Enzymes and the digestive system


Enzymes and Digestion

The Oesophagus- carries food from the mouth to the stomach.

  • Adapted for transport- made up of a thick muscular wall 
  • Peristalsis- wave of movement to pass the food down the oesophagus.

The Stomach- muscular sac that produces enzymes.

  • Stores and digests food
  • glands that secrete enzymes to digest proteins (pepsin/pepsinogen)
  • other glands secretes mucus- stops stomach being digested by enzymes.

The Small Intestines- long muscular tube

  • Enzymes that are produced by its walls and glands that pour secretions into it.
  • Inner walls folded into villi- increases SA. 
  • SA further increased by microvilli on epithelium of cells of each villus. 
  • Rate of absorption increased- from lumen to cells to bloodstream.

The Large Instestine-  absorbs water

  • food becomes drier and thicker and forms faeces.
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Enzymes and Digestion

The Rectum

  • where the faeces is stored before being removed via the 'bum' (egestion)

The Salivary Glands-  

  • Pass secretions via a duct in the mouth.
  • Secretions contain amylase: breaks down starch into maltose.

The Pancreas- 

  • Produces pancreatic juice- contains proteases, lipases and amylase
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What is digestion?

Physical Breakdown- 

  • The teeth breaks down large insoluble molecules into smaller soluble ones.
  • This creates a larger SA for chemical digestion.
  • Food churned by muscles in the stomach (peristalsis)

Chemical Digestion-

  • Hydrolysis- splits up molecules by adding water to the chemical bonds that hold it together.
  • Carbohydrases- break down carbohydrates into monosacharrides.
  • Lipases- break down lipids into glycerol and fatty acids.
  • Proteases- break down proteins into amino acids

Once these molecules have been broken down they travel through the small intestine in a thick liquid called chyme where they are absorbed into the blood---> they get carried to different parts of the body where they are built up again and associated into body tissues (ASSIMILATION)

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Carbohydrates- monosaccharides

  • The general formula of a monosaccharide is (CH2O)n


Testing for Reducing Sugars

  • Reduction- molecules gain electrons
  • A reducing sugar is a molecule that can donate their electrons to another chemical (Benedicts reagent (copper sulphate)) 
  • When sugar heated with Benedict's reagent--> forms an insoluble red precipitate of copper oxide
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  • When monosaccharides join a glycosidic bond is formed in a condensation reaction(http://www.mrothery.co.uk/images/Image6.gif)
  • Test for non-reducing sugars (disaccharides):
  • first step is same for testing reducing sugars
  • If colour change doesn't occur, add HCL (hydrolyses disaccharide) to food sample
  • heat
  • add sodium hydrogencarbonate (neutralises HCL)
  • heat
  • If non-reducing sugar present reagent will turn orange-brown- polymers been broken down into monomers.
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  • polysaccharides are insoluble- suitable for storageTest for starch:
  • add iodine solution to food sample
  • shake/stir
  • presence of starch indicated by blue-black colouration.


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Starch Digestion

  • Amylase( found in the salivary glands and the pancreatic juice of the pancreas) break down(hydrolyse) the alternate glycosidic bonds of starch into maltose.
  • Maltose is then hydrolysed into alpha glucose by MALTASE, which is found in the lining of the intestine.
  • Food is chewed by the teeth in the mouth= SA of food increased (food broken down into smaller pieces)
  • Saliva mixes w/ food- salivary amylase hydrolyses any starch found in food to maltose. 
  • Mineral salts keep the pH at around neutral (which is optimum working conditions for salivary amylase)
  • Food swallowed--> enters stomach (which is acidic--> denatures salivary amylase= stops the hydrolysis of starch.
  • Peristalsis in the stomach churns food up further= SA ↑  of food for chemical digestion (chyme)
  • Chyme passed into S.Intestine where pancreatic juices containing pancreatic amylase hydrolyses any remaining starch into maltose.
  • Muscles in the instesine move food along the S.Intestine (peristalsis)
  • Epithelial lining produces MALTASE→ hydrolyses maltose in α-glucose→absorbed into epithelial cells of S.Intestine→Assimilation.
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Disaccharide Digestion


  • Sucrose usually contained in cells of food
  • Released by chewing in the mouth
  • Passes through stomach into S.Intestines
  • Epithelial lining produces enzyme SUCRASE→ hydrolyses glycosidic bond between fructose and α-glucose.


  • Epithelial lining in the S.Intestine produces LACTASE→ hydrolyses glycosidic bond to break down lactose into galactose and glucose.


  • undigested lactose reaches L.Intestines where microorganisms break it down into smaller, soluble molecules and large volume of gas→ Ψ ↓= Ψ potential gradient created= causes water to move out of cells into lumen of intestines= smelly diarrhoea (eww)
  • This can be solved by drinking milk with enzyme lactase in it or taking foods rich in calcium.
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Structure of an Amino Acid

  • Amino acids are the basic monomer units which combine to make the polymer, polypeptide.
  • Polypeptides combine to form proteins
  • Every amino acid has a central carbon:
  • amino group (-NH2)
  • carboxy group (-COOH)
  • R group- variety of chemical groups- each A.A has a different R group

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The formation of a peptide bond

  • (http://www.phschool.com/science/biology_place/biocoach/images/translation/peptbond.gif)
  • The -OH from the carboxyl group and the -H from the amino group are removed (makes water during a condensation reaction) to form a new peptide bond between the carbon atom of one A.A and nitrogen of the other A.A.
  • Can be broken down by hydrolysis.
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The primary structure of a protein

  • Many A.A can be joined together in a process called polymerisation
  • Polypeptide chains  are formed from the binding of hundreds of A.A's 
  • The sequence of these chains is called the PRIMARY STRUCTURE
  • The primary structure determines the shape of the protein- so a single change in an amino acid in the chain could cause the shape of the protein to change→proteins will function less well.


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The secondary structure of proteins

  • H of amino group has a overall +ve charge
  • O of the carboxyl group has a overall -ve charge
  • These two groups readily form HYDROGEN BONDS
  • Causes polypeptide chain to be twisted into a 3-D shape: α-helix or ß-pleated sheets.


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Tertiary structure of proteins

  • α-helices can be folded even more to give a more complex and unique 3-D shape/structure of each protein.
  • Maintained by:
  • Disulfide bonds- strong, not easily broken down
  • Ionic bonds- formed between carboxyl and amino groups not involved in making peptide bonds. Weaker, easily broken by changes in pH.
  • Hydrogen bonds- weak, but there are lots of them.
  • Hydrophobic interactions


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Quartenary structure of proteins

  • Large proteins formed containing numerous individual polypeptide chains that are linked in various ways (haemoglobin)
  • There may be a prosthetic group, which is a non-protein (ferrous group) associated with the molecules.

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Test for proteins

  • The Biuret test- detects peptide bonds;
  • add sodium hydroxide to sample solution at room temp.
  • add copper sulphate solution and mix gently
  • purple colouration indicated presence of peptide bonds, hence a protein--- if no protein present, solution stays a blue colour.


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Enzymes as catalysts

  • The reactant molecules must collide with energy to break it down.
  • The product molecules have to have lower energy than the reactants.
  • ACTIVATION ENERGY- minimum energy required for a reaction to take place.
  • Enzymes lower the Ea→ allows reactions to take place at a lower temperature= reactions can take place at body temperature.

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Enzyme structure

  • Active site- only functional part of the tertiary structure of an enzyme
  • This is made up of relatively small numbers of amino acids.
  • Substrate binds to active site of enzyme to form an ENZYME-SUBSTRATE COMPLEX- which is held together by bonds formed by various amino acids.
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Enzyme-substrate complexes...

  • The key (substrate) has a specific shape that fits only a single lock (enzyme)
  • scientists observed that other molecules can bind to sites of the enzyme other than the active site which could change the shape of the active site- so the structure can't be rigid like a lock and key, it has to be flexible
  • The enzyme changes its shape to fit the shape of the substrate- the enzyme is flexible and can mould its way around the substrate (like the way a glove moulds around the hand).
  • As the enzyme changes shape it puts a strain on substrate molecules-- this distorts a certain bond and lowers the Ea needed to break the bond.
  • It is a better explanation:
  • it shows how other molecules can affect enzyme activity
  • how the Ea is lowered.
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Effect on temperature on enzyme action

  • ↑ in temperature= more kinetic energy in molecules= molecules move more rapidly and collide more often= ↑ E-S complexes= rate of reaction ↑.
  • ↑ temp.= hydrogen bonds begin to break= enzyme structure changes shape= substrate bind less easily = ↓ rate of reaction.
  • ↑↑↑ temp.= enzymes to denature= permanent change= enzymes can't function again


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Effect of pH on enzyme action

  • A change in pH alters the charges on the amino acids that makes up the active site= substrate can no longer become attached to active site= E-S complexes can't be formed.
  • Could cause bonds that maintain tertiary structure to break= enzyme changes shape= substrate no longer fits= enzyme denatured.


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Effects of substrate concentration on the rate of

  •  at a low substrate concentration- there are too few substrate molecules to bind the the active sites of the enzymes= rate of reaction is half of the maximum
  • intermediate substrate concentration- all active sites are occupied, maximum rate of reaction is formed.
  • high substrate concentration- addition of more substrate molecules has no effect on the rate of enzyme action because all E-S complexes have been formed.


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Enzyme inhibition


  • Similar molecular shape that of the substrate.
  • It occupies active site of enzyme= compete for active site with original substrate.
  • If substrate concentration ↑= effect of inhibitor ↓
  • Inhibitor not permenantly bound to active site so when it leaves another molecule can take its place.
  • Eventually all substrate molecules will occupy active site of the enzyme but it's the concentration of the inhibitor molecules that determine how long that will take.


  • Attach themselve to a site other than the active site.
  • Alters the shape of the active site of the enzyme.
  • Substrate molecules can no longer occupy the active site- enzyme cannot function.
  • ↑ in substrate concentration doesn't reduce the effect of the inhibitor.
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