Digestion and absorption of the products of digestion Flashcards
How does the digestion of starch work?
1) Starch enters the buccal cavity.
2) Salivary amylase breaks down the starch into maltose (a disaccharide) but some still remained as starch.
The salvia contains minerals which maintain the pH at 6.8-neutral because this is the optimum pH for salivary amylase to work.
3) Food is swallowed and enters the stomach, the pH here would be acidic 2/3. This acidic condition would denature the amylase so the active site changes shape and it’s no longer complementary to the substance so the starch is not broken down, therefore, there’s no digestion in the stomach.
4) Then it enters the small intestine where it mixes with the pancreatic juice produced by the pancreas.
5) The pancreatic juice contains pancreatic amylase which will break down any remaining starch to maltose. Alkaline salts are produced by the pancreas and the intestinal wall to maintain the pH at a neutral condition so the amylase can function.
6) Muscles in the intestine wall push the food along the ileum. Ileum has villi which are made up of small intestine epithelial cells which produce maltase which breaks down the maltose into 2 alpha glucose. Maltase is a brush border enzyme which is locked in place on the villi and it’s a disaccharidase.
What is the function of bile salts?
Emulsifies fat so more SA for lipase to break down the lipids.
Also neutralises the acidity of food that comes from the stomach.
Why are some enzymes e.g. maltase are membrane-bounded to the microvilli?
- They are bounded so they don’t end up in the rectum so it can be used over and over again.
- Also when it’s membrane-bounded it can increase the rate of diffusion because there’s a greater concentration difference - e.g. X wants to be absorbed but it has to be broken down into forms of Y first, and by going near the membrane, it can easily and rapidly break down X into Y and it only has a short distance to travel therefore a quicker diffusion rate.
Which membrane-bound disaccharidase breaks down sucrose?
Sucrase - it hydrolyses the single glycosidic bond in the sucrose and produces the 2 monosaccharides of glucose and fructose.
Which membrane-bound disaccharidase breaks down lactose?
Lactase - it hydrolyses the single glycosidic bond in the lactose and produces the 2 monosaccharides of glucose and galactose.
How does the digestion of lipids work?
1) Triglycerides are converted into monoglycerides and fatty acids by enzyme lipase where it hydrolyses the ester bond between the glycerol and the fatty acids.
2) Bile salts which are produced by the liver and stored in the gall bladder bind onto the lipids and emulsifies them into tiny droplets.
3) Once they are broken down, the monoglyceride and the fatty acids bind with the bile salt to form micelles.
4) Micelles help the monoglyceride and fatty acids to move towards the epithelium.
5) When they reach the epithelium, the micelles detach away from them.
6) Monoglycerides and fatty acids are soluble and non-polar so they just diffuse across the epithelial cell membrane.
7) When they pass the endoplasmic reticulum and then into the Golgi apparatus, they are converted back into triglycerides.
8) Then the triglycerides are combined with cholesterols and lipoproteins to make chylomicrons. (Chylomicrons are special particles adapted for the transport of lipids).
9) Then the chylomicrons are removed by exocytosis and into the lacteal.
10) Chylomicrons pass to lymphatic vessels and then the bloodstream. The triglycerides in the chylomicrons are hydrolysed by an enzyme in the epithelial cell of the blood capillaries and diffused into cells.
What enzyme breaks down proteins?
Peptidases.
What is an endopeptidase?
An enzyme which hydrolyses the peptide bonds between amino acids in the middle of a protein molecule.
What is an exopeptidase?
An enzyme which hydrolyses the peptide bonds on the terminal (each end) of the amino acid of the peptide molecule, maybe formed from endopeptidases.
This allows dipeptides and single amino acids to be formed.
What is dipeptidase?
An enzyme which hydrolyse the bond between 2 amino acids of a dipeptide. They are membrane bound so it’s part of the cell-surface membrane of the epithelial cells lining the ileum.
How is the ileum adapted for absorbing the products of digestion?
- The walls are folded with ‘finger-like projections’ called villi.
- Thin walls, lined with the epithelial cells and has a rich network of blood capillaries.
- The villi increase the SA of the ileum, therefore, it can increase the rate of absorption.
Where are the villi located?
Between the lumen of the intestine and the blood and other tissues inside the body.
How do villi increase the efficiency of absorption?
- Increases the SA for diffusion.
- Has very thin walls, therefore, reduced the diffusion distance.
- They contain muscles which maintain a diffusion concentration gradient. As a product is absorbed, the move can move and replace that product with a new material rich on the product of digestion.
- Well supplied with blood vessels so the blood can carry away the absorbed molecules and maintain a diffusion gradient.
- The epithelial cells lining the villi have microvilli which are lined on the cell-surface membrane and further increases the SA for absorption.
How are glucose, galactose, and fructose absorbed after it’s been digested?
Glucose and galactose - by active transport with sodium ion by a co-transporter protein.
Fructose - by facilitated diffusion.
