B1. Digestion and Absorption Flashcards
Digestion basics
Why is digestion necessary?
What happens in digestion and what is the effect?
The large biological molecules (e.g. starch, proteins) in food are too big to cross cell membranes. This means they can’t be absorbed from the gut into the blood. During digestion, these large molecules are broken down into smaller molecules (e.g. glucose, amino acids), which can move across cell membranes. This means they can be easily absorbed from the gut into the blood, to be transported around the body for use by the body cells.
The digestion of carbohydrates in 3 steps
1) Amylase works by catalysing hydrolysis reactions that break the glycosidic bonds in starch to produce maltose (a disaccharide).
2) Maltase (membrane-bound disaccharidase) breaks down maltose (disaccharide) into 2 alpha glucose (monosaccharide) by hydrolysing glycosidic bonds.
3) The alpha glucoses (monosaccharides) can be transported across the epithelial cell membranes in the ileum via specific cotransporter proteins alongside sodium ions
The digestion of carbohydrates - Extra questions
What is amylase?
What is starch?
Where is amylase produced?
What are membrane-bound disaccharidases?
- 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 is produced by the salivary glands, which release amylase into the mouth, and also by the pancreas, which releases amylase into the small intestine
- 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.
Figure 2: The breakdown of sucrose.
The digestion of lipids in 3 steps
1) Bile salts are produced by the liver and emulsify lipids meaning they cause the lipids to form small droplets to increase the surface area for lipases to work on.
2) Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids involving the hydrolysis of the ester bonds.
3) The monoglycerides and fatty acids stick with the bile salts to form micelles which help the products to be absorbed
Figure 5: The emulsification of lipids by bile salts.
Figure 4: The hydrolysis of ester bonds in a triglyceride.
The digestion of proteins in 3 steps
1) Endopeptidases hydrolyse peptide bonds within a polypeptide
2) Exopeptidases hydrolyse peptide bonds at the ends of polypeptides removing single amino acids
3) Dipeptidases separate the two amino acids that make up a dipeptide by hydrolysing the peptide bond between them
The digestion of proteins - Where are dipeptidases found?
Dipeptidases are often located in the cell-surface membrane of epithelial cells in the small intestine.
Absorption of the products of digestion
The products of digestion are…
The products of digestion are absorbed across the ileum epithelium into the bloodstream
Absorption of the products of digestion - Monosaccharides (3)
- 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.
Absorption of the products of digestion - Monoglycerides and fatty acids (3)
- Micelles help to move monoglycerides and fatty acids towards the epithelium.
- Whole micelles are not taken up across the epithelium - Because micelles constantly break up and reform they can ‘release’ monoglycerides and fatty acids, allowing them to be absorbed.
- Monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the epithelial cell membrane.
Absorption of the products of digestion - Amino acids (2)
Amino acids are absorbed in a similar way to glucose and galactose
1) Sodium ions are actively transported out of the epithelial cells into the ileum itself.
2) They then diffuse back into the cells through sodium-dependent transporter proteins in the epithelial cell membranes, carrying the amino acids with them.
