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Biochemistry Unit 6: Carbohydrates and Lipids
Carbohydrates and lipids store energy and build structures such as cell walls and membranes. This unit covers sugar stereochemistry, glycosidic bonds, polysaccharides, fatty acids and the main lipid classes.
Covers: Monosaccharides and stereochemistry, Ring forms, anomers and mutarotation, Glycosidic bonds and disaccharides, Polysaccharides, Fatty acids and saturation, Triacylglycerols and saponification, Phospholipids, sphingolipids and steroids
Study guide
Monosaccharides are aldoses (aldehyde at C1) or ketoses (ketone, usually at C2), named by carbon number: trioses, pentoses, hexoses. A sugar with n stereocenters has 2ⁿ stereoisomers; aldohexoses have four stereocenters and 16 isomers. D or L is set by the stereocenter farthest from the carbonyl, compared with glyceraldehyde; nearly all natural sugars are D. Epimers differ at only one stereocenter, such as glucose and galactose (C4) or glucose and mannose (C2). Sugars differing at more than one but not all centers are diastereomers.
In water, sugars with five or more carbons cyclize. A hydroxyl attacks the carbonyl to form a hemiacetal (aldoses) or hemiketal (ketoses). Glucose's C5 –OH attacks C1, giving a six-membered pyranose; fructose's C5 –OH attacks C2, giving a five-membered furanose. Ring closure creates a new stereocenter, the anomeric carbon. In the α anomer of D-glucose, its –OH is trans to the CH₂OH (axial, down in a Haworth projection); in β it is cis (equatorial). The two anomers interconvert through the open chain, a process called mutarotation; β-glucose is favored at about 64%.
A glycosidic bond joins the anomeric carbon of one sugar to a hydroxyl of another, releasing water. Maltose is glucose-α(1→4)-glucose, lactose is galactose-β(1→4)-glucose, and sucrose is glucose-α1→β2-fructose. A reducing sugar has a free anomeric carbon that can open and reduce Cu²⁺ in Benedict's test; sucrose is non-reducing because both anomeric carbons are bonded. Starch (amylose and branched amylopectin) and glycogen are α-linked glucose stores; glycogen branches every 8 to 12 residues by α(1→6) bonds, giving many ends for fast release. Cellulose and chitin are β(1→4)-linked structural polymers.
Fatty acids are carboxylic acids with long hydrocarbon chains, usually 12 to 24 carbons. Notation 18:2 cis-Δ⁹,¹² means 18 carbons and two cis double bonds starting at C9 and C12; the ω number counts from the methyl end. Saturated chains pack tightly and have higher melting points; cis double bonds create kinks that lower them, while trans bonds keep chains straight. Humans cannot add double bonds beyond C9, so linoleic (ω-6) and α-linolenic (ω-3) acids are essential. Triacylglycerols, glycerol esterified to three fatty acids, are compact, anhydrous energy stores. Saponification with base hydrolyzes their esters into glycerol and soap.
Membrane lipids are amphipathic. Glycerophospholipids have two fatty acids on glycerol plus a phosphate-linked head group such as choline, ethanolamine, serine or inositol. Sphingolipids are built on sphingosine: ceramide adds a fatty acid, sphingomyelin adds phosphocholine, cerebrosides add one sugar and gangliosides add complex oligosaccharides. Defects in their lysosomal breakdown cause storage diseases such as Gaucher and Tay-Sachs. Steroids, including cholesterol, have four fused rings; cholesterol buffers membrane fluidity and is the precursor of bile acids, steroid hormones and vitamin D.
Key ideas
- Sugars with n stereocenters have 2ⁿ stereoisomers; D/L is set by the farthest stereocenter.
- Cyclization creates an anomeric carbon with α and β forms that interconvert by mutarotation.
- Reducing sugars have a free anomeric carbon; sucrose is non-reducing.
- α linkages make digestible storage polymers; β linkages make structural fibers.
- Saturation and double-bond geometry control fatty acid packing and melting point.
- Triacylglycerols are dense, anhydrous energy stores.
- Phospholipids and sphingolipids form bilayers; cholesterol buffers membrane fluidity.
Formulas and rules
Common mistakes
- Confusing anomers (differ at the anomeric carbon) with epimers (differ at any one stereocenter).
- Using the anomeric carbon to assign D or L.
- Calling sucrose a reducing sugar.
- Counting ω positions from the carboxyl end instead of the methyl end.
- Thinking cis double bonds raise melting point.
Before the test, make sure you can
- Classify sugars as aldoses or ketoses and count their stereoisomers.
- Distinguish enantiomers, epimers, diastereomers and anomers.
- Explain ring formation, anomers and mutarotation.
- Identify reducing versus non-reducing disaccharides and their linkages.
- Compare starch, glycogen and cellulose in structure and function.
- Interpret fatty acid notation and predict melting point trends.
- Calculate saponification amounts and disaccharide masses.
- Describe phospholipids, sphingolipids and cholesterol and their roles.
Words to know
- Aldose
- A sugar with an aldehyde group at C1, such as glucose.
- Ketose
- A sugar with a ketone group, usually at C2, such as fructose.
- Epimers
- Sugars that differ in configuration at exactly one stereocenter.
- Anomeric carbon
- The former carbonyl carbon that becomes a stereocenter on ring closure.
- Anomers
- Cyclic sugars differing only at the anomeric carbon (α and β).
- Mutarotation
- Interconversion of α and β anomers in solution via the open-chain form.
- Pyranose
- A six-membered sugar ring containing one oxygen.
- Furanose
- A five-membered sugar ring containing one oxygen.
- Glycosidic bond
- A bond from a sugar's anomeric carbon to another group, formed with loss of water.
- Reducing sugar
- A sugar with a free anomeric carbon that can be oxidized.
- Glycogen
- A highly branched α(1→4), α(1→6) glucose polymer stored in liver and muscle.
- Cellulose
- An unbranched β(1→4) glucose polymer forming plant cell walls.
- Essential fatty acids
- Linoleic (ω-6) and α-linolenic (ω-3) acids, which humans must eat.
- Saponification
- Base hydrolysis of ester bonds in fats, giving glycerol and soap.
- Glycerophospholipid
- Glycerol with two fatty acids and a phosphate-linked head group.
- Sphingolipid
- A lipid built on sphingosine, such as sphingomyelin or gangliosides.
- Cholesterol
- A four-ring steroid that buffers membrane fluidity and is a hormone precursor.
- Trans fat
- An unsaturated fat with trans double bonds that packs like a saturated fat.
Sample practice questions
30 more questions like these, from level 1 to 5, are in the practice set.
Glucose is an aldohexose. What does this name tell you?
- It has six carbons and a ketone group
- It has six rings and an alcohol group
- It has six carbons and an aldehyde group
- It has five carbons and an aldehyde group
Show the answer and steps
Answer: C) It has six carbons and an aldehyde group
- 'Aldo' means an aldehyde carbonyl at C1.
- 'Hexose' means six carbons.
- Fructose, by contrast, is a ketohexose with a ketone at C2.
The α and β forms of cyclic glucose are called:
- Anomers
- Epimers at C4
- Structural isomers
- Enantiomers
Show the answer and steps
Answer: A) Anomers
- Ring closure makes C1 a new stereocenter, the anomeric carbon.
- α and β differ only in the configuration at this carbon.
- Isomers differing only at the anomeric carbon are anomers.
Why does glycogen, which has α(1→6) branches every 8 to 12 residues, suit rapid glucose release better than unbranched amylose?
- α(1→6) bonds store more energy than α(1→4) bonds
- Branches make glycogen insoluble so it cannot be lost
- Branches allow β linkages that humans digest faster
- Branches give many nonreducing ends for enzymes
Show the answer and steps
Answer: D) Branches give many nonreducing ends for enzymes
- Glycogen phosphorylase removes glucose from nonreducing ends.
- Each branch adds another nonreducing end, so many enzymes can act at once.
- Branching also increases solubility, the opposite of making it insoluble.