2.2.1 The human digestive system

• What is digestion?

  ● Breaking down large, insoluble molecules in food

  ● Into smaller, soluble molecules that can be absorbed into blood
• What is an enzyme?

  ● A type of protein molecule

  ● Biological catalyst → speeds up reactions in organisms without being used up / changed

  ● Each type catalyses a specific reaction
• How do enzymes work?

  1. Substrate (substance on which an enzyme acts) fits into / binds to enzyme active site ● Because shapes of active site and substrate are complementary
  2. This causes a chemical reaction (eg. bonds in substrate broken to produce smaller molecules)
  3. Products released from the unchanged enzyme
• Why can each enzyme only catalyse a specific reaction? Eg. Why can amylase (enzyme) only break down starch (substrate)?

  ● Each enzyme has an active site with a specific (3D) shape

  ● Only a substrate molecule with a complementary shape to this active site can bind / fit
• How are enzymes related to metabolism?

  ● Metabolism is the sum of all the reactions in a cell or the body

  ● Different enzymes catalyse specific metabolic reactions , eg. respiration

  ● Reactions may join smaller molecules to form larger molecules, or break down molecules
• Enzyme: Lipase

  what does it break down?

  • Lipids (fats) into fatty acids + glycerol

  Where is it produced?

  • Pancreas & small intestine

  Where does it act?

  • Small intestine
• Enzyme: Carbohydrase eg. amylase

  What does it break down

  • Carbohydrates into simple sugars eg. starch into glucose

  Where is it produced?

  • Salivary glands, pancreas & small intestine

  Where does it act?

  
  

  *mouth and small intestine
• Enzyme: Protease

  what does it break down?

  Proteins into amino acids

  where is it produced?

  Stomach, pancreas & small intestine
• Enzyme: Protease

  what does it break down?

  • Proteins into amino acids

  where is it produced?

  • Stomach, pancreas & small intestine

  Where does it act?

  Stomach and small intestine
• How are the products of digestion used in the body once absorbed?

  -Fatty acids & glycerol

  ● To build new lipids (fats)
• How are the products of digestion used in the body once absorbed?

  -Simple sugars eg. glucose

  ● To build new carbohydrates

  ● Some glucose used for respiration to release energy
• How are the products of digestion used in the body once absorbed?

  -Amino acids

  ● To build new proteins
• Where is bile produced, stored and released into?

  Produced - liver

  Stored - gall bladder

  Released into - small intestine
• What is the role of bile in digestion?

  1. Alkaline so neutralises hydrochloric acid from stomach

  ● To create optimum conditions for small intestine enzymes

  2. Emulsifies lipids (fat) by converting larger lipid droplets into smaller droplets

  ● To increase surface area for lipase enzyme to work on

  ● To break down / digest fat faster
• What does it mean when an enzyme denatures?

  ● Active site changes shape (eg. due to too high temp. or too high / low pH)

  ● So substrate no longer binds / fits and reaction can no longer be catalysed
• Describe & explain how temperature affects enzyme activity

  ● Describe - as temperature increases up to an optimum, rate of reaction increases

  ○ Explain - molecules gain kinetic energy so more frequent enzyme-substrate collisions

  ● Describe - as temperature increases above optimum, rate of reaction decreases to 0

  ○ Explain - enzymes denature so substrate no longer fits
• Describe & explain how pH affects enzyme activity

  ● Describe - enzyme activity is at a maximum at an optimum pH

  ● Describe - as pH is increased above or decreased below this, enzyme activity decreases

  ○ Explain - above / below optimum pH enzymes denature so substrate no longer fits
• what is this statement wrong?

  “Enzymes are cells.”

  Enzymes are protein molecules, not cells
• why is this statement wrong?

  “Bile is an enzyme.”

  Bile is not an enzyme. It is a substance produced by the liver that helps the enzyme lipase by emulsifying fats and neutralising acid.
• why is this statement wrong?

  "Enzymes die at high temperatures.”

  Enzymes are not living, so they cannot die. At high temperatures they can denature, meaning their active site changes shape.
• The enzyme amylase can only digest starch. Explain why amylase cannot digest other substances. (2)

  ● Amylase has an active site

  ● (With a specific / unique shape) which will only fit / bind to starch / the substrate
• Amylase is an enzyme that breaks down starch. Explain how amylase breaks down starch. Answer in terms of the ‘lock and key theory’. (3)

  ● Starch / substrate binds to active site (of enzyme)

  ● Shape of active site and substrate are complementary

  ● A chemical reaction occurs to produce smaller molecules OR bonds between the (starch) molecules are broken to produce smaller molecules
• A person has a tumour blocking the tube leading from the gall bladder to the small intestine. Explain why this person would have difficulty digesting fat. (4)

  ● No / less bile reaches the small intestine

  ● So less / no emulsification of fat

  ● So smaller surface area for lipase to break down fat

  ● pH of small intestine not neutralised

  ● So lipase is not at its optimum pH to break down fat
• One symptom of pancreatic cancer is weight loss. Explain how pancreatic cancer may cause a person to lose weight. Do not refer to hormones in your answer. (4)

  ● Reduced / no enzyme production from pancreas

  ● Food is not broken down / digested fully

  ● Eg. less glucose absorbed

  ● So less glucose available for respiration so more fat used up in metabolism / respiration

  ● Eg. fewer fatty acids absorbed

  ● So fewer fatty acids available so less fat is stored