digestive system

• Nutrition in humans


  1. Ingestion

  2. Digestion

  3. Absorption

  4. Assimilation

  5. Egestion
• Nutrition


  The process by which organisms obtain food and energy for growth, repair and maintenance of the body
• Components of human nutrition

  • Ingestion - food being taken into the body
  • Digestion - large food molecules are broken down into simpler, soluble molecules that can be absorbed into body cells
  • Absorption - nutrients move from the small intestines into the bloodstream
  • Assimilation - nutrients are used by cells to provide energy or to make new cytoplasm for growth
  • Egestion - undigested matter is removed from the body as faeces
• Parts of the human digestive system

  • Mouth/buccal cavity
  • Pharynx
  • Oesophagus
  • Small intestines (duodenum, jejunum, ileum)
  • Large intestine (colon, rectum)
• Accessory organs of the human digestive system

  • Salivary glands
  • Liver
  • Pancreas
  • Gall bladder
• Physical digestion in the mouth

  1. Teeth break down larger pieces of food into smaller pieces by chewing

  2. Increases surface area-to-volume ratio for the enzyme, salivary amylase to act on
• Chemical digestion in the mouth

  1. Saliva secreted by the salivary glands into the buccal cavity contains salivary amylase

  2. Salivary amylase catalyses the breakdown of starch into maltose
• Other processes in the mouth

  Tongue mixes the food with saliva and rolls it into a bolus, which then moves to the back of the mouth to be swallowed
• Pharynx


  Part of the alimentary canal which connects the buccal cavity to the oesophagus and larynx (voice-box) and trachea (windpipe)
• Peristalsis in the oesophagus

  1. Wall of the oesophagus contains 2 layers of antagonistic muscles: longitudinal muscles on the outer side and circular muscles on the inner side

  2. Contraction and relaxation of the muscles moves food along the oesophagus
• Peristalsis


  Rhythmic, wave-like muscular contractions in the wall of the alimentary canal that enable food to be mixed with digestive juices
• Physical digestion in the stomach

  1. Peristalsis in the stomach churns and physically breaks down the food into smaller pieces

  2. Helps the food to mix with the gastric juices
• Chemical digestion in the stomach

  1. Gastric juice containing hydrochloric acid and enzyme protease, pepsin is secreted into the stomach cavity from the gastric glands

  2. Pepsin catalyses the breakdown of proteins into polypeptides
• Gastric juice


  Provides an acidic environment that is optimal for enzyme activity and kills potentially harmful microorganisms in food
• Sections of the small intestine

  • Duodenum
  • Jejunum
  • Ileum
• Digestion and absorption in the small intestine

  1. Bile produced by the liver and stored in the gall bladder is released into the duodenum

  2. Bile salts break down fats into smaller fat droplets, increasing surface area for enzymes to act on

  3. Pancreatic juice from the pancreas containing enzymes (amylase, protease, lipase) is secreted into the duodenum

  4. Intestinal juice from the small intestine containing enzymes (maltase, protease, lipase) is secreted into the duodenum

  5. Enzymes catalyse the breakdown of starch, proteins and fats
• Alkaline environment in the small intestine

  Provides the optimal pH for the various digestive enzymes
• Adaptations of the small intestine for efficient absorption

  • Inner surface has extensive folding and numerous finger-like projections called villi to increase surface area-to-volume ratio
  • Epithelium of the villus is one cell thick to provide a short diffusion distance for nutrients
  • Epithelial cells have microvilli to further increase surface area-to-volume ratio
  • Each villus has extensive blood capillaries and a lacteal to transport absorbed nutrients away and maintain a steep concentration gradient
  • Epithelial cells have numerous mitochondria to provide energy for active transport of nutrients
• Pancreas


  Produces special group of cells known as the islets of Langerhans which secretes hormones
• Insulin


  Stimulates the conversion of glucose into glycogen in the liver
• Glucagon


  Stimulates the conversion of glycogen into glucose in the liver
• Insulin and glucagon


  Both are involved in controlling the blood sugar level in the body
• Absorption of nutrients into blood capillaries

  1. Blood rich in nutrients will flow through the hepatic portal vein to the liver

  2. Assimilation occurs
• Glucose


  • All cells use it as a source of energy
  • Assimilated and broken-down during respiration to release energy for the cells
• Amino acids


  Converted into new cytoplasm that is used for growth and repair of worn out tissues
• Functions of the liver

  • Produces bile
  • Deamination of amino acids
  • Regulation of blood sugar concentration
  • Detoxification
• Bile production


  1. Liver cells produce bile

  2. Bile emulsifies fats into smaller fat droplets with larger surface area-to-volume ratio for enzyme lipase to act on

  3. Bile is temporarily stored in the gall bladder

  4. When the gall bladder contracts, bile flows into the small intestines via the bile duct
• Deamination of amino acids

  1. Amino groups are removed from the amino acids

  2. Converted to urea

  3. Urea is removed from the body by urine

  4. Remains of the deaminated amino acids are converted into glucose in the liver

  5. Excess glucose will be stored as glycogen
• Regulation of blood sugar concentration

  1. Insulin stimulates liver cells to convert excess glucose into glycogen

  2. Glycogen acts as a temporary storage for glucose

  3. Glucagon stimulates the liver cells to convert glycogen into glucose

  4. Glucose is then transported by the blood to the cells
• Detoxification

  1. The liver converts harmful substances into harmless ones

  2. Example: alcohol is converted by dehydrogenase from harmful acetaldehyde into harmless acetate which can be used in respiration to provide energy for the cells