Enzymes: 11. CHEMICAL DIGESTION
Aspects of the chemistry of digestion -
enzymes breaking down big molecules to small molecules e.g. protein to amino
acids (protease), complex carbohydrates to sugars (amylase) and fats and oils to
fatty acids and glycerol (lipase)
Doc Brown's Biology exam study revision notes
There are various sections to work through.
of biology notes on enzymes and digestion
examples of enzyme controlled reactions: explaining how we
digestion is all about big molecules to small molecules!
When we take in food it
contains many large molecules, like carbohydrates, fats and proteins, all of
which must be broken down by digestive enzymes to produce the useful smaller
molecules to supply cells with necessary nutrients.
Enzymes control the chemical processes of
digestion, but there are important physical processes too.
In the mouth, the action of your teeth cut and
grind up your food into a pulp of smaller pieces and moistened with
saliva containing enzymes.
This allows the food to move more easily
through the digestive system.
It also increases the surface for the enzymes
to react with the food particles.
Even the tiniest bits of food cannot pass
through permeable membranes into the blood.
Therefore the food must be broken down at the
molecular level ...
AND so to digestive enzymes!
Enzymes are produced at certain points in the
digestive system to break the food down into small soluble molecules that
can be absorbed into the bloodstream - the process of chemical digestion.
Digestive enzymes break down e.g.
like starch into sugars by carbohydrase enzymes like amylase,
(ii) animal fats and vegetable oils are broken down into glycerol and long
acids by lipase enzymes
(iii) proteins are broken down into amino acids
by protease enzymes.
Apart from fatty acids, sugars, amino acids and
glycerol are all soluble in water and readily pass through the
walls of the digestive system and so easily absorbed into the bloodstream for
the body to use.
The smaller molecules can now pass through cell
membranes for the cells to use.
The small digested molecules can then used for a variety purposes,
all involving enzyme catalysed reactions e.g.
Muscle tissue is built from protein synthesised from
amino acids in the ribosomes (examples of growth).
Fatty tissue is made from newly
synthesised lipid molecules, these are used in building cell
membranes (examples of growth). Lipid molecules are made from fatty
acids and glycerol. Fat molecules are used as an chemical energy
store and in synthesising hormone molecules.
Glycogen, made from glucose, can be used as a
energy store in the body, needed for ATP production in respiration.
On hydrolysis (enzyme catalysed), glycogen breaks
down to reform the smaller molecule glucose - the main 'fuel' for
In plants, the carbohydrate starch is used as
an energy store. When a plant needs energy, the starch is broken
down by enzymes and converted to small sugar molecules
The sugars are
then used to provide energy for the cells from respiration.
The simple sugars can also be converted into
cellulose, the infrastructure of the plant.
Fats and fatty acids are not soluble in water,
but they are essential nutrients.
The body uses bile to neutralise stomach acid and
aid the emulsification of fat.
Bile is produced in the liver and stored in
the gall bladder prior to release into the small intestine.
The stomach acid, hydrochloric acid (HCl)
makes the pH too low, too acidic, for most enzymes to operate
efficiently in the small intestine.
However, bile is alkaline, and neutralises the
stomach acid and makes the ambient pH over 7, so the digestion medium is made
The enzymes in the small intestine work best under
The bile helps emulsify the fats by
reducing them to tiny droplets which are readily suspended and dispersed in the digestion
The emulsification into tiny fat drops greatly
increases the surface for the lipase enzymes to act on, and so
increases the rate of enzyme reaction - increases the speed of
For more theory see
reaction on changing the surface area
Some examples of digestion chemical reactions - ALL
catalysed by specific enzymes
Three examples of 'big' molecules
to 'little' molecules
Carbohydrates are compounds containing the
elements carbon, hydrogen and oxygen e.g. C6H12O6.
They range in size from small simple sugar
molecules like glucose, fructose, sucrose etc. to the huge complex
carbohydrate polymer molecules of glycogen, starch and cellulose.
In many respects, analogous to synthetic
polymers like poly(ethene) or nylon, simple sugars can be
considered the monomer molecules and the complex carbohydrates
the 'natural' polymer molecules
Carbohydrases break down carbohydrates into
Carbohydrase enzymes are made in salivary
glands, pancreas and small intestine.
The carbohydrase enzyme amylase, breaks down starch into
small sugar molecules
- an important digestion reaction.
Enzyme reaction word equation: starch +
water === amylase enzyme ==> maltose, glucose (dextrose)
+ nH2O ====> nC12H22O11
+ nH2O ====> nC6H12O6
(n is a very large
To effect this conversion, the amylase enzyme
is produced in salivary glands, small intestine and the pancreas,
they work best close to a neutral pH with an optimum around pH 6 to
It is important in a living organism,
complex carbohydrates like starch can be broken down to provide
small molecules like glucose - used up in respiration
chemistry to power the life of cells - energy source and facilitate
the synthesis of other molecules.
Lipids, like many organic molecules, only
contain the elements carbon, hydrogen and oxygen.
Lipase enzymes break down lipids like
natural fats and oils (triglyceride esters) into glycerol and long
chain fatty acids. Lipids are NOT polymers because they are not very
long chain molecules.
Lipase enzymes are made in the pancreas.
Enzyme reaction word equation:
lipid == lipase enzymes ==> glycerol + long chain
To effect this conversion, the lipase enzymes
are produced in the pancreas and the small intestine.
(Fats and oils are a sub-group of a class
of molecules called lipids)
The sort of molecular change that takes place -
details you do not need to know for GCSE level biology.
'decay' investigation using milk and lipase gcse
biology revision notes
Its part of the web page
nitrogen cycle, water cycle, decomposition - decay investigation
All amino acid molecules contain carbon,
hydrogen, oxygen and nitrogen and are the 'monomer' molecules for
making the natural 'polymers' we call protein.
Proteases break down proteins into
amino acids - they can work at a very low optimum pH of 2 - caused by the
presence of the strong stomach
acid (hydrochloric acid) which is important because it kills most bacteria
in the stomach.
Note that your stomach produces a thick
mucous to coat the lining wall of the stomach to protect the
tissue being irritated or harmed by the hydrochloric acid.
Protease enzymes are made in the stomach,
pancreas and small intestine.
Enzyme reaction word equation:
one protein molecule === protease ===>
several different amino acids
To effect this conversion, the protease
enzymes are produced in the stomach (here the protease enzyme is
called pepsin), the pancreas and the small intestine.
It is important that in a living organism,
proteins from meat, fish and plant foods can be broken down into
amino acids, which are required to make the specific proteins
required by that organism.
Summary of learning objectives and key words or phrases
Be able to describe the chemistry of digestion by enzymes
that break down big molecules
to small molecules e.g. proteins to amino acids by the enzyme protease,
breaking down complex carbohydrates to
sugars with enzymes like amylase and animal fats or vegetable oils to fatty acids
and glycerol by lipase enzymes.
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