Digestion through the human body and digestive diseases

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large organic moledules are broken down into small, soluble, organic molecules and are absorbed into the cells

the raw materials are used to synthesise molecules which can beused by the body

breakdown of food is carried out by the digestive system, a long tube from mouth to **** known as the alimentary canal, or gut

physical digestions (mechanical digestion) happens by crushing action of teeth; action of stomach; peristaltic action of muscle layers in gut wall

this is muscular action which pounds food into a semi-soid state

it increases surface are of food over which enzymes may act to chemically digest food

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Main stages of digestion


large food particles taken into the mouth and broken down by teeth, salive and tongue (mastication), so it may then move into gut (mechanical digestion)


the chemical breakdown (enzymes) of large food molecules into small soluble molecules


small, soluble molecules move from the small intestine into the bloodstream, which transports them to cells


undigested food moves out of the body as faeces via the colon, rectum and ****

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Main Layers of the Gut

Mucosa: inner surface surrounding lumen consisting of epithelial cells.  Contains goblet cells, enzyme secreting glands and connective tissue lying underneath

Muscularis mucosa: smooth muscle which contracts without tiring (involuntary conractions called peristalsis) and move the villi allowing greater contact with food

Submucosa: connective tissue, blood and lymph vessels with a high proportion of collagen and elastin fibres.  Nerve fibres in the ileum control peristaltic muscle contraction and release of secretions from glands

Muscularis externa: 2 layers of smooth muscle: longitudinal muscle which lies lengthwise along the wall of the gut; and circular muscle which lies around the gut

Serosa: thin layer of loose connective tissue covered by a simple squamous epithelium (mesothelium).  Serosa present in parts of intestinal tract that are present in peritoneal cavity; the regions not present in the peritoneal cavity are held in place by outer layer of loose connective tissue (adrentitia)

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Buccal Cavity

food broken up into smaller pieces in the buccal cavity by mastication, and moistened by saliva

saliva: water, mineral salts, salivary amylase (starch to maltose), mucus (lubricant) and lysozyme (kills bacteria)

saliva has pH of 6.5-7.5 due to presence of mineral salts

this is mechanical breakdown and food is now a bolus

bolus is forced through the pharynx to the oesophagus by the act of swallowing

bolus moves to the stomach by a series of rhythmic contractions-peristalsis

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sack with 5L capacity

ridges (rugae) to help with mechanical breakdown of food

muscles at upper and lower ends of the stomach: cardiac sphincter relaxes at upper end to allow food to enter; pyloric sphincter and lower end allows food to move into duodenum

food remains in stomach for 2-4 hours

3 thick layers of muscle allow it to contract and relax to grind food

stomach absorbs simple chemicals e.g. alcohol, aspirin, water and salts

food mixed with gastric juice: pepsin (hydrolyse protein to polypeptides), rennin (curdle milk), lipase (hydrolyse lipids), HCl (kills germs on food, provides optimum pH for enzymes, activates peptidases)

stomach has pH 1-2 and mucus protects the stomach cells from acid

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Gastric Glands

glands in the stomach are simple tubular glands containing:

peptic (chief) cells to produce pepsin

oxyntic (parietal) cells to produce hydrochloric acid

goblet cells to produce mucus

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Peptic (Chief) cells

secretes pepsin-digests proteins to polypeptides

pepsin secreted as inactive precursor (pepsinogen) to prevent autolysis

activated by HCl and enterokinase

mucus protectc pepsin from damaging stomach wall

pro-rennin (precursor) becomes rennin and coagulates the soluble protein in milk

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Oxyntic (Parietal) cells

secrete HCl which:

kills germs on food

makes stomach contents acid-optimum conditions for function of enzymes

activates pepsinogen to pepsin

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Goblet cells

secrete mucus which:

forms protective layer, preventing HCl and pepsin from breaking down stomach wall

aids movement of food within stomach

mucus is alkaline due to presence of HCO3 salts

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Stomach to Duodenum

food stays in stomach for 2-4 hours to maximise chemical and mechanical digestion

semi-liquid material now called chyme flows into duodenum through pyloric sphincter

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Small Intestine

responsible for a great deal of digestions and absorption

2 main parts: duodenum and ileum

small intestine-5m long, duodenum-25cm long

central cavity called lumen

Brunners glands: found in wall of duodenum and secretes mucus, which is alkali and neutralises the stomach acid and protects duodenal lining

Crypts of Leiberkühn: between the villi and glands called Crypts of Leiberkühn

epithelial cells lining the glands produce enzymes

goblet produces mucus

small intestine secretes large amounts of water in which chemical digestion takes place

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Digestive Enzymes

come from pancreas, duodenum, ileum and stomach

liver produces bile which is stored in the gall bladder and flows along the bile duct into the duodenum

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any type of enzyme which digests (hydrolyses) proteins

secreted as inactive precursors e.g. trypsinogen converted to trypsin by the enzyme enterokinase, trypsin then activates other proteases

endopeptidase: any enzyme which digests peptide bonds within the protein molecule to equal small polypeptide chains (pepsin)

exopeptidase: any enzyme which digests the polypeptide chains to form either dipeptides and/or amino acids (trypsin)

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Enzymes produced by Walls of Small Intestine

maltase-maltose to glucose

sucrase-sucrose to glucose and fructose

enterokinase-activates trypsin, pepsin, rennin

endopeptidase-proteins within the molecule

exopeptidase-shorter polypeptide chains to amino acids

dipeptidase-dipeptides to amino acids

amino peptidase-peptides to amino acids


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pancratic juice flows into the duodenum from pancreas via pancreatic duct

main enzymes are pancreatic amylase, lipase and endopeptidase

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produces bile stored in gall bladder and flows along bile duct into duodenum

bile breaks fat into tiny droplets which increase surface area so enzymes can digest them more readily (emulsification)

bicarbonate salts help neutralise stomach acid-pH of the small intestine becomes more alkaline which favours action of various enzymes

bile stimulates peristalsis in the duodenum and ileum

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digested food absorbed by the ileum into the bloodstream

monosaccharides, amino acids, minerals and vitamins are absorbed through the plasma membrane of epithelial cells on the villi to the bloodstream

fatty acids and glycerol absorbed into the lacteals (lymph system)

molecules are absorbed by passive and facilitated diffusion and active transport

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Sugars and Amino Acids

end products of carbohydrate digestion are all monosaccharides

any disacchradies absorbed by the plasma membrane of epithelial cells and broken down into monosaccharides in the cells (intracellular)

glucose and amino acids are absorbed across epithelium of villi by facilitated diffusion and active transport (transport proteins)

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Fatty Acids and Glycerol

passed into lacteal

lacteal is a lymph capillary in the centre of each villus and leads to lymphatic system which opens into the bloodstream at the thoracic duct

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Water, Ions and Mineral Salts

pass through epithelial membrane via facilitated diffusion through transport proteins

fat soluble vitamins and minerals diffuse through phospholipid bilayer

water moves down the water potential gradient by osmosis

all products from digestion move into the hepatic portal vein to the liver where toxins are broken down

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Adaptations of Small Intestine for Absorption

in humans, ileum is very long -5m

lining is folded and on each fold and finger-like projections called villi

epithelial cells on villi have microscopic projections called microvilli

features increase surface area for increased absorption

epithelial cells contain many mitochondria-absorption is active and ATP required

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Large Intestine (Colon)

egestion of undigested food occurs after going through the large intestine

4 sections: caecum, colon, rectum, **** and 1.5m long from ileum to ****

many folds increase surface area

appendix found at bottom of caecum-no longer used in humans but may become inflamed in appendicitis


absorption of water, sodium ions and mucus

manufacture of vitamin K and folic acid via microorganisms living in the large intestine

forms and expels undigested food e.g. cellulose, bacteria and sloughed celld

undigested food known as faeces and egested from **** (defaecation(

assisted by large amounts of mucus produced from goblet cells

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cellulose fibre from plant cell walls required to provide bulk and stimulate peristalsis

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digestion and absorption now complete

through the bloodstream, nutrients are distributed to the stomach to the tissues where they are assimilated (built up into complex molecules) or used in respiration for production of energy

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Uses of products of digestion

glucose absorbed into cells from the bloodstream for energy in respiration

excess glucose converted into glycogen and stored in liver

amino acids absorbed for protein synthesis

excess amino acids deaminated in liver-amino groups removed and converted into urea excreted by kidneys; remainder converted into carbohydrates and stored

lipids are used for membranes and hormones

excess lipids stored as fat

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Ileum adaptations for absorption

major site of absorption of food monomers

large surface area-folds, long, 1mm longs multicellular projections (villi), epithelial cells have 1um long projections called microvilli (brush border)

good capillary circulation to carry away polar molecules (sugars, amino acids, minerals, water) and lymph vessels (lacteals) to carry away lipids and fat soluble vitamins (AKED)

constant good stirring via peristalsis and movement of micro/villi

small separation of food and blood (2 cells layers)

lots of mitochondria in epithelial cells supply ATP for active transport

lipids in lacteals transport to the subclavian vein where they join general circulation

food monomers go in hepatic portal vein to liver where levels are controlls

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Colon Cancer

progressive and uncontrolled growth of abnormal cells in the colon


change in bowel habits

blood in faeces

growth, bleeding or diruption of colon function

rapid weight loss

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Coeliac Disease

chronic disease in which an individual can't tolerate foods containing gluten or wheat protein


tiredness and lethargy

weight loss

acute illness

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condition where diverticula become inflamed/infected





If infection:





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Peptic Ulcer

erosion of lining of the wall of the stomach cause by H. pylori bacteria or long term use of anti-inflammatory drugs causing increase in acid or damage to mucus lining


constant stomach pain made worse by eating

weight loss


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