B5 DIGESTION AND ABSORPTION

What is the definition of digestion?

the process which large molecules are hydrolysed by enzymes into small molecules which can be absorbed and assilimilated

What is the definition of hydrolysis?

the splitting up of large molecules into smaller molecules by the addition of water

What is the definition of absorption?

the process by which small, soluble molecules produced by digestion are taken up from the small intestines and into the blood

What is the definition of assimilation?

the process by which substances absorbed from the small intestine after digestion are taken into the cells of the body and are built up into useful substances

What do the salivary glands release?

amylase

What does the pancreas release?

carbohydrase, lipase and protease

What does the stomach release?

protease

What’re the steps to hydrolyses starch?

• amylase is produced in the mouth and hydrolyses glycosidic bonds found in starch, to form maltose

• maltose is hydrolysed into glucose by maltase in the ileum

What forms maltose?

2 glucose molecules

What forms sucrose?

a glucose and a fructose molecule

What forms lactose?

a glucose and a galactose molecule

Where is amylase produced and what reaction is catalysed by it?

• produced in the salivary glands and in the pancreas

• starch → maltose

Where is maltase produced and what reaction is catalysed by it?

• produced in the epithelium cell membrane of the small intestines

• maltose → glucose

How are glucose and monosaccharides absorbed?

by co-transport, through the epithelial cells of the ileum and through the capillary endothelium. plasma transports them around the body to the cells

What enzymes are involved in digesting proteins?

• endopeptidase

• exopeptidase

• dipeptidase

What is the function of endopeptidase enzymes?

they act in the middle of the protein, to produce short polypeptides which increases the number of ends

What is the function of exopeptidase enzymes?

act on the end of the protein, to produce dipeptides

What is the function of dipeptidase enzymes?

acts between dipeptides to produce single amino acids

How’re amino acids absorbed?

by co-transport, through the epithelial cells of the ileum through the capillary endothelium

What’re bile salts?

they’re added to the lipid before the action of lipase to emulsify it

Where are bile salts made and stored?

• made in the liver

• stored in the gall bladder

What is involved in emulsification?

the breaking up of large globules of lipid into tiny droplets which increases the surface area of lipid available for lipase to act upon

Where is lipase produced and released?

• produced in the pancreas

• released in the small intestine

What happens after the lipid has been hydrolysed by lipase?

the monoglycerides and 2 fatty acids remain in association with bile salts, known as micelles, which help absorb the lipids into the blood

What are villi?

the ileum is folded into many finger like projections which have thin walls made from a single layer of specialised epithelial cells

What’re properties of the villi?

• increased surface area

• only one cell thick walls so short diffusion distance

• contain muscle which causes peristalsis, which maintains a concentration gradient

• blood capillaries maintain a concentration gradient

• large number of carrier and channel proteins

What occurs in co-transport?

1. Na+ ions in the epithelial cell cytoplasm are actively transported out of the cell and into the capillary against their concentration gradient using ATP

2. this creates a concentration gradient

3. Na+ diffuses into the epithelial cell from the ileum by facilitated diffusion through a carrier protein which carries glucose against the glucose concentration gradient

4. glucose passes into blood plasma by facilitated diffusion

What’re micelles?

• when monoglycerides and fatty acids remain associated with bile salts and phospholipids

• small droplets which transport monoglycerides and fatty acids to the surface of the epithelial cell membrane

• constantly break down and reform, meaning the monoglycerides and fatty acids can diffuse into the epithelial cells

• these are then recombined at the endoplasmic reticulum to form triglycerides

• these are modified by the golgi body to form chylomicrons which are packaged into vesicles

• these are released by exocytosis