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The Digestive System

Digestion breaks down large molecules into smaller ones:

  • Molecules in our food are polymers (large complex molecules composed of long chains of monomers -small basic molecular units)
  • Proteins and some carbohydrates are polymers. Contain the elements C,H & O    -in proteins the monomers are amino acids (C,H,O & N)
  • Polymers in our food are insoluble (cant be directly absorbed into the blood stream or made into new products
  • Polymers are hydrolysed (add H2O) into smaller, more soluble molecules
  • This occurs during digestion
  • Hydrolysis is catalysed by digestive enzymes.
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Each Part of the digestive system & specific funct

OESOPHAGUS: -Tube takes food from mouth to stomach using muscle contrations (peristalsis)                              -Mucus secreated from tissues in the walls lubricates the foods passage

STOMACH: -Small sac with lots of small folds that allows the stomach to expand                               -The entry and exit of the stomach is controlled by the sphincter muscles.                         -Walls produce gastric juice which helps break down food, it consists of HCl, Pepsin (enzyme) and mucus.       -Pepsin hydrolyses proteins into smaller polypeptide chains. (only works in acidic conditions hence the HCl)     -Peristalsis of the stomach turns the food into chyme. 

SMALL INTESTINE: -DUODENUM and the ILEUM.    -Chye moves along the small intestine by preistalsis.     -In the Duodenum, bile(alkaline) and pancreatic juice neautralise the acidity of chyme and break it down into smaller molecules.    -In the Ileum small, soluble molecule (e.e glucose and amino acids) are absorbed through villi that line the gut wall.    -Absorbed by diffusion, F diffusion and active transport.

LARGE INTESTINE: -Absorbs H2O, Satls and minerals.   -Has folded walls for larger surface area for absorption.   -Some bacteria decmpose undigested material found here

RECTUM: -Faeces stores here.   -Pass through via sphincter muscles and are pooped out. :)

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Pancreas and Salivary Glands

Glands along the digestive system release enzymes to help break down food. 

SALIVARY: -There are 3 main pairs in the mouth.    -They secrete saliva (consists of mucus, mineral salts and salivary amylase)     -Salivary amylase breaks down starch into maltose, which is a disaccharide. 

PANCREASE: -Releases pancreatic juice into the duodenum through the pancreatic duct.                   -Pancreatic juice consists on amylase, trypsin, chymotrypsin and lipase. (also contains sodium hydrogencarbonate which neutralises the acidity of the stomach)


 Carbohydrases catalyses hydrolysis of Carbohydrates

 Protease catalyses hydrolysis of Proteins

 Lipases catalyses hydrolysis of Lipids

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Proteins Intro

  • Made up of long chains of amino acids (Its monomer is and amino acid)
  • Dipeptide is formed when 2 amino acid join together
  • Polypeptide is when more than 2 join together
  • Proteins are made up of 1 or more polypeptides

All amino acids have the same general structure:

-The differences between amino acids is their variable group(R)

Polypeptides are formed by condensation reactions.

  • Amino acids are linked together by condensation reactions to form polypeptides
  • Water is released
  • Bonds are formed between amino acids, PEPTIDE BOND                                                                          
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Proteins Structures

PRIMARY: sequence of amino acids in the peptide chain

SECONDARY: Hydrogen bonds form between the amino acids in the chain. Making it coil into an alpha helix or fold into a beta pleated sheet.

TERTIARY: Coiled or folded chain is often coiled or folded further. More bonds are formed at different parts of the polypeptide chain.    -Proteins made of a single polypeptide chain, the tertiary structure forms their final 3D structure.

QUATERNARY: Some proteins are madde of sereval different poly peptide chains held together by bonds.  The quaternary structure are the way these polypeptide chains are assembled together.

The shape determines the function of the protein.

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Proteins functions and Test!


  • Have a variety of functions
  • They have different structures and shapes, making them specialised to carry out particular jobs. 

Enzymes, Antibodies, Transport proteins and structural proteins.

Biuret Test

 1. The test solution needs to be alkaline, so you add a few drops of NaOH solution

 2. Add some copper (2) sulfate solution 

 -If a protein is present a PURPLE layer forms

-If there is no protein it will remain blue.

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Carbohydrates Introduction

  • Made from monosaccharides   -contain C,H & O
  • Monomer that its made from are monosaccharides e.g. glucose, fructose & galactose.


Hexose Sugar- 6 C's

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Carbohydrates, formation of Di&Polysaccharides

  • Monosaccharides joined together by condensation reactions
  • H2O is released
  • Glycosidic bonds are formed between 2 monosaccharides

DISACCHARIDE: 2 monosaccharides joined together

POLYSACCHARIDE: more than 2 monosaccharides join together.

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Break down of Di/Polysaccharides

  • Disaccharides and Polysaccharides are broken down during digestion
  • Enzymes in the Intestinal Epithelium hydrolyse the break down.

DISACCHARIDE                    HYDROLYSED BY...                      INTO........

Maltose                                      Maltase                                 Glucose + Glucose

Sucrose                                     Sucrase                                 Glucose + Fructose

Lactose                                      Lactase                                 Glucose + Galactose     


  • Lactose is a sugar found in milk, It is digested by lactase, found in the intestines.
  • If you don't have enough lactse, you can't break down lactose in milk
  • The undigested lactose is fermented by bacteria and can cause intestinal complaints like cramps, flactulence and diarrhoea
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Benedicts test for Sugars

  • Sugar is a general term for monosaccharides and disaccharides
  • This test differs depending on which test tou are testing for.


  • All monosaccharides, but there are some disaccharides e.g. maltose
  • Add Benedicts reagent (blue) to a sample and heat
  • If the sample cotains reducing sugars it will gradually turn a brick red colour.


  • non-reducing sugars like sucrose, you have to first break them down into monosaccharides
  • Boil the test solution with dilute HCl and then neutralise using sodium hydrogen carbonate
  • You then have to carry out benedicts test as before.
  • If the result is positive it can either be reducing and non-reducing sugar, so you have to under go benedicts test initially.
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Made up of 2 polysaccharides: AMYLOSE and AMYLOPECTIN

  • They are both composed of long chains of alpha-glucose linked by glycosidic bonds, formed in the condensation reactions.
  • When starch is digested, it is first broken down into maltose and amylase (enzme released in the salivary glands and pancreas)
  • Maltose is then broken down into alpha-glucose molecules by maltase. Which is released by the Intestinal epithelium

Test for starch

Use Iodine to test if there is any starch left.

  • Add Iodine dissolved in Potassium iodide solution to the test sample.
  • If there is any starch present there will be a colour change:

Brown/Orange --->  Blue/Black

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Enzyme Action

  • Enzymes are biological catalysts -they catalyse the metabolic reactions in out body
  • They are Proteins
  • They have an active site that is a specific shape -> this is where the substrate molecule will bind to
  • It is highly specific due to it's tertiary structure.

Enzymes lower the activation energy of a reaction by forming an enzyme-substrate complex

  • If 2 substrate molcules need to be joined, being attached to the enzyme holds them closer together, reducing the repulsion so bonds can be more easily formed.
  • Fitting into the active site puts strains on the bonds in the substrate, so the substrate molecule breaks up more eaisly
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Enzyme Models



  • It is a better theory, as it explains that enzymes are so specific and only bond to one particular substrate.
  • The substrate isn't only the right shape for the active site, but it has to make it change shape.
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Enzymes Properties (tertiary structure)

  • Very Specific --> only catalyse one reaction e.g. maltosase only breaks down moltose
  • Only 1 substrate will fit into the active site
  • Active site is determined by enzymes tertiary structure
  • each enzyme has a different tertiary structure, so a different active site
  • If the tertiary structure of a protein is altered the shape of the active site changes. So a substrate can no longer fit & an enzyme no longer carries out its function
  • the tertiary shape altered by changes in pH ot Temperature
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Factors affecting enzyme activity

TEMPERATURE: -More heat, more kinetic energy --> more successsful collisions, resulting in a reaction.     

  • If temperature gets too high, the reaction stops --> molecules vibrate more, as the temperatures get higher, stronger vibrations breaking some bonds that hold the enzymes shape.
  • Active site changes shape, so the substrate no longer fits
  • Enzyme is now denatured

pH: -All enzymes work at an optimum pH value, above or below this the H+ and OH- ions mess up the ionicc bonds and hydrogen bonds --> causing the active site to change shape

  • The enzyme is now denatured

SUBSTRATE CONC.: -A higher substrate concentration means a faster reaction -->more substrate molecules there are, a collision between substrate and enzyme is more likely and so more active sites will be used.

  • Only true until 'saturation point' after that all the active sites are full.
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Enzyme activity can be inhibited by enzyme inhibitors


  • They are similar in shape to the substarte molecule
  • They compete with the substrate to bind to the active site
  • They block the active site, so no substrate can fit in
  • If there is a high concentration of inhibitor it'll take up nearly all the active site and hardly any substrate will get to the enzyme


  • Binds to the enzyme away from the active site
  • causes the active site to change shape, substrate can no longer bind.
  • Dont compete with the substrate for the active site as they are a different shape
  • increasing the concentration will not make a difference as the enzyme will still be inhibited.
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