digestive

• Digestion


  1. Hydrolysis of large, insoluble biological molecules in food into smaller, soluble molecules

  2. Absorption of smaller molecules across cell membranes into bloodstream

  3. Delivery of smaller molecules to cells in the body
• Enzymes


  Essential for the process of digestion as they catalyse the hydrolysis
• Small soluble molecules


  Used to provide cells with energy via respiration or to build other molecules for cell growth, repair and function
• Hydrolysis of food molecules

  1. Proteins into amino acids

  2. Carbohydrates into simple sugars

  3. Lipids into glycerol and fatty acids
• Human digestive system


  • Glands (salivary and pancreas) produce digestive juices
  • Stomach and small intestine are sites of digestion
  • Liver produces bile
  • Small intestine is site of absorption
  • Large intestine is site of water reabsorption
• Holozoic nutrition in mammals


  • Ingestion
  • Digestion
  • Absorption
  • Assimilation
  • Egestion
• The blue whale (Balaenoptera musculus) is the largest mammal that has ever existed
• During the summer, each blue whale can take in over 4 tonnes of krill per day
• Holozoic nutrition in mammals

  1. Ingestion

  2. Digestion

  3. Absorption

  4. Assimilation

  5. Egestion
• Ingestion


  The act of eating: taking food into the gut, where it is processed
• Food ingested by humpback whale


  • Small fish
  • Crustacea called krill
• Filter feeding


  Straining food out from the water using a baleen, a very large comb-like array of plates on either side of the upper jaw
• Digestion


  Chemical digestion takes place as a series of hydrolytic reactions in different regions of the gut, each with its own specific types of enzymes
• Absorption


  Useful digested products and other soluble substances such as vitamins and minerals are transported across the cells lining the gut wall into the bloodstream and lymph
• Egestion


  Undigested food is eliminated from the gut through the anus
• Assimilation


  Digested food molecules are carried by the bloodstream to the whale's body cells, where they may be stored, broken down further, or used by cells
• A humpback whale can be more than 16 m long and can weigh up to 65 000 kg
• Alimentary canal


  A tubular passage or gut divided into specific regions where different digestive processes take place
• Main parts of the human alimentary canal


  • Mouth (oral cavity or buccal cavity)
  • Tongue
  • Pharynx
  • Oesophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Rectum
  • Anus
• Structure of the alimentary canal wall


  • Epithelial tissue lines the lumen of the gut
  • Glandular tissue secretes chemicals into the gut lumen
  • Mucus and specific enzymes are secreted in some regions
  • Highly folded lining (villi) increases surface area for digestion and absorption
  • Circular and longitudinal muscles propel food along the canal by peristalsis
• Tongue


  Organ of taste, with four types of taste buds: sweet, salt, sour, and bitter
• Saliva


  • Consists mainly of water and mucoproteins which lubricate the food
  • Contains salivary amylase enzyme which hydrolyses starch to maltose
  • Contains buffers and antibacterial agents
• Teeth


  • Living structures with their own supply of nerves and blood vessels
  • Premolars and molars mechanically digest the food, grinding and crushing it into smaller particles
• Swallowing


  1. Larynx moves upwards and pushes the epiglottis over the trachea opening

  2. Circular and longitudinal muscles in the oesophagus contract to move the food to the stomach
• It usually takes about 12 hours for food to travel the full length of the human gut, although the time varies enormously
• Swallowing


  1. Larynx moves downwards

  2. Trachea opens

  3. Circular muscles contract, narrowing the oesophagus and pushing the bolus down

  4. Longitudinal muscles contract, shortening the oesophagus ahead of the bolus
• Oesophagus


  • Contains two layers of smooth muscle which contract involuntarily during the swallowing reflex to move the food quickly to the stomach
  • Inner layer consists of circular muscle, when this contracts it makes the oesophagus narrower (constricted) and longer
  • Outer layer consists of longitudinal muscle, when this contracts it makes the oesophagus wider (dilated) and shorter
  • Muscles work antagonistically
• Peristalsis


  Wave-like contractions that travel down the oesophagus to squeeze the bolus of food down to the stomach
• It is difficult to swallow when there is not much saliva in the buccal cavity
• When food touches the back of the pharynx, it is difficult not to swallow
• The stomach may start to secrete gastric juices before any food reaches it
• When fully stretched, a human stomach can contain about 5 dm³ of food or drink
• The stomach is not usually thought of as an organ of absorption. However, very small lipid-soluble molecules such as alcohol can pass through the phospholipid bilayer of cell membranes and enter the bloodstream. Aspirin is also lipid soluble in acidic conditions and can therefore be absorbed directly from the stomach into the bloodstream.
• Stomach


  • Highly elastic and muscular organ which can expand easily to hold a large meal
  • Wall is highly folded and dotted with pits leading to tubular gastric glands which secrete gastric juices
• Gastric juices


  • Mucus from goblet cells
  • Hydrochloric acid from oxyntic cells
  • Pepsinogen from chief cells
• Pepsinogen


  Precursor, an inactive form of an enzyme which is converted by hydrochloric acid to pepsin, the active enzyme
• Pepsin


  Endopeptidase that hydrolyses large polypeptide chains into smaller polypeptides, works most efficiently in acidic conditions
• Prorennin is important in young mammals
• Control of gastric secretion


  1. Nervous phase: Sight, smell, or taste of food triggers a reflex that causes gastric glands to release secretions

  2. Gastric phase: Substances from food in the stomach stimulate endocrine cells to secrete gastrin, which increases gastric secretions

  3. Intestinal phase: Partially digested food in the duodenum stimulates the release of gastric inhibitory peptide, which inhibits gastric secretion and reduces stomach movements
• Sphincters


  • Cardiac sphincter guards the opening from the oesophagus
  • Pyloric sphincter guards the opening to the small intestine
  • Sphincters are closed most of the time, open briefly to allow food to pass