epithelial cells fatty acids & monoglycerides enter
smooth endoplasmic reticulum (here they are reformed into triacylglycerols by enzymes located within the sER)
Triacylglycerol droplets coated within amphiphatic protein =
Emulsification
Triacylglycerol droplets transported through cell in
Vesicles formed from sER membrane and then is processed by Golgi & exocytosed into ECF as chylomicrons
Extracellular fat droplets =
Chylomicrons (also contain phospholipids, cholesterol & fat-soluble vitamins)
Chylomicrons pass into lacteals between
endothelial cells
Fat-soluble vitamins:
A,D,E,K
Water-soluble vitamins:
B,C & folic acid
Vitamin B12
large charged molecule
Vitamin B12 binds to
intrinsic factor in stomach to form complex (which is absorbed via specific transport mechanism in distal ileum)
B12 deficiency =
Pernicious anaemia (failure of RBC maturation)
Iron enters duodenum cells via
divalent metal transporter (DMT1)
Iron ions incorporated into
ferritin (protein-iron complex –> intracellular iron store)
Unbound iron moved into blood & binds with
transferrin (protein)
Hyperaemia leads to
increase ferritin, more bound in cell and wont be in blood
Anaemia leads to
decrease ferritin, more iron released to blood
