14: The human digestive system

• Macronutrients

  Carbohydrate, protein, fats
• Structure of carbohydrates

  • family of biological compounds containing the elements carbon, hydrogen & oxygen.
• Simplest type of carbohydrate

  Monosaccharide (simple sugar) , e.g. glucose
• Types of carbohydrates

  Monosaccharides (single sugar) can be chemically combined together in pairs to form disaccharide ( double sugar). Many monosaccharide units can be chemically combined together in long chains to form polysaccharides ( complex carbohydrate )
• Structure of proteins

  Family of biological compounds containing the elements carbon, hydrogen, oxygen and nitrogen
• Types of proteins


  Proteins are formed from small molecules called amino acids , which are chemically combined in long chains to form polypeptides, which are folded into a specific three-dimensional shape to form a functional protein.
• Structure of fats

  Family of biological compounds containing the elements carbon, hydrogen and oxygen, but with different ratios compared to carbohydrates.
• Some types of fats are made of

  1 glycerol unit and 3 fatty acid units chemically combined together
• What does calcium give

  Strong teeth and bones
• Vitamin D

  Aid in absorption of calcium
• Vitamin c


  Healthy skin and gums
• Fibre

  Stimulate peristalsis to prevent constipation
• Iron

  For synthesis of harmonic of red blood cells
• In order for nutrient molecules to be absorbed into the bloodstream, they must be


  small, soluble and diffusable enough to cross

  the cell surface membranes of the cells making up the intestinal wall; and the walls of the blood capillaries at the intestine.
• Can complex molecules cross into the bloodstream

  Complex molecules such as starch, proteins and fats are too large to cross into the bloodstream.
• Can simple molecules cross into the bloodstream

  Simple molecules such as monosaccharides, amino acids, glycerol and fatty acids are able to cross into the bloodstream.
• The need for digestion

  the digestion in the human body (and many animal bodies) is to break down complex nutrient molecules into small, soluble and diffusible molecules for absorption.
• What are the two types of digestion that are carried out in the body

  Physical & chemical
• What is phys digestion

  breaking down of large pieces of food into

  smaller pieces (e.g. chewing) * no new substances are formed
• What is chem digestion

  Chemical digestion: breaking down of complex molecules into simpler molecules (e.g. digestion of starch into maltose) * new substances are formed
• Physical digestion is made possible by

  the teeth (cuts food into smaller pieces) & the process of peristalsis, especially in the stomach (churns food and mixes it with digestive juices)
• Chemical digestion is made possible by enzymes which are

  biological catalysts made of protein. They speed up chemical reactions without themselves being chemically changed at the end of the reaction.
• the breakdown of complex nutrient molecules into simpler molecules is

  Extremely slow but is sped up by enzymes produced by the body.
• Carbohydrates are digested by 


  a group of enzymes called carbohydrases
• Proteins are digested by a

  group of enzymes called proteases.
• Fats (lipids) are digested by


  a group of enzymes called lipases.
• What is the alimentary canal

  The alimentary canal refers to the path taken by food between the point of ingestion (eating) and egestion (removal of undigested/unabsorbed food).
• Where does digestion occur

  mouth

  oesophagus (due to saliva still being mixed with food)

  stomach

  duodenum of small intestine
• Absorption occurs in

  Ileum and colon
• The mouth

  chewing breaks up large pieces of food into smaller pieces and mixes it with saliva. This increases surface area to volume ratio of food particles for faster digestion by enzymes

  chem digestion : a carbohydrase enzyme found in saliva digests some starch (polysaccharide) into maltose (disaccharide)
• Oesophagus

  Muscular tube connecting the mouth to the stomach

  Food is moved by peristalsis
• Stomach

  Phys: peristalsis continues in the stomach, churning and mixing food with gastric juice

  Chem: acidic gastric juice contains a protease enzyme which digests protein molecules into polypeptides
• Duodenum (phys)

  the liver produces a detergent known as bile, which is stored in the gall bladder and released into the duodenum. Bile acts on fats, emulsifying large globules of fats into smaller droplets.

  After emulsification, smaller droplets of fats can be chemically digested more efficiently by enzymes.
• Duodenum( chem)

  Carbohydrase enzymes in pancreatic and intestinal juices break down. starch into maltose, and maltose into glucose (monosaccharide).
• Digestion in the duodenum

  (monosaccharide).

  carbohydrase enzymes digest sucrose into glucose and fructos

  Carbohydrase enzymes digest lactose into glucose and galactose.

  Lipids will be digested by lipase to form glycerol and fatty acids in the ratio of 1:3

  Protease enzymes will digest proteins into polypeptides

  Peptidase enzymes will digest polypeptides into amino acids.
• Final product of digestion in the mouth

  Maltose from starch
• Final product of digestion in the stomach

  Polypeptides from proteins
• Final product of digestion in the duodenum

  Maltose(starch) digested by carbohydrates
• Final product of digestion in duodenum

  • Maltose from starch digested by carbohydrase
  • glucose from maltose digested by carbohydrase
  • glucose + galactose from lactose digested by carbohydrase
  • glucose + fructose from sucrose digested by carbohydrase
  • polypeptides from proteins digested by protease
  • amino acids from polypeptides digested by peptidase
  • glycerol + fatty acids from lipids digested by lipase
• Final product of digestion in the stomach

  Polypeptides from proteins digested by protease