Heterotrophic Nutrition

• Autotrophic Nutrition


  Fixing inorganic sources into organic molecules by capturing light energy and converting it to stored chemical energy or using energy from redox reactions
• Heterotrophic Nutrition


  Cannot synthesise their own food, feed on complex organic compounds and obtain chemical energy by breaking them down
• Types of Heterotrophic Nutrition

  • Holozoic
  • Saprophytic
  • Symbiotic
• Holozoic


  • Ingest food into body/alimentary canal and digest it, shown by many organisms like predators, detrivores, substrate feeders, filter feeders, fluid feeders, absorptive feeders
• Saprophytic


  • Saprophytes/saprobes secrete digestive enzymes exterior to body, leading to external digestion of complex biomolecules, then absorb the soluble, simple products of digestion, mainly feed on dead and decaying organic matter (decomposers), mostly protoctists and fungi
• Symbiotic


  • Two or more organisms of different species living together in close association (long-term), types include mutualism, commensalism, parasitism
• Stages of Nutrition in Holozoic Animals

  1. Ingestion

  2. Digestion

  3. Absorption

  4. Assimilation

  5. Egestion
• Body Plans of Holozoic Animals

  • Sac-like body (invertebrates with 1 opening)
  • Tube-within-a-tube (food enters through 1 opening and exits from another, e.g. vertebrates)
• Intracellular Digestion


  • Food taken into cell by phagocytosis, food vacuole fuses with lysosome containing hydrolytic enzymes, digestion within cell, e.g. Amoeba, heterotrophic Protoctists
• Extracellular Digestion


  • Cells release hydrolytic enzymes into gut, food digested in lumen of alimentary canal, digestive products absorbed by cells and pass into blood stream, e.g. chordates, annelids, arthropods
• Main Components of Human Digestive System

  • Mouth
  • Pharynx
  • Oesophagus
  • Stomach
  • Small Intestine
  • Large Intestine
  • Anus
• Accessory Organs of Human Digestive System

  • Salivary Glands
  • Pancreas
  • Liver
  • Gall Bladder
• Histology of Alimentary Canal

  • Mucosa (innermost layer lining lumen, contains glandular epithelium, connective tissue, thin smooth muscle layer)
  • Submucosa (layer of connective tissue with blood/lymph vessels and nerves)
  • Muscularis Externa (composed of inner circular and outer longitudinal muscle layers)
  • Serosa (outermost layer of fibrous connective tissue covered by peritoneum)
• Passage of Food through Alimentary Canal

  1. Mouth & Buccal Cavity

  2. Oesophagus

  3. Stomach

  4. Small Intestine
• Functions of Saliva


  • Lubrication and protection from digestive enzymes
  • Contain enzymes like salivary amylase and lysozyme for chemical digestion
• Functions of Stomach


  • Temporary storage of food
  • Mechanical digestion by churning action
  • Chemical digestion by gastric juices containing pepsin, rennin, hydrochloric acid
• Gastric Juice Components


  • Mucus (protects stomach lining)
  • Pepsinogen (inactive precursor of pepsin)
  • Rennin (coagulates milk protein)
  • Hydrochloric acid (creates optimal pH for enzymes, destroys bacteria)
• Control of Gastric Secretions

  • Nervous Control (sight, smell, taste, swallowing reflex)
  • Mechanical Control (stretching of stomach wall)
  • Hormonal Control (gastrin secreted by G-cells)
• Inhibition of Gastric Secretion

  • Secretin (inhibits gastric juice secretion)
  • Cholecystokinin (inhibits release of chyme into duodenum)
• Functions of Small Intestine

  • Main site of digestive processes due to accessory organ secretions
  • Absorption of digested nutrients
• Regions of Small Intestine

  • Duodenum
  • Jejunum
  • Ileum
• Secretions in Duodenum


  • Intestinal Juice (contains enzymes, mucus, water)
  • Pancreatic Juice (contains enzymes, bicarbonate)
  • Bile (contains bile salts, bile pigments)
• Functions of Intestinal Juice Enzymes

  • Enterokinase (activates trypsinogen)
  • Nuclease (breaks down nucleic acids)
  • Glycosidases (break down oligosaccharides)
• Functions of Pancreatic Juice Components

  • Bicarbonate (neutralizes acidic chyme)
  • Pancreatic amylase (breaks down starch)
  • Pancreatic lipase (breaks down triglycerides)
  • Trypsinogen, chymotrypsinogen, nuclease, elastase, carboxypeptidase (break down proteins)
• Functions of Bile


  • Bile salts (emulsify fats)
  • Bile pigments
• Crypts of Lieberkuhn


  Depressions between adjacent villi
• Immobilized enzymes


  On microvilli cell membrane
• Contents of Pancreatic Juice

  • HCO3
  • Pancreatic amylase
  • Pancreatic triglycerides
  • Trypsinogen
  • Chymotrypsinogen
  • Nuclease
  • Elastase
  • Carboxypeptidase
• HCO3


  Alkalinity neutralises acidic chyme, provides optimum pH for enzymes, released first to neutralise acid and establish pH before enzymes released
• Pancreatic amylase


  Breaks down starch to maltose
• Triglycerides

  Broken down to monoglyceride and 2 fatty acids by lipase
• Trypsinogen


  Activated to trypsin by enterokinase
• Chymotrypsinogen


  Activated to chymotrypsin by trypsin
• Nuclease


  Breaks down nucleic acids to nucleotides
• Elastase


  Breaks down protein elastin
• Carboxypeptidase


  Removes amino acids from C-terminus of polypeptides (exopeptidase)
• Contents of Bile


  • Bile salts
  • Bile pigments
• Bile salts


  Emulsify fats, making them into smaller micelles with increased surface area for lipase action. Amphipathic nature means they surround small droplets and prevent them from aggregating, providing mechanical digestion.
• Bile pigments


  Bilirubin (brown) and biliverdin (green) derived from breakdown of haemoglobin, give colour to faeces
• Bile is a yellow/green mucous fluid produced by the liver, stored in the gallbladder and released into the duodenum via the bile duct. It has no digestive enzymes and no role in chemical digestion.