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Here's a simple breakdown of everything in your BIO-201 Carbohydrates lecture (from Lehninger Principles of Biochemistry, Chapter 7):
π What Are Carbohydrates?
Carbohydrates are the most abundant biomolecules on Earth. Chemically, they are molecules made of carbon, hydrogen, and oxygen - specifically polyhydroxy aldehydes or ketones. You find them in bread, potatoes, milk, pasta, fruits, and vegetables.
Their 4 main jobs in the body:
- Provide energy (immediate fuel)
- Store energy (for later use)
- Build other large molecules (like DNA, cell walls)
- Spare proteins and fats from being burned for energy
π¦ The 3 Big Classes of Carbohydrates
| Class | What it is | Examples |
|---|---|---|
| Monosaccharides | Single sugar unit | Glucose, fructose |
| Oligosaccharides (especially disaccharides) | 2 sugar units joined together | Sucrose (table sugar), lactose (milk sugar), maltose |
| Polysaccharides | Hundreds/thousands of sugar units chained together | Starch, glycogen, cellulose |
π¬ Monosaccharides in Detail
These are the building blocks of all carbohydrates. Key facts:
- Water-soluble, sweet-tasting
- Two families:
- Aldoses - the carbonyl (C=O) group is at the end of the chain (an aldehyde)
- Ketoses - the carbonyl group is in the middle of the chain (a ketone)
- Named by how many carbons they have: triose (3C), tetrose (4C), pentose (5C), hexose (6C)
Special terms to know:
- Epimers - two sugars that are identical except at ONE carbon. Example: glucose vs. galactose (differ at C-4), glucose vs. mannose (differ at C-2).
- Anomers - cyclic sugars that differ only at the anomeric carbon (C-1 for aldoses). Example: Ξ±-D-glucopyranose vs. Ξ²-D-glucopyranose.
- Mutarotation - when a sugar in solution spontaneously switches between its Ξ± and Ξ² forms until it reaches an equilibrium mixture.
- Pyranoses - 6-membered ring sugars (like glucose in ring form)
- Furanoses - 5-membered ring sugars (like fructose in ring form)
- Hemiacetal/Hemiketal - the chemical reaction that forms these rings: an aldehyde + alcohol β hemiacetal; a ketone + alcohol β hemiketal.
- Chair conformation - the actual 3D shape of a pyranose ring (it's not flat - it looks like a chair). The 3D shape matters for biological function.
π Disaccharides
Two monosaccharides joined by a glycosidic bond (specifically an O-glycosidic bond - formed when a hydroxyl group reacts with the anomeric carbon of another sugar).
| Disaccharide | Made of | Found in |
|---|---|---|
| Sucrose | Glucose + Fructose | Table sugar |
| Lactose | Glucose + Galactose | Milk |
| Maltose | Glucose + Glucose | Malt/germinating grains |
πΎ Polysaccharides
Long chains of sugars. Two types:
- Homopolysaccharides - made of only ONE type of sugar
- Heteropolysaccharides - made of TWO or more different sugars
Key Homopolysaccharides (all made of glucose!):
| Polysaccharide | Where found | Branching | Function |
|---|---|---|---|
| Starch (amylose + amylopectin) | Plants | Moderately branched | Energy storage |
| Glycogen | Animals (liver, muscle) | Highly branched | Energy storage |
| Cellulose | Plant cell walls | None (unbranched) | Structural support |
- Humans can digest starch and glycogen but NOT cellulose (we lack the enzyme). Ruminants (cows, etc.) can digest it.
- Chitin - a linear polysaccharide made of N-acetylglucosamine. It's the second most abundant polysaccharide in nature (after cellulose) and makes up insect exoskeletons and fungal cell walls.
Key Heteropolysaccharide:
- Peptidoglycan - found in bacterial cell walls. Made of alternating GlcNAc and MurNAc units. It gives bacteria their rigid shape and protects them. (This is actually the target of penicillin-type antibiotics!)
π Glycoconjugates - Carbs Linked to Other Molecules
Carbohydrates don't always act alone - they can be covalently attached to proteins or lipids:
| Type | What it is | Example |
|---|---|---|
| Proteoglycans | Protein + many glycosaminoglycan (GAG) chains | Aggrecan (in cartilage) |
| Glycoproteins | Protein + one or a few oligosaccharide chains | Mucin (in mucus), Collagen |
| Glycolipids | Lipid + carbohydrate attached by glycosidic bond | Gangliosides (in cell membrane) |
Their roles include cell recognition, membrane stability, and structural support.
π Quick Exam Tips (the slides tell you directly what's on the exam!)
The lecturer explicitly flagged these as important:
- Definition and examples of mono-, di-, and polysaccharides
- Drawing structures of glucose, fructose, and disaccharides (Haworth formulas)
- Epimers vs. anomers (know the difference!)
- What mutarotation is
- Chair conformation of Ξ±-D-glucopyranose
- Homo vs. heteropolysaccharides with examples
- Differences between starch, glycogen, and cellulose (bonding and branching)
- Glycoconjugates (proteoglycans, glycoproteins, glycolipids) with examples
Some slides specifically said "No question in the exam from the images of this slide" for polysaccharide images, the pyranose/furanose formation from glucose, and the chitin polymer structure - so those diagrams are lower priority.
The next lecture covers Lipids. Hope this breakdown helps!
explain the difference between aldotriose and ketotriose
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Here's a clear explanation, with the actual textbook diagram:
Aldotriose vs. Ketotriose
Both are 3-carbon sugars (trioses). The only difference is where the carbonyl group (C=O) is located on the chain.
Aldotriose - Glyceraldehyde
The carbonyl group is an aldehyde - it sits at carbon 1, at the very end of the chain. An aldehyde always has a hydrogen attached to it (C=O-H).
C=O β aldehyde group at the END (C-1)
|
H-C-OH
|
CHβOH
Name: Glyceraldehyde
Ketotriose - Dihydroxyacetone
The carbonyl group is a ketone - it sits at carbon 2, in the middle of the chain. A ketone has carbons on both sides of it (no hydrogen).
CHβOH
|
C=O β keto group in the MIDDLE (C-2)
|
CHβOH
Name: Dihydroxyacetone
Here's the textbook diagram showing both side by side:
(Lippincott's Illustrated Reviews: Biochemistry, 8th Ed., Fig. 7.2)
Summary Table
| Feature | Aldotriose | Ketotriose |
|---|---|---|
| Carbonyl type | Aldehyde (C=O with H) | Ketone (C=O between 2 carbons) |
| Position of C=O | Carbon 1 (end of chain) | Carbon 2 (middle of chain) |
| Name | Glyceraldehyde | Dihydroxyacetone |
| Has a chiral center? | Yes (C-2 is chiral) | No (no asymmetric carbon) |
The last point is important: glyceraldehyde has a chiral carbon (C-2), so it can exist as D- or L-glyceraldehyde. Dihydroxyacetone is symmetric, so it has no D/L forms - it's the simplest ketose and the only monosaccharide with no chiral center.
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