lecture recording on 06 February 2025 at 18.22.46 PM

Introduction to Carbohydrates

  • Sugars are the primary fuel stores in our bodies.

  • Most carbohydrates in our diet come from plants, primarily fruits and vegetables, produced through photosynthesis.

  • Carbon dioxide (CO2) exhaled by humans is fixed by plants into carbohydrates, which we consume.

  • Carbohydrates are critical components of nucleic acids, such as DNA and RNA.

  • Carbohydrates are structurally diverse and follow a general chemical formula

    • (C_x(H2O)_y).

Basic Structure of Carbohydrates

  • Simplest carbohydrates have the formula C3H6O3, indicating a consistent ratio of carbon, oxygen, and hydrogen atoms:

    • For every carbon atom, there are two hydrogen atoms and one oxygen atom.

  • Example carbohydrates:

    • Glyceraldehyde (an aldose with an aldehyde functional group)

    • Dihydroxyacetone (a ketose with a ketone group).

Classification of Carbohydrates

1. By Functional Groups

  • Aldoses: Sugars with an aldehyde group; e.g., Glyceraldehyde.

  • Ketoses: Sugars with a ketone group; e.g., Dihydroxyacetone.

2. By Number of Carbons

  • Classified into groups based on carbon count:

    • Triose: 3 carbons (e.g., Glyceraldehyde, Dihydroxyacetone).

    • Tetrose: 4 carbons.

    • Pentose: 5 carbons.

    • Hexose: 6 carbons.

  • Example: Glyceraldehyde is an aldo-triose, and Dihydroxyacetone is a keto-triose.

Common Sugars in Carbohydrates

  • Aldose Examples:

    • Glyceraldehyde, Ribose, Glucose, Galactose.

  • Ketose Examples:

    • Dihydroxyacetone, Fructose.

  • Specific sugar names can be remembered through tables, but memorization isn't required for all.

Fisher Projections of Sugars

  • Fisher Projection: Two-dimensional representation that shows the arrangement of atoms in sugars.

  • Carbon atoms are indicated at each intersection; horizontal groups project out of the plane while vertical groups are directed into the plane.

Chiral Carbons in Sugars

  • Sugars contain chiral centers, leading to different isomers:

    • D and L Isomers: Determined by the orientation of the hydroxyl group on the lowest-numbered chiral carbon.

  • Enantiomers: Non-superimposable mirror images, e.g., D-glucose vs L-glucose.

  • Epimers: Sugars that differ at one specific carbon atom, e.g., D-glucose and D-galactose differ at carbon 4.

Cyclic Structures of Carbohydrates

  • Carbohydrates often exist in cyclic forms instead of linear forms.

  • Cyclic Form Formation: Occurs when a hydroxyl group reacts with a carbonyl carbon, leading to the formation of hemiacetals.

  • Anomeric Carbon: The newly formed chiral center in the cyclic structure; for aldoses, it's carbon 1, for ketoses, it’s carbon 2.

Alpha and Beta Anomers

  • Alpha Anomer: Hydroxyl group on the anomeric carbon is on the opposite side from the highest-numbered exocyclic carbon.

  • Beta Anomer: Hydroxyl group on the same side as the highest-numbered exocyclic carbon.

Examples of Important Sugars

  1. Glucose: An aldose with a ring structure, found in many foods.

  2. Fructose: A keto sugar (ketose), present in many fruits and processed foods.

  3. Galactose: A C-4 epimer of glucose, key in lactose.

  4. Ribose: A key pentose in nucleic acids, specifically RNA.

Modifications of Carbohydrates

  • Carbohydrates can be modified through oxidation, reduction, and by adding functional groups.

  • Examples:

    • Oxidation of glucose can lead to glucuronic acid or glucoic acid.

    • Introduction of amine groups leads to sugars like D-galactosamine from galactose.

    • Sulfonation gives rise to molecules such as D-glucose-1-sulfate.

    • Phosphorylation leads to compounds like D-mannose-6-phosphate, relevant in metabolic pathways.

Reducing and Non-Reducing Sugars

  • Reducing Sugars: Sugars that can revert to their linear forms and participate in reduction reactions due to free aldehyde groups.

  • Example: Glucose can reduce silver ions in a solution—this test is known as the silver mirror test.

  • Non-reducing sugars are those with no free carbonyl, which can’t participate in such reactions.

Summary of Carbohydrates and Their Features

  • To recognize, classify, and name carbohydrates effectively:

    • Understand their structural variations and classifications.

    • Know the rules of chiral centers and nomenclature for D and L designations.

    • Be familiar with the significance of cyclic versus linear forms in terms of stability and function.