Topic 7

Carbohydrates are often referred to as sugars.
Monosaccharides are single carbohydrate units.
Polysaccharides are multiple monosaccharide units joined together.
Nutrition Labels: "Sugars" typically refer to mono- and disaccharides (simple sugars), such as glucose, fructose, sucrose, and lactose. What is often missing from this breakdown is starch, which is also a carbohydrate.

2.1 Basic Terminology (Exam Focus)

Pentose: A sugar with 5 carbon atoms.
Hexose: A sugar with 6 carbon atoms.
Ketose: A sugar where the carbonyl (C=O) group is in the middle of the carbon chain (e.g., fructose).
Aldose: A sugar where the carbonyl (C=O) group is at the end of the carbon chain, forming an aldehyde group (e.g., glucose, mannose).
Stereochemistry (L or D Sugars):
- Determined by the O-H group on the highest-numbered chiral carbon.
- D-sugar: O-H group is on the right side in a Fischer projection.
- L-sugar: O-H group is on the left side in a Fischer projection.
- For exams, focus primarily on D-sugars.

2.2 Fischer and Haworth Projections (Crucial Exam Topic)

Fischer Projection: A linear representation of a carbohydrate structure.
- Horizontal lines indicate groups coming out towards you; vertical lines indicate groups going back.
- Emil Fischer won the Nobel Prize for determining the structure of glucose and defining the D/L convention.
Cyclization of Sugars: Monosaccharides spontaneously cycle in aqueous solution to form stable 5- or 6-membered rings (pyranose for 6-membered, furanose for 5-membered). This is a reversible equilibrium, favoring the cyclic form.
- In glucose, the O-H of carbon 5 attacks the carbonyl of carbon 1.
Haworth Projection: A cyclic representation of a carbohydrate structure, typically depicted as a planar ring (though in reality, they adopt chair/boat conformations).
- Sir Walter Norman Haworth won the Nobel Prize for his work on carbohydrate structures and introduced this projection.
- Interconverting between Fischer and Haworth Projections (Key Exam Skill):
1. Rotate the Fischer projection 90 degrees clockwise.
2. O-H groups on the right in the Fischer projection will be down in the Haworth projection.
3. O-H groups on the left in the Fischer projection will be up in the Haworth projection.
4. The $CH_2OH$ group (carbon 6 in hexoses) will typically be above the plane of the ring for D-sugars when the anomeric carbon is on the right.

2.3 Anomeric Carbon and Alpha/Beta Configuration (Key Exam Topic)

Anomeric Carbon: The carbon atom that was originally the carbonyl carbon in the linear form and is now bonded to two oxygens in the cyclic form. It's unique in having two bonds to oxygen.
Anomers: The two possible stereoisomers formed at the anomeric carbon during cyclization (e.g., $\alpha$ -glucose and $\beta$ -glucose).
Determining Alpha ( $\alpha$ ) or Beta ( $\beta$ ) (Crucial Exam Skill):
- Compare the position of the O-H group on the anomeric carbon with the highest-numbered carbon group (often $CH_2OH$ ) in the ring.
- Alpha ( $\alpha$ ): The O-H group on the anomeric carbon and the highest-numbered carbon group are on opposite sides (one up, one down). Alpha and opposite both start with vowels.
- Beta ( $\beta$ ): The O-H group on the anomeric carbon and the highest-numbered carbon group are on the same side (both up or both down). Beta and same both start with consonants.

Formed by a condensation reaction between two monosaccharides, removing water to create an O-glycosidic bond.
An O-glycosidic bond typically involves at least one anomeric carbon.
Consequence of Glycosidic Bond Formation: Once formed, the monosaccharide ring involved in the glycosidic bond is locked in its cyclic form and can no longer open or close (i.e., it can't interconvert between its $\alpha$ and $\beta$ forms).
Naming Linkages: Described by the anomeric configuration (alpha or beta) of the first sugar and the carbon numbers involved.
- Lactose: Formed from $\beta$ -galactose and glucose via a $\beta$ -1,4 linkage.
- The galactose unit remains in the $\beta$ configuration due to the linkage.
- `1The glucose unit's anomeric carbon is free to interconvert between $\alpha$ and $\beta$ forms (resulting in $\alpha$ -lactose and $\beta$ -lactose).
- Sucrose: Formed from glucose and fructose via an $\alpha$ -1, $\beta$ -2 linkage.
- Carbon Numbering in Fructose's Haworth Projection: Carbon 1 is the terminal $CH_2OH$ group. Carbon 2 is the anomeric carbon (involved in the linkage with glucose).
- Both anomeric carbons are involved in the linkage, meaning sucrose cannot readily open to a linear form.

4.1 Energy Storage (in animals and plants)

Glycogen (Animals):
- Highly branched polymer of glucose units.
- Main chain linkages: $\alpha$ -1,4 glycosidic bonds.
- Branch point linkages: $\alpha$ -1,6 glycosidic bonds (approximately every 13 glucose units).
- Major storage sites: Muscle cells (for rapid energy use) and liver (to maintain blood glucose levels).
Starch (Plants):
- Two main forms: Amylose and Amylopectin.
- Amylose: Long, linear chains of glucose units with primarily $\alpha$ -1,4 linkages; very few branch points.
- Amylopectin: Similar to amylose but more branched, with $\alpha$ -1,6 branch points every ~25 glucose units (less regular than glycogen).
- Digestibility: Human enzymes can efficiently break down glycogen and starch due to their $\alpha$ -linkages.

4.2 Structural Molecules and Dietary Fiber

Cellulose (Plants):
- Forms the basis of plant cell walls.
- Linear polymer of glucose units linked by $\beta$ -1,4 glycosidic bonds.
- Digestibility: Humans cannot digest cellulose because they lack enzymes to break $\beta$ -1,4 linkages. It is a major component of dietary fiber.
Pectin (Fruits):
- Found in fruits like apples.
- Composed of D-galacturonic acid units linked by $\alpha$ -1,4 linkages.
- The presence of D-galacturonic acid (an acid derivative of galactose) makes it difficult for human enzymes to digest, thus classifying it as dietary fiber.
Chitin (Insects):
- Forms the hard exoskeleton of insects and crustaceans (e.g., in a crunchy cockroach).
- A polysaccharide that is also considered dietary fiber for humans.
Inulin (Root Vegetables):
- A branched polyfructose, found in plants like camas and sunchokes.
- Classified as dietary fiber because human digestive enzymes cannot efficiently break it down.
- Traditional cooking methods (slow roasting) can chemically break down inulin into digestible fructose.
Dietary Fiber: Any polysaccharide that humans cannot digest efficiently. It contributes to gut health but not a direct energy source for the human body.

4.3 Oligosaccharides on Cell Surfaces

Functions:
- Provide physical protection for cells.
- Are very hydrophilic, attracting water and contributing to slippery surfaces (e.g., mucus).
- Crucial role in cell-cell recognition and signaling (e.g., sperm-egg interaction, immune responses).