Carbohydrates and Their Importance

Carbohydrates

• Carbohydrates include sugars, starches, and fibers found in foods such as bread, pasta, and rice.
• The term "carbohydrate" comes from the observation that when sugars are heated, they produce carbon and water (hydrate of carbon).

Importance of Carbohydrates

• Primary source of energy for the body.
• Important structural components in animal and plant cells.
• Key components of nucleic acids, free nucleotides, and coenzymes.
• Major antigens are carbohydrates in nature, e.g., blood group substance.
• Play a biological role as part of hormones and their receptors and enzymes.

Classification of Carbohydrates

• Monosaccharides (simple sugars): contain one sugar unit, cannot be further hydrolyzed, and represent the end products of carbohydrate digestion in the human body.
• Oligosaccharides: contain 2-10 monosaccharide units per molecule and give monosaccharides on acid hydrolysis.
• Polysaccharides: contain more than 10 monosaccharide units per molecule and give monosaccharides on acid hydrolysis.

Monosaccharides

• Classified according to the number of carbon atoms into five important groups.
• Hexoses: monosaccharides containing 6 carbon atoms, help maintain blood glucose, and are an important source of "quick energy" during exercise.

Fiber

• Mostly indigestible carbohydrate.
• Component of plant cell walls.
• Classified according to solubility in water.
• Abundant in whole grains, legumes, fruits, and vegetables.

Carbohydrate Metabolism

• Includes digestion, absorption, utilization, and excretion of carbohydrates.
• Digestion involves the breakdown of carbohydrates into simpler molecules.
• Absorption involves the uptake of digested carbohydrates into the bloodstream.
• Utilization involves the conversion of absorbed carbohydrates into energy or storage forms.
• Excretion involves the removal of excess glucose from the body.

Digestion and Absorption of Carbohydrates

• Digestion starts in the mouth with salivary amylase, which breaks down starch into dextrin and maltose.
• In the stomach, digestion halts temporarily due to the high acidity, which inactivates salivary amylase.
• In the small intestine, pancreatic amylase completes the digestion of carbohydrates into monosaccharides.
• Monosaccharides are absorbed into the bloodstream through active transport against a concentration gradient.

Fate of Absorbed Monosaccharides

• In the liver, fructose and galactose are converted to glucose.
• Glucose is utilized by the body through oxidation to produce energy, conversion to other substances, storage as glycogen or fat, or excretion in urine.

Sources of Glucose to the Human Body

• Carbohydrates in the diet are the primary source of glucose for the body.
• Glycogen stored in the liver and muscles can be degraded to yield glucose during fasting or exercise.
• Glucose can also be synthesized from non-carbohydrate sources through gluconeogenesis.

ATP Production

• Complete oxidation of one glucose molecule results in a net production of 38 ATP molecules.
• Aerobic glycolysis produces 8 ATP, oxidative decarboxylation produces 6 ATP, and the Krebs cycle produces 24 ATP.

Krebs Cycle

• Final common pathway for the oxidation of carbohydrates, fatty acids, and amino acids.
• Occurs in the mitochondria and is aerobic.

Glycogenesis and Glycogenolysis

• Glycogenesis: synthesis of glycogen from glucose in the liver and muscles.
• Glycogenolysis: degradation of glycogen into glucose in the liver and muscles.

Hexose Monophosphate Pathway (HMP) or Pentose Phosphate Pathway (PPP)

• Important source of NADPH for reductive syntheses.
• Source of metabolically active ribose for nucleic acid synthesis.

Regulation of Carbohydrate Metabolism

• The liver, pancreas, and other endocrine glands are involved in controlling blood glucose concentrations.
• During fasting, glucose is supplied to the extracellular fluid from the liver through glycogenolysis and gluconeogenesis.