Digestion in humans (revised)

Question 1

Types of digestion

Answer 1

1. Physical

2. Chemical

Question 2

Physical digestion

Answer 2

• mechanical breakup of food into smaller particles
• chewing
• peristalsis
• occurs in mouth and stomach

Question 3

Chemical digestion

Answer 3

• Involves break down of large complex molecules into small soluble molecules
• hydrolytic rxns catalysed by digestive enzymes

Question 4

Hydrolysis`

Answer 4

• rxn where water molecules needed to break up complex molecule into smaller one
• e.g. maltose + water –> glucose + glucose
• catalysed by maltase enzyme

Question 5

Condensation rxn

Answer 5

• chemical rxn
• 2 simple molecules are joined tgt
• form larger molecule
• removal of 1 molecule of water
• glucose + glucose –> maltose + water

Question 6

Physical digestion (Mouth)

Answer 6

1. Chewing action of teeth
   - breaks up larger pieces of food into smaller ones
   - increase SA:V ratio for amylase to work on
2. Tongue
   - roles food into boli
   - salivary glands secrete saliva, mixed with food by tongue
3. Saliva contain mucin
   - softens food

Question 7

Chemical digestion (Mouth)

Answer 7

• salivary amylase

- digests starch to maltose

Question 8

Physical digestion (Oesophagus)

Answer 8

• boli swallowed
• passed down into oesophagus via pharynx
• peristalsis in walls & gravity push bolus into stomach

Question 9

Physical digestion (Stomach)

Answer 9

• bolus enters stomach, stimulates release of gastric juice by gastric glands
• peristalsis in walls churns, breaks up & mixes food with gastric juice

Question 10

Composition of gastric juice

Answer 10

• Dilute soln of HCl, 2 enzymes, mucus
• denatures salivary amylase
• converts pepsinogen (inactive) into pepsin, prorennin into rennin
• acidic medium for action of pepsin and rennin (opt pH=2)
• kills harmful microorganisms in food

Question 11

Pepsin

Answer 11

• digests proteins into polypeptides

Question 12

Rennin

Answer 12

• converts soluble caseinogen

- into insoluble casein

Question 13

How long food stay in stomach

Answer 13

• 3 to 4 hrs
• partially digested food known as chyme
• passes into duodenum as plyoric sphincter relaxes and opens

Question 14

Small intestine

Answer 14

• chyme enters duodenum, stimulates release of alkaline fluids:
• intestinal juice (by intestinal glands)
• pancreatic juice (by pancreas)
• bile (by gall bladder)
• all 3 neutralise acidic chyme to pH 8
• alkaline medium needed for action of intestinal and pancreatic enzymes

Question 15

Physical digestion in small intestine

Answer 15

• gall bladder releases bile (produced in liver )
• bile passes thru the bile duct into duodenum
• bile salts speed up rate of digestion
• emulsification of fat droplets suspended in water

Question 16

Pancreatic juice composition

Answer 16

• pancreatic amylase
• pancreatic lipase
• trypsinogen (inactive trypsin)

Question 17

Intestinal juice composition

Answer 17

• peptidases (short polypeptide chains to amino acids)
• maltase, sucrase, lactase
• intestinal lipase

Question 18

How carbohydrates are digested (small intestine)

Answer 18

• starch –> maltose (pancreatic amylase)
• maltose –> glucose (maltase)
• lactose –> glucose + galactose (lactase)
• sucrose –> glucose + fructose (sucrase)
• simple sugars/monosaccharides can be absorbed by bloodstream

Question 19

How proteins are digested (small intestine)

Answer 19

• inactive trypsinogen –> active trypsin (enterokinase) [activation]
• proteins –> polypeptides (trypsin)
• polypeptides –> amino acids (peptidases)
• amino acids can be absorbed

Question 20

How fats are digested

Answer 20

• bile salts emulsify fats
• reduce attractive forces btwn fat molecules
• break down fat into small fat droplets suspended in water to form emulsion
• increase SA:V of fats, speed up digestion by lipase
• fats –> fatty acids + glycerol (pancreatic, intestinal lipase)