Nutrient Digestion Flashcards
What are monosaccharides? Name them.
Hexose sugars (6 carbons) which are broken down from complex carbs and absorbed by the small intestine
Glucose, fructose and galactose
What are disaccharides?
2 monosacs linked via a glycosidic bond
Where are disaccharides broken down?
Broken down into their constituent monosacs by the brush border enzymes in the small intestine
Name the disaccharides, their respective monomers and the enzymes which break them down
Lactose = glucose + galactose (lactase) Sucrose = glucose + fructose (sucrase) Maltose = glucose + glucose (maltase)
List the polysaccharides.
Starch
Cellulose
Glycogen
What is starch?
Plant storage form of glucose
What are the types of starch and what is the differences between them?
Alpha-amylose - glucose linked in straight chains
Amylopectin - glucose chains that are highly branched
What are glucose monomers linked by? What are branches linked by?
Linked by alpha-1,4 glycosidic bonds
Branches are alpha 1,6
What enzyme breaks down starch/polysaccharides into disaccharides?
Alpha-Amylase
2 types - one in saliva and one from the pancreas
What is cellulose?
A plant wall constituent its made of straight glucose chains linked by beta-1,4 glycosidic bonds
How is cellulose digested?
Need cellulase to digest but we do not have this, therefore it is digested by gut bacteria
Explain the structure and use of glycogen.
Glucose linked by alpha-1,4,-glycosidic bonds
Storage of glucose in the liver
What are intestinal villi?
Finger-like folds of columnar epithelium which increase the surface area in the gut
What do villi have on them that increase surface area even more?
Microvilli
How is glucose and galactose absorbed?
Absorbed via SGLT1 (a sodium glucose transport protein) along with Sodium from the lumen
Na+ leaves the cell as K+ enters
Glucose then leaves via GLUT-2 protein into the blood
How is fructose absorbed?
It doesn’t pain with Na+
Enters from lumen with GLUT-5 and leaves using GLUT-2
Explain the membranes of villi.
The epithelial cells that make up villi have two cell membranes:
- Apical at the top
- Basolateral on the sides/base
They are connected by a tight junction which also connects with adjacent villi to form a tight junctional complex
Explain molecule transport across intestinal villi.
Transcellular:
- in the top of the cell and out the bottom
- Since it goes through 2 different membranes it requires at least two transport proteins
Paracellular:
- Transport out the lumen between the villi
What are proteins and peptides?
Polymers of amino acids bound by peptide bonds
Small ones (i.e. 3-10 monomers) are called peptides rather than proteins
How are glycoproteins and lipoproteins etc produced?
Proteins undergo post-translational modification
What do we call enzymes that break down proteins and peptides?
Proteases for proteins
Peptidases for peptides
Exopeptidases are split into aminopeptidases and carboxypeptidases based on whether they act on the amino terminal end or the carboxy terminal end
Difference between an endo and an exo peptidase?
Endopeptidases break down internal peptide bonds creating two smaller peptides
Exopeptidases break the terminal peptide bond reducing the peptide by one monomer
How are amino acids transported out the gut lumen into the blood?
Exactly the same as glucose just with different transporters:
SAAT1 instead of SGLT1 etc
Can polypeptides be transported out the gut without being broken down to amino acids? If so how?
Some di and tripeptides can.
- The surface of the gut around the villi is kept in an acidic microclimate to produce a H+ conc. gradient into the epithelial cell
- This gradient drives the movements of peptides into the cell
- H+ is then pumped back into the gut lumen via a transporter swapping it with Na+
- The sodium gradient provides the driving force for this which is in turn provided by a Na+K+ATPase transporter between the epithelium and blood
- Another transporter pumps the peptides into the blood from the epithelial cell.
What is the majority of ingested fat?
Triacylglycerides
What is TAg made up of?
A glycerol and 3 strearic acids
Where is all fat digested?
In the small intestine by pancreatic lipase - a water soluble enzyme
-Turns TAG into monoglyceride and 2 FFAs
TAG is insoluble in water - therefore forms very large lipid balls in solution making it hard to digest - how does the body get around this?
Emulsification - divide large lipid droplets into smaller droplets
1 - Smooth muscle in the muscularis extrena cause mechanical disruption and mixes luminal contents stopping formation of big droplets
2 - Bile is the main emulsifying agent
Where is bile coming from and what does bile have in it?
Comes from liver, stored in gallbladder
Contains bile salts and phospholipids
How do bile salts emulsify lipids?
Bile are amphipathic - having both hydrophobic and philic parts
This means he non-polar part attracts TAG and the polar part repels and stops the TAG reforming into large droplets
What enhances absorption of emulsified TAG further?
Micelles
Made up of - bile salt, monoglyceride, FFA and phospholipids
Descibe the polarity of micelles
The polar portion of molecules are at the micelle surface and the non-polar portions from the micelle core
Does the micelle enter the cell?
No - it is extracellular
It is NOT absorbed
It serves to aid absorption and protect products being further digested by lipase
What happens when micelles break down?
Releases small amounts of free fatty acid and monoglyceride into solution which diffuses across membrane and into cell
There is a dynamic equilibrium between FFAs and monoglycerides in solution and in micelles
Why do micelles breakdown?
Acid microclimate near the microvilli destabilises the micelle and releases fatty acids and monoglyceride
There is a dynamic equilibrium between FFAs and monoglycerides in solution and in micelles - why?
Allows for retention of most of fat digestion products in solution while constantly replenishing micelle supply of free molecules for absorption
What happens to the monoglycerides and FFAs once reabsorbed by the epithelial cells?
They enter the smooth ER and are reformed into TAG by enzymes
TAG then coated with an amphiphatic protein for emulsification and then transported through the cell by vesicles formed by the sER and then exocytosed into the ECF at the serosal membrane
What happens to TAG once it is in the ECF?
Extracellular fat droplets become chlyomicrons - which also contain phospholipids, cholesterol and fat soluble vitamins
What happens to the chylomicrons?
These go into the lacteals pass into lacteals between endothelial cells as they cannot pass between the capiliary basement membrane
Go to the liver
How are fat soluble vitamins absorbed? Name them.
Same as fat
Vits. ADEK
What vitamins are water soulble and how are they absorbed?
B group
Vit. C
Folic acid
Absorbed via passive diffusion or mainly carrier mediated transport
Describe Vit. B12 and how it is absorbed?
It is a large charged molcule
Binds to intrinsic factor in stomach to form a complex
This complex is then absorbed via specific transport mechanisms in the distal ileum
Why is it odd that the Vit. B12 complex is absorbed in the distal ileum?
as most molecules absorbed at the start of the small intestine
What does Vit. B12 deficiency cause?
Pernicious anaemia - failure of RBC maturation
How is iron absorbed?
Needs to be reduced from Fe3+ to Fe2+
Is then transported across the brush borded by DMT1 into duodenal enterocytes
Once in cell it becomes ferritin due to dangers with reacting to O2 free radicals
What is ferritin ?
A protein lattice structure with 12 Fe2+ molecules inside
Once its locked in it can’t get back out at all, once its there its trapped only the iron not put in is used. Anything not attached to ferritin is transported to blood
What happens to iron not bound to ferritin?
Becomes transferrin which is then used around the body
What regulates ferritin expression?
depending on body’s iron status/needs
What would ferritin be like if there was hyperaemia of iron?
High levels to bind more iron stopping it being used in body
What happens to ferritin if it isn’t used?
Every 5 days the entire small intestine surfaces replenishes itself – so the ferritin just dies off and is sent out in the faeces.
