Enzymes and the digestive system

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Digestion

There are two types:

  • Physical: Broken down down mechanical action of the teeth or muscles.
  • ChemicalThe hydrolysis of large molecules by enzymes to produce smaller molecules which can be absorbed by the body.

Enzymes and their locations:

  • Amylase is produced in the salivary glands and pancreas and breaks down starch.
  • Protease is produced in the pancreas and stomach and breaks down proteins.
  • Lipase is produced in the pancreas and breaks down fats and lipids.
  • Maltase is produced in the lining of the small intestine and breaks down maltose.
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Tests for Molecules

Reducing sugarsMake sure that the solution is in liquid form, if not, dissolve it. Then add an equal amount of the sample and benedicts solution in a test tube and heat for 5 minutes. If a reducing sugar is present, a brick red colour change will occur.

Non-reducing sugarsAdd equal amounts of the food sample and HCl in a test tube and heat it for 5 minutes to hydrolyse the disaccharides. Then add sodium hydrogencarbonate to neutralise the acid (benedicts does not work in acid) and test with pH paper to ensure it is alkaline. Retest the solution with the reducing sugars test.

StarchAdd iodine and watch for blue-black colour.

Protein: Add equal amount of the solution and sodium hydroxide then a few drops of copper sulfate and watch for a purple colour.

Lipids: Add ethanol to the solution then shake to dissolve the lipid. After that add water and shake again, a cloudy solution indicates a lipid.

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Carbohydrates

Monosaccharides have the general formula (CH2O)n where n is a number between 3 and 7.

Disaccharides are two monosaccharides joined together by a glycosidic bond in a condensation reaction in which a molecules of water is removed. The opposite reaction of this is hydrolysis in which water is added, breaking the glycosidic bond.

Combinations of monosaccharides:

Glucose + Glucose = Maltose

Glucose + Fructose = Sucrose

Glucose + Galactose = Lactose

Mono and disaccharides are soluble but polysaccharides like starch are insoluble.

Lactose Intolerance: Having a diet low in lactose will result in your body producing less lactase (the enzyme that breaks it down). This means that when you ingest lactose you cannot break it down and is passes through to the small intestine where microorganisms break it down and creates large volumes of gas, resulting in bloating and diarrhoea.

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Proteins

Base unit of proteins is called the amino acid has several groups within it, the amino group (NH2), the carboxyl group (COOH), the central Carbon and hydrogen and the R group which varies between different proteins giving them different properties.

They are joined in a condensation reaction and form a peptide bond to make a polypeptide.

Primary Structure: The basic sequence of amino acids bonded together. 

Secondary Structure: This is the shape of an alpha helix and is formed by hydrogen bonding between amino and carboxyl groups which causes the whole structure to twist and coil.

Tertiary Structure: The helix is then twisted further to make a complex shape and is held together by 3 types of bond:

  • Disulfide bonds which are strong bonds
  • Ionic bonds which form between carboxyl and amino groups but are broken by pH changes.
  • Hydrogen bonds which are weak but numerous.

Quaternary Structure: Several polypeptide chains join together along with non-protein groups.

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Enzymes

Enzymes are proteins which act as catalysts for reactions (reduce activation energy) and are not used up in the reaction. The active site of an enzymes is the part that the substrate binds to forming an enzyme-substrate complex and is specific to a certain substrate. Some of the amino acids temporarily bond to the substrate to hold it in place while it is broken down. When the active site of an enzyme changes shape, it becomes useless and is said to be denatured.

Factors affecting enzyme action:

  • Temperature: Raising the temperature will increase the number of collisions between enzymes and molecules, increasing the rate of reaction. If the temperature goes above its optimum temperature, the hydrogen bonds begin to break and they are denatured.
  • pH: All enzymes have an optimum pH but if the pH changes too much then the ionic bonds break and they become denatured.
  • Competitive inhibitors: These have a similar shape to the substrate and fit in the active site but are not broken down. This slows the reaction down but increasing substrate concentration can offset this. 
  • Non-competitive inhibitors: These bond to the enzyme at a place other than the active site and break its bonds, denaturing it which slows down the reaction.
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