Digestion (complete)

Question 1

Describe what happens during digestion in the body?
- Physical vs Chemical digestion?

Answer 1

large biological molecules are hydrolysed to small molecules that can be absorbed across the cell membranes.
- Physical - chewing/mixing by muscle contractions around each organ.
- Chemical - hydrolysis reactions by enzymes and lipid digestion/absorption by bile salts.

Question 2

Name the three types of protease enzymes and where on the polypeptide chain they work.

Answer 2

• Endopeptidases - work on the peptide bonds in the middle of the polypeptide chain.
• Exopeptidases - work on the peptide bonds on the ends of the polypeptide chain.
• Dipeptidases - work on the peptide bond within a dipeptide (two amino acids)

Question 3

Describe the adaptations of the villi for digestion.

Answer 3

• folded membrane (micro-villi) - increase surface area to increase number of carrier proteins in a given space to increase rate of diffusion.
• rich blood supply - maintains a concentration gradient for efficient diffusion.
• muscle layer - contracts and relaxes to mix the digestion food and helps maintain a concentration gradient.

Question 4

Name the site of production, site of action and description of action for these chemicals involved in digestion:
- Amylases
- Membrane-bound disaccharides
- Lipase
- Bile salts
- Endopeptidases
- Exopeptidases
- Membrane-bound dipeptidases

Answer 4

• Amylases - produced in the salivary gland, pancreas and wall of ileum, act in the mouth/oesophagus and small intestine, and catalyse the hydrolysis starch into maltose.
• Membrane-bound disaccharides - produced in the cell membrane of ileum epithelial cells, act in the small intestine, and catalyse the hydrolysis of disaccharides into monosaccharides.
• Lipase - produced in the pancreas, act in the small intestine, and catalyse the hydrolysis of lipids into fatty acids and monoglycerides.
• Bile salts - produced in the liver, act in the small intestine, and help digest lipids by forming micelles which allow the monoglycerides and fatty acids to be absorbed by the ileum epithelial cells.
• Endopeptidases - produced in the pancreas, act in the small intestine and catalyse the hydrolysis of peptide bonds within a polypeptide chain.
• Exopeptidases - produced in the pancreas, act in the small intestine and catalyse the hydrolysis of peptide bonds on the ends of polypeptide chains.
• Membrane-bound dipeptidases - produced in the cell membranes of ileum epithelial cells, act in the stomach and small intestine and catalyse the hydrolysis of peptide bonds between dipeptides into amino acids.

Question 5

Describe the process of the absorption of triglycerides (lipids).

Answer 5

1- A large lipid droplet is emulsified by bile salts into smaller lipid droplets which are easier to be absorbed.
2 - A smaller lipid droplet is hydrolysed by lipase into a monoglyceride and two fatty acids, which are then associated with bile salts to form micelles.
3 - The micelles diffuse into the epithelial cells in the ileum, where they move to the smooth endoplasmic reticulum to be reformed into triglycerides.
4 - The triglycerides move to the golgi apparatus to be modified using proteins to form chylomicrons.
5 - The modified triglycerides undergo exocytosis to exit the epithelial cell and diffuse into the lacteal tract found in the centre of each villi. Here they are transported to the cells which require them.

Question 6

How do:
- Chylomicrons
- Micelles
: aid in lipid digestion/absorption?

Answer 6

• Chylomicrons - special particles adapted first lipid transport, made of phospholipids and lipoproteins that hold the triglyceride inside, to hide it from the aqueous environment of the cytoplasm and blood.
• Micelles - aid in the transport of fatty acids and monoglycerides to the surface of the epithelial cells