Digestion
Digestion through the human body and digestive diseases
- Created by: Katrina
- Created on: 14-05-12 19:17
Digestion
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 anus 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
Main stages of digestion
INGESTION
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)
DIGESTION
the chemical breakdown (enzymes) of large food molecules into small soluble molecules
ABSORPTION
small, soluble molecules move from the small intestine into the bloodstream, which transports them to cells
EGESTION (ELIMINATION)
undigested food moves out of the body as faeces via the colon, rectum and anus
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)
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
Stomach
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
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
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
Oxyntic (Parietal) cells
secrete HCl which:
kills germs on food
makes stomach contents acid-optimum conditions for function of enzymes
activates pepsinogen to pepsin
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
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
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
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
Proteases
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)
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
nucleotidase-nucleotides
Pancreas
pancratic juice flows into the duodenum from pancreas via pancreatic duct
main enzymes are pancreatic amylase, lipase and endopeptidase
Liver
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
Absorption
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
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)
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
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
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
Large Intestine (Colon)
egestion of undigested food occurs after going through the large intestine
4 sections: caecum, colon, rectum, anus and 1.5m long from ileum to anus
many folds increase surface area
appendix found at bottom of caecum-no longer used in humans but may become inflamed in appendicitis
FUNCTIONS OF LARGE INTESTINE
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 anus (defaecation(
assisted by large amounts of mucus produced from goblet cells
Fibre
cellulose fibre from plant cell walls required to provide bulk and stimulate peristalsis
Assimilation
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
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
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
Colon Cancer
progressive and uncontrolled growth of abnormal cells in the colon
symptoms:
change in bowel habits
blood in faeces
growth, bleeding or diruption of colon function
rapid weight loss
Coeliac Disease
chronic disease in which an individual can't tolerate foods containing gluten or wheat protein
symptoms:
tiredness and lethargy
weight loss
acute illness
Diverticulosis
condition where diverticula become inflamed/infected
symptoms:
cramps
bloating
constipation
If infection:
nausea/vomiting
chills
cramping
constipation
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
symptoms:
constant stomach pain made worse by eating
weight loss
nausea/vomiting
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