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THE MOUTH |
- Chewing makes a larger surface area of
the food for the enzymes to attack.
- Salivery amylase hydrolyses some starch
to maltose.
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THE STOMACH |
- the walls of the stomach contain layers
of muscle. the functions of which include:
- churning, mechanical digestion, mixing,
and peristalsis.
- the gastric glands in the stomach wall
secrete endopeptidase pepsin. however, it is secreted in its
inactive form, pepsinogen. HCl in the stomach activates the enzyme.
- the enzyme is secreted in its inactive
form in order to prevent it from digesting the walls of the stomach,
while it is in storage in the gastric glands.
- once the enzyme has been activated, mucus,
which coats the stomach walls, prevents them from being digested,
and also protects the walls from acid.
- HCl in the stomach kills bacteria which
are ingested along with food, and also created a low pH environment
in which stomach enzymes work at their optimum rate.
- endopeptidases digest proteins into polypeptide
chains
- food is released from the stomach by periodic
relaxation of the pyloric sphincter muscle at the lower end of
the stomach.
- after being released from the stomach,
food enters the first part of the small intestine, known as the
duodenum.
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THE SMALL INTESTINE |
- Large Surface area
- Moist surface
- Thin (epithelial) surface/ short absorption
pathway
- Long/ folds (increasing surface area)
- Villi
- Microvilli
- Lacteal
- Capillary network in villus/ good blood
supply
- Mitochondria to supply ATP/ energy for
active transport
- Carrier proteins in membranes.
- the duodenum contains the following enzymes:
- lipase – for the digestion of lipids.
lipids are hydrolysed to fatty acids and glycerol.
- maltases – the small intestine contains
maltase as part of the intestinal fluid which forms a secretion
which coats the walls of the small intestine epithelial cells.
maltase acts on the disaccharide sugar maltose and hydrolyses
the glycoside bonds between the units of glucose. the sugar is
broken down to its simplest form glucose, and can then be absorbed.
- endopeptidases – for the digestion
of proteins. these are hydrolysed to polypeptides.
- exopeptidases digest polypeptide chains
to amino acids.
- both endo and exopeptidases are required
for efficient digestion of polypeptides and proteins because
endopeptidases act on the centre of polypeptide chains within
proteins and hydrolyse them to smaller chains.
- this means that more ‘ends’
are created for the exopeptidases to act upon, in order to break
down polypeptide chains to amino acids.
- enzymes in the duodenum are secreted from
the pancreas, and are carried to the duodenum by the hepato-pancreatic
duct which also brings bile from the liver.
- the duodenum is the main site of absorption
of all components of digestion, except water.
- food is moved along the duodenum by peristalsis
(rhythmic contraction of the muscles of the intestinal wall,
cause food to be pushed along the duodenum)
- Segmentation in the duodenum produces
a to and fro movement that causes mixing of the contents of the
gut and digestive juices. causing acid chyme which has a creamy
consistency to be converted to chyle which is alkaline with a
watery consistency.
- segmentation also aids digestion by bringing
products into contact with the mucosa –hence enabling absorption
to occur.
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ABSORPTION IN THE SMALL INTESTINE |
- Diffusion in capillaries
- Active transport/ facilitated diffusion
involved
- ATP used by active transport
- Disaccharides/ enzymes in cell surface
membrane
- Glucose/ monomers/ monosaccharides actively
transported into epithelial cells via protein carriers/ channels
(in membrane)
- Facilitated diffusion from epithelial
cell/ towards blood
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THE ROLE OF THE LIVER IN DIGESTION |
- bile is a biological detergent, which
is produced in the liver.
- bile reduces the surface tension of the
contents of the gut and increases the surface area. this allows
enzymes to work. e.g. lipase.
- in order for lipids to act upon triglycerides,
the triglycerides must first be broken down into minute droplets
to enable then to mix with lipases present in the pancreatic
juice within the duodenum.
- in order to do this bile is secreted from
the gall bladder.
- bile reduces the surface tension and increases
the surface area /volume ratio. i.e., fats are emulsified.
- therefore, lipases act on a larger volume
of material in a shorter time, ensuring that enzymes operate
at their optimum rate.
- bile also neutralizes stomach acid, and
provides the optimum pH for pancreatic digestive enzymes to work.
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THE ROLE OF THE PANCREAS IN DIGESTION |
- produces pancreatic juice.
- pancreatic juice is rich in sodium hydrogencarbonate,
which:
- neutralizes acid chyme from the stomach.
- raises the pH to enable enzymes in the
pancreatic juice to work.
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THE LARGE INTESTINE |
- the large intestine is made up of the
following parts:
- ceacum and appendix – these are sack-like
structures t the junction of the small and large intestines.
- the appendix is vestigial – i.e.
it has no function.
- the colon and rectum- this is a muscular
tube which contains large amounts of bacteria.
- peristalsis moves contents along the colon,
and also compacts faeces.
- faeces are stored in the rectum.
- mucosa in the colon secretes mucus which
lubricates the mucosa and protects it from enzymes action.
- the colon absorbs water and other soluble
compounds.
- the colon absorbs vitamins and ions.
- bacteria contained in the colon, break
down undigested food. this food is then absorbed of excreted
as faeces.
- these bacteria synthesize vitamins b and
k.
- faeces excreted via the anus. main components
are:
- undigested food, bile pigments, bacteria,
and dead cells from the small intestine.
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HORMONAL CONTROL OF DIGESTION
The following underlined words show the
hormones involved in digestion, which must be learnt for the
BYO3 exam. The bullet points underneath give the information
that is asked in the exams, which you will need to know. |
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ROLE OF GASTRIN IN CONTROL OF GASTRIC
JUICE SECRETION
- Arrival of food in stomach
- Causes cells to secrete gastrin
- Gastrin transported by blood to gastric
glands
- Enzymes / acids created
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GASTRIN
- Gastrin is found in the stomach.
- It is produced in the stomachs Gastric
juice.
- The contact of food with the stomachs
lining causes the production of gastrin.
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CHOLECYSTOKININ
- The stimulus that triggers the release
of cholecystokinin is the contact of food with the lining of
the small intestine/duodenum.
- It is secreted in the pancreatic juice
and in bile.
- The release of the hormone cholecystokinin
causes the gall bladder to contract.
- Certain drugs such as loxiglumide may
stop the action of the hormone cholecystokinin on the gall bladder.
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SECRETIN
- Secretin is also secreted when food makes
contact with the lining of the duodenum.
- It is secreted in an alkaline fluid from
the pancreas.
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