Exchange: Digestion And Absorption Flashcards
(13 cards)
How is Amylase used to break down starch?
- Amylase is a digestive enzyme that catalyses the breakdown of starch.
- Starch is a mixture of two polysaccharides, each made from long chains of alpha-glucose molecules
- Amylase works by catalysing hydrolysis reactions that break the glycosidic bonds in starch to produce maltose (a disaccharide).
- Amylase is produced by the salivary glands, which release amylase into the mouth, and also by the pancreas, which releases amylase into the small intestine
How are membrane-bound disaccharides used to digest starch?
Membrane-bound disaccharidases are enzymes that are attached to the cell membranes of epithelial cells lining the ileum (the final part of the small intestine). They help to break down disaccharides into monosaccharides.
Again, this involves the hydrolysis of glycosidic bonds. The disaccharides maltose and lactose are broken down in a similar way to sucrose: via hydrolysis of the glycosidic bonds.
How does the digestion of lipids occur?
Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids. This involves the hydrolysis of the ester bonds in lipids. Lipases are mainly made in the pancreas — they’re then secreted into the small intestine where they act.
How do Bile salts help to digest lipids?
- Bile salts are produced by the liver and emulsify lipids — this means they cause the lipids to form small droplets.
- Although bile salts are not enzymes they are really important in the process of lipid digestion. Several small lipid droplets have a bigger surface area than a single large droplet (for the same volume of lipid). So the formation of small droplets greatly increases the surface area of lipid that’s available for lipases to work on.
What happens after the lipid has been broken down by lipase?
Once the lipid has been broken down by lipase, the monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles. Micelles help the products of lipid digestion to be absorbed
How are proteins broken down?
Proteins are broken down by a combination of different peptidases. These are enzymes that catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between amine acids. You need to know about endopeptidases and exopeptidases (including dipeptidases).
What is an endopeptidase?
Endopeptidases act to hydrolyse peptide bonds within a protein:
• Trypsin and chymotrypsin are both endopeptidases. They’re synthesised in the pancreas and secreted into the small intestine.
• Pepsin is another endopeptidase. It’s released into the stomach by cells in the stomach lining. Pepsin only works in acidic conditions — these are provided by hydrochloric acid in the stomach.
What is an exopeptidase?
Exopeptidases act to hydrolyse peptide bonds at the ends of protein molecules. They remove single amino acids from proteins
What are dipeptidases?
Dipeptidases are exopeptidases that work specifically on dipeptides. They act to separate the two amino acids that make up a dipeptide by hydrolysing the the cell-surface membrane of epithelial cells in the small intestine.
peptide bond between them. Dipeptidases are often located in the cell-surface membrane of epithelial cells in the small intestine
Where are the products of digestion absorbed across?
The products of digestion are absorbed across the ileum epithelium into the bloodstream.
How are monosaccharides absorbed?
Glucose is absorbed by active transport with sodium ions via a co-transporter protein. Galactose is absorbed in the same way using the same co-transporter protein. Fructose is absorbed via facilitated diffusion through a different transporter protein.
How are monoglycerides and fatty acids absorbed?
Micelles help to move monoglycerides and fatty acids towards the epithelium. Because micelles constantly break up and reform they can ‘release’ monoglycerides and fatty acids, allowing them to be absorbed - whole micelles are not taken up across the epithelium.
Monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the epithelial cell membrane.
How are amino acids absorbed?
Amino acids are absorbed in a similar way to glucose and galactose. Sodium ions are actively transported out of the epithelial cells into the ileum itself. They then diffuse back into the cells through sodium-dependent transporter proteins in the epithelial cell membranes, carrying the amino acids with them.
