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).
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).
Aldoses: Sugars with an aldehyde group; e.g., Glyceraldehyde.
Ketoses: Sugars with a ketone group; e.g., Dihydroxyacetone.
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.
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 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.
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.
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 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.
Glucose: An aldose with a ring structure, found in many foods.
Fructose: A keto sugar (ketose), present in many fruits and processed foods.
Galactose: A C-4 epimer of glucose, key in lactose.
Ribose: A key pentose in nucleic acids, specifically RNA.
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 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.
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.