Digestion + Absorption

Question 1

What happens during digestion?

Answer 1

• large biological molecules are hydrolysed into smaller molecules that can be absorbed across cell membranes + into bloodstream

Question 2

What does the human digestive system include?

Answer 2

• mouth: breaks food into smaller pieces + inc SA:V ratio
• oesophagus: tube connecting mouth to stomach
• stomach: tissue churns food, mixing it w enzymes + acid, digests proteins
• small intestine: ileum lined w villi to inc SA for absorption of food molecules into bloodstream
• large intestine: absorbs water, vitamins + minerals
• rectum: stores faeces until is removed via anus during egestion

Question 3

Describe the digestion of carbohydrates in mammals.

Answer 3

• takes place in mouth + small intestine
• requires amylase + membrane-bound disaccharidases
• amylase (produced by salivary glands + pancreas): hydrolyses starch into maltose by hydrolysing glycosidic bonds
• membrane-bound disaccharidases (maltase, sucrase + lactase): hydrolyse maltose, sucrose + lactose into monosaccharides so can be absorbed into epithelial cells of small intestine + into bloodstream

Question 4

Describe the digestion of proteins in mammals.

Answer 4

• starts in lumen of stomach, continues in duodenum + is fully digested in ileum
• requires 3 protease enzymes
• endopeptidases: hydrolyse peptide bonds between AAs in middle of polypeptide chains to form dipeptides
• exopeptidases: hydrolyse peptide bonds between AAs at ends of polypeptide chains to form dipeptides
• membrane-bound dipeptidases: hydrolyse peptide bonds between only 2 AAs

Question 5

Describe the digestion of lipids in mammals.

Answer 5

• takes place in lumen of small intestine
• physical breakdown: bile salts (produced in liver) coat + emulsify lipids to form tiny droplets called micelles which inc SA so lipase can hydrolyse lipids faster
• chemical digestion: lipase (produced in pancreas) hydrolyse ester bonds in lipids to form glycerol + fatty acids (+ some monoglycerides)

Question 6

Describe a method to investigate the effect of pH on enzyme reaction rates.

Answer 6

• add a drop of iodine solution to each depression in spotting tile
• add 2cm3 of amylase to a test tube using a syringe + then add 1cm3 of buffer solution
• add 2cm3 of starch solution + start stopwatch whilst mixing w a pipette
• after 10s, place 1 drop of mixture to first drop of iodine (will turn blue/black to show starch is present)
• repeat at 10s time intervals until iodine solution remains orange-brown
• repeat experiment at diff pH values

Question 7

Describe the absorption of AAs + monosaccharides in mammals.

Answer 7

• co-transport proteins, found in cell membrane of epithelial cells in ileum, transport AAs/glucose into cell along with Na+
• AAs/glucose diffuse across epithelial cells + then pass into capillaries via facilitated diffusion
• conc gradient of Na+ is maintained by active transport of Na+ out of epithelial cell into blood via a sodium-potassium pump

Question 8

Describe the absorption of lipids in mammals.

Answer 8

• lipids are digested into monoglycerides + fatty acids by lipase + bile salts which form micelles (water soluble vesicles)
• they then simply diffuse across cell surface membrane + enter ileum epithelial cells
• once in cell, they’re modified back into triglycerides inside of endoplasmic reticulum + golgi body
• sometimes a protein combines w lipid to form a chylomicron
• vesicles containing triglycerides are released, move towards cell membrane + are released from cell by exocytosis which enters bloodstream via lacteal

Question 9

Describe a method to investigate the absorption of products of digestion.

Answer 9

• fill section of visking tube w mixture of starch + amylase solutions
• suspend tube in a beaker of water for 20 mins
• take samples from liquid outside visking tube at regular intervals + test for presence of starch + glucose
• over time, conc of glucose in liquid outside visking tube inc as more starch is digested by amylase