BIO 220 (quiz 2)

• Three major types of macromolecules

  • Carbohydrates
  • Lipids
  • Proteins
• Digestion


  1. Starts in the mouth

  2. Continues in the stomach

  3. Finishes in the small intestine
• Distinct chemical changes occur as food moves through each compartment in the digestive tract
• The small molecules resulting from digestion are absorbed in the small intestine, along with water, vitamins, and ions
• More water is absorbed in the large intestine, producing feces that are eventually eliminated from the body through the anus
• Digestive tract


  Also known as the alimentary canal or gastrointestinal (GI) tract, begins at the mouth and ends at the anus
• What happens in the mouth

  1. Enzymes in saliva start to break down some components of the food

  2. A wide variety of mouthpart structures have evolved to facilitate capturing and processing of food

  3. Salivary amylase catalyzes the breakdown of carbohydrates

  4. Cells in the tongue secrete lipase, which begins the breakdown of lipids

  5. Salivary glands release water and glycoproteins called mucins, forming the slimy substance called mucus
• Peristalsis


  Coordinated waves of muscle contractions that occur in response to nerve signals, as a reflexive, autonomic reaction to swallowing, propelling food through the esophagus to the stomach
• Esophagus


  • The upper third consists of skeletal muscle, the lower third of smooth muscle, and the middle third of a combination of skeletal and smooth muscles
• Stomach


  A tough, muscular pouch bracketed on both ends by ring-like muscles called sphincters, with a highly acidic lumen that mechanically churns the food with gastric juice
• Secretory cells in the stomach epithelium

  • Parietal cells (source of HCl in gastric juice)
  • Chief cells (contain pepsinogen, a pepsin precursor)
  • Mucous cells (secrete mucus that protects the stomach)
• HCl


  The predominant acid in the stomach, formed by the enzyme carbonic anhydrase in parietal cells
• Digestion in the small intestine

  1. Partially digested food passes from the stomach into the small intestine, where it mixes with secretions from the pancreas and liver and begins to move through the tube

  2. At the end of the small intestine, digestion is complete and most nutrients—along with water—has been absorbed
• Small intestine


  • Has an enormous surface area for absorption of nutrients due to projections called villi, which in turn have projections called microvilli
  • Each villus contains blood vessels and a lymphatic vessel called a lacteal, allowing nutrients to pass quickly from epithelial cells into the body's transport systems
• Protein processing by pancreatic enzymes

  1. Proteases in the small intestine digest polypeptides to monomers

  2. Proteases are synthesized in inactive form by the pancreas, transferred through the pancreatic duct to the small intestine, and activated there by another enzyme known as enterokinase

  3. After activation, each enzyme begins cleaving specific peptide bonds, eventually breaking up polypeptides into amino acid monomers
• Digestion is under both neural and hormonal control


  • Hormones are involved in stomach function (e.g. gastrin stimulates HCl secretion)
  • Hormones are involved in intestinal function (e.g. secretin induces flow of bicarbonate ions, cholecystokinin stimulates secretion of digestive enzymes and molecules involved in lipid digestion)
• Carbohydrate digestion and transport

  1. The pancreas produces nucleases, which digest the RNA and DNA in food, and pancreatic amylase, which continues the digestion of carbohydrates

  2. Pancreatic secretions include digestive enzymes that act on fats, proteins and carbohydrates

  3. Pancreatic lipase breaks certain bonds in complex fats, releasing fatty acids and other small lipids
• Digesting lipids: bile and transport


  1. Hydrophobic fats must be emulsified before digestion can begin

  2. Bile salts, small lipids synthesized in the liver and secreted in bile, raise the pH and emulsify the fat into small globules

  3. Following emulsification, pancreatic lipase breaks bonds in the complex fats, releasing fatty acids and other small lipids

  4. Monoglycerides and fatty acids enter the epithelial cells attached to a protein, then are processed into protein-coated globules called chylomicrons which diffuse into lacteals
• Glucose uptake and production

  • Glucose is the body's primary energy source, resulting from the digestion of foods containing carbohydrates
  • Glucose from digested food circulates in the blood as a ready energy source for cells
• Nutritional homeostasis


  • When digestion is complete, nutrients enter the bloodstream and are delivered to the cells that need them
  • Too much or too little of a nutrient can be problematic or even fatal
• Insulin and glucagon's roles in homeostasis

  • Insulin, produced in the pancreas when blood glucose is high, allows cells to increase glucose uptake and processing, decreasing blood glucose levels
  • Glucagon, secreted by the pancreas if blood glucose falls too low, causes the liver to convert stored glycogen into glucose, raising blood glucose levels
• Diabetes mellitus results when glucose stays in the blood rather than entering the cells, which can happen if not enough insulin is produced or the insulin receptor no longer responds properly
• An epidemic of type II diabetes is currently underway, correlated with increased obesity due to factors like high fat intake and decreased physical activity
• Type 1 diabetes is treated with insulin injections and careful attention to diet, while type 2 diabetes is managed through prescribed diets, monitoring blood glucose, and drugs that increase cellular responsiveness to insulin
• Homeostasis of Blood Glucose Levels - Glucoregulation


  Regulation and counter-regulation
• An epidemic of type II diabetes is currently under way in certain human populations
• Although some individuals have a genetic predisposition for type II diabetes, there is strong evidence that environmental conditions, such as increased fat intake and decreased physical activity, have an important impact
• The incidence of type 2 diabetes is correlated with obesity. A person who has a body mass index greater than or equal to 30 is considered obese. The body mass index is calculated as weight (kg) divided by height (m) squared
• Nutrition-related diseases have become a major public health concern
• Type I diabetes mellitus

  Diabetes develops in people who do not synthesize insulin
• Type II diabetes mellitus


  Diabetes develops in people who have defective versions of the insulin receptor
• Currently, type 1 diabetes mellitus is treated with insulin injections and careful attention to diet; type 2 diabetes is managed through prescribed diets, monitoring blood glucose levels, and drugs that increase cellular responsiveness to insulin
• The challenge is to achieve blood glucose homeostasis in the absence of the body's normal regulatory mechanisms
• Reasons why diabetes mellitus results when glucose stays in the blood rather entering the cells

  • Not enough insulin produced
  • The receptor no longer responds properly to insulin
• Cells must obtain oxygen and expel carbon dioxide continuously to support ATP production by mitochondria, permitting body processes such as growth, movement, digestion, and other functions
• Oxygen and carbon dioxide must be continuously exchanged with the environment
• O2 and CO2, along with wastes, nutrients, and other types of molecules, must be transported throughout the body
• Animals have to take in oxygen and expel carbon dioxide to sustain cellular respiration. Different animal species face different challenges in performing gas exchange
• Gas-exchange organs


  • Maximize the rate of O2 and CO2 diffusion by (1) presenting a large, thin surface area to the environment, and (2) maintaining a steep partial-pressure gradient that favors entry of O2 and elimination of CO2
• Blood


  • Transports gases, along with nutrients and wastes. Hemoglobin is an oxygen-carrying protein that is extremely efficient at taking up oxygen in the lungs and delivering it to tissues