IB Biology SL

Revision Notes

6.1.1 Digestion

Introduction to Digestion

  • The digestive system is an example of an organ system in which several organs work together to digest and absorb food
  • Digestion is a process in which relatively large, insoluble molecules in food (such as starch, proteins) are broken down into smaller, soluble molecules that can be absorbed into the bloodstream and delivered to cells in the body
  • These small soluble molecules (such as glucose and amino acids) are used either to provide cells with energy (via respiration), or with materials with which they can build other molecules to grow, repair and function
  • The human digestive system is made up of the organs that form the alimentary canal and accessory organs
    • The alimentary canal is the channel or passage through which food flows through the body, starting at the mouth and ending at the anus
    • Digestion occurs within the alimentary canal
    • Accessory organs produce substances that are needed for digestion to occur (such as enzymes and bile) but food does not pass directly through these organs

The human digestive system, IGCSE & GCSE Biology revision notes

The human digestive system includes the organs of the alimentary canal and accessory organs that work together to break large insoluble molecules into small soluble molecules

Alimentary Canal & Accessory Structures Table

The Alimentary Canal table, IGCSE & GCSE Biology revision notes


  • Peristalsis is a mechanism that helps moves food along the alimentary canal
  • Muscles in the walls of the small intestines create waves of contractions which force the bolus along
    • These contractions are controlled unconsciously by the autonomic nervous system
  • Peristalsis is controlled by circular and longitudinal muscles
  • These muscles are smooth muscle (not striated)
    • Circular muscles contract to reduce the diameter of the small intestine
      • This prevents the food moving backwards towards the mouth
    • Longitudinal muscles contract to reduce the length of that section of the small intestine
      • This forces the food forwards through the alimentary canal
  • Mucus is produced to lubricate the bolus and reduce friction
  • Dietary fibre provides the roughage required for the muscles to push against during peristalsis
  • In the small intestine, peristalsis is slow (compared to in the oesophagus), it aids digestion by churning up the food with enzymes as it pushes it along the gut

The mechanism of peristalsis, downloadable IGCSE & GCSE Biology revision notes

Circular and longitudinal muscles in the alimentary canal contract rhythmically to move the bolus along in a wave-like action

Pancreatic Juices

  • The pancreas is a gland made up of two types of tissue
    • The first type of tissue secretes hormones into the blood
    • The second type of tissue synthesizes and secretes digestive enzymes into the lumen of the small intestine
      • Enzymes are synthesized in the ribosomes of the rough endoplasmic reticulum. They are then processed within the Golgi apparatus before being secreted by exocytosis into the lumen of the small intestine
      • The enzymes found in pancreatic juice include:
        • Amylase – for digestion of starch into maltose
        • Lipase – for digestion of triglycerides into fatty acids and glycerol or monoglycerides
        • Phospholipase – for digestion of phospholipids into fatty acids, glycerol and phosphate
        • Protease – for digestion of proteins and polypeptides into shorter peptides

The structure of the pancreas

  • Digestive enzymes are produced in specialised gland cells which are known as acinar cells
  • These cells are located in clusters around the ends of tubes called ducts
  • Ducts join together to form larger ducts and eventually, one pancreatic duct
    • This is where the pancreatic juices, containing enzymes, are secreted into the duodenum of the small intestine

Enzymes in Small Intestine Digestion

  • Enzymes are required to carry out the hydrolysis reactions required to digest large insoluble macromolecules into small soluble monomers
  • As well as those enzymes found in pancreatic juices, enzymes are also produced in the walls of the small intestine
    • Some enzymes are secreted from the epithelial cells into the intestinal lumen with partially digested food
    • Other enzymes e.g. maltase, are attached to the membrane of the epithelial cells where they digest substrate molecules as the food is forced through the small intestine
  • Some substances that we consume, such as cellulose, may remain undigested as humans are unable to produce the enzymes required

Membrane bound maltase, downloadable AS & A Level Biology revision notes

Image showing maltase enzyme attached to the cell-surface membrane of an epithelial cell

Table of Digestive Enzymes

Table of Digestive Enzymes, downloadable AS & A Level Biology revision notes

Small Intestine Digestion

Digestion of proteins

  • Pancreatic juice contains endopeptidases and exopeptidases
    • Endopeptidases hydrolyse peptide bonds within polypeptide chains to produce dipeptides
    • Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains to produce dipeptides
  • Lastly, there are dipeptidase enzymes found within the cell surface membrane of the epithelial cells in the small intestine. These enzymes hydrolyse dipeptides into amino acids which are released into the cytoplasm of the cell

Digestion of Protein, downloadable AS & A Level Biology revision notes

Image showing the digestion of protein by several enzymes

Membrane bound dipeptidase, downloadable AS & A Level Biology revision notes

Image showing dipeptidase inside the cell-surface membrane of an epithelial cell

Emulsification of lipids

  • There are several stages of breakdown that lipids go through prior to digestion
  • In the stomach, solid lipids are turned into a fatty liquid consisting of fat droplets – but this is not digestion
  • When the fatty liquid arrives in the small intestine, bile (containing bile salts) which has been made in the liver and stored in the gallbladder is secreted
  • The bile salts bind to the fatty liquid and breaks the fatty droplets into smaller ones via emulsification
  • Emulsification helps to increase the surface area of the fatty droplets for action of digestive enzymes

Digestion of lipids

  • The digestion of lipids takes place solely in the lumen of the small intestine
  • Lipase enzymes break down lipids (fats) to glycerol and fatty acids
  • Lipase enzymes are produced in the pancreas and secreted into the small intestine by the pancreas

Digestion of Lipids, downloadable AS & A Level Biology revision notes

Image showing the digestion of lipids by lipase enzymes in the lumen of the gut.

Digestion of starch

  • Starch is a macromolecule made up of many α-glucose bonded together in condensation reactions
  • There are two main types of starch
    • Amylose which is an unbranched molecule containing 1,4 glycosidic bonds only
    • Amylopectin which is a branched molecule with 1,4 and 1,6 glycosidic bonds
  • The digestion of starch begins in the mouth and the small intestine with the enzyme amylase
    • Amylase is a carbohydrase that is made in the salivary glands, the pancreas and the small intestine
    • It hydrolyses (breaks down) the 1,4 glycosidic bonds found in both amylose and amylopectin
    • Amylase action breaks starch down into maltose and maltotriose (a chain of 3 glucose molecules)
    • Amylase is unable to digest the 1,6 bonds found in amylopectin as a result, short strands of amylopectin (containing these bonds) are produced. These are called dextrins
  • The next stage of starch digestion involves three enzymes immobilised in the membranes of the microvilli
    • Maltase is a disaccharidase which hydrolyses maltose into glucose
    • Glucosidase digests maltotriose into glucose
    • Dextrinase digests the 1,6 glycosidic bonds found in dextrins to produce glucose
  • There are also other disaccharidases in the cell-surface membrane of the epithelial cells in the small intestine: sucrase and lactase that hydrolysed sucrose and lactose respectively
  • After digestion, the monosaccharides can be absorbed into epithelial cells of the small intestine which pass them into the blood stream
    • Glucose is absorbed by co-transport with sodium ions into the epithelium cells
    • It then moves by facilitated diffusion into the interstitial spaces before entering the capillaries through enlarged pores in the single layer of cells
  • Note that the lining of the small intestine is folded and there are microvilli present. This increases the surface area substantially allowing more membrane-bound disaccharases to fit and more absorption to take place

Digestion of Starch, downloadable AS & A Level Biology revision notesImage showing the digestion of starch by enzymes. Amylase is a carbohydrase enzyme while maltase is a disaccharidase enzyme.

The products of digestion

  • The products of digestion are absorbed into the blood stream which flows into the venules found in the sub-mucosa layer of the small intestine and then through the hepatic portal vein into the liver
  • The liver absorbs excess glucose and stores it as glycogen
    • glycogen has a similar branched structure to amylopectin but is more branched due to having a higher proportion of 1,6 glycosidic bonds

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