HUMAN HEALTH AND PHYSIOLOGY
DIGESTION
Assessment Statements
Explain why the digestion of large food molecules is essential
Explain the need for enzymes in digestion
State the source, substrate, products and optimum pH conditions
for one amylase, one protease and one lipase.
Draw and label a diagram of the digestive system.
Outline the function of the stomach, small intestine and large
intestine.
Distinguish between ‘absorption’ and ‘assimilation’.
Explain how the structure of the villus is related to its role in
absorption and transport of the products of digestion.
Why is Digestion Necessary?
When we eat, we take in food consisting of
large, complex organic molecules, which are
not suitable to be used since they are too
large.
Large molecules can’t pass through
membranes to enter the cells that line the
intestine or pass on into the bloodstream
Digestion- the biochemical breakdown of large, insoluble food
molecules into small, soluble molecules.
This process is essential because only small molecules can enter cells
and be used in the body.
Molecules produced by digestion pass through the wall of the intestine
by diffusion, facilitated diffusion or active transport.
They enter the bloodstream and travel to the cells, where they are
reassembled into new structures.-
Three main types of food molecule that must be digested are
carbohydrates, proteins and lipids.
Type of Molecule
Form of molecule
In ingested food
Product of
Digestion
Carbohydrates
Monosaccharides,
disaccharides,
polysaccharides
Monosaccharides
(glucose)
Proteins
Proteins
Amino acids
Lipids
Triglycerides
Fatty acids and
glycerol
Nucleic acids
DNA, RNA
Nucleotides
Digestion of large molecules occurs very slowly at body temperature.
Enzymes are essential to speed up the rate of digestion so that it is fast
enough to process nutrients to supply our needs.
There are many different enzymes in the human digestive system.
Different enzymes are released in different sections of the digestive
system and each one is specific for one type of food.
Some enzymes are specific to the different carbohydrates that we eat, and
others work one after another to digest foods such as proteins in a series
of stages.
All digestive enzymes help to catalyze hydrolysis reactions and work best
at 37C.
Enzyme
Type
Example
Source
Substrate
Products
Optimum
pH
Amylase
Salivary
glands
Salivary glands
Starch
Maltose
7
Protease
Pepsin
Gastric glands
Protein
in stomach wall
Polypeptides
2
Lipase
Pancreatic
Lipase
Pancreas
Fatty acids
and glycerol
7
Triglycerides
Water molecules are used to break the links between the
subunits of the large molecules.
The Digestive System
The digestive system consists of long, muscular tubes, also
called the gut or alimentary canal.
Associated with it are a number of glands that secrete enzymes
and other digestive juices.
The gut extends from the mouth to the anus and is specialized
for the movement, digestion and absorption of food.
Mouth, Oesophagus and Stomach
In the mouth, food is broken down into small pieces by the jaws and
teeth, and mixed with saliva containing the enzyme salivary
amylase, which begins the digestion of any starch the food contains.
The food is then passed down the oesophagus to the stomach by
a sequence of muscle contractions known as peristalsis.
http://www.youtube.com/watch?v=o18UycWRsaA
The stomach is a muscular sac that holds food for up to four hours
while digestion proceeds inside it.
As the muscles of the stomach contract, food and enzymes are
mixed- this gives maximum contact between food and enzyme
molecules, and speeds up the digestive process.
http://www.youtube.com/watch?v=URHBBE3RKEs
Digestion of proteins begin here, catalyzed by the enzyme pepsin, which is
secreted in gastric juice produced by millions of gastric glands in the stomach
wall.
Gastric juice contains pepsin in an inactive form.
Hydrochloric acid activates the pepsin and maintains a pH of 1.5-2 in the
stomach.
This pH is optimum for protein digestion and also kills many of the bacteria
present in the food we eat.
Goblet cells in the stomach lining
secrete mucus to protect the interior
of the stomach from the acid and
enzymes, which would otherwise
digest it.
Food is transformed in the stomach
to a semi-liquid called chyme and is
ready to the small intestine.
Roles of the Small Intestine
Little by little, chyme leaves the stomach through a valve into the 5
meter long small intestine.
Digestion is completed in the first section of the small intestine.
Digestive juices are secreted from the liver, gall bladder, pancreas
and intestine walls.
Bile is added from the liver and gall bladder, and the pancreas
secretes pancreatic juice containing trypsin, lipase, amylase and
bicarbonate ions.
The acidity of the chyme is reduced by these ions, allowing the
enzymes to work at their optimum pH
http://www.youtube.com/watch?v=umPJNAF4VKo
The inner surface of the small intestine is greatly folded to form
thousands of villi.
Each villus contains a network of capillaries and a lacteal
(vessels for the lymphatic system).
Villi greatly increases the surface area of the small intestine
improving its ability to absorb nutrients.
As small molecules such as glucose, Amino acids, fatty acids and glycerol, come
into contact with villus, they are absorbed, either passively or by active
transport, into the single layer of epithelial cells.
Amino acids and glucose then enter capillaries and are carried away in the
bloodstream. Fatty acids and glycerol are taken into the lacteal and travel in
the lymphatic system.
After digested food has been absorbed, it is assimilated into the body
And enters cells to become part of the body’s tissues or reserves.
Glucose is transported to the liver, which maintains constant level of
blood sugar.
Amino acids form part of the reserve of amino acids used to build new
proteins in cells all over the body, and fatty acids and glycerol enter the
bloodstream from lymph vessels near the heart to be used as an energy
source or to build larger molecules.
Absorption- the process by which small molecules are taken
through the cells of the intestine and pass into the
bloodstream.
Assimilation- the process by which products of digestion are
used or stored by the body.
Role of the Large Intestine
By the time food reaches the end of the small intestine, most useful
substances have been removed.
Any remaining undigested material passes into the large intestine,
which also contains mucus, dead cells from intestine lining and
large number of naturally occurring bacteria.
Bacteria here are mutualistic organisms, gaining nutrients and a
suitable habitat, while synthesizing vitamin K for the benefit of the
human host.
The main role of the large intestine is reabsorbing water and mineral ions
such as NA+ and chloride ClWater in the gut contents comes not only from our diet, but also from the
many additional liters that are added into the intestine in digestive juices.
What remains of the original food now is now feces and is egested, or
eliminated from the body via the anus.
Egestion- the process by which undigested material leaves the body at the
end of the gut.
Find 5 diseases or complications of the digestive system. What does
it do? What causes it? What is the treatment?
List he reasons why digestion is essential.
Distinguish between absorption and assimilation
State why enzymes are needed in digestion.
List the ways in which the villus is adapted to increase the efficiency
of absorption of nutrients.