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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)

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There are various sections to work through.

Sub-index of biology notes on enzymes and digestion

11. More examples of enzyme controlled reactions: explaining how we digest food

Enzymes and 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.

(i) carbohydrates like starch into sugars by carbohydrase enzymes like amylase,

(ii) animal fats and vegetable oils are broken down into glycerol and long chain fatty 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 chemical 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 respiration.

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 alkaline.

The enzymes in the small intestine work best under alkaline conditions.

The bile helps emulsify the fats by reducing them to tiny droplets which are readily suspended and dispersed in the digestion fluids.

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 digestion.

For more theory see Effect on rate of 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

1. Carbohydrases

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 simple sugars.

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) etc.

2(C6H10O5)n  +  nH2O  ====>  nC12H22O11

or  (C6H10O5)n  +  nH2O  ====>  nC6H12O6  

(n is a very large number!)

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 pH 7.

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.


2. lipases

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 fatty acids

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.

See a 'decay' investigation using milk and lipase  gcse biology revision notes

Its part of the web page Carbon cycle, nitrogen cycle, water cycle, decomposition - decay investigation 


3. proteases

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.



INDEX of biology notes on enzymes

(Enzymes are also dealt with in my GCSE chemistry notes chemistry - biotechnology)

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INDEX of biology notes on enzymes and digestion

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