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Enzymes: 9. Details of how the human digestive system
works - what goes on in the alimentary canal
Doc Brown's Biology exam study revision notes
There are various sections to work through, after 1 they can be read and studied in any order.
Sub-index
of biology notes on enzymes and digestion
9.
Summary of the digestive system
The
parts and function of the parts of the alimentary canal
and the role of gut bacteria
Some important
definitions
Absorption is the movement of small food molecules and ions through
the wall of the intestine into the blood.
Assimilation is the movement of digested food molecules into the
cells of the body where they are used and become part of the cells
and tissue.
Chemical digestion is the breakdown of large, insoluble molecules
into small, soluble molecules.
Diarrhoea is the 'urgent' loss of watery faeces.
Egestion is the passing out of food that has not been digested or
absorbed, as faeces, through the anus
Ingestion is the taking of substances, e.g. food and drink, into the
body through the mouth.
Mechanical digestion is the breakdown of food into smaller pieces
without chemical change to the food molecules.
Peristalsis is the involuntary constriction and relaxation
of the muscles of the intestine or oesophagus canals, creating a
wave-like movements that pushes the contents forward (i.e.
undigested or digested food).
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You need to know about the structure and functions
of the tissues and organs of the human digestive
system, including adaptations to function and how the digestive
system digests food using enzymes as biological catalysts.
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Through the digestive organs food and liquids travel as they are
swallowed, digested, absorbed, and leave the body as faeces.
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You should also know the importance of bacteria in the human digestive
system
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These organs include the mouth, pharynx (throat), oesophagus,
stomach, small intestine, large intestine, rectum, and anus.
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The alimentary tract is part of the digestive system and is also
called digestive tract and gastrointestinal tract.
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Know that organs are made of
different tissues acting together to perform some particular function.
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Know that the stomach is an organ that
contains:
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muscular tissue, to churn the contents
and break up the food into smaller chunks to aid digestion,
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glandular tissue, to
produce digestive juices containing enzymes to break food down at the
molecular level,
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epithelial tissue, to cover the outside and the
inside of the stomach.
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Know that organ systems are
groups of organs that work together to perform a particular function.
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Know that the digestive system is one
example of a system in which humans and other mammals exchange substances
with the environment.
-
The digestion process requires a
variety of enzymes to breakdown food into soluble products we can absorb
from the digestive system, and these enzymes are produced by specialised
cells in the glands and gut system.
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Large insoluble molecules like
proteins, starch like carbohydrates and oils/fats cannot pass through the
membranes of the cell walls of the gut system.
-
However, smaller soluble
molecules like amino acids, sugars and fatty acids can pass through the
walls of the digestive system.
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Some examples of the enzymes
responsible for the breakdown of large insoluble molecules into small
soluble absorbable molecules are ...
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protease enzymes like pepsin
convert proteins to amino acids,
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carbohydrase enzymes like
amylase convert carbohydrates like starch to sugars such as glucose,
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lipase enzymes convert oils/fats
to fatty acids and glycerol,
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and these smaller molecules can now pass from the small
intestine into the blood capillaries.
 There
are 8 sections to the function of the alimentary canal in the human
digestive system
You need to be able to identify the main regions of the
alimentary canal and associated organs, including mouth, salivary
glands, oesophagus, stomach, small intestine (duodenum and ileum),
pancreas, liver, gall bladder and large intestine (colon, rectum and
anus).
Be able to describe the functions of the regions of the
alimentary canal listed above, in relation to ingestion, digestion,
absorption, assimilation and egestion of food
Know that the digestive system includes,
and their functions:
(1) After ingestion, in the mouth,
salivary glands produce the enzyme amylase which can break down
carbohydrates like starch into maltose. Further down in the intestine this
continues and the enzyme maltase breaks down the maltose into
glucose. The saliva also moistens the food and, together
with the chewing action (mastication) of the mouth muscles, balls of food
are formed that are easily swallowed. The chewing action of the teeth mashes
the food and increases the surface area the enzymes can act on -
mechanical digestion.
(2) The oesophagus
(gullet) is a tube that connects the mouth to the stomach and its lined with
muscles that help move the balls of food along (this action is an example of
peristalsis).
(3) In the stomach the food is churned and broken up into
smaller chunks by the muscles of the stomach wall.
The protease enzyme
pepsin is secreted from the pancreas and breaks down proteins to amino acids.
At the same
time hydrochloric acid is produced in the gastric juice:
(i) killing most of the bacteria present
by denaturing their enzymes.
(ii) creates the right acidic pH conditions (~pH 2) for the
protease enzyme pepsin,
which works
best in these acid conditions to break down proteins into amino acids.
This
is the start of the chemical digestion process of protein.
Muscles in the stomach wall compact the food as
well as churning and mixing everything up.
Other muscles in the digestive system squeeze
the food along by the process of peristalsis.
The contraction and relaxation of the tissue
lining muscles in the wall of the digestive system produce a
wave-like movement that pushes the balls of food along the digestive
system, a process known as peristalsis.
(4) The liver produces alkaline bile, which
neutralises excess
stomach acid in the mixture of churned up food and gastric juices
entering the duodenum of the small intestine.
Apart from the likes of
pepsin, most enzymes can't work in very acid conditions of the
stomach, and also bile helps to emulsify oils/fats which
speeds up their breakdown by lipase enzymes into fatty acids and
glycerol.
The emulsification is
essential for the efficient faster digestion of oils/fats, the oils/fats are more
dispersed giving greater surface area (greater surface area - think of the
oil/fat as broken down into smaller droplets/particles).
A lacteal is a lymphatic capillary that
absorbs dietary fats in the villi of the small intestine.
(5) The gall bladder
stores bile before its released into the small intestine to help with
digestion.
(6) The pancreas gland
tissue produces digestive juices
containing the enzymes
(i) protease enzyme pepsin breaks down proteins
to amino acids,
(ii)
amylase enzyme breaks down starches to sugars.
(iii) lipase enzymes breaks down oils/fats,
which are released into the small intestine.
(7) The small intestine is where
most of the digestion process continues with the release of the enzymes from the
pancreas.
The small intestine consists
of two sections:
The duodenum is fed into
from the stomach.
The ileum is the last and
longest section of the small intestine where the walls begin
to thin and most nutrients and water are absorbed here and
the waste is passed on to the large intestine.
The small intestine is the
longest segment of the gastrointestinal tract where most food is
broken down into a fluid containing soluble small molecules e.g.
The enzyme amylase breaks
down carbohydrates like starch into maltose (soluble) and
the enzyme maltase breaks down the maltose into glucose
(soluble) for respiration. This reaction happens on
the epithelial lining of the small intestine.
The enzyme protease breaks down protein
(polypeptides) into the much smaller soluble amino acids.
Lipase enzymes break down lipids (saturated
animal fats or unsaturated vegetable oils) into fatty acids
and glycerol.
The small intestine
is where the absorption of soluble food into the blood stream
occurs from the digestive system e.g. smaller molecules like
amino acids, sugars,
fatty acids and ions - all the products of enzyme
breakdown of larger food molecules.
These smaller soluble molecules can then be absorbed by
diffusion through the gut wall into the bloodstream and
transported to wherever they are needed in the body i.e the
cells and tissues of the organ systems.
Note that large insoluble molecules cannot be
absorbed into the body.
Water is absorbed mostly in the small
intestine and much less so in the colon
For more details on the structure and function
of the small intestine see
The small
intestine and villi
(8) In the
large intestine excess water is absorbed from the undigested food, producing
faeces which are initially stored in the rectum before release through the
anus - the process of egestion.
The colon is the longest part of the
large intestine.
The colon receives almost completely digested food
from the cecum, absorbs water and nutrients, and passes waste
(stool or faeces) to the rectum which is passed out through the
anus.
The large intestine technically consists of
three sections - the colon, rectum and anus.
(9) The role of gut bacteria and fungi (known
as the microbiome)
The microbiome, or gut flora, are the
microorganisms, including bacteria, archaea, fungi, and viruses
that live in the digestive tracts of animals. The gut is the
main location of the microbiome in human beings.
Bacteria are unicellular organisms and there are
over 100 million bacterial cells in our alimentary canal.
Most of these are in the lower end of the small
intestine and in the large intestine.
These bacteria are naturally found in your body
and are not usually harmful to you.
These gut bacteria ...
(i) produce enzymes that help digest food,
(ii) synthesise important vitamins such as
vitamin K,
(iii) produce important useful hormones,
(iv) reduce the possibility of harmful
bacteria growing in your intestine and causing illness.
You should be able to
recognise the organs of the digestive system on a diagram and
know their function where described.
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eg salivary glands, stomach,
gall bladder, liver, large intestine, pancreas, small intestine, rectum
See also
Surfaces for the exchange of substances in
animal organisms (includes small intestine)
Summary of learning objectives and key words or phrases
Be able to describe, with the aid of a diagram of the
alimentary canal, how human digestive system functions, including, the small intestine,
importance of gut bacteria and fungi (the microbiome), describe the function
of the oesophagus, gall bladder, liver, the
absorption of digested food, know the difference between egestion and ingestion,
know how digested food is assimilation into the blood stream. know the
action of peristalsis in the alimentary canal.
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