Polizaharide: Test de cunoștințe

Carbohydrates - General Aspects

• Carbohydrates, also known as sugars, are a crucial class of natural substances found in all living organisms.
• They primarily consist of carbon, hydrogen, and oxygen, often with a hydrogen-oxygen ratio similar to water.
• Carbohydrates constitute over 50% of the dry matter in most plant organisms.
• They provide 50-60% of the total energy produced in animal organisms.
• Many complex carbohydrates play a structural role, forming cell walls in plants and bacteria.
• In animal tissues, carbohydrates frequently combine with proteins.

Classification of Carbohydrates

• Simple sugars (monosaccharides):
  • Trioses (e.g., glyceraldehyde)
  • Tetroses
  • Pentoses (e.g., ribose, xylose)
  • Hexoses (e.g., glucose, fructose, mannose)
• Disaccharides: Combining two monosaccharides (e.g., sucrose, maltose, lactose)
• Oligosaccharides: Short chains of monosaccharides (e.g., trisaccharides, tetrasaccharides)
• Complex sugars (polysaccharides): Larger chains of monosaccharides (e.g., starch, glycogen, cellulose)

Monosaccharides - Structural Properties

• Monosaccharides can be seen as polyalcohol derivatives; oxidation can form them.
• Glyceraldehyde and dihydroxyacetone, arising from glycerol oxidation, are crucial in cell metabolism.
• Oxidation of a primary alcohol group in monosaccharides produces aldoses, while oxidation of a secondary alcohol group gives rise to ketoses.
• Monosaccharides contain chiral (asymmetric) carbons, leading to optical isomers identified as D- or L-isomers.
• A mixture with equal amounts of D- and L-isomers is called a racemic mixture and is optically inactive.

Monosaccharides - Properties

• Monosaccharides are crystalline, white solids soluble in water and slightly soluble in alcohol; they are insoluble in most organic solvents.
• Solutions exhibit sweetness, with variations in intensity depending on the specific sugar.
• They are classified as aldoses (having an aldehyde group) or ketoses (having a ketone group).
• Monosaccharides can exist in linear (open-chain) or cyclic forms, primarily ring structures.
• The ring structures can assume alpha (α) and beta (β) forms, which interconvert in a process known as mutarotation.
• Different forms have varied properties, like optical rotation and reactivity.

Monosaccharides - Reactions

• Oxidation: Leads to aldonic, uronic, or aldaric acids, depending on the oxidation site.
• Reduction: Forms polyalcohols.
• Esterification: Reaction with acids, like glucose-6-phosphate or fructose-1,6-diphosphate.
• Formation of glycosides: Formation of bonds between monosaccharides, leading to disaccharides and polysaccharides.

Important Monosaccharides

• Glucose: Significant energy source for cells; structural component in various polysaccharides (starch, glycogen).
• Fructose: Known for its high sweetness; component of sucrose and inulin.
• Galactose: Component of lactose; found in glycoproteins and glycolipids.
• Ribose: Part of RNA and nucleotides.

Disaccharides and Oligosaccharides

• Disaccharides and oligosaccharides are formed by glycosidic bonds between two or more monosaccharides.
• Common disaccharides include sucrose (glucose and fructose), maltose (two glucose molecules), and lactose (glucose and galactose).
• Their properties, reactivity, and biological functions vary based on the monosaccharide components and the type of glycosidic bond.

Polysaccharides

• Polysaccharides are polymers consisting of numerous monosaccharides linked by glycosidic bonds.
• Important polysaccharides include starch (energy storage in plants, composed of amylose and amylopectin), glycogen (energy storage in animals), and cellulose (structural component in plant cell walls).
• Other polysaccharides including inulin, gums, mucilages, hemicelluloses, and chitin have distinct characteristics in their structure and function.

Homo- and Heteropolysaccharides

• Homo-polysaccharides contain a single type of monosaccharide unit. Examples: starch, glycogen, cellulose.
• Hetero-polysaccharides are composed of two or more different types of monosaccharides. Key examples are hemicellulose, gums, mucilages, hyaluronic acid.

Polysaccharides in Organisms

• These macromolecules are essential components in plants and animals, serving as energy reserves or structural building blocks.
• Starch in plants and glycogen in animals store glucose for later use.
• Cellulose provides structural support in plant cell walls.