Table of Contents
- Introduction to Carbohydrates
- Classification of Carbohydrates
- Monosaccharides
- Chemical Reactions of Monosaccharides
- Disaccharides
- 5.1 Glycosidic Bond Formation
- 5.2 Maltose
- 5.3 Lactose
- 5.4 Sucrose
- Polysaccharides
- Hexose Derivatives
- Heteropolysaccharides and Glycoconjugates
- Structures to Memorize for the Exam
- Summary and Key Takeaways
1. Introduction to Carbohydrates
Carbohydrates are one of the four major classes of biological macromolecules, alongside lipids, proteins, and nucleic acids. Their name derives from the fact that they are, in a chemical sense, “hydrates of carbon” — their general formula is Cₓ(H₂O)ᵧ.
All carbohydrates share two fundamental functional groups:
- A carbonyl group (C=O) — either an aldehyde or a ketone
- One or more hydroxyl groups (–OH)
Biological Functions of Carbohydrates
| Function | Description | Example |
|---|---|---|
| Energetic (immediate) | Instant energy source; 1 g = 4 kcal | Glucose (primary fuel for brain & nervous system) |
| Energetic (reserve) | Long-term energy backup | Glycogen (animals), Starch (plants) |
| Structural | Building blocks of nucleotides, glycoproteins, proteoglycans | Ribose (RNA), Deoxyribose (DNA) |
| Signaling | Cell–cell communication and surface identification | Oligosaccharides on plasma membranes |
| Excretion | Detoxification and urinary excretion of waste compounds | Glucuronic acid (glucuronidation in liver) |
Key concept: Glucose (C₆H₁₂O₆) is the central molecule of carbohydrate metabolism. It fuels virtually every cell in the body and its blood concentration is tightly regulated by the hormones insulin (lowers blood glucose by stimulating cellular uptake) and glucagon (raises blood glucose by stimulating hepatic glucose release). Normal blood glucose is 100–120 mg/100 mL blood.
2. Classification of Carbohydrates
Carbohydrates are classified based on the number of sugar units they contain:
| Class | Units | Bond | Examples |
|---|---|---|---|
| Monosaccharides | 1 | — | Glucose, Fructose, Ribose |
| Disaccharides | 2 | Glycosidic bond | Sucrose, Lactose, Maltose |
| Oligosaccharides | 2–10 | Glycosidic bond | Cell-surface recognition oligosaccharides |
| Polysaccharides | > 10 (often thousands) | Glycosidic bond | Cellulose, Starch, Glycogen |
Sugar units in all di-, oligo-, and polysaccharides are linked by O-glycosidic bonds, which form an oxygen bridge (–C–O–C–) between two units, with loss of a water molecule.
3. Monosaccharides
Monosaccharides are the simplest, non-hydrolyzable carbohydrates — a single sugar unit. They can be further classified by:
- The nature of the carbonyl group (aldehyde or ketone)
- The number of carbon atoms
- The stereochemistry (D- or L- configuration)
3.1 Functional Groups: Aldoses vs. Ketoses
- Aldose: contains an aldehyde group (–CHO) at carbon-1 (e.g., glucose, ribose, galactose)
- Ketose: contains a ketone group (C=O) at carbon-2 (e.g., fructose)
Because they also contain multiple hydroxyl groups, monosaccharides are sometimes called polyhydroxyaldehydes or polyhydroxyketones.
Monosaccharides are also named by the number of carbons they contain:
| Name | Carbon count | Examples |
|---|---|---|
| Triose | 3C | Glyceraldehyde |
| Tetrose | 4C | Erythrose |
| Pentose | 5C | Ribose, Deoxyribose |
| Hexose | 6C | Glucose, Fructose, Galactose |
| Heptose | 7C | Sedoheptulose |
Combining both criteria gives names like aldohexose (e.g., D-glucose) or ketohexose (e.g., D-fructose).
3.2 Stereochemistry: Chirality and Isomerism
A chiral carbon (asymmetric carbon) is bonded to four different groups, giving rise to two non-superimposable mirror images. Molecules with at least one chiral carbon are called chiral molecules.
Types of stereoisomers:
- Enantiomers: a pair of non-superimposable mirror images; they rotate plane-polarized light by the same degree but in opposite directions. For example, D-glyceraldehyde and L-glyceraldehyde are enantiomers.
- Diastereomers: stereoisomers with two or more chiral centers that are not mirror images of each other. They differ in their chemical and physical properties.
- Racemic mixture (racemate): a 50:50 mixture of two enantiomers, designated with the prefix (±). The net optical rotation is zero because the rotations cancel out.
- Epimers: a special case of diastereomers that differ in configuration at only one chiral carbon (e.g., D-glucose and D-galactose differ at C-4).
Important formula: A molecule with n chiral centers can have a maximum of 2ⁿ stereoisomers. For example, aldohexoses have 4 chiral carbons → 2⁴ = 16 possible stereoisomers, split into 8 D- and 8 L-forms.
3.3 D- and L- Configuration
The D/L designation is based on the configuration of the chiral carbon farthest from the carbonyl group (the one with the highest number), using the molecule drawn in Fischer projection with the most oxidized carbon at the top.
- If the –OH on the bottom chiral center points to the right → D-sugar
- If the –OH on the bottom chiral center points to the left → L-sugar
Biological relevance: Almost all carbohydrates in living systems are D-sugars. A few exceptions include L-arabinose and L-fucose. Similarly, all amino acids in proteins are L-stereoisomers. The reason only one stereoisomer was selected during evolution remains unknown, but once selected, evolving enzymes retained their stereospecificity.
The same D/L system applies to amino acids (e.g., L-alanine and D-alanine are enantiomers).
3.4 Fischer Projection Formulas
The Fischer projection is a 2D convention to represent chiral molecules:
- The chiral carbon is placed at the intersection of two lines.
- Horizontal bonds project toward the viewer (like solid wedges).
- Vertical bonds project away from the viewer (like dashed wedges).
- By convention, the most oxidized carbon (aldehyde or carboxylic acid) is placed at the top.
This makes it easy to quickly assign D/L configuration: look at the bottom chiral carbon — –OH right = D, –OH left = L.
3.5 Epimers
Epimers are diastereomers that differ in configuration at exactly one chiral carbon.
Key examples:
- D-Glucose and D-Mannose are epimers at C-2
- D-Glucose and D-Galactose are epimers at C-4
These subtle differences in structure have significant biological consequences, as enzymes are exquisitely sensitive to the exact stereochemistry of their substrates.
3.6 Cyclization: Hemiacetals and Hemiketals
In aqueous solution, monosaccharides do not exist primarily in their open-chain form. Instead, they undergo intramolecular cyclization:
Hemiacetal formation (aldoses): An aldehyde reacts with an alcohol → hemiacetal
- In glucose (an aldohexose), the aldehyde at C-1 reacts with the hydroxyl at C-5, forming a 6-membered ring(pyranose form)
Hemiketal formation (ketoses): A ketone reacts with an alcohol → hemiketal
- In fructose (a ketohexose), the ketone at C-2 reacts with the hydroxyl at C-5, forming a 5-membered ring(furanose form; this is the biologically active form in sucrose and fructans)
Hemiacetals and hemiketals are in equilibrium with their open-chain forms in solution — this equilibrium is important for the reducing properties of sugars.
Ring naming:
- Pyranose: 6-membered ring (resembles pyran)
- Furanose: 5-membered ring (resembles furan)
For D-glucose at equilibrium in water:
- Pyranose forms (α + β): ~99%
- Open-chain form: ~0.25%
- Furanose forms (α + β): < 1%
3.7 Anomers, Mutarotation, and Ring Conformations
When the ring closes, a new chiral center is created at the former carbonyl carbon (C-1 for aldoses, C-2 for ketoses). This carbon is called the anomeric carbon, and the two possible configurations are called anomers:
- α-anomer: the –OH on the anomeric carbon is axial (below the ring in Haworth/chair projection for D-sugars)
- β-anomer: the –OH on the anomeric carbon is equatorial (above the ring in Haworth/chair projection for D-sugars)
In D-glucose: the β-anomer (64%) predominates over the α-anomer (36%) at equilibrium because the equatorial –OH is more energetically stable than the axial –OH.
Mutarotation is the spontaneous interconversion between α and β anomers through the open-chain intermediate in aqueous solution:
- Pure α-D-glucopyranose: specific rotation = +112° → equilibrium value of +52.5°
- Pure β-D-glucopyranose: specific rotation = +18.7° → equilibrium value of +52.5°
Chair vs. Boat Conformation: Six-membered rings (pyranoses) can adopt different 3D conformations:
- Chair conformation: most stable — bond angle between carbons is ~109.5° (tetrahedral), atoms are maximally spaced to minimize electron repulsion. In β-D-glucopyranose, all substituents are equatorial.
- Boat conformation: much less stable — atoms are more crowded, creating greater repulsion.
3.8 Biologically Important Monosaccharides
D-Glucose (Aldohexose)
- The most important sugar in the human body; also called grape sugar or blood sugar
- Primary energy source for the brain and nervous system
- Oxidized in glycolysis and other metabolic pathways for ATP production
- Blood concentration (100–120 mg/100 mL) regulated by insulin and glucagon
- Exists predominantly in β-D-glucopyranose form in solution
D-Fructose (Ketohexose)
- Also called fruit sugar; the sweetest of all simple sugars
- Found in honey, corn syrup, and sweet fruits
- Isomer of glucose (same molecular formula C₆H₁₂O₆, different structure)
- Free fructose exists as fructopyranose (6-membered ring)
- When part of sucrose or fructans (e.g., inulin), exists in furanose form (5-membered ring)
- When sucrose is hydrolyzed by invertase, fructose converts from furanose to pyranose for hepatic fructolysis
D-Galactose (Aldohexose)
- Found in milk as a component of the disaccharide lactose
- Epimer of glucose at C-4
- β-D-Galactose and its modified form β-D-N-acetylgalactosamine are components of blood group antigens
D-Ribose and 2-Deoxy-D-Ribose (Aldopentoses)
- Ribose: component of RNA, many coenzymes (NAD⁺, FAD, CoA), and ATP
- ATP = adenine + ribose + 3 phosphate groups; provides energy for muscle contraction, nerve impulse propagation, chemical synthesis
- 2-Deoxyribose: component of DNA; identical to ribose except the –OH at C-2 is replaced by –H
4. Chemical Reactions of Monosaccharides
4.1 Oxidation: Reducing Sugars and Uronic Acids
Reducing sugars are sugars that can act as reducing agents (electron donors) in oxidation–reduction reactions. A sugar is a reducing sugar if it has a free hemiacetal (or hemiketal) group — i.e., its ring can open to expose a free aldehyde (or in the case of ketoses, a tautomeric aldehyde via an enediol intermediate).
The aldehyde is oxidized to a carboxylic acid; the oxidant is reduced. This is the basis of the Fehling test (blue → brick-red precipitate).
Aldonic acids result from oxidation at C-1:
- D-Glucose → D-Gluconate (gluconic acid anion)
Uronic acids result from oxidation at C-6:
- D-Glucose → D-Glucuronic acid (UDP-D-glucuronate)
- Component of glycosaminoglycans (e.g., heparin, chondroitin sulfate, hyaluronic acid) and glycolipids
- In the liver, used in glucuronidation: conjugation with endo- and xenobiotics (toxins, drugs) to form glucuronides, which are water-soluble and excreted in urine
Note on ketoses: C-1 of a ketose (a –CH₂OH group) is not directly oxidized. However, under alkaline conditions of the Fehling test, a 2-ketose like fructose tautomerizes to an aldose via an enediol intermediate, and the resulting aldose is then oxidized. This is why fructose is also a reducing sugar.
4.2 Reduction: Alditols
The carbonyl group of a monosaccharide can be reduced to a hydroxyl group (using H₂ with a metal catalyst or NaBH₄). The resulting polyalcohols are called alditols.
Key examples:
- Sorbitol (from glucose): found in cherries, plums, pears, apples, seaweed; ~60% as sweet as sucrose; used in candies and as a sugar substitute for diabetics
- D-Mannitol: from mannose; used medically as an osmotic diuretic
- Xylitol: from xylose; sweetening agent in “sugarless” gum, candy, and cereals
- Erythritol: from erythrose
5. Disaccharides
Disaccharides form when the hemiacetal (or hemiketal) group of one monosaccharide reacts with a hydroxyl group of another to form an O-glycosidic bond (an acetal bond), releasing one molecule of water.
5.1 Glycosidic Bond Formation
The glycosidic bond:
- Creates a C–O–C bridge (oxygen bridge) between two monosaccharides
- Can be α or β depending on the configuration of the anomeric carbon involved
- In biological systems, specific disaccharides are produced because reactions are enzyme-catalyzed — each enzyme is specific for one pair of hydroxyl groups
Reducing vs. non-reducing disaccharides: A disaccharide is a reducing sugar if one of the anomeric carbons is not involved in the glycosidic bond (i.e., it still has a free hemiacetal –OH). If both anomeric carbons are joined (as in sucrose), the disaccharide is non-reducing.
5.2 Maltose
- Two α-D-glucose units linked by an α(1→4) glycosidic bond
- Reducing sugar (C-1 of the second glucose has a free hemiacetal –OH)
- Not common in nature except in germinating grains (from starch hydrolysis)
- Hydrolyzed by the enzyme maltase (present in the human intestinal brush border)
- Intermediate in the hydrolysis of starch
Diagram description: Maltose has an acetal bond on the left glucose unit (C-1 involved in bond) and a hemiacetal hydroxyl on the right glucose unit (free –OH at C-1), which allows the ring to open and act as a reducing agent.
5.3 Lactose
- β-D-Galactose + D-Glucose linked by a β(1→4) glycosidic bond
- The galactose C-1 (β) is connected to the C-4 hydroxyl of glucose
- Reducing sugar (glucose’s C-1 hemiacetal is free)
- Principal sugar found in the milk of most mammals
- Hydrolyzed by the enzyme lactase (β-galactosidase) in the intestinal brush border
Lactose intolerance: Many adults and some children cannot hydrolyze lactose because they do not produce sufficient lactase. Prevalence varies widely: ~20% of the US population, ~95% in Japan. Undigested lactose passes to the colon, where it is fermented by bacteria, producing gas, bloating, and diarrhea.
5.4 Sucrose
- α-D-Glucose (C-1) + β-D-Fructose (C-2) linked by an α,β(1→2) glycosidic bond
- Both anomeric carbons are involved in the bond → non-reducing sugar (no free hemiacetal)
- Cannot be synthesized by animals; transported through plant circulatory systems
- High concentrations create high osmotic pressure → inhibits microbial growth → used as a food preservative
- Widely used as a sweetener, but associated with:
- “Empty calories” (no vitamins or minerals)
- Increased risk of cardiovascular disease mortality (excess consumption)
- Dental caries (scientifically proven link)
6. Polysaccharides
Polysaccharides are polymers of more than 10 (often hundreds to thousands of) monosaccharide units. They are the most abundant carbohydrates in nature, making up 50–90% of plant dry weight.
Unlike mono- and disaccharides, polysaccharides are nearly insoluble in water due to their large molecular size (only the surface can be hydrated).
Classification:
- Homopolysaccharides: composed of a single type of monosaccharide
- Storage: starch (plants), glycogen (animals)
- Structural: cellulose
- Heteropolysaccharides: composed of two or more different monosaccharides
- Glycosaminoglycans, proteoglycans, glycoproteins
Types of glycosidic bonds in polysaccharides:
- Linear polymer: one type of glycosidic bond (e.g., α(1→4)) links all units in a chain
- Branched polymer: some internal residues also form additional glycosidic bonds (e.g., α(1→6)) creating branch points
6.1 Starch (Amylose and Amylopectin)
Starch is the main energy storage polysaccharide in plants and the most important dietary carbohydrate for humans. It consists of two components:
Amylose
- Linear polymer of α-D-glucose
- Linkage: α(1→4) glycosidic bonds throughout
- Accounts for ~20% of plant starch
- Soluble in hot water
- Forms a helical secondary structure; binds iodine to give a characteristic blue-black color (used as a diagnostic test)
Amylopectin
- Branched polymer of α-D-glucose
- Main chain: α(1→4) glycosidic bonds
- Branch points: α(1→6) glycosidic bonds approximately every 24–30 glucose units
- Accounts for ~70–90% of plant starch
- More compact and complex structure than amylose
6.2 Glycogen
Glycogen is the primary glucose storage polysaccharide in animals, sometimes called “animal starch.”
- Main chain: α(1→4) glycosidic bonds
- Branch points: α(1→6) glycosidic bonds, occurring approximately every 8–12 glucose units (more frequent than in amylopectin)
- Contains the primer protein glycogenin at the core of each granule, linked to glucose residues — this protein acts as the initiator for glycogen synthesis and reconstitution
- Stored mainly in the liver (regulates blood glucose) and skeletal muscle (local energy reserve)
- Synthesis and degradation are tightly regulated by insulin and glucagon
Why branching matters: The highly branched structure of glycogen provides numerous free non-reducing ends from which glucose can be simultaneously mobilized — allowing rapid glucose release during metabolic demand (e.g., exercise, fasting). The more branch points, the faster the mobilization.
Amylopectin vs. Glycogen comparison:
| Feature | Amylopectin | Glycogen |
|---|---|---|
| Organism | Plants | Animals |
| Branch frequency | Every ~24–30 units | Every ~8–12 units |
| Branches | Less frequent | More frequent |
| Primer protein | None | Glycogenin (present) |
6.3 Cellulose
Cellulose is the most abundant polysaccharide in nature and the principal structural component of plant cell walls.
- Linear polymer of β-D-glucose
- Linkage: β(1→4) glycosidic bonds throughout
- The β-configuration forces each glucose to be flipped 180° relative to its neighbors, creating a perfectly straight, rigid chain
- Parallel chains form fibrils held together by hydrogen bonds between the –OH groups — this creates extraordinary tensile strength
Why can’t we digest cellulose? Humans (and most vertebrates) lack the enzyme cellulase, which hydrolyzes β(1→4) glycosidic bonds. Dietary cellulose therefore functions as insoluble fiber — it adds bulk to stool and supports gut motility.
- Termites can digest cellulose because their intestinal tract contains the symbiotic microorganism Trichonympha, which secretes cellulase.
- Ruminants (cows, sheep, goats) have symbiotic microorganisms in the rumen (first of four stomach compartments) that hydrolyze cellulose from soft grasses, but not from woody plants.
Cellobiose: the disaccharide unit obtained from cellulose hydrolysis — two β-D-glucose units linked by a β(1→4) bond. The enzyme cellulase catalyzes its production from cellulose.
6.4 Comparison Table: Cellulose, Starch, and Glycogen
| Feature | Cellulose | Amylose (Starch) | Amylopectin (Starch) | Glycogen |
|---|---|---|---|---|
| Source | Plants | Plants | Plants | Animals |
| Subunit | β-D-Glucose | α-D-Glucose | α-D-Glucose | α-D-Glucose |
| Bonds | β(1→4) | α(1→4) | α(1→4) + α(1→6) | α(1→4) + α(1→6) |
| Branches | No | No | Yes (~every 24–30) | Yes (~every 8–12) |
| Shape | Linear, rigid fibrils | Linear coil | Branched | Highly branched |
| Function | Structural | Energy storage | Energy storage | Energy storage |
| Human digestion | No (no cellulase) | Yes (amylases) | Yes (amylases + glucosidases) | Yes (glycogen phosphorylase) |
6.5 Digestion and Degradation of Polysaccharides
Dietary starch (amylose, amylopectin) and glycogen are hydrolyzed by a series of enzymes:
-
α-Amylase (in saliva and pancreatic juice):
- Cleaves α(1→4) glycosidic bonds at random internal points in the chain
- Produces shorter oligosaccharide fragments (dextrins)
-
β-Amylase (mainly in plants and some bacteria):
- Sequentially cleaves from the reducing end
- Releases the disaccharide maltose (cleaves between the 2nd and 3rd glucose from the end)
-
Maltase (intestinal brush border):
- Hydrolyzes maltose → 2 molecules of D-glucose
-
Amyloglucosidases (glucoamylases) (intestinal mucosa):
- Hydrolyze both α(1→4) and α(1→6) bonds
- Release individual glucose molecules from branched chains
The glucose released is rapidly absorbed by intestinal cells and distributed to tissues for energy.
7. Hexose Derivatives
Living organisms contain a variety of monosaccharide derivatives, formed by modification of the hydroxyl groups or other portions of the hexose skeleton. These are crucial building blocks of many complex biological molecules.
Phosphate Esters
- Formed by condensation of phosphoric acid with a hydroxyl group of a sugar
- Glucose 6-phosphate: the first metabolite formed in glycolysis (phosphorylation of glucose at C-6 by hexokinase/glucokinase)
- Phosphorylation traps glucose inside the cell
Uronic Acids
- Glucuronic acid (from glucose oxidation at C-6):
- Component of glycosaminoglycans (heparin, chondroitin sulfate, hyaluronic acid) and glycolipids
- Used in hepatic glucuronidation for detoxification and excretion (linked via UDP to form UDP-D-glucuronic acid)
- Gluconate and D-Glucono-δ-lactone (from C-1 oxidation of glucose)
Amino Sugars
Formed by replacement of the –OH at C-2 with an amino group (–NH₂):
- Glucosamine (2-amino-D-glucose)
- Galactosamine (2-amino-D-galactose)
- Mannosamine (2-amino-D-mannose)
- When the amino group is acetylated with acetic acid, N-acetyl sugars are formed:
- N-Acetyl-β-D-glucosamine (GlcNAc)
- N-Acetyl-β-D-galactosamine (GalNAc) — blood group antigen component
- Muramic acid and N-Acetylmuramic acid — components of bacterial cell walls
Deoxy Sugars
Formed by replacement of an –OH with –H:
- 2-Deoxy-D-ribose: the sugar in DNA (–OH at C-2 replaced by –H)
- L-Fucose (6-deoxy-L-galactose): found in complex oligosaccharides of glycoproteins and glycolipids, including blood group antigens and selectin ligands
- L-Rhamnose (6-deoxy-L-mannose): found in plant and bacterial polysaccharides
8. Heteropolysaccharides and Glycoconjugates
8.1 Glycosaminoglycans
Glycosaminoglycans (GAGs), formerly called mucopolysaccharides, are long-chain heteropolysaccharides containing two alternating monosaccharide units, typically an amino sugar and a uronic acid. Their many negative charges (from sulfate and carboxylate groups) attract water molecules, giving them viscous, gel-like properties.
GAGs are major components of the extracellular matrix of connective tissues (cartilage, synovial fluid, skin, tendons, blood vessels, intervertebral disks, cornea). They are typically found covalently attached to core proteins to form proteoglycans.
Key glycosaminoglycans:
Hyaluronic Acid
- Composed of alternating D-glucuronic acid and N-acetyl-D-glucosamine
- No sulfate groups (only GAG that is non-sulfated)
- Major component of:
- Synovial fluid (joint lubrication — increases viscosity)
- Articular cartilage (coats each chondrocyte)
- Muscular connective tissues (facilitates sliding between tissue layers)
- Skin (involved in tissue repair)
Chondroitin Sulfate
- Important component of cartilage, aorta, connective tissue, bone, and skin
- Provides compressive strength to cartilage
Heparin
- A heterogeneous mixture of variably sulfated polysaccharide chains (MW 6,000–30,000 g/mol)
- Synthesized and stored in mast cells (immune cells found especially in liver, lungs, and gut)
- Best known function: anticoagulant activity — binds strongly to antithrombin III (a plasma protein that terminates blood clotting), thereby inhibiting coagulation
- Widely used clinically as an anticoagulant (e.g., prevention of deep vein thrombosis, pulmonary embolism)
8.2 Glycoconjugates and Blood Group Antigens
The outer face of plasma cell membranes is literally “sugar-coated” — carbohydrates are covalently attached to membrane proteins (glycoproteins) and lipids (glycolipids), forming the glycocalyx.
These surface carbohydrates serve as:
- Biochemical markers for cell–cell recognition
- Receptor ligands for extracellular signals
- Determinants of blood group antigens
Common monosaccharide components of glycoconjugates: D-galactose, D-mannose, L-fucose, N-acetyl-D-glucosamine, N-acetyl-D-galactosamine (4–17 units typical)
ABO Blood Group System
Whether an individual belongs to blood type A, B, AB, or O is genetically determined and depends on which sugar is attached to the surface of red blood cells (RBCs).
The blood carries antibodies against foreign blood group antigens — transfusion of incompatible blood causes the antibodies to agglutinate (clump) the foreign RBCs, which can be fatal.
| Blood Type | Surface Antigen (Sugar) | Antibodies in Plasma | Compatible Donor |
|---|---|---|---|
| A | N-acetyl-D-galactosamine (GalNAc) | Anti-B | A, O |
| B | D-galactose | Anti-A | B, O |
| AB | Both GalNAc and D-galactose | Neither anti-A nor anti-B | A, B, AB, O (universal recipient) |
| O | Neither | Both anti-A and anti-B | O (universal donor) |
Clinical note: Type O is the universal donor (no antigens on RBCs), and type AB is the universal recipient (no antibodies in plasma). Transfusing type A blood into a type B individual (or vice versa) can cause a fatal hemolytic transfusion reaction.
9. Structures to Memorize for the Exam
The following open-chain and cyclic structures must be memorized for the biochemistry exam:
| Molecule | Type | Notes |
|---|---|---|
| Glyceraldehyde | Open chain only | Simplest aldose; reference for D/L configuration |
| D-Glucose | Open chain + cyclic (α and β pyranose) | Aldohexose; most important sugar |
| D-Fructose | Open chain + cyclic (pyranose + furanose) | Ketohexose; furanose in sucrose |
| D-Ribose | Open chain + cyclic (furanose) | Component of RNA, ATP |
| D-Deoxyribose | Open chain + cyclic (furanose) | Component of DNA |
| D-Galactose | Open chain + cyclic | Epimer of glucose at C-4; in lactose |
| D-Mannose | Open chain + cyclic | Epimer of glucose at C-2 |
| Sucrose | Cyclic (α-glucose + β-fructose) | α,β(1→2) bond; non-reducing |
| Lactose | Cyclic (β-galactose + glucose) | β(1→4) bond; reducing |
| Maltose | Cyclic (α-glucose + α-glucose) | α(1→4) bond; reducing |
| L-Fucose | Cyclic | 6-deoxy-L-galactose; blood group antigens |
| β-D-Glucuronate | Cyclic | C-6 carboxylate; liver detoxification |
| β-D-Galactosamine | Cyclic | 2-amino-D-galactose; GAGs, blood groups |
| β-D-Glucose 6-phosphate | Cyclic | First metabolite in glycolysis |
10. Summary and Key Takeaways
-
Carbohydrates have the general formula Cₓ(H₂O)ᵧ and all contain carbonyl and hydroxyl groups. They serve as energy sources, structural materials, signaling molecules, and participants in detoxification.
-
Monosaccharides are the building blocks. Aldoses have an aldehyde at C-1; ketoses have a ketone at C-2. They are classified by carbon number and by D/L stereochemistry based on the bottommost chiral center in Fischer projection.
-
Chirality is central to biochemistry. D- and L-sugars are enantiomers; diastereomers differ at more than one chiral center; epimers differ at exactly one. Living organisms use almost exclusively D-sugars and L-amino acids.
-
Cyclization occurs in solution: aldoses form hemiacetals (pyranose or furanose rings), ketoses form hemiketals. The new chiral center at the anomeric carbon gives rise to α and β anomers, which interconvert via mutarotation through the open-chain form.
-
Reducing sugars (those with a free hemiacetal) can be oxidized. Oxidation at C-1 gives aldonic acids; at C-6 gives uronic acids (critical for detoxification and ECM structure). Reduction of the carbonyl gives alditols (e.g., sorbitol).
-
Glycosidic bonds link monosaccharides into di-, oligo-, and polysaccharides. The type (α or β) and position (1→2, 1→4, 1→6) of the bond determine the structure and function of the resulting polymer.
-
Key disaccharides: Maltose (α1→4, reducing), Lactose (β1→4, reducing), Sucrose (α,β1→2, non-reducing).
-
Polysaccharides: Starch (amylose: linear α1→4; amylopectin: branched α1→4 + α1→6) and glycogen (more branched than amylopectin, contains glycogenin) serve as energy storage. Cellulose (β1→4) is a structural polymer; humans cannot digest it due to lack of cellulase.
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Hexose derivatives including amino sugars, phosphate esters, uronic acids, and deoxy sugars are essential components of nucleic acids, glycosaminoglycans, glycoproteins, glycolipids, and metabolic intermediates.
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Heteropolysaccharides like hyaluronic acid, chondroitin sulfate, and heparin are major components of the ECM. Heparin is an important anticoagulant. Glycoconjugates on cell surfaces mediate recognition — including the ABO blood group system.
Prepared for the General Biochemistry course, University of Bologna. Reference textbook: Lehninger Principles of Biochemistry, 7th or 8th edition (Nelson, Cox, Hoskins).