Digestion Flashcards
Digestion in mammals of:
Carbohydrates by amylases and membrane-bound disaccharidases
The digestion of carbohydrates takes place in the mouth and the small intestine
Amylase is a carbohydrase that hydrolyses (breaks down) starch into maltose
Maltose is then hydrolysed into glucose by the enzyme maltase
Amylase is made in the salivary glands, the pancreas and the small intestine
Maltase is a disaccharidase which is found in cell-surface membranes of the epithelial cells lining the small intestine
There are also other disaccharidases in the cell-surface membrane of the epithelial cells in the small intestine:
sucrase and lactase, that hydrolyse sucrose and lactose respectively
This allows the absorption of monosaccharides into epithelial cells of the small intestine which pass them into the blood stream
Digestion in mammals of:
Lipids by lipase, including the action of bile salts
Emulsification:
There are several stages of breakdown that lipids go through prior to digestion
In the stomach, solid lipids are turned into a fatty liquid consisting of fat droplets - but this is not digestion
When the fatty liquid arrives in the small intestine, bile (containing bile salts) which has been made in the liver and stored in the gallbladder is secreted
The bile salts bind to the fatty liquid and breaks the fatty droplets into smaller ones via emulsification
Emulsification helps to increase the surface area of the fatty droplets for action of digestive enzymes
Digestion of Lipids:
The digestion of lipids takes place solely in the lumen of the small intestine
Lipase enzymes break down lipids (fats) to glycerol and fatty acids
Lipase enzymes are produced in the pancreas and secreted into the small intestine by the pancreas
Digestion in mammals of:
Proteins by endopeptidases, exopeptidases and membrane-bound dipeptidases
Protein digestion begins in the lumen of the stomach by protease enzymes
A protease enzyme called an endopeptidase hydrolyses peptide bonds within proteins, creating smaller sized protein “chunks”
This enzyme is secreted along with hydrochloric acid, meaning the pH in the stomach is low and therefore acidic
The partially digested food moves from the stomach into the small intestine
Fluid secreted by the pancreas travels to the small intestine and helps to neutralize the acidic mixture and increase the pH
This pancreatic juice contains endopeptidases and exopeptidases
Endopeptidases hydrolyse peptide bonds within polypeptide chains to produce dipeptides
Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains to produce dipeptides
Lastly, there are dipeptidase enzymes found within the cell surface membrane of the epithelial cells in the small intestine
These enzymes hydrolyse dipeptides into amino acids which are released into the cytoplasm of the cell
Mechanisms for the absorption of the products of digestion by cells lining the ileum of mammals, to include:
Co-transport mechanisms for the absorption of amino acids and of monosaccharides
Specific amino acid co-transport proteins (carrier molecules) are found within the cell-surface membrane of the epithelial cells in the ileum
They transport amino acids only when there are sodium ions present
For every sodium ion that is transported into the cell, an amino acid is transported in
This occurs via facilitated diffusion, which requires the movement of molecules down their concentration gradient (from high concentration to low concentration)
Amino acids diffuse across the epithelial cell and then pass into the capillaries via facilitated diffusion
The concentration gradient of sodium ions from the lumen of the ileum into the epithelial cell is maintained by the active transport of sodium ions out of the cell and into the blood via a sodium-potassium pump at the other end of the cell
Mechanisms for the absorption of the products of digestion by cells lining the ileum of mammals, to include:
The role of micelles in the absorption of lipids
The products of lipid digestion are fatty acids, monoglycerides and glycerol
Absorption of these products is significantly different from the absorption of carbohydrates and proteins
The monoglycerides and fatty acids associate with phospholipids and bile salts to form micelles (very small droplets)
Monoglycerides and fatty acids are not very soluble so the micelles aid the transport of these molecules to the surface of the epithelial cells
The micelles break down and add to a pool of fatty acids and monoglycerides that are dissolved in the small intestine solution surrounding the epithelial cells
These freely dissolved molecules enter the epithelial cell by diffusion
They are non-polar molecules so they can diffuse through the phospholipid bilayer of the cell membrane
The short fatty acid chains within the epithelial cells can move directly into the blood via diffusion but the longer chains are unable to do so
The longer fatty acid chains recombine with monoglycerides and glycerol to form triglycerides in the endoplasmic reticulum
The triglycerides are packaged into lipoproteins called chylomicrons
Chylomicron structure is a spherical ‘case’ made of phospholipids and lipoproteins that hold the non-polar triglycerides inside, hidden from the aqueous environment of the cytoplasm and blood
Cholesterol and phospholipids are also found within chylomicrons
These droplets are transported to a lacteal (a lymph vessel within the villus) via exocytosis
Eventually, chylomicrons enter the bloodstream
