Biomolecules: Carbohydrates Overview

Biomolecules

• Biomolecules are macromolecules, including carbohydrates, lipids, proteins, and nucleic acids.

Carbohydrates: Definition

• Carbohydrates are organic compounds made of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio (Cn(H₂O)n).
• They are vital biological macromolecules found in all living organisms, serving as a primary energy source, structural components, and are involved in various biochemical processes.
• The term "carbohydrate" comes from their general formula, which suggests they are hydrates of carbon.
• Key roles of carbohydrates include energy storage, signaling, and forming building blocks for more complex molecules.
• Carbohydrates are composed of polyhydroxy aldehydes or ketones, or compounds that yield these derivatives via hydrolysis.

General Properties of Carbohydrates

• Carbohydrates are organic compounds containing carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio.
• They are essential for life and play diverse roles like energy storage, structural support, and signaling.

Energy Source

• Monosaccharides like glucose are rapidly metabolized to provide energy in the form of ATP during cellular respiration.
• Polysaccharides (starch and glycogen) are long-term energy storage molecules, broken down into glucose when energy is required.
• Carbohydrates provide 4 kcal/gram of energy, playing a crucial role in the human diet.

Structural Role

• Certain polysaccharides act as structural components in living organisms.
• Cellulose is a major component of plant cell walls providing rigidity and strength. Humans cannot digest cellulose due to lack of cellulase enzymes.
• Chitin is a structural polysaccharide found in the exoskeletons of arthropods and cell walls of fungi.

Biological Functions

• Carbohydrates are stored as glycogen in animals and starch in plants, broken down to glucose for energy.
• Oligosaccharides on cell surfaces are involved in cell signaling and recognition.
• Mucopolysaccharides contribute to tissue lubrication and protection, especially in joints and the extracellular matrix.

Classification of Carbohydrates

• Carbohydrates are categorized based on the number of sugar units and their complexity.
• The main types are monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
• Monosaccharides are simple sugars (e.g., glucose, fructose, galactose).
• Disaccharides are composed of two monosaccharides linked together (e.g., sucrose, lactose, maltose).
• Oligosaccharides are composed of 3-10 monosaccharides.
• Polysaccharides are composed of more than 10 monosaccharides. They can be linear or branched.

Structure of Carbohydrates

• Monosaccharide structure is based on the number of carbon atoms (e.g., trioses, pentoses, hexoses).
• In aqueous solutions, monosaccharides frequently exist in cyclic forms (hemiacetals or hemiketals).

Important Terms

• Asymmetric carbon atom: A carbon atom attached to four different groups or atoms.
• Penultimate (reference) carbon atom: The carbon atom adjacent to the terminal carbon, having an -OH group.
• Anomeric carbon: The carbon atom (C1 in aldoses, C2 in ketoses) that becomes part of the ring structure in cyclic sugars.

Isomerism

• Stereoisomers: Identical molecular formula, but differing in spatial arrangement. Includes enantiomers, diastereomers, and anomers.
• Enantiomers: Non-superimposable mirror images of each other.
• Diastereomers: Stereoisomers that are not mirror images.
• Anomers: Different positions of a hydroxyl group (-OH) on the anomeric carbon (C1 or C2).

Solubility

• Carbohydrates are usually soluble in water due to numerous hydroxyl groups forming hydrogen bonds with water molecules.
• Solubility generally decreases with increasing size. Small carbohydrates are more soluble.

Taste

• Simple carbohydrates (monosaccharides and disaccharides) are typically sweet.
• Fructose is the sweetest sugar.
• Larger carbohydrates like starch and cellulose lack sweetness.

Optical Activity

• Carbohydrates can rotate plane-polarized light.
• Optical activity is dependent on chiral centers (asymmetric carbons) present in the molecule.
• Classified as dextrorotatory (D) or levorotatory (L) based on direction of light rotation.

Reducing Properties

• Some monosaccharides and disaccharides are reducing agents, due to a free aldehyde group.
• Reducing agents cause color change in solutions like Benedict's solution or Fehling's solution.
• Disaccharides like sucrose lack a free aldehyde or ketone group, therefore are non-reducing sugars.

Polymer Formation

• Carbohydrates form glycosidic bonds between monosaccharides through condensation reactions.
• Release of water molecule in the linkage process forms disaccharides, oligosaccharides, and polysaccharides (e.g. starch, cellulose, glycogen).
• Some polysaccharides, like glycogen and amylopectin, have a branched structure due to 1,6-glycosidic bonds in addition to 1,4-glycosidic bonds.

Functions of Monosaccharides

• Glucose is a vital energy source for humans and plants.
• Plants use carbon dioxide and water to produce glucose via photosynthesis.
• Galactose is a component of milk sugar.
• Fructose is found in fruits and honey, and contributes sweetness.
• Ribose and mannose form part of nucleic acids and other co-enzymes/proteins.

Disaccharides

• Disaccharides are composed of two monosaccharide units linked by glycosidic bonds (alpha or beta).
• Common examples are sucrose, lactose, and maltose.
• Results from dehydration synthesis, (loss of water molecule).

Oligosaccharides

• Oligosaccharides are composed of 3-10 monosaccharide units.
• Often attached to proteins and lipids, playing roles in cell recognition and signaling.
• Do not usually exist on their own; they are part of larger structures such as glycoconjugates.

Polysaccharides

• Polysaccharides are composed of more than 10 monosaccharide units.
• They are classified as homopolysaccharides and heteropolysaccharides based on the type of monomer units.
• Can be branched or unbranched, depending on the type of glycosidic bonds.
• Important forms include starch (plants), glycogen (animals), cellulose (plants), and chitin (exoskeletons and fungi).

Heteropolysaccharides

• Composed of two or more different types of monosaccharide units . Usually found linked to proteins and peptides.
• Important types include glycosaminoglycans like hyaluronic acid, chondroitin sulfate, and heparin.

Functions of Polysaccharides

• Energy storage: Starch and glycogen, for example.
• Structural support: Cellulose and chitin.
• Protection: Hyaluronic acid and other mucopolysaccharides.

Additional Notes

• The specific glycosidic bonds determine the structure and function of the polysaccharide.
• The different forms of isomerism (structural, stereoisomerism, and anomerism) are critical for the function of carbohydrate molecules.

This quiz covers the essential aspects of carbohydrates, including their definition, composition, and general properties. Learn about the vital roles carbohydrates play in biological processes and their significance as macromolecules. Test your knowledge of these organic compounds and their importance for life.