Digestive System

• Food sustains life
• Digestion in less complex animals


  • Simple due to the absence of a digestive tube with accessory glands
  • In cnidarians like hydras, extracellular digestion occurs throughout the gastrovascular cavity
  • They have a single opening that acts as a mouth and anus
• The most primitive type of gut or digestive tract is found in nematodes
• Parasitic roundworms


  • Feed on the nutrients from the host's partially digested food
  • The food passes through the esophagus into the intestine, where it is further digested by enzymes secreted by the worm
  • Nutrients are absorbed through the intestine's lining and utilized by the worm for energy, growth, and reproduction
  • Some parasitic roundworms like hookworms feed on blood sucked from the host through their hooks
• Pathway of Food in Humans

  Food is ingested through the mouth, chewed/ground into smaller pieces with the help of the teeth, mixed with saliva, and pushed backwards by the tongue->bolus is swallowed and moved to the oropharynx->from the oropharynx, the bolus now enters the esophagus and passes down by peristalsis->cardiac sphincter->the stomach holds the food for about 3 to 4 hours to mix it with gastric juices and digestive enzymes->pyloric sphincter->then, the chyme enters the duodenum where various enzymes completely breakdown the carbs, proteins, and fats->the food molecules are now small enough to be absorbed by the jejunum and ileum->undigested food is channeled into the large intestine or colon where water and ions are reabsorbed and the bulk that remains is now called stool->stool is stored for a while in the rectum until movements into the anus occur and the defecation reflex is initiated
• Enzymes and secretions in the digestive system

  • Salivary amylase, oral cavity
  • Lingual lipase, oral cavity
  • Hydrochloric acid (HCl), stomach
  • Pepsinogen, stomach
  • Pepsin, stomach
  • Intrinsic factor (a protein), stomach
  • Trypsinogen, small intestine, duodenum (produced by the pancreas)
  • Trypsin, small intestine, duodenum (produced by the pancreas)
  • Chymotrypsinogen (produced by the pancreas)
  • Pancreatic lipase (produced by the pancreas)
  • Nucleases (produced by the pancreas)
  • Pancreatic amylase (produced by the pancreas)
  • Sucrase
  • Lactase
  • Maltase
• Salivary amylase


  Initiates carb breakdown
• Lingual lipase


  Starts fat digestion
• Hydrochloric acid (HCl)


  Secreted by parietal cells, maintains acidic pH in stomach
• Pepsinogen


  Secreted by chief cells, transformed by HCl into its active form pepsin
• Intrinsic factor


  Secreted by parietal cells, protects vitamin B12 from digestion
• Trypsinogen


  Converted by enterokinase to its active form trypsin
• Trypsin


  Breaks down proteins into amino acids
• Chymotrypsinogen


  Breaks down proteins into their aromatic amino acids
• Pancreatic lipase


  Breaks down triglycerides into fatty acids and glycerol
• Nucleases


  Breaks down nucleic acids into nucleotides
• Pancreatic amylase


  Breaks down starch, glycogen, and most carbs except cellulose into monosaccharides
• Sucrase, Lactase, Maltase


  "Brush border" enzymes (secreted and found along the entire lining of the small intestine) that break down disaccharides
• Accessory glands of the digestive system

  • Liver
  • Gallbladder
  • Pancreas
• Liver


  • Consists of 4 major cell types: hepatocytes, hepatic stellate cells, Kupffer cells, and sinusoidal endothelial cells
  • Consists of 4 lobes: right, left, quadrate, and caudate
  • Blood circulates through the liver by the portal vein and the hepatic vein
  • Neutralizes and eliminates toxic substances from the blood
  • Stores vitamins, iron, and glucose
  • Synthesizes proteins such as albumin and fibrinogen
  • Converts highly toxic ammonia into urea
  • Produces bile, a fluid that contains cholesterol, bile acids, and bilirubin which aid in lipid digestion
• Pathway of bile


  Bile canaliculi in the liver merge to form bile ducts or intrahepatic ducts->the bile ducts drain into the right and left hepatic ducts->the right and left HDs then merge to form the common hepatic duct->the CHD and the cystic duct (of the gall bladder) join to form the common bile duct (CBD)->the CBD and the pancreatic duct (from the pancreas) or the duct of wirsung then merge to form the ampulla of vater, which finally drains into the duodenum
• Bile may flow into the gall bladder via the cystic duct to be stored, concentrated, and used later, OR it may go directly into the CBD, to the ampulla of vater, and then to the duodenum
• A bile canaliculus (plural: canaliculi; also called bile capillaries) is a thin tube that collects bile secreted by hepatocytes
• Pancreas


  • Both an endocrine and an exocrine gland
  • The islets of Langerhans are responsible for the secretion of insulin and glucagon
  • Digestive enzymes such as trypsinogen, chymotrypsinogen, and carboxypeptidase are secreted into the duodenum by the pancreatic duct and the ampulla of vater
• Soil is a mixture of many ingredients which may include organic compost, minerals, water, air, and microorganisms
• Mineral composition of soil varies depending on the composition of the preexisting/parent rocks
• Microorganisms are very important in recycling nutrients
• Macronutrients


  • C, H, O, N, K, Ca, Mg, P, S
• Micronutrients


  • Cl, Fe, Mn, B, Zn, Mo, Cu
• A deficiency in any of the micronutrients can generate severe effects on plant growth and development
• Magnesium


  A micronutrient that serves as a component of the chlorophyll pigment, which if deficient may lead to chlorosis (a condition in which leaves produce insufficient chlorophyll)
• The Light-Dependent Reactions of Photosynthesis

  1. Occurs in the thylakoid membrane

  2. Water is split into hydrogen ions and oxygen

  3. There are two reaction centers: photosystem (PS) II and PS I

  4. When chlorophyll absorbs a photon, one of the electrons in chlorophyll is boosted to a higher energy level where it becomes reactive or unstable

  5. The energy is converted to ATP and an electron carrier called NADPH

  6. Each photosystem has only one chlorophyll a, hundreds of chlorophyll b, and carotenoids that help collect light energy
• The Light-Independent Reactions (Calvin Cycle) of Photosynthesis


  1. Uses the products of light-dependent reactions, ATP and NADPH, to reduce CO2 to glucose through redox reactions

  2. These reactions occur in the stroma, the fluid-filled part of a chloroplast, outside of the thylakoid membrane
• Plant cells also utilize the sugar from photosynthesis by carrying out cellular respiration
• Comparison of Light-Dependent and Light-Independent Reactions

  • Light-Dependent Reactions: Requires sunlight, occurs in thylakoid membranes, captures light energy and converts it into chemical energy (ATP & NADPH), uses water & sunlight as main reactants, produces ATP, NADPH, & Oxygen
  • Calvin Cycle: Does not require sunlight, occurs in stroma, uses ATP & NADPH from light-dependent reactions to fix carbon dioxide and build sugars, uses carbon dioxide, ATP & NADPH as main reactants, produces glucose (and other sugars)
• Nitrogen-fixing bacteria


  • Rhizobium bacteria live symbiotically with plant roots, help plants fix atmospheric nitrogen into usable forms
  • Rhizobium bacteria move into the cortex of the root and take control of cell division, transforming into bacteroids that produce nitrogenase enzymes
• Mycorrhizal fungi


  • Found in nearly 90% of vascular plants, play an important role in the transfer of phosphorus and in the uptake of some micronutrients, increase the surface area of roots available for nutrient uptake
• Carnivorous plants


  • Grow in nitrogen-poor soils, catch insects and small animals to obtain an adequate supply of nitrogen
• Parasitic plants


  • Tap the resources of other plants by growing close with the host plant, siphon nutrients through the sap of their host
  • Examples: Rafflesia leonardi, Rafflesia speciosa, Balanophora spp.