the human digestive system

why must food be digested

bc the molecules r:

-insoluble and too big to cross membranes and be absorbed into the blood

-polymers must be converted to their monomers so they can be rebuilt into molecules needed by body cells

ingestion=

taking food into the body through the mouth

digestion=

mechanical and chemical breakdown of large molecules; carbohydrates, fats and proteins into smaller products so theyre small enough to be absorbed in the blood

absorption=

the passage of molecules and ions through the gut wall into the capillaries or lacteals

egestion=

e.g.

the elimination of undigested waste not made by the body

-cellulose

assimilation=

+e.g.

using digested food molecules un the body

-e.g. to make new proteins or use in respiration

digestion and absorption occur where

the gut

perilstalsis=

rhythmic wave of coordinated muscular contractions in the circular and longitudinal muscle of the wall, passing food along the gut in one direction only

function of:

1-mouth

2-oesophagus

3-stomach

4-duodenum

5-ileum

6-colon

7-rectum

8-anus

1-ingestion, digestion of starch and glycogen

2-carriage of food to the stomach

3-digestion of protein

4-digestion of carbohydrates, fats and proteins

5-digestion of carbs, fats and proteins. absorption of digested food and water

6-absorption of water

7-storage of faeces

8-egestion

what tissue layers is the gut made of

1- the serosa

2- muscle

3-submucosa

3-mucosa

describe the serosa

-function

-outermost layer of gut

-tough connective tissue protecting the gut wall

-the gut moves while processing food and the serosa reduces friction with other abdominal organs

describe the muscle tissue in the gut

-function

-comprises 2 layers in diff directions, the inner circular muscles and the outer longitudinal muscles

0makes coordinated waves of contractions, peristalsis.

-behind the bolus, circular muscles contract and longitudinal muscles relax, pushing the food along

describe the submucosa tissue in the gut

-function

-connective tissue containing blood and lymph vessels which remove absorbed products of digestion, and nerves that co-ordinate perilstalsis

describe the mucosa tissue in the gut

-function

-innermost layer and lines the gut wall

-its epithelium secretes mucus, lubricating and protecting the mucosa

-in some regions of the gut, it secretes digestive juices and in others, absorbs digested food

-has large folds called rugae, which allow the stomach to expand after a meal, and specialised structures called gastric pits which contain specialised cells to secrete gastric juice

chemical digestion is..

the hydrolysis of larger molecules into smaller, more soluble ones, by the action of enzymes

saliva contains

-what does it do

-amylase which begins the hydrolysis of starch + glycogen to maltose

-HCO₃ ^- + CO₃^2-ions to maintain a slightly alkaline pH (7)

-mucus which lubricates the food

what is the stomach

expanded sac-like region bounded by sphincters (rings of muscles) at either end keeping the food there for up to several hours

extra layer of muscle between the longitudinal and circular in stomach wall called..

-purpose

-oblique later

-enables churning which further increases the SA of food

amylase hydrolyses starch and glycogen into..

which is then digested to..

the disaccharide maltose

-maltase digests maltose to glucose

endopeptidase function

catalyse the hydrolysis of peptide bonds within the polypeptide chain

exopeptidase function

catalyse the hydrolysis of peptide bonds at the ends of short polypeptides

what do gastric juices contain

-secreted by what

-purpose

• peptidases, secreted by zymogen/ chief cells, at the base of the gastric pit

  -pepsinogen, an inactive enzyme, is secreted and activated by H⁺ ions to pepsin, an endopeptidase which hydrolyses protein to polypeptides

• hcl acid, secreted by oxyntic cells, lowers the pH of the stomach contents to about pH 2 →optimum pH for the enzymes + kills most bacteria in food

• mucus, secreted by goblet cells, at the top of gastric pit. mucus forms a lining which protects the stomach wall from the enzymes and lubricates the food

structure of small intestine

-6.5 m long, pH 7.8

-duodenum: 25cm

-ileum

what happns in the duodenum

-secretions from the Brunners glands, pancreas and gall bladder enter here

-digestion of all major food groups occurs

-some absorption occurs

what happens in the ileum

-digestion of all molecules is completed

-absorption of small soluble products occurs

bile made, stored and passed where

-,ade in liver, stored in gall bladder, passes through the bile duct into the duodenum

features of bile

-contains no enzymes

-lipids r non polar and form large globules in the aqueous chyme

-bile contains bile salts which r amphipathic (molecules hv both hydrophilic/obic parts)

-bile is alkaline and neutralises the acid in food coming from the stomach, it provides a suitable pH for the enzymes in the small intestine

what do bile salts do

they emulsify lipids in food by lowering their surface tension and breaking up large globules into smaller globules preventing it recoalescing

- increases the SA making digestion by lipase more efficient

the pancreas is what kind of glands

endocrine + exocrine

endocrine gland does what

secretes hormones into the blood which circulate affecting cells that have receptors in their membreanes

exocrine gland does what

secretions delivered via ducts to the site of the action

pancreatic juice contains

endopeptidases including trypsinogen

what is trypsinogen

inactive trypsin

how is inactive trypsinogen converted to active trypsin

by enterokinase enzyme found on the external surface of the epithelial cells lining the duodenum

hydrolysis of starch (amylose + amylopectin)

amylase hydrolyses starch to maltose

what hydrolyses triglycerides

lipase hydrolyses tri to monoglyrcerides + fatty acids

while bile needed to emulsify the lipids

so that lipase is able to complete digestion within the time that the food is in the gut

enterocytes=

epithelial cells lining the gut

how is brush border formed

-enterocytes form a single layer on the outer surface of the mucosa layer

-the membrane of the enterocyte adjacent to the lumen is folded into microvilli forming a brush border →inc SA

-cells at the tips of the villi r constantly being sloughed off; theyre replaced by the cells originally formed in the Crypts

-membrane of brush border contains exopeptidase + dipeptidase enzymes

what do exopeptidases do

continue the digestion of the short peptides that were formed by the action of the endopeptidases to releasee amino acids + dipeptides

what do dipeptidases do

digest dipeptides into amino acids

brush border + disaccharides

disaccharides r actively transported into the enterocyte and then digested by disaccharidase enzymes in the cell membrane

hydrolysation of maltose, sucrose, lactose

• maltose —maltase—> a-glucose

• sucrose —sucrase—> glucose+ fructose

• lactose —lactase—> glucose + galactose

6 adaptions for absorption in villus

1- a v large SA due to folds of the intestinal wall, villi + microvilli

2-a short diffusion distance due to the single layer of permeable epithelial cells

3-blood capillary network close to epithelium 4 rapid absorption + to carry away the products of protein + carbohydrate digestion

4-muscle fibres in each villus contracts to move the villus (peristalsis) and mixes the gut contents—> increased contact between food + the enzymes of microvilli membranes

5-many mitochondria present in epithelial cells for active transport

6-a lacteal in each villus to carry away absorbed lipids + absorbs lipoproteins

how does glucose move into the enterocyte

by co-transport with sodium ions

how are sodium ions transported out of the enterocyte + into the capillary

-why

-actively transported out and enter the capillary by facilitated diffusion

-to maintain conc gradient

how does glucose leave the enterocyte + enter capillary

by facilitated diffusion + enters the capillary dissolving in the blood plasma

how does the enterocyte ensure all of glucose is absorbed

some glucose may also be actively absorbed from the lumen

how r amino acids + dipeptides absorbed into the enterocyte

-how do amino acids enter the capillary + leave the enterocyte

by active transport; dipeptides r digested to amino acids

-amino acids leave + enter by facilitated diffusion

how r minerals, water soluble vitamins + water enter thee enterocyte

-minerals absorbed by facilitated diffusion

-water soluble vits by diffusion

-water follows the absorption of solutes by osmosis

how r absorbed nurients carried first to the liver

the capillaries drain into the hepatic portal vein

how is digested food passed into the duodenum a little at a time

relaxation of the pyloric sphincter muscle at the base of the stomach allows partially digested food to pass a little at a time into the duodenum

the duodenum recieves secretions from the

liver + prancreas

pancreatic juice is secreted by what exocrine glands

+how it enters duodenum

islet cells in the pancreas

-enters duodenum through pancreatic duct

what enzymes does pancreatic juice contain +their function

-trypsinogen: inactive enzyme converted into the endopeptidase trypsin by the duodenal enyzme, enterokinase

-endopeptidases: hydrolyse proteins + polypeptides to peptides

-amylase: digests any remaining starch to maltose

-lipase: hydrolyses lipids into fatty acids and monoglycerides

other than enzymes what else does pancreatic juice contain + its function

sodium hydrogen carbonate: raises the pH to make pancreatic juice slightly alkaline + contributes to neutralising acid from the stomach + providing the appropriate pH for the pancreatic enzymes to work efficiently

describe the absorption of lipids in the lacteal

-fatty acids + glycerol diffuse into the enterocytes and into the lacteals

-fat soluble vitamins and cholesterol r also absorbed into the lacteal

-the lacteal drains into the lymphatic system which eventually empties into the subclavian vein near the heart

what passes into the colon (large intestine) + what is absorbed along it

-undigested food, mucus, dead cells + bacteria is passed

-water is absorbed along w vitamin K and folic acid produced by gut microbes

what is the large insteastine made up of

the caecum, appendix, colon + rectum

describe defacation process

as material passes along the colon, water is absorbed, and by the time it reaches the rectum the material is semi-solid.

-passes along the rectum and is egested as faeces

which vitamins r water/lipid soluble + where they absorbed

-vitamins B +C r water-soluble, absorbed into the blood

-vitamins A, D + E r fat soluble, absorbed into lacteals

look at transport mechanisms on p239

y

what is excess glucose stored as

fat

what happens if theyre excess amino acids

cannot be stored so the liver deaminates the amino acids + converts the -NH₂ groups to urea, which is carried in the blood + excreted at the kidney

-the remains of the amino acid molecules r converted into carbohydrates for storage or conversion to fat

where r glucose + amino acids absorbed

absorbed by diffusion + active transport across the epithelium of villi into the capillaries, then to venules and then travel via the hepatic portal vein to the liver

assimilation=

The synthesis of biological compounds from absorbed simpler molecules