Carbohydrates in Human Biology

Dietary Carbohydrates

• Include sucrose (table sugar), lactose (in milk) and starch (found in potatoes, rice, corn and wheat)
• Cellulose is present in the cell wall of some plants but is not digested by humans because it contains β-glucosidic linkage, which human enzymes cannot hydrolyze
• Cellulose increases the bulk of stools and prevents constipation.

Digestion of Carbohydrates

• Mouth: Salivary amylase starts the digestion of starch
• Stomach: HCl partially hydrolyzes carbohydrates to monosaccharides.
• Small Intestines: Pancreatic amylase completes the digestion of starch
• Intestinal disaccharidases digest disaccharides:
  • Maltase digests maltose into 2 glucose molecules
  • Lactase digests lactose into glucose and galactose
  • Sucrase digests sucrose into glucose and fructose

Absorption of Carbohydrates

• Absorbed by active transport, which requires energy (ATP) and a transporter.

Fate of Absorbed Glucose

• Uptake and Utilization by Different Tissues:
  • Oxidation to produce energy: glycolysis, then Krebs’ cycle, and electron transport chain
  • Conversion to other substances, including:
    • Other carbohydrates: such as ribose for nucleic acids synthesis
    • Lipids
    • Proteins
• Storage of Excess Glucose:
  • As glycogen in liver and muscles (glycogenesis)
  • Excess glucose is converted to triacylglycerols in the adipose tissue (lipogenesis)
• Excretion in Urine:
  • Normally, there is no glucose detectable in urine
  • If the blood glucose exceeds the maximum capacity of the kidneys to reabsorb glucose (>180 mg/dL), it will be excreted in urine

Glycolysis

• A group of reactions by which glucose is converted to pyruvate (in aerobic conditions) or lactate (in anaerobic conditions).
• Glucose is the major source of energy for all cells.
• Glycolysis itself liberates energy from glucose oxidation, and it also provides the mitochondria with pyruvate, which gives acetyl CoA to start the Krebs' cycle.

Glycogen Metabolism

• Introduction:
  • Glycogen is the main storage form of carbohydrates in animals.
  • It is stored mainly in the form of liver glycogen and muscle glycogen.
• Liver glycogen:
  • Maintains blood glucose, especially between meals
  • Is depleted after 12-18 hours of fasting
• Muscle glycogen:
  • Supplies glucose within muscles during contraction
  • Is only depleted after prolonged exercise
• Definitions:
  • Glycogenesis: synthesis of glycogen from glucose; by glycogen synthase
  • Glycogenolysis: breakdown of glycogen to glucose; by glycogen phosphorylase

Gluconeogenesis

• Synthesis of glucose from non-carbohydrate sources (mainly in the liver)
• Occurs by reversal of glycolysis
• Gluconeogenic precursors include lactate, glycerol, and some amino acids
• The main function is to supply blood glucose in case of carbohydrate deficiency (fasting, starvation, and low carbohydrate diet) for more than 18 hours (after glycogen depletion)

Sources of Blood Glucose

• Liver glycogen: Provides glucose between meals.
• Diet: Provides glucose from carbohydrates.
• Gluconeogenesis: Provides glucose from non-carbohydrate sources during prolonged fasting or starvation.

This quiz covers key concepts related to dietary carbohydrates, their digestion, and absorption in the human body. Explore the roles of different carbohydrates, enzymes involved, and the process of glucose utilization. Perfect for students looking to deepen their understanding of nutrition and human physiology.