Chapter 3: Carbohydrates and Lipids
What are carbohydrates?
Polyhydroxy aldehydes (aldoses) or ketones (ketoses).
How are carbons in a carbohydrate numbered?
Beginning from the end containing the functional group (aldehyde or ketone).
How are carbohydrates designated as D or L?
Based on the configuration around the highest numbered asymmetric center (D if the OH group is on the right, L if it is on the left).
What confirmation do nearly all sugars in the body have?
How many possible stereoisomers would an aldohexose (e.g. glucose) have?
An aldohexose has four asymmetric centers, so that there are 16 possible stereoisomers (24).
How are carbohydrates designated as (+) or (−)?
Based on optical activity; rotation of plane-polarized light may be dextrorotatory (+) or levorotatory (−).
Why might the reason for selection of glucose as a blood sugar?
Of all D-sugars, D-glucose exists to the greatest extent in the cyclic conformation, making it the least oxidizable and least reactive with protein.
What are the two possible cyclical conformations of glucose?
It may form furanose (five-carbon) or pyranose (six-carbon) hemiacetal ring structures.
What is the anomeric carbon?
The asymmetric center produced at C-1, produced by the cyclization reaction of a carbohydrate.
What are the two possible anomers for a cyclic carbohydrate?
They can exist as the α-anomer, OH group at C-1 is on the opposite face as the CH2OH group, or as the β-anomer, where it is on the same face.
What is the preferred anomeric configuration for glucose and why?
The β-anomer is preferred (~65%) because all of the hydroxyl groups are oriented equatorially in the plane of the ring, minimizing steric interactions.
What is the only sugar found to a significant extent as a free sugar (blood sugar) in the body?
What are glycans?
Complex molecules composed of sugars joined by glycosidic bonds to form disaccharides, trisaccharides, oligosaccharides, and polysaccharides.
What are homoglycans?
Polysaccharides composed of a single sugar (e.g. cellulose).
What are heteroglycans?
Polysaccharides composed of different sugars (e.g. hyaluronic acid)
Why do amylose (starch) and cellulose have such different biochemical properties, despite both being polysaccharides of glucose?
Because amylose is an α-1→4-linked linear glucan, whereas cellulose is a β-1→4-linked linear glucan.
How do glycosidic linkages form?
They form between a hemiacetal carbon of one sugar and a hydroxyl group of another sugar.
How was the Fehling and Benedict assay used to measure blood glucose?
Based on its reducing activity; oxidation of the sugar reduces the cupric ion (blue-green color) to cuprous ion (orange-red color); the color yield is directly proportional to the glucose content of the sample (cannot distinguish glucose from other reducing sugars, such as fructose or glactose).
What are the three main forms of lipids in the body?
- fatty acids in the plasma
- triglycerides in adipose tissue
- phospholipids in cell membranes
What are fatty acids?
Long, straight-chain alkanoic acids, commonly with 16 or 18 carbons that may exist in free form and as components of more complex lipids.
What is the difference between unsaturated and saturated fatty acids?
Unsaturated fatty acids contain one or more double bonds; saturated fatty acids contain no double bonds.
What is the difference between monounsaturated and polyunsaturated fatty acids?
Monounsaturated fatty acids contain one double bond; polyunsaturated fatty acids contain two or more double bonds.
How does cis geometry of the double bonds in fatty acids influence its physical properties?
They place a kink in the linear structure of the fatty acid chain that interferes with close packing, and thus lowers the freezing temperature (lower melting point).
How are trans-fatty acids produces and why are they of concern?
They are formed during the partial hydrogenation of vegetable oils; they are atherogenic.
What does the ω designation indicate for unsaturated fatty acids?
The location of the first double bond from the methyl end of the molecule, commonly 3 or 6.
What do the Δ superscripts indicate for unsaturated fatty acids?
The location of the double bonds from the carboxyl end of the molecule.
What are triglycerides?
Lipids formed by the esterification of fatty acids to glycerol, forming triacylglycerols that are stored in solid form in adipose tissue as fat.
What is saponification?
Hydrolyzation of the ester bond of triglycerides by a strong base (e.g. NaOH) to forming glycerol and free fatty acids, the sodium salt of of which is soap.
What are phospholipids?
Polar lipids derived from phosphatidic acid that are the are the major lipids in biological membranes.
What is the unique biochemical property of phospholipids?
In aqueous solution, they spontaneously form closed vesicular structures termed liposomes, which are the model for the structure of a biological membrane.
What is the role of cholesterol in biomembranes?
It is a modulator of membrane fluidity; at lower temperatures, it increases fluidity, but at higher temperatures, it decreases fluidity.
What is platelet-activating factor (PAF)?
A phospholipid that mediates hypersensitivity reactions and increases platelet aggregation.
How do biomembranes differ throughout the body?
In their lipid composition and protein-to-lipid ratio.
What is the fluid mosaic model?
A biomembrane structure that represents the membrane as a fluidlike phospholipid bilayer into which other lipids and proteins are embedded.
What are integral membrane proteins?
Membrane proteins that are embedded in the lipid bilayer.
What are transmembrane proteins?
Integral membrane proteins traverse the membrane, having internal and external polypeptide segments that participate in regulatory processes.
What are peripheral membrane proteins?
Membrane proteins that are bound to membrane lipids or integral membrane proteins, but are not embedded in the lipid bilayer.
What is the function of the cell membranes?
They maintain structural integrity, cellular recognition processes, and transport functions of the cell.
What are membrane patches?
Plasma membrane microdomains (e.g. caveolae or lipid rafts) rich in cholesterol and sphingolipids, which are important for signal transduction and endocytosis.
What is bacteriorhodopsin?
A light-driven proton pump in bacteria that generates an H+-concentration gradient across the membrane, which is used to produce energy.
What are gap junctions?
High-capacity channels between cells that permit coordinated functions (e.g. contraction of the uterus during labor).
What are spectrin and ankyrin?
Membrane-anchoring proteins that act to stabilize band 3 and glycophorin in erythrocyte membranes.
What can occur due to defects in spectrin?
Diseases of altered red cell morphology, such as hereditary spherocytosis or elliptocytosis.
What can occur due to defects in ankyrin?
Delocalization of plasma-membrane proteins in cardiac muscle, causing cardiac arrhythmia.