(L3) carbohydrates

What are the two categories of carbohydrate?

Sugars and polysaccharides

What is the general formula for carbohydrates?

$$\text{C}_n\text{H}_{2n}\text{O}_n$$

What are the two subcategories of sugars?

Monosaccharides and disaccharides

What are the two kinds of sugar in monosaccharides?

Glucose and pentose sugars

Define monosaccharides, disaccharides, and polysaccharides

Monosaccharides are single sugar units, disaccharides consist of two monosaccharide units joined together, and polysaccharides are long chains of monosaccharide units.

What are the two kinds of glucose in monosaccharides?

$\alpha$-glucose and $\beta$-glucose

Draw an alpha and beta glucose on a whiteboard

Both are six-carbon hexose rings: in $\alpha$-glucose, the hydroxyl group ($\text{-OH}$) on carbon-1 points downward below the ring; in $\beta$-glucose, the hydroxyl group on carbon-1 points upward above the ring.

What are the two kinds of pentose sugar in monosaccharides?

Ribose and deoxyribose

Draw ribose and deoxyribose on a whiteboard

Both are five-carbon pentose sugars: ribose has a hydroxyl group ($\text{-OH}$) attached to carbon-2, whereas deoxyribose has a single hydrogen atom ($\text{-H}$) at carbon-2.

Where are the two kinds of pentose sugar found?

In DNA and RNA nucleotides

What are the three categories of disaccharides?

Maltose, sucrose, and lactose

What is the reaction for maltose formation?

$\text{Glucose} + \text{glucose} \rightarrow \text{maltose} + \text{water}$ (condensation reaction forming a glycosidic bond)

What is the reaction formula for sucrose?

$\text{Glucose} + \text{fructose} \rightarrow \text{sucrose} + \text{water}$ (condensation reaction forming a glycosidic bond)

What is the reaction formula for lactose?

$\text{Glucose} + \text{galactose} \rightarrow \text{lactose} + \text{water}$ (condensation reaction forming a glycosidic bond)

What are the two categories of polysaccharides?

Storage polysaccharides and structural polysaccharides

Why is glucose stored as starch and glycogen rather than free glucose?

Glucose is soluble and lowers water potential, which would cause water to enter the cell by osmosis and risk lysis. Polysaccharides are insoluble and exert no osmotic effect.

What is starch made of, and where is it found?

Made of amylose and amylopectin; found in plants.

Compare amylose and amylopectin

Amylose consists of $\alpha$-glucose with $1,4$-glycosidic bonds in an unbranched helical coil for compact storage. Amylopectin consists of $\alpha$-glucose with $1,4$- and $1,6$-glycosidic bonds in a branched structure, giving a large surface area for rapid enzymatic hydrolysis.

Outline the structural features and storage function of glycogen

Composed of $\alpha$-glucose linked by $1,4$- and $1,6$-glycosidic bonds; its highly branched structure provides numerous terminal ends for enzymes to rapidly hydrolyze glycogen into glucose.

What are the four kinds of structural polysaccharides?

Cellulose, microfibrils, chitin, and peptidoglycan

Cellulose: location and primary function

Located in plant cell walls to provide mechanical strength and prevent the cell from bursting under turgor pressure.

Cellulose: formation and bond type

Formed by condensation reactions between $\beta$-glucose monomers to produce $1,4$-glycosidic bonds.

Cellulose: key structural features

Long, straight, unbranched chains where alternating $\beta$-glucose molecules are inverted by $180^\circ$, allowing cross-linking hydrogen bonds to form between adjacent chains.

Draw a cellulose diagram on a whiteboard

Parallel unbranched chains of $\beta$-glucose units linked by $1,4$-glycosidic bonds with alternating monomer inversion, cross-linked by hydrogen bonds.

Microfibrils: key facts

Formed when numerous cellulose chains bundle together via cross-linking hydrogen bonds, providing high tensile strength to plant cell walls.

Peptidoglycan: key facts

Found in bacterial cell walls, consisting of parallel polysaccharide chains cross-linked by short peptide chains.

Chitin: key facts

Found in fungal cell walls and arthropod exoskeletons, composed of chains of $N$-acetylglucosamine with cross-links that provide strength and lightness.