- chewing makes a larger surface
area of the food for the enzymes to attack.
- salivery amylase hydrolyses
some starch to maltose.
- the walls of the stomach
contain layers of muscle. the functions of which
mechanical digestion, mixing, and peristalsis.
- the gastric glands in the
stomach wall secrete endopeptidase pepsin.
however, it is secreted in its inactive form 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 by hydrolysing bonds in
the centre of the protein molecule.
- 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.
|THE SMALL INTESTINE
- large Surface area
- moist surface
- thin (epithelial) surface/
short absorption pathway
- long/ folds (increasing
- capillary network in villus/
good blood supply
- mitochondria to supply ATP/
energy for active transport
- carrier proteins in membranes.
- the duodenum contains the
- amylase (from pancreas)
hydrolyses starch to maltose.
- lipase (from pancreas) - for
the digestion of lipids. lipids are hydrolysed to
fatty acids and glycerol.
- endopeptidases (from pancreas)
- for the digestion of proteins. these are
hydrolysed to polypeptides.
- exopeptidases (from pancreas)
- 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.
- many 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.
- 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.
- dipeptidases - the small
intestine contains dipeptidases as part of the
intestinal fluid which forms a secretion which
coats the walls of the small intestine epithelial
cells. Dipeptidases hydrolyses the peptide bonds
between amino acids. the dipeptide is broken down
into 2 amino acids, and can then be absorbed.
- the duodenum is the main site
of absorption of all components of digestion,
- 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.
- segmentation also aids
digestion by bringing products into contact with
the mucosa hence enabling absorption to
|ABSORPTION IN THE
- diffusion in capillaries
- active transport/ facilitated
- ATP used by active transport
in cell surface membrane
- glucose/ monomers/
monosaccharides actively transported into
epithelial cells via protein carriers/ channels
- facilitated diffusion from
epithelial cell/ towards blood
|THE ROLE OF THE LIVER
- bile is a biological
detergent, which is produced in the liver.
- 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
- bile also neutralizes stomach
acid, and provides the optimum pH for pancreatic
digestive enzymes to work.
|THE ROLE OF THE
PANCREAS IN DIGESTION
- produces pancreatic juice.
- pancreatic juice contains many
enzymes as detailled above.
- pancreatic juice is rich in
sodium hydrogencarbonate, which:
- neutralizes acid chyme from
- raises the pH to enable
enzymes in the pancreatic juice to work.
|THE LARGE INTESTINE
- the large intestine is made up
of the following parts:
- ceacum and appendix
these are sack-like structures are at the
junction of the small and large intestines.
- the colon and rectum- this is
a muscular tube which contains large amounts of
- peristalsis moves contents
along the colon, and also compacts faeces.
- faeces are stored in the
- 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
- 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