6.1.1 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 and 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 release energy (via respiration) to the cells, or to provide cells 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 includes the organs of the alimentary canal and accessory organs that work together to break large insoluble molecules into small soluble molecules

• Peristalsis is series of muscle contractions in the walls of the oesophagus or small intestine that pass like a wave along the alimentary canal
  • This wave forces the bolus of food along the alimentary canal
  • 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 lumen of the oesophagus or small intestine
    • This prevents the food moving backwards towards the mouth
  • Longitudinal muscles contract to reduce the length of that section the oesophagus or the small intestine
    • This forces the food forwards through the alimentary canal

• Once the bolus has reached the stomach, it is churned into a less solid form, called chyme, which continues on to the small intestine
• Mucus is produced to continually lubricate the food mass and reduce friction
• In the small intestine peristalsis is slow compared to the peristalsis that occurs in the oesophagus. It also aids digestion by churning up the food with enzymes as it pushes it along the gut

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

• The pancreas is a gland made up of two types of tissue
  • The first type of tissue secretes the hormones insulin and glucagon into the blood
  • The second type of tissue synthesises and secretes digestive enzymes into the lumen of the small intestine
    • Enzymes are synthesised on 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 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

Acinar cells synthesise and secrete pancreatic enzymes into the ducts and the pancreatic duct secretes the pancreatic juice into the small intestine

• Enzymes are required to carry out the hydrolysis reactions required to digest large insoluble macromolecules into small, soluble, monomers
• The enzymes found in pancreatic juice include:
  • Amylase - for the partial digestion of starch into maltose
  • Lipase - for digestion of triglycerides into fatty acids and glycerol/monoglycerides
  • Phospholipase - for digestion of phospholipids into fatty acids, glycerol and phosphate
  • Protease - for the partial digestion of proteins and polypeptides into shorter peptides

• As well as those enzymes found in pancreatic juices, enzymes are also produced in the walls of the small intestine
  • These enzymes break the products of pancreatic enzyme digestion down into monomers, e.g.
    • Nucleases break down nucleic acids
    • Lactase digests lactose
    • Sucrase digests sucrose
    • Maltase digests maltose
    • Dipeptidase digests dipeptides
• Some enzymes are secreted from the epithelial cells into the intestinal lumen with partially digested food
• Other enzymes e.g. maltase, are immobilised and are attached to the membrane of the epithelial cells where they digest substrate molecules as the food is forced through the small intestine
  • These enzymes are examples of integral proteins

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

Enzymes of the Digestive System Table

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

Image showing the digestion of protein by several enzymes

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

Emulsification of lipids

• When 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 break 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 to glycerol and fatty acids
  • Lipids can also be broken down into monoglycerides and fatty acids
• Lipase enzymes are produced in the pancreas and secreted into the small intestine 

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 molecules bonded together in condensation reactions
• There are two main types of starch
  • Amylose - an unbranched molecule containing [popover id="tpMGiuKDIISd8vkG" label="1,4 glycosidic bonds" only]
  • Amylopectin - 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 the 1,4 glycosidic bonds found in both amylose and amylopectin
  • Amylase action breaks starch down into maltose 
  • 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 short strands are called dextrins.
• The next stage of starch digestion involves enzymes immobilised in the membranes of the microvilli e.g.
  • Maltase - a disaccharidase which hydrolyses maltose into glucose
  • Dextrinase - digests the 1,6 glycosidic bonds found in dextrins
• 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 spaces between villus cells, before entering the villus capillaries
• Note that the lining of the small intestine is folded and there are microvilli present. This increases the surface area for proteins such as membrane-bound disaccharidases and co-transporters

Image showing the digestion of starch by enzymes. Amylase is a carbohydrase enzyme and maltase is a disaccharidase enzyme.

Digestion of nucleic acids

• Nucleases are enzymes which break down DNA and RNA into nucleotides
• They break the phosphodiester bonds between the nucleotide bases
• These can then be absorbed into the blood

The products of digestion

• The products of digestion travel via 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