L03

• Digestion

  Digestion is the breakdown of food into molecules that can be absorbed into the blood and dissolve into the plasma. There are two types:
• Mechanical Digestion

  •Starts when we chew food in the mouth making food small enough to swallow (bolus)

  •Most happens in the stomach. Muscles churn the food, breaking it up, making chyme.

   

  •This is to increase surface area so enzymes have better access to the nutrients in food
• Chemical Digestion

  •Uses specialised proteins (enzymes) to break large insoluble molecules of protein, carbohydrates and fats into smaller, soluble and easily absorbed molecules. 

   

  •Enzymes are ‘chemical scissors’. 

  •Enzymes are specific and need the right conditions (temperature and pH) to work efficiently.
• Describe two functions of each part of the digestive

  system given below.

  Buccal cavity

  • mechanical digestion by chewing

  • chemical digestion of starch by salivary amylase

  • dissolving food in saliva to allow tasting

  Stomach

  • mechanical digestion by muscular churning

  • chemical digestion of proteins by pepsin

  • some denaturing of bacteria by stomach acid

  (b)
• Pepsin in stomach [1] protein polypeptides [1]

  Peptidases in duodenum[1] polypeptides  peptides and then peptides  amino acids [1]

  Lipases in duodenum[1] fats  fatty acids and glycerol [1]

  2. Liver release bile and the pancreas makes pancreatic juices [1] contain alkaline salts [1] that raise the pH of chime [1]
• Digestion


  1. Begins in the mouth

  2. Tongue and teeth start mechanical digestion

  3. Saliva moistens food

  4. Mechanical digestion in stomach

  5. Muscles churn food into chyme

  6. Chemical digestion starts in mouth
• Mechanical digestion


  Breaking down food into smaller pieces to increase surface area for enzymes
• Chemical digestion


  Using enzymes to break down large insoluble molecules into smaller, soluble and easily absorbed molecules
• Enzymes


  'Chemical scissors' that break down food molecules
• Enzymes


  • Specific and need right conditions (temperature and pH) to work efficiently
• What is absorption?

  Digested food molecules are absorbed in the small intestine . This means that they pass through the wall of the small intestine and into our bloodstream. Once there, the digested food molecules are carried around the body to where they are needed.

  https://www.bbc.co.uk/bitesize/guides/z9pv34j/revision/1 Follow this link to revise and test yourself 

   

  How does this happen?

   

  ◼Enzymes in the gut convert large molecules to small molecules.

  ◼The digested food molecules are transported across the lining of the gut into the blood.
• Carbohydrate digestion


  1. Amylase breaks down starch into maltose

  2. Maltase converts maltose into glucose

  3. Sucrase breaks down sucrose into glucose
• Pepsin


  Enzyme that breaks down proteins into polypeptides in the stomach
• Where does absorption occur?

  ◼Absorption occurs mostly in the Ileum which is in the small intestine.

  ◼The Ileum is very long and is about 6 m in an adult.

  ◼The Ileum has a very large surface area which is necessary for absorption. 

  ◼The lining of the Ileum can be folded into tiny structures called Villi.
• Protein digestion


  1. Pepsin breaks down proteins into polypeptides

  2. Peptidases break polypeptides into peptides

  3. Peptidases break peptides into amino acids
• How is the villus adapted for its job?

  ◼They have microvilli which increase surface area 20 times. 

  ◼It has an epithelium which is one cell thick so there is a short distance for absorption. 

  ◼It has a goblet cell which produces mucus that protects the gut lining against digestion by the body’s own enzymes. 

  ◼It contains capillaries which transports glucose and amino acids. 

  ◼It has a lacteal which transports fatty acids and glycerol. 

  ◼It has a circular muscle layer  which is responsible for peristalsis.
• Fat and nucleic acid digestion

  1. Lipases break down fats into fatty acids and glycerol

  2. Nucleases break down DNA into nucleotides
• Assimilation 

  ◼It is when food molecules are moved into the cells where they are used. 

  ◼Assimilation mainly occurs in the liver.

  ◼The liver sorts out digested food molecules and the food absorbed into the capillaries are sent to the liver.

  ◼The veins leaving the villi join together to form the hepatic portal vein.
• Role of liver and pancreas

  1. Stomach adds gastric juices with hydrochloric acid

  2. Duodenum neutralises acid with alkaline salts from bile and pancreatic juices

  3. Bile emulsifies fats to allow lipases to work
• The main functions of the liver 

  ◼It produces bile which helps in digesting fat. 

  ◼It stores glucose as glycogen.

  ◼The liver converts amino acids  into products important for the body by transamination. Transimination is a chemical reaction that transfers amino groups to form new amino  acids.

  ◼The liver excretes excess amino acids in a process called deamination. 

  ◼It also helps to breakdown alcohol in a process called detoxification.
• By the time chyme reaches the ileum, most nutrients can be easily absorbed
• How the consumption of alcohol affects the liver?

  ◼The liver receives every molecule that the gut absorbs from food. 

  ◼Alcohol is a harmful molecule that a liver can absorb.

  ◼Alcohol is a drug that affects the way the body works. 

  ◼The cells in the liver convert the alcohol to another substance that does not pass to the body’s circulation.

  ◼The substance can be dangerous to the liver  cells.

  ◼If taken in huge amounts a disease will be caused called cirrhosis.

  ◼This can affect the whole body because the blood concentration can’t be controlled. 
• Products of Digestion

  ◼Small molecules (aspirin, alcohol and glucose) absorbed through stomach wall into bloodstream.

  ◼Food in ileum will take 6-12 hours for molecules to be small enough to be absorbed.  They are then absorbed into the bloodstream or lymphatic fluid (fluid between tissues spaces). 

  ▪Carbohydrates broken down into simple sugars, mainly glucose.

  ▪Pips broken down into fatty acids and monoglycerides.

  ▪Proteins broken down into amino acids. 

  ▪Nucleic acids broken down to their constituent molecules ( sugars, phosphates, nitrogenous bases)
• Ileum wall


  • Tightly folded - large surface area for absorption
• Villi


  • Many "finger-like" projections - increase surface area
• Epithelial cells of each villi

  • Have microvilli on membrane - increased surface area