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The Digestive System
Enzymes are biological catalysts.
 They are natural substances, which speed up the
breakdown of food substances and other materials.
 They work by combining with the substrate
forming the enzymes – substrate complex.
 The enzymes – substrate complex breaks down
and forms the product and releases the enzyme.
 The enzyme can then be used again.
 Enzymes are very specific and will only work on one
substrate.
 Enzymes are very important in the breakdown of
food in our digestive system.
 Enzymes work at specific pH’s. Most at pH 7 but
pepsin works only at pH 2 & 3.
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Induced fit theory
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The alimentary canal is a long tube going
from the mouth to the anus.
Along the tube are glands where substances
called enzymes are produced.
The intake of food has five steps:
Ingestion
Digestion
Absorption
Assimilation
Egestion
 Ingestion
is the in take of food.
 Digestion is the physical or chemical
breakdown of food, so that it can be
absorbed into the blood stream.
 Absorption is the release of digested food
into the blood stream.
 Assimilation is the use of digested food in
the organisms cells.
 Egestion is the removal of indigestible
waste materials from the body.
 Digestion
in the human occurs in distinct
areas:
 Mouth (Physical and Chemical)
 Stomach (Physical and Chemical)
 Duodenum (Chemical)
 Ileum (Chemical)
 Large Intestine (Chemical)
 Food
is mechanically broken down by the
teeth, increasing the surface area for the
enzyme to work.
 Saliva, containing the enzyme Salivary
Amylase, breaks down Starch into
Maltose.
 Food is brought down to the stomach, in a
structure called a bolus through the
oesophagus by peristalsis.
 The
stomach contains Hydrochloric Acid
giving it a pH of 1 – 2 .
 Hydrochloric acid loosens food, kills bacteria,
triggers stomach enzymes and turns off
amylase.
 Inactive Pepsinogen is activated by the low
pH turning it into Pepsin (Proteinase).
 Proteinase breaks down proteins into
peptides.
 Peptides are short chains of Amino Acids.
 The
stomach churns the food for a
number of hours until the food is a thick
soupy liquid called Chyme.
 Sphincter
muscles relax and the chyme
enters the duodenum.
 Physical
digestion by the churning
process, chemical digestion by the action
of enzymes.
 Lots
of enzymes are secreted from the wall
of the duodenum, the pancreas and the
Gall bladder.
 Enzymes including lipase, proteinase
(trypsin), amylase, maltase, sucrase,
lactase, are found there.
 Bile is also secreted from Gall Bladder.
 Bile emulsifies fat, so that it can be
digested.
 Bile also neutralises the stomach acids.
Intestinal juice secreted from the walls of the small
intestine contains the same enzymes as the
duodenum.
 Most food is digested here, as it is 5.5 metres long.
 The wall of the intestine contains millions of finger
like projections called villi (villus in singular) so the
surface area is increased.
 Sugars, Amino acids, vitamins, minerals and water are
absorbed into the blood stream.
 This material travels to the liver via the Hepatic Portal
Vein and then onto the heart.
 Fatty acids and glycerol are transported by the lymph
vessels within the villus itself.
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 Here
any remaining broken down food is
absorbed.
 Water is removed from the waste.
 Excess substances are moved to other parts of
the body for storage.
 Excess Glucose is stored when too much is
ingested than needed.
 Glucose can be stored in the short term as
Glycogen. It is stored in the liver.
 The long-term storage of glucose fat.
Because cows, sheep and horses feed on fibrous
materials like grass and hay, their stomachs are
adapted to suit their needs.
 The ruminant has four compartments in their
stomach, the abomasum, the rumen, the
reticulum and the omasum.
 In the largest compartment, the rumen, there are
millions of bacteria and protozoans.
 These organisms produce enzymes that break down
cellulose into starch.
 The organism get what they want and the cows gets
the energy value.
 This kind of relationship is called a symbiotic
relationship.
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The organisms can also make all the amino acids needed
for the ruminant so there are no essential amino acids.
These bacteria can make all B vitamins as well.
The rumen is nearly 250 litres in volume and is far
bigger than the other compartments. Breakdown of
cellulose occurs here.
The reticulum is the smallest of the compartments and
is used to bring food back to the mouth to be re-chewed
(chewing the cud).
The omasum squeezes the semi – digested food and
increases the surface area for the bacteria.
In the abomasum all the processes that take place in
the stomach of the human occur here.
No amylase is found in the saliva of cows, so chewing the
grass only breaks the food down physically.
This increases the surface area for chemical breakdown
in the stomach compartments.