⚡️ 2. Central Role of Carbohydrate Metabolism

• What are the final products of carbohydrate digestion in the alimentary tract?

  Glucose, fructose, and galactose
• What percentage of carbohydrate digestion products is glucose on average?

  About 80 percent
• What happens to fructose and galactose after absorption from the intestinal tract?

  They are rapidly converted into glucose in the liver
• What is the primary monosaccharide present in circulating blood?

  Glucose
• What role does the liver play in carbohydrate metabolism?

  The liver converts fructose and galactose into glucose
• How do liver cells promote interconversions among monosaccharides?

  By using appropriate enzymes
• What is the final product when the liver releases monosaccharides back into the blood?

  Almost entirely glucose
• Why is glucose the predominant monosaccharide in circulation?

  The liver contains large amounts of glucose phosphatase
• What happens to glucose-6-phosphate in the liver cells?

  It can be degraded to glucose and phosphate
• What percentage of monosaccharides in the blood are glucose?

  More than 95 percent
• What are the key processes involved in glucose metabolism in the liver?

  • Conversion of fructose and galactose to glucose
  • Interconversion among glucose, fructose, and galactose
  • Release of glucose into the bloodstream
  • Degradation of glucose-6-phosphate to glucose
• What must glucose do before being used by the body’s tissue cells?

  It must be transported through the cell membrane
• Why can't glucose easily diffuse through the cell membrane?

  Because its molecular weight is 180
• What mechanism allows glucose to pass into cells despite its size?

  Facilitated diffusion
• What is the maximum molecular weight of particles that can diffuse readily?

  About 100
• How do protein carrier molecules assist in glucose transport?

  They bind with glucose to transport it
• What happens to glucose transport when its concentration is higher on one side of the membrane?

  More glucose is transported to the lower concentration
• How does glucose transport differ in tissue cells compared to gastrointestinal and renal tubule membranes?

  Active sodium-glucose co-transport is used
• What provides energy for absorbing glucose against a concentration difference?

  Active transport of sodium
• In which cells does the sodium-glucose co-transport mechanism function?

  Special epithelial cells adapted for absorption
• What increases the rate of glucose transport into most cells?

  Insulin
• How much does insulin increase the rate of glucose transport?

  To 10 or more times the normal rate
• What happens to glucose transport in the absence of insulin?

  It is far too little for energy metabolism
• What controls the rate of carbohydrate utilization by most cells?

  The rate of insulin secretion and tissue sensitivity
• What happens to glucose immediately upon entry into the cells?

  It combines with a phosphate radical
• Which enzymes promote the phosphorylation of glucose?

  Glucokinase and hexokinase
• What is the reaction for the phosphorylation of glucose?

  Glucose + ATP → Glucose-6-phosphate
• Why is the phosphorylation of glucose almost completely irreversible?

  Because of its binding with phosphate
• In which cells can the phosphorylation of glucose be reversed?

  Liver, renal tubular, and intestinal epithelial cells
• What enzyme can reverse the phosphorylation of glucose in certain cells?

  Glucose phosphatase
• What is the primary purpose of glucose phosphorylation in most tissues?

  To capture glucose in the cell
• Which cells have phosphatase that allows glucose to diffuse back out?

  Liver cells
• What happens to glucose in most tissues due to phosphorylation?

  It will not diffuse back out