biology-the GI tract

• how does food travel in the body

  oral cavity

  pharynx

  oesophagus

  stomach

  small/large intestine
• what are the five processes of digestion

  mechanical-chewing and mixing. occurs in the mouth

  secretion-digestive enzymes and hormones done by the accessory organs

  digestion-breaking down the food to smallest absorable units. occurs in the stomach

  absorption-into the blood or lymph vessels this occure in the intestines

  elimination-the removal of waste and undigested material. occurs in the colon.
• why does statch and other macroneutrients need breaking down before they are absorbed

  they are too large to pass through the membrane
• what happens at the oral cavity

  -physical and chemical digestion of food begins in the mouth. this begins by the chewing of the food to break it up into smaller pieces

  -this gives them a larger surface area for enzyme action

  -some of the enzymes that aid digestion at this time are amalyase and lingual lipase.
• how is the mouth protected

  -mucin (glycoproteins) protects the lining of the mouth and lubricated the food

  -the buffers in saliva help to prevent tooth decay be neutralising the acid
• what happens at the pharynx

  -the tounge pushes food into the pharynx

  -the trachea move so that it is covered by the epiglottis. this prevents food from going down the wrong hole.
• what happens at the oesophagus

  -conducts food from the pharynx down to the stomach

  -peristalis squeezes the food along the oesophagus

  -salivary amalyse continues to hydrolyse the starch to glucose.
• what occurs at the stomach

  -the epithelium secretes gastric juice and digestive fluid that mixes with the food.

  -a coating of mucus helps to protect the stomach from being digested.

  -every three days the new stomach cells replace the stomach lining.

  -when the stomach churns and the food mixing with enzymes

  -it is now known as acid chyme.
• what is pepsin 


  an enzyme that acts in the stomach

  the ideal PH is PH2

  begins the hydrolysis of proteins into smaller polypeptides.

  pepsin is secreated as inactive it is activated by hydrochloric acid in the stomach.
• what happens in the small intestine

  -where most macromolecules are hydrolysed by enzymes.

  -enzymes and bile are produced by the pancreas,liver and bile is stored in the gallbladder.
• what are the functions of the small intestine

  absorption of minerals back into the bloodstream and lymph system.

  -in the duodenum the acid chyme is mixed with enzymes from the pancreas and bile from both the liver and gallbladder
• how does the pancreas aid digestion

  secretes pancreatic juice, which

  contains numerous hydrolytic enzymes and an alkaline solution rich in bicarbonate. The bicarbonate acts as a buffer, offsetting the acidity of the chyme
• how does the liver and gallbladder aid digestion

  -The liver- secrets bile, stored in

  the gall bladder, and released

  through the bile duct into the

  duodenum.

  -Although bile contains no

  enzymes, it contains substances

  that help prepare fats for digestion. Fats tend to clump together into globs, making it difficult for enzymes to reach

  the molecules.

  -Bile separates small fat droplets,

  preventing them from clumping

  into globs. This enables digestive

  enzymes to break the fats down

  more efficiently.
• how are carbohydrates digested

  -amalyse hydrolyses starch and glycogen into disaccharides

  -maltase digests maltose into glucose

  -sucrase digests sucrose into glucose and fructose.

  -lactase digestes lactose into glucose and galactose
• how are proteins digested

  -trypsin and chymotrypsin complete the hydrolysis of proteins by pepsin

  -they are hydrolysed into amino acids
• how are fats digested

  -digested in the small intestine

  -bile salts from the gallbladder oat the fat droplets preventing them from clumping together.

  -hydrolysed into fatty acids and monoglycerides by lipase.
• how are proteins and carbohydrates absorbed

  Transport may be

  • Passive eg simple sugars (fructose)

  • Active eg peptides, vitamins

  ■The hepatic portal system carries blood rich in nutrients from digestion

  in the small intestine to the liver, the organ that monitors the composition

  of the blood.
• how are fats absorbed

  Glycerol and fatty acids recombine in the epithelial cell layer to form fats again

  • They are then mixed with cholesterol and coated with special proteins to form chylomicrons (small globules)

  • They are then transported by exocytosis into the lacteal
• how do hormones help regulate digestion

  Hormones released ensure digestive secretions are only present when needed ie when we see, smell or taste food.

  •Gastrin: stimulates release of gastric juice

  • Secretin: stimulates pancreas to secrete water and bicarbonate

  •Cholecystokinin (CCK): signals pancreas to secrete digestive enzymes
• what happens at the large intestine

  The colon removes the remaining water, about 90% of the water entering the canal is reabsorbed

  • The waste (faeces) becomes more solid and is moved along the colon by peristalsis

  • Faeces are stored in the rectum until they can be eliminated by a strong contractions of the colon
• what are some disorders of the digestive system

  pancreatitis

  cancer

  chrons

  colitis
• what is peristalsis


  the contraction and relaxation of muscles.