Bio unit 6.1

• Research into human physiology is the foundation of modern medicine
• Body functions


  Carried out by specialized organ systems
• Structure of the wall of the small intestine

  • Allows it to move, digest and absorb food
• Blood system


  • Continuously transports substances to cells and simultaneously collects waste products
• Skin and immune system

  • Resist the continuous threat of invasion by pathogens
• Lungs


  • Actively ventilated to ensure that gas exchange can occur passively
• Neurons


  • Transmit the message
• Synapses


  • Modulate the message
• Hormones


  • Used when signals need to be widely distributed
• Digestion and absorption in the small intestine

  1. Contraction of circular and longitudinal muscle layers mixes the food with enzymes and moves it along the gut

  2. Pancreas secretes enzymes into the lumen of the small intestine

  3. Enzymes digest most macromolecules in food into monomers

  4. Villi increase the surface area of epithelium over which absorption is carried out

  5. Villi absorb monomers formed by digestion as well as mineral ions and vitamins

  6. Different methods of membrane transport are required to absorb different nutrients
• The part of the human body used for digestion can be described in simple terms as a tube through which food passes from the mouth to the anus
• The role of the digestive system is to break down the diverse mixture of large carbon compounds in food, to yield ions and smaller compounds that can be absorbed
• Digestion requires surfactants to break up lipid droplets and enzymes to catalyse reactions
• Glandular cells in the lining of the stomach and intestines produce some of the enzymes
• Surfactants and other enzymes are secreted by accessory glands that have ducts leading to the digestive system
• Controlled, selective absorption of the nutrients released by digestion takes place in the small intestine and colon, but some small molecules, notably alcohol, diffuse through the stomach lining before reaching the small intestine
• Functions of different parts of the digestive system

  • Mouth: Voluntary control of eating and swallowing. Mechanical digestion of food by chewing and mixing with saliva, which contains lubricants and enzymes that start starch digestion
  • Esophagus: Movement of food by peristalsis from the mouth to the stomach
  • Stomach: Churning and mixing with secreted water and acid which kills foreign bacteria and other pathogens in food, plus initial stages of protein digestion
  • Small intestine: Final stages of digestion of lipids, carbohydrates, proteins and nucleic acids, neutralizing stomach acid, plus absorption of nutrients
  • Pancreas: Secretion of lipase, amylase and protease
  • Liver: Secretion of surfactants in bile to break up lipid droplets
  • Gall bladder: Storage and regulated release of bile
  • Large intestine: Re-absorption of water, further digestion especially of carbohydrates by symbiotic bacteria, plus formation and storage of feces
• The wall of the small intestine is made of layers of living tissues, which are usually quite easy to distinguish in sections of the wall
• From the outside of the wall going inwards there are four layers: serosa, muscle layers, sub-mucosa, and mucosa
• Peristalsis


  Contraction of circular and longitudinal muscle layers of the small intestine mixes the food with enzymes and moves it along the gut
• The circular and longitudinal muscle in the wall of the gut is smooth muscle rather than striated muscle
• Waves of muscle contraction, called peristalsis, pass along the intestine
• Contraction of circular muscles behind the food constricts the gut to prevent it from being pushed back towards the mouth
• Contraction of longitudinal muscle where the food is located moves it on along the gut
• The contractions are controlled unconsciously not by the brain but by the enteric nervous system, which is extensive and complex
• Swallowed food moves quickly down the esophagus to the stomach in one continuous peristaltic wave
• Peristalsis only occurs in one direction, away from the mouth
• When food is returned to the mouth from the stomach during vomiting, abdominal muscles are used rather than the circular and longitudinal muscle in the gut wall
• In the intestines the food is moved only a few centimetres at a time so the overall progression through the intestine is much slower, allowing time for digestion
• The main function of peristalsis in the intestine is churning of the semi-digested food to mix it with enzymes and thus speed up the process of digestion
• The pancreas contains two types of gland tissue: small groups of cells that secrete the hormones insulin and glucagon, and the remainder of the pancreas that synthesizes and secretes digestive enzymes into the gut
• The digestive enzymes are synthesized in pancreatic gland cells on ribosomes on the rough endoplasmic reticulum, then processed in the Golgi apparatus and secreted by exocytosis
• Pancreatic juice contains enzymes that digest all the three main types of macromolecule found in food: amylase to digest starch, lipases to digest triglycerides and phospholipids, and proteases to digest proteins and peptides
• Digestion in the small intestine

  1. Enzymes secreted by the pancreas into the lumen of the small intestine carry out hydrolysis reactions to digest starch to maltose, triglycerides to fatty acids and glycerol or fatty acids and monoglycerides, phospholipids to fatty acids, glycerol and phosphate, and proteins and polypeptides to shorter peptides

  2. The wall of the small intestine produces additional enzymes that further digest substances, including nucleases to digest DNA and RNA, maltase to digest maltose, lactase to digest lactose, sucrase to digest sucrose, exopeptidases to digest peptides, and dipeptidases to digest dipeptides
• Because of the great length of the small intestine, food takes hours to pass through, allowing time for digestion of most macromolecules to be completed
• Some substances remain largely undigested, because humans cannot synthesize the necessary enzymes, such as cellulose which passes on to the large intestine as one of the main components of dietary fibre
• Absorption


  The process of taking substances into cells and the blood
• Nutrients are absorbed principally in the small intestine
• The rate of absorption depends on the surface area of the epithelium that carries out the process
• The small intestine in adults is approximately seven metres long and 25-30 millimetres wide and there are folds on its inner surface, giving a large surface area