Transcript Chemistry of Digestion

Chemistry of Digestion
To be able to identify the sites of production and
action of:
• amylases;
• endopeptidases;
• exopeptidases;
• lipase;
• maltase;
• bile.
And describe mechanisms for the absorption of
food by the ileum, including the roles of diffusion,
facilitated diffusion and active transport.
Testing enzyme activity
• If a source of enzyme is placed in an agar plate, the
enzyme will diffuse out through the agar, turning the
substrate into product as it goes.
• There must be a way to distinguish the substrate from
the product, and the reaction will then show up as a
ring around the enzyme source.
• The higher the concentration of enzyme, the higher the
diffusion gradient, so the faster the enzyme diffuses
through the agar, so the larger the ring in a given time.
• The diameter of the ring is therefore proportional to the
enzyme concentration.
• This can be done for many enzymes, e.g. a protein agar
plate can be used for a protease enzyme, or a starch
agar plate can be used for the enzyme amylase.2
wells containing
different amounts
of amylase
starch
agar
blue where starch
is still present
incubate for a few hours
at 40°C, then flood with
iodine solution
A
clear where starch
is absent
B
petri dish
well B has more
amylase than well A
Digestion of Carbohydrates
• By far the most abundant carbohydrate in the human
diet is starch (in bread, potatoes, cereal, rice, pasta,
biscuits, cake, etc)
• There may also be a lot of sugar (mainly sucrose)
and some glycogen (in meat).
Salivary amylase
• Starts the digestion of starch.
• Very little digestion actually takes place,
since amylase is quickly denatured in the
stomach
• It does help to clean mouth and reduce
bacterial infection.
Pancreatic amylase
• Digests all the remaining starch in the
duodenum.
• Amylase digests starch molecules from the
ends of the chains in two-glucose units,
forming the disaccharide maltose.
• Glycogen is also digested here.
Disaccharidases
• In the membrane of the ileum epithelial cells
complete the digestion of disaccharides to
monosaccharides.
• This includes maltose from starch digestion as well
as any sucrose and lactose in the diet.
• There are three important disaccharidase enzymes:
maltose
sucrose
lactose
maltase
sucrase
lactase
glucose
glucose + fructose
glucose + galactose
Monsaccharides
• The monsaccharides (glucose, fructose and galactose)
are absorbed by active transport into the epithelial cells
of the ileum, whence they diffuse into the blood
capillaries of the villi.
• Active transport requires energy in the form of ATP, but
it allows very rapid absorption, even against a
concentration gradient.
• The membrane-bound disaccharidases and the
monosaccharide pumps are often closely associated:
monosaccharides
disaccharide
specific
membrane pump
disaccharidase
epithelial cell
of villus
ATP
blood capillary
ADP+Pi
• The carbohydrates that make up plant
fibres (cellulose, hemicellulose, lignin,
etc) cannot be digested, so pass
through the digestive system as fibre.
Digestion of Proteins
• Protease enzymes are potentially dangerous
because they can break down other enzymes
(including themselves!) and other proteins in
cells.
• To prevent this they are synthesised in the
RER of their secretory cells as inactive
forms, called zymogens.
• These are quite safe inside cells, and the
enzymes are only activated in the lumen of
the intestine when they are required.
Rennin (in gastric juice)
• This converts the soluble milk protein
caesin into its insoluble calcium salt.
• This keeps it in the stomach longer so
that pepsin can digest it.
• Rennin is normally only produced by
infant mammals.
• It is used commercially to make
cheese.
Pepsin (in gastric juice)
• This digests proteins to peptides, 6-12
amino acids long.
• Pepsin is an endopeptidase, which
means it hydrolyses peptide bonds in
the middle of a polypeptide chain.
• It is unusual in that it has an optimum
pH of about 2 and stops working at
neutral pH.
Pancreatic endopeptidases
• These continue to digest proteins and
peptides to short peptides in the duodenum.
• Different endopeptidase enzymes cut at
different places on a peptide chain because
they have different target amino acid
sequences
• This is an efficient way to cut a long chain up
into many short fragments, and it provides
many free ends for the next enzymes to work
on.
Exopeptidases
• These are found in the membrane of the ileum
epithelial cells and complete the digestion of the
short peptides to individual amino acids.
• Exopeptidases remove amino acids one by one
from the ends of peptide chains.
• Carboxypeptidases work from the C-terminal end
• Aminopeptidases work from the N-terminal end,
• Dipeptidases cut dipeptides in half.
• The amino acids are absorbed by active
transport into the epithelial cells of the
ileum.
• From here they diffuse into the blood
capillaries of the villi.
• The membrane-bound peptidases and
the amino acid transporters are closely
associated.
Inactive  active
• Pepsin is synthesised as inactive pepsinogen, and
activated by the acid in the stomach
• Rennin is synthesised as inactive prorennin, and
activated by pepsin in the stomach
• The pancreatic endopeptidases are activated by
specific enzymes in the duodenum
• The membrane-bound peptidase enzymes do not
have this problem since they are fixed, so cannot
come into contact with cell proteins.
• The lining of mucus between the stomach wall and
the food also protects the cells from the protease
enzymes once they are activated.
Digestion of Triglycerides
• Fats are emulsified by bile salts to form small
oil droplets called micelles, which have a
large surface area.
• Pancreatic lipase enzymes digest
triglycerides to fatty acids and glycerol in the
duodenum.
• Fatty acids and glycerol are lipid soluble and
diffuse across the membrane (by lipid
diffusion) into the epithelial cells of the villi
in the ileum.
• In the epithelial cells of the ileum triglycerides are resynthesised and combine with proteins to form tiny
lipoprotein particles called chylomicrons.
• The chylomicrons diffuse into the lacteal - the lymph
vessel inside each villus. The emulsified fatty
droplets give lymph its milky colour, hence name
lacteal.
• The chylomicrons are carried through the lymphatic
system to enter the bloodstream at the vena cava,
and are then carried in the blood to all parts of the
body. They are stored as triglycerides in adipose
(fat) tissue.
• Fats are not properly broken down until they are
used for respiration in liver or muscle cells.
Digestion of Nucleic acids
• Pancreatic nuclease enzymes digest nucleic
acids (DNA and RNA) to nucleotides in the
duodenum.
• Membrane-bound nucleotidase enzymes in
the epithelial cells of the ileum digest the
nucleotides to sugar, base and phosphate,
which are absorbed.
Other substances
• Many substances in the diet are composed of small
molecules that need little or no digestion.
• These include sugars, mineral ions, vitamins and water.
These are absorbed by different transport mechanisms:
• Cholesterol and the fat-soluble vitamins (A, D, E, K) are
absorbed into the epithelial cells of the ileum by lipid
diffusion
• Mineral ions and water-soluble vitamins are absorbed by
passive transport in the ileum
• Dietary monosaccharides are absorbed by active
transport in the ileum
• Water is absorbed by osmosis in the ileum and colon.
Digestion in Fungi
• Fungi are not consumers like animals, but
are either saprophytes (decomposers), or
pathogens.
• They therefore use saprophytic nutrition,(as
opposed to holozoic) which means they do
not ingest their food, but use extracellular
digestion.
• Fungi secrete digestive enzymes
(carbohydrases, proteases and lipases) into
the material that surrounds them and then
absorb the soluble products (sugars, amino
acids, etc).
• Fungi are usually composed of long thin threads called
hyphae.
• These grow quickly, penetrating dead material such as
leaves, as well as growing underground throughout soil.
• The cotton wool appearance of bread mould growing on
decaying bread is typical of a mass of hyphae, called a
fungal mycelium.
• These thin hyphae give fungi a large surface area to
volume ratio.
• They contain many nuclei, since they are formed from
the fusion of many cells.