Digestion

• Digestion;

  • digestion is the process by which the large, insoluble molecules in the food we eat are hydrolysed into smaller, more soluble molecules that can be absorbed across cell membranes and assimilated
  • all digestive enzymes function by hydrolysis - the splitting of molecules by breaking bonds and adding water
• Stages of digestion;

  • physical (mechanical) breakdown - large food molecules are broken down into smaller ones by the action of the teeth and then by churning food in the stomach, which increases the surface area for chemical digestion
  • chemical breakdown - where the larger insoluble molecules are broken down into smaller soluble ones by the action of enzymes
• The digestive system;

  the digestive system is made up of a long muscular tube and associated glands. These glands produce digestive enzymes to hydrolyse food into smaller molecules, ready for absorption. The digestive is therefore an exchange surface through which food substances are absorbed
• The digestive system;

  • the salivary glands are situated nearer the mouth. They pass their secretions through a duct into the mouth. The secretions contain the enzyme amylase, which hydrolyses starch into maltose
  • the oesophagus carries food from the mouth to the stomach. Rings of muscle throughout allow for waves of muscle contractions that push the food along, called peristalsis
• The digestive system;

  • the stomach is a muscular sac with an inner layer that produces enzymes. Its role is to store and digest food, especially proteins. It has glands that that produce enzymes to hydrolyse proteins. The stomach juices are highly acidic due to the presence of hydrochloric acid. A thick mucus lining prevents damage to the stomach lining by the gastric juice
• The digestive system;

  • the duodenum is the first, short section of the small intestine, leading from the stomach. Chemicals and enzymes from the gall bladder and pancreas are secreted here
  • the pancreas is a large gland situated below the stomach. It produces a secretion called pancreatic juice. The secretion contains proteases, lipase and amylase
• The digestive system;

  • the gall bladder is a small pouch which stores bile which is produced by the liver
  • the ileum is a long, final section of the small intestine. Food is digested further in the ileum by enzymes that are produced by its walls and by glands. The inner walls of the ileum are folded into villi, giving them a large surface area. This is further increased by millions of microvilli, adapting the ileum for absorption
• The digestive system;

  • the large intestine (colon) absorbs water
  • the rectum is the final section of the intestines. Faeces is stored here before being removed via the anus in a process called egestion
• Digestion of carbohydrates;

  • during digestion, carbohydrase enzymes are responsible for the stepwise breakdown of starch (polysaccharide) into molecules of maltose (disaccharide) and then into molecules of a-glucose (monosaccharide)
  • several enzymes are required as carbohydrates like starch tend to be larger molecules
• Digestion of carbohydrates in the mouth;

  • the hydrolysis of starch begins in the mouth with salivary amylase. Saliva enters the mouth from the salivary glands and is thoroughly mixed with the food during chewing. Saliva contains salivary amylase. This hydrolyses alternate glyosidic bonds in the starch molecule, producing maltose
  • other components of saliva keep the pH around neutral, the optimum pH for salivary amylase to work
• Digestion of carbohydrates in the small intestine;

  • once the food is passed into the small intestine, it mixes with pancreatic juice, secreted from the pancreas. This contains pancreatic amylase, continuing the hydrolysis of starch to maltose
  • muscles in the small intestine push the food along to the ileum. The epithelium lining contains the disaccharide enzyme maltase. It is a membrane-bound disaccharidase
• Digestion of carbohydrates in the small intestine;

  • the maltase hydrolyses the glyosidic bond in the maltose into a-glucose, which is now small enough to be absorbed into the blood by co transport
  • there are also two other common disaccharides in the diet that requires hydrolysis - sucrose (found in fruits) and lactose (found in milk)
• Digestion of disaccharides;

  • sucrose is hydrolysed by a membrane-bound disaccharidase enzyme called sucrase into the monosaccharides fructose and glucose
  • lactose is hydrolysed by another membrane-bound disaccharidase enzyme called lactase into the monosaccharides galactose and glucose
• Digestion of proteins;

  • proteins are large, complex molecules that are hydrolysed by a group of enzymes called peptidases (or proteases)
  • proteins digestion begins in the stomach and continues in the small intestine
  • there are several different peptidases
• Endopeptidases;

  • endopeptidases hydrolyse the peptide bonds between amino acids in the middle of the protein molecule, forming a series of smaller of smaller peptide molecules
  • these are produced by the stomach (pepsin) and the pancreas
• Exopeptidases;

  • exopeptidases hydrolyse the peptide bonds between the terminal amino acids of the peptide molecules formed by endopeptidases, to release dipeptides or single amino acids
  • some of these protease enzymes are produced by the pancreas, whereas others are found in the small intestine wall
• Dipeptidases;

  • dipeptidases hydrolyse the peptide bond between two amino acids of a dipeptide
  • these enzymes are membrane-bound, found on the cell surface membrane of the small intestine epithelial cells lining the ileum
  • the amino acids are then small enough to be absorbed into the bloodstream by co-transport
• Digestion of lipids;

  • lipids are hydrolysed by enzymes called lipases
  • lipases are produced in the pancreas and work in the small intestine, hydrolysing ester bonds in triglycerides to form fatty acids and monoglycerides (a glycerol molecule with a single fatty acid attached)
• Emulsification;

  • firstly, lipids are changed from big droplets into tiny droplets by bile salts which are produced in the liver. This process is called emulsification and increases the surface area for lipases to work, which begin to hydrolyse the lipids into monoglycerides and fatty acids
  • bile is alkaline, which helps to neutralise stomach acid that enters the small intestine and provide the optimum pH for lipase enzyme
• Digestion of lipids;

  • once the lipid has been hydrolysed by lipase, the monoglycerides and fatty acids stick together with the bile in small, spherical structures called micelles, which helps the products of lipid digestion to be absorbed across the epithelial lining of the small intestine
• Absorption of the products of digestion;

  • the ileum is adapted for absorption: it is 6 metres long with a highly folded inner lining. The finger-like projections formed from the folding of the epithelium are called villi
  • the properties of the villi increase the efficiency of absorption
• Adaption of the ileum;

  • many villi - increases the surface area for diffusion
  • villi have very thin walls (one epithelial cell thick) - short diffusion pathway
  • rich blood supply due to each individual villus being supplied by capillaries - maintains the diffusion gradient between the lumen of the ileum and the blood
• Adaptation of the ileum;

  • individual epithelial cells lining the villi have finger-like projections of their cell-surface membranes called microvilli - further increases the surface area for diffusion
  • epithelial cells linking the ileum contain many mitochondria - can provide large amounts of ATP from respiration for active transport of substances into the blood

  absorption of the products of digestion occurs mainly by diffusion, facilitated diffusion and active transport
• Absorption of monosaccharides;

  • glucose is absorbed by active transport with sodium ions via a co-transporter protein
  • galactose is absorbed in the same way using a co-transporter protein
  • fructose is absorbed via facilitated diffusion through a different transporter protein
• Absorption of amino acids;

  • amino acids are absorbed in a similar way to glucose using co-transport with sodium ions
  • it is the sodium ion concentration, rather than the ATP directly, that powers the movement of glucose/amino acids into the cell
• Absorption of monoglycerides and fatty acids;

  • monoglycerides and fatty acids remain in association with the bile salts that initially emulsified the lipid droplets in structures called micelles
  • micelles come into contact with the epithelial cells lining the ileum, where they break down, releasing the monoglycerides and fatty acids
  • as these are non-polar molecules, they easily diffuse across the cell-surface membrane into the epithelial cells
• Absorption of monoglycerides and fatty acids;

  • once inside, monoglycerides and fatty acids are transported to the smooth endoplasmic reticulum where they are recombined to form triglycerides
  • starting in the endoplasmic reticulum and continuing in the Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins to from structures called chylomicrons. Chylomicrons are special particles adapted for the transport of lipids
• Absorption of monoglycerides and fatty acids;

  • chylomicrons move out of the epithelial cells by exocytosis. They enter lymphatic capillaries called lacteals that are found at the centre of each villus, then into the bloodstream
  • when they arrive at a cell, the triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries, from where they diffuse into cells