Carbohydrates Quiz

Carbohydrates

• Carbohydrates are polyhydroxy aldehydes or ketones, or compounds yielding these on hydrolysis.
• They are a source of energy, intermediates in the biosynthesis of other biochemical entities (fats and proteins), associated with other entities (glycosides, vitamins, antibiotics), form structural tissues in plants and microorganisms (cellulose, lignin, murein), participate in biological transport, cell-cell recognition, activation of growth factors, and modulation of the immune system.
• Glucose provides energy for the brain and 1 ½ of energy for muscles and tissues.
• Glycogen is stored glucose and serves as reserve energy, while glucose is immediate energy.
• Carbohydrates have a basic composition of (CH₂O)_n or H-C-OH.

Classification of Carbohydrates

• Carbohydrates are also called sugars or saccharides.
• Monosaccharides: cannot be hydrolyzed; examples: glucose, fructose.
• Disaccharides: hydrolyzed into 2 monosaccharides; examples: sucrose, hydrolyzed into glucose and fructose.
• Oligosaccharides: hydrolyzed into few monosaccharides.
• Polysaccharides: polymeric sugars; examples: starch, cellulose.

Characteristics of Carbohydrates

• Carbohydrates consist of carbon, hydrogen, and oxygen.
• They are energy-containing molecules and some provide structure.
• The basic building block is a monosaccharide (CH₂O)_n with n = 3, 5, or 6.
• Two monosaccharides form a disaccharide.

Monosaccharides

• Monosaccharides are also known as simple sugars.
• They are classified by the number of carbons (triose, tetrose, pentose, hexose) and whether they are aldoses or ketoses.
• Most (99%) are straight chain compounds.
• D-glyceraldehyde is the simplest aldose (aldotriose).
• All other sugars end in -ose (glucose, galactose, ribose, lactose, etc.).

Glucose

• The chemical formula for glucose is C₆H₁₂O₆.
• Glucose is a six-sided ring.
• The structure on the left of the diagram is a simplified structure of glucose.

Relative Sweetness of Different Sugars

• Provides a table of relative sweetness of different sugars, including sucrose (100), glucose (74), fructose (174), lactose (16), invert sugar (126), maltose (32), and galactose (32).

Aldoses and Ketoses

• Aldoses have an aldehyde group at one end.
• Ketoses have a keto group, usually at C2.

Stereoisomers

• Chiral centers are carbon atoms with four different atoms or groups bonded to them.
• Compounds with the same structural formula but different spatial configurations are stereoisomers.
• The number of possible isomers (2ⁿ) is determined by the number of asymmetric carbon atoms (n).
• Reference C atom is the penultimate C atom around which mirror images are formed.

Sugar Nomenclature

• For sugars with more than one chiral center, D or L refers to the asymmetric carbon farthest from the aldehyde or keto group.
• Most naturally occurring sugars are D isomers.
• D and L sugars are mirror images of one another.

D and L Designations

• D and L designations are based on the configuration about the single asymmetric C in glyceraldehyde.
• The lower representations are Fischer Projections.

Enantiomers

• Enantiomers are stereoisomers that are mirror images of each other.
• Stereoisomers that differ in configuration only around one chiral center are called epimers.

Hemiacetal and Hemiketal Formation

• An aldehyde can react with an alcohol to form a hemiacetal.
• A ketone can react with an alcohol to form a hemiketal.

Anomers

• Anomers are stereoisomers formed when a ring is formed.
• α is same side with the ring.
• For D-sugars, the OH at the anomeric position is drawn down for α and up for β.

Rules for Drawing Haworth Projections

• For D-sugars, the highest-numbered carbon (furthest from the carbonyl) is drawn up.
• For L-sugars, it is drawn down.
• For D-sugars, the OH group at the anomeric position is drawn down for α and up for β.
• For L-sugars, α is up and β is down.

Pentoses and Hexoses

• Pentoses and hexoses can cyclize as the aldehyde or ketone reacts with a distal OH.
• Glucose forms an intramolecular hemiacetal, as the C1 aldehyde and C5 OH react, to form a six-member pyranose ring, and five-member furanose ring .
• The named representations of the cyclic sugars are called Haworth projections.

Structural Representation of Sugars

• Fischer projection: straight chain representation.
• Haworth projection: simple ring in perspective.
• Conformational representation: chair and boat configurations.

Deoxy Sugars

• Oxygen of the hydroxyl group is removed to form deoxy sugars.
• They are non-reducing and non-osazone forming.
• They are an important part of nucleic acids.

Three Monosaccharides

• Glucose, galactose, and fructose are the three monosaccharides with the formula C₆H₁₂O₆. Individual structures are provided.

Simple Carbs

• All monosaccharides are six-carbon hexoses.
• Each has 6 carbons, 12 hydrogens, and 6 oxygens.
• Arrangements differ, accounting for varying sweetness.

Disaccharides

• Maltose is formed from two a-D-glucose units by an alpha (1→4) glycosidic bond.
• Cellobiose is formed from two beta-D-glucose units joined by beta (1→4) glycosidic bond.

Sucrose

• Sucrose is common table sugar consisting of glucose and fructose.
• Joined by alpha(1->2), beta (2->1), glycosidic bond.
• Sucrose is a non-reducing sugar.

Lactose

• Lactose is milk sugar consisting of galactose and glucose joined by beta(1->4), glycosidic bond.

Reducing and Non-Reducing

• Maltose and Lactose are reducing sugars (with free aldehyde or keto group).
• Sucrose and Trehalose are non-reducing sugars (no free aldehyde or keto group).

Invert Sugar

• Sucrose in solution changes from dextro-rotatory (+66.5) to levo-rotatory (-19.8).
• This is called invert sugar.

Dehydration Synthesis of a Disaccharide

• Provides diagrams of the chemical reactions for the formation of disaccharides (eg: between glucose and fructose) showing the removal of water to form a glycosidic bond.

Formation of Disaccharides

• Provides detailed diagrams showing step-by-step formation of disaccharides (eg; between glucose and fructose).

Starches

• Starches, stored in plant cells, are hydrolyzed by the body to glucose.

Starch

• Starch (composed of amylose and amylopectin) is the most common storage polysaccharide in plants.
• Amylose and Amylopectin are glucose polymers linked by a(1->4); with amylopectin having a(1->6) branches.
• Molecular weights range from thousands to half a million.

Polysaccharides

• Plants store glucose as amylose or amylopectin (collectively called starch).
• Glucose storage in a polymeric form minimizes osmotic effects.
• Amylose is a glucose polymer with α(1→4) linkages.
• The end of the polysaccharide with an anomeric carbon not involved in a glycosidic bond is called the reducing end.

Amylose and Amylopectin

• Amylose and amylopectin are the two forms of starch.
• Amylopectin is highly branched (with branches occurring every 12–30 residues).

Glycogen

• Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin but with more α(1→6) branches.
• The highly branched structure permits rapid glucose release from glycogen stores, essential to animals during exercise.

Glycogen

• Also known as animal starch.
• Stored in muscle and liver (mostly).
• Present in cells as granules (high MW).
• Branches at every 8 or 12 glucose unit (more frequent than in starch).
• Complete hydrolysis yields glucose.
• Glycogen and iodine gives a red-violet color.
• Hydrolyzed by both α and β-amylases and by glycogen phosphorylase.

Cellulose

• Cellulose is a polymer of β-D-glucose attached by β(1,4) linkages.
• Only digested and utilized by ruminants.
• A structural polysaccharide.
• Yields glucose upon complete hydrolysis.
• Partial hydrolysis yields cellobiose.
• Most abundant of all carbohydrates, found in cotton, flax, and wood.
• It gives no color with iodine.
• Held together with lignin in woody plant tissues.
• Cellulose consists of long linear chains of glucose with β(1→4) linkages.
• Every other glucose is flipped over due to β linkage/Waals interaction, meaning the chains can be straight and rigid and pack with a crystalline arrangement in thick bundles called microfibrils.

Special Monosaccharides (Amino Sugars)

• Amino sugars (e.g., glucosamine, galactosamine) have an amino group substituted for a hydroxyl group.
• They are constituents of mucopolysaccharides.

Test your knowledge on carbohydrates with this quiz covering their classification, structure, and functions. Answer questions about monosaccharides, oligosaccharides, and their roles in energy production for the brain. Perfect for students studying biology or nutrition.