Carbohydrates & Lipids
World D — Polymers & Biomolecules
44.A — CARBOHYDRATES (THE FUEL)
R44.01 Monosaccharides
- Simple sugars. General formula: $(CH_2O)_n$.
- Aldoses: Contain an aldehyde group (e.g., Glucose - a 6-membered pyranose ring).
- Ketoses: Contain a ketone group (e.g., Fructose - a 5-membered furanose ring).
R44.02 Chirality & D/L Forms
- Sugars contain multiple chiral centers (Carbon attached to 4 different groups).
- Creates non-superimposable mirror images (enantiomers).
- Nature almost exclusively uses D-sugars (OH on the right of the lowest chiral center).
OH
⇌
OH
R44.03 Anomers & Mutarotation
- When the straight chain closes into a ring, the carbonyl carbon becomes a new chiral center (Anomeric Carbon).
- $\alpha$-anomer: -OH group points DOWN.
- $\beta$-anomer: -OH group points UP.
- In solution, they spontaneously flip back and forth (Mutarotation).
R44.04 The Glycosidic Linkage
- Covalent bond joining two monosaccharides to form Disaccharides or Polysaccharides.
- Formed via a condensation reaction (dehydration synthesis), eliminating $H_2O$.
- Sucrose = Glucose + Fructose. Lactose = Glucose + Galactose.
R44.05 Polysaccharides ($\alpha$ vs $\beta$)
- Starch/Glycogen: $\alpha(1\rightarrow4)$ linkages. Forms a loose, helical coil. Easily digested for energy.
- Cellulose: $\beta(1\rightarrow4)$ linkages. Every other glucose is flipped. Forms straight, rigid, tightly packed fibers (structural).
44.B — LIPIDS (FATS & MEMBRANES)
R44.06 Fatty Acids
- Hydrophobic hydrocarbon tail + Hydrophilic Carboxyl (-COOH) head.
- Saturated: No double bonds. Pack tightly into solids (Fats/Butter).
- Unsaturated: Cis-double bonds create "kinks" preventing packing. Liquids at room temp (Oils).
R44.07 Triglycerides (Esters)
- One Glycerol backbone attached to three fatty acid tails.
- Joined by Ester linkages (-COO-).
- Nature's most efficient, dense energy storage molecule.
R44.08 Phospholipids
- Similar to triglycerides, but one fatty acid is replaced by a polar Phosphate group.
- Amphipathic: Highly polar head, highly non-polar tails.
- Spontaneously assemble into the lipid bilayers of all cell membranes.
R44.09 Saponification (Soap)
- Alkaline hydrolysis of triglycerides using NaOH or KOH.
- Breaks the ester bonds, yielding Glycerol and Fatty Acid Salts (Soap).
- Soap forms Micelles: hydrophobic tails trap oil/dirt, hydrophilic heads dissolve in water.
HO-
R44.10 Steroids & Cholesterol
- Lipids characterized by a core of 4 fused carbon rings (3 hexagons, 1 pentagon).
- Cholesterol: Crucial for maintaining membrane fluidity and acting as a precursor for steroid hormones (Testosterone, Estrogen).
44.C — INTERSECTIONS & METABOLISM
R44.11 Glycolipids
- Lipids with a carbohydrate chain attached.
- Found on the outer surface of cell membranes.
- Crucial for cellular recognition, immune response, and tissue formation (e.g., ABO blood group antigens).
R44.12 Energy Density
- Carbohydrates: ~4 kcal/g. Already partially oxidized (contain lots of Oxygen). Fast energy release.
- Lipids: ~9 kcal/g. Highly reduced (mostly C-H bonds). Yields over twice the energy upon full oxidation/respiration.