The Digestive System

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  • Created by: ernily
  • Created on: 22-03-15 19:48

The Digestive System

  • Digestion breaks down large molecules into smaller molecules.
  • Polymers in our food are insoluble, they have to be hydrolysed into smaller, soluble molecules by adding water.
  • Enzymes help us to digest food molecules;
    • Carbohydrases (e.g. amylase) catalyse the hydrolysis of carbohydrates.
    • Proteases (e.g. pepsin) catalyse the hydrolysis of proteins.
    • Lipases (e.g. lipase) catalyse the hydrolysis of lipids.
  • Oesophagus: Takes food to the stomach using muscle contractions called peristalsis. Mucus is secreted to lubricate the food's passage.
  • Stomach: Has lots of folds, allowing it to expand. It's walls produce gastric juice, helping to break down food. Peristalsis turns food into chyme.
  • Small Intestine: Chyme is moved by peristalsis. In the duodenum, bile & pancreatic juice neutralise the chyme and break it down. In the ileum, small molecules are absorbed through villi, which line the gut wall.
  • Large Intestine: Absorbs water, salts & minerals. Has a folded wall which provides a large surface area for absorption.
  • Rectum: Faeces are stored & then pass through the sphincter.
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The Digestive System

  • The Salivary Glands:
    • Secrete saliva which consists of mucus, mineral salts & salivary amylase.
    • Salivary amylase breaks down starch into maltose.
  • The Pancreas:
    • Releases pancreatic juice into the duodenum through the pancreatic duct.
    • Pancreatic juice contains amylase, trypsin, lipase & chrymotrypsin.
    • Also contians sodium hydrogencarbonate which neutralises stomach acid.
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Proteins

  • All amino acids have a carboxyl group and an amino group attached to a carbon atom.
  • Amino acids are linked by condensation reactions to form polypeptides.
  • The bons between amino acids are peptide bonds.
  • Proteins have 4 strucural levels:
    • Primary Structure: Sequence of amino acids.
    • Secondary Structure: Hydrogen bonds form between the amino acids, causing it to coil into an alpha helix, or fold into a beta pleated sheet.
    • Tertiary Structure: More bonds form between different parts of the polypeptide chain. If the protein only consists of one polypeptide chain then this forms it's final 3D structure.
    • Quaternary Structure: The way several different polypeptide chains are assembled together. If the protein consists of more than one polypeptide chain then this forms it's final 3D structure.
  • Functions:
    • Enzymes: Usually spherical due to tight folding, soluble, can break down or synthesise large molecules.
    • Antibodies: Made up of 2 light and heavy polypeptide chains.
    • Transport Proteins: Long polypeptide chains lying parallel to each other with cross links.
  • Lipids Test: Add NaOH to food sample, then add Cu(ii)SO4. 
  • If solution turns purple, protein is present. If it stays blue, there is no protein.
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Carbohydrates

  • Carbohydrates are made from monosaccharides.
  • Monosaccharides join together to form disaccharides and polysaccharides.
    • Monosaccharides are joined by condensation reactions.
    • A water molecule is released and glycosidic bond forms between monosccharides.
  • Disaccharides and polysaccharides are broken down during digestion.
  • Lactose - Intolerance is caused by a lack of lactase.
  • If you don't have enough lactase, you can't properly break down lactose.
  • This is lactose intolerance.
  • Benedict's Test For Sugars - Reducing Sugars:
  • Add Benedict's Reagent to a sample and heat it.
  • If it turns brick red, there are reducing sugars.
  • Benedict's Test For Sugars - Non Reducing Sugars:
  • Boil the test tube with dilute HCl, then neutralise it with sodium hydrogencarbonate.
  • Then carry out the Benedict's test (as above).
  • Starch is made from amylose and amylopectin.
  • Use the iodine test for starch.
  • Add iodine dissolved in potassium iodide solution.
  • If starch is present, the sample goes blue-black.
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Enzyme Action

  • Enzymes are biological catalysts.
  • They are proteins.
  • They have an active site with a specific shape.
  • Substrate molecules bind to the active site.
  • Enzymes are highly specific due to their tertiary structure.
  • Enzymes lower the activation energy of a reaction.
  • An enzyme-substrate complex forms when a substrate fits into the enzyme's active site.
  • This lowers the activation energy because:
    • If two substrate molecules need to be joined, being attached to the enzyme holds them closer together, reducing repulsions so molecules can bond more easily.
    • Fitting into the active site strains the bonds, so molecules break up easily.
  • Lock & Key Model:
    • Substrate fits into active site.
    • Enzyme-substrate complex formed.
    • Products formed, enzyme is unchanged.
  • Induced Fit Model:
    • Substrate fits into active site.
    • Substrate binds, the active site changes shape slightly.
    • Enzyme-substrate complex formed, along with products.
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Enzyme Action

  • Enzymes are very specific, they usually only catalyse one reaction.
  • This is because only one substrate will fit the active site.
  • The shape of the active site is determined by the enzyme's tertiary structure.
  • If the tertiary structure is altered, the shape of the active site will change, meaning that the substrate won't fit.
  • The enzyme will no longer be able to carry out it's function.
  • The tertiary structure can be altered by changes in pH or temperature.
  • The primary structure of a protein is determined by a gene. If a mutation occurs in that gene, the tertiary structure could be changed.
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Factors Affecting Enzyme Activity

  • Temperature:
    • Rise in temperature causes the enzyme's molecules to vibrate more.
    • If the temperature goes above a certain level, the vibration breaks some of the bonds that hold the enzyme's shape.
    • The active site changes shape, so the enzyme and substrate no longer fit.
    • The enzyme is denatured.
  • pH:
    • Above and below the optimum pH, the H+ and OH- ions can mess up the ionic and hydrogen bonds, which will denature the enzyme because the active site will change shape.
  • Substrate Concentration:
    • Higher substrate concentration = faster reaction.
    • More substrate molecules = more frequent collisions.
    • Until saturation point, after which all the active sites are full, so adding more substrate makes no difference.
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Factors Affecting Enzyme Activity

  • Competitive Inhibition:
    • Competitive Inhibitor molecules have a similar shape to the substrate molecules.
    • They compete with substrates and bind to the active site, preventing a reaction from taking place.
    • They block the active site, so no substrate can fit.
    • Enzyme inhibition depends on the relative concentration of the inhibitor and substrate.
    • If there's a high concentration of inhibitor, it takes up nearly all the active sites.
  • Non Competitive Inhibition:
    • Non Competitive Inhibitor molecules bind to the enzyme away from the active site.
    • This causes the active site to change shape.
    • The substrate molecules can no longer fit.
    • They don't compete with substrate molecules because they are a different shape.
    • Increasing the concentration of substrate won't make a difference.
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