3.3.3 digestion and absorption

Question 1

digestive system

Answer 1

made up of a long muscular tube and it’s associated glands/organs

tube is known as the alimentary canal and is 6m long from mouth to anus

glands produce hydrolytic enzymes that break down large molecules into small, soluable molecules ready for absorbtion

Question 2

oesophagus

Answer 2

carries food from the mouth to the stomach by peristalis (involunatry contraction)

adapted for transported rather than for digestion or absorption

thick muscular wall

Question 3

what is the stomach?

Answer 3

average capacity of 4 litres

muscular sac with an inner layer that produces enzymes

stores and digests food especially proteins

has glands to produce enzymes which digest proteins (protease) and mucus

mucus= prevents stomach from being digested by it’s own enzymes (protective lining)

Question 4

describe the small intestine = ileum, final 3m of small intestine

Answer 4

long muscular tube and multicellular

food is further digested in the small intestine by enzymes, produced by it’s walls and by glands that pour secretions into it

inner walls are folded into villi (microvilli = epithelial cells of each villus) which gives a larger SA

adaptation = absorbing the products of digestion into the bloodstream

Question 5

what is the large intestine?

Answer 5

absorbs water + nothing useful left over is passed into the large intestine

most of the water that is reabsorbed comes from the secretions of the digestion glands

undigested food within the large intestine becomes drier and thicker in consistency and forms faeces

Question 6

rectum

Answer 6

faeces are stored here before periodically being removed via the anus = egestion

(not ecretion = waste products in metabolic reactions)

Question 7

examiner hints

Answer 7

contents of intestines are not inside the body in tubes (intestines)

molecules and ions only truly enter the body when they cross the cells and cell surface membrane of the epithlial lining of the intestines

Question 8

salivary glands

Answer 8

produces saliva amylase which hydrolyses starch into maltose

pass their secretions via a duct in the mouth + situates near the mouth

Question 9

what is the pancreas?

Answer 9

large gland situated below the stomach

produces a secretion called pancreatic juice

secretion contains:
protease (digests proteins)
lipase (digests lipids)
pancreatic amylase (digests starch)

bile = liver, no enzymes, alkaline, emulsifies fats

Question 10

describe the process of physical breakdown of food

Answer 10

large pieces of food are broken down into small pieces of food by the teeth = mastication

enables us to ingest food and provides a large SA for chemical digestion

food is churned up by stomach wall muscles

Question 11

describe the process of chemical breakdown of food

Answer 11

breaks down large, insoluable molecules into smaller soluable ones

carried out by enzymes = which function by hydrolysis (splitting of molecules by adding water)

usually one enzyme hydrolyses a large molecule and a further enzyme then hydrolyses this section into monomers

Question 12

what are the two stages of digestion?

Answer 12

1. physical breakdown
2. chemical digestion

Question 13

chemical breakdown products

Answer 13

carbohydrases = carbohydrates > monosaccharides
lipases = lipids > monoglycerides (glycerol + fatty acids)
proteases = proteins > amino acids

Question 14

assimilation

Answer 14

small molecules such as amino acids, fatty acids, glycerol and monosaccarides are absorbed into the blood through the small intestine

carried to different parts of the body and may be built up again into larger molecules to be used for body tissues

Question 15

starch digestion process

Answer 15

starch > maltose > glucose + glucose

amylase is produced in mouth + pancreas

hydrolyses alternate glycosidic bonds of starch to produce the disaccharide maltose

maltose is then hydrolysed into the monosaccharide a- glucose by maltase

maltose = produced in lining of small intestine

Question 16

process

Answer 16

1. mastication (chewing) occurs - breaking down food into smaller pieces giving a large SA
2. saliva enters the mouth from saliva glands and is mixed with food during chewing, saliva contains amalyse which starts hydrolysing the food into maltose
3. saliva also contains mineral salts which allows the pH to maintain at neutral (optimum pH)
4. food is swallowed and enters the stomach. conditions are acidic, this acid denatures the amalyse and prevents further digestion of starch
5. food is passed through small intestine where it mixes with a secretion from the pancreas called pancreatic juice ( containing pancreatic amylase) which continues to hydrolyse starch
6. alkaline salts are produced by the pancreas and intestinal wall to maintain pH at neutral
7. muscles in the intestine wall push food along small intestine

Question 17

disaccharide digestion = sucrose

Answer 17

often contained within cells which must be physically broken down by the teeth to release it

sucrose passes through the stomach and into the small intestine, whose epithelia lining produces the enzyme sucrase

sucrase hydrolyses the single glycosidic bond in the sucrose molecule to produce glucose + fructose

Question 18

describe disaccharides being digested into lactose

Answer 18

sugar found in milk and milk products eg cheese

digested in the small intestine whose epithelial lining produces the enzyme lactase

lactase hydrolses the single glycosidic bond that links the glucose and galactose together

Question 19

protein digestion

Answer 19

digested by a number of proteases called peptidases

1. endopeptidases
2. exopeptidases
3. dipeptidases

Question 20

endopeptidases

Answer 20

hydrolyse peptide bonds within large polypeptides, breaking them down into smaller segments

Question 21

exopeptidases

Answer 21

hydrolyse peptide bonds near the end of the smaller segments,
releasing dipeptides
(2 amino acids joined by a peptide bond)

Question 22

dipeptidases

Answer 22

hydrolyse peptide bonds between the amino acids of the dipeptides, releasing individual amino acids

found within the membrane of cells lining the villi
as amino acids are released they are absorbed straight into the blood

Question 23

lipid digestion

Answer 23

1. lipids are split into tiny droplets called micelles by bile salts (produced in liver)
   emulsification and increases SA for lipid action
2. two of the ester bonds of individual triglycerides are hydrolysed by lipases to produce monosaccharides and 2 fatty acids

Question 24

active transport in small intestine

Answer 24

happens via active transport , cotransport and facilitated diffusion - glucose and amino acids

1. Na+ ions actively transported into blood via carrier protein (sodium-potassium pump) requires ATP
2. Na+ ions concentration in intestinal cell is reduced
3. Na+ ions concentration in intestinal lumen (higher conc) into cell (lower conc) carrying glucose molecules via co-transport
   protein (type of carrier protein)
4. glucose concentration increases in the cell
5. glucose diffuses into the blood via carrier protein (facilitated diffusion)

Question 25

what enzymes are present in cell membranes?

Answer 25

maltase, lactase and sucrase are situated in the cell
membrane of the epithelial cells which digest hydrolyse
disaccharides into their monosaccharides

Question 26

describe the absorption of triglycerides

Answer 26

after digestion, monoglycerides and fatty acids remain in association with bile salts. HELPS MAKE THEM SOLUABLE AS THEY ARE HYDROPHOBIC

non polar = diffuse through the phospholipid bilayer membrane of intestinal cells

recombined to form triglycerides in the SER

transported to golgi apparatus and combined with cholesterol and lipoproteins to form chylomicrons

Question 27

what is the structure of the ileum?

Answer 27

wall of the ileum is folded and possesses finger like projections = villi
thin walls lined with epithelial cells which has a rich network of blood capillaries

villa increase SA for diffusion + thin walled
contain muscle allowing movement and maintains diffusion gradients
well supplied with blood vessels
villi posses microvilli increasing rate of absorption