Chapter 5: Carbohydrates: Life’s Sweet Molecules
Carbohydrate – sugar composed of C, H, and O. A primary source of energy in a normal diet.
Most carbohydrate names end in –ose.Glucose, fructose, lactose, etc
Simple carbs are called “simple sugars” and often taste sweet.
Simple sugars break down quickly; if we consume too many, they are converted to fat.
Complex sugars take longer to break down.
- Not just used for energy
- –Found on surfaces of cells and allow molecules to distinguish one cell from another
–ABO blood markers are carbs found on red blood cells which allow us to distinguish our body’s blood type from another.
Heparin – a carb used by the body to prevent blood clots
Carbs also found in DNA and RNA
Monosaccharides – simplest carbs, often sweet-tasting. Cannot be broken down into smaller carbs.
Glucose – C6H12O6
Disaccharides – two monosaccharide units joined together. Can be split into two monosaccharide units.
Sucrose – C12H22O11 – can be split (hydrolyzed) into two monosaccharides: glucose and fructose
Oligosaccharides – carbs containing anywhere from 3 to 9 monosaccharide units
–ABO blood-typing groups are oligosaccharides.
Polysaccharides – 10 or more monosaccharide units joined together
–Sugar units can be connected in one continuous chain or the chain can be branched.
Section 5.2: Monosaccharides
- General formula: Cn(H2O)n, n ≧ 3
- Two kinds of functional groups
- –Hydroxyl (f.c. = alcohol): -OH
- –Carbonyl (f.c. = aldehyde or ketone): C=O
- Classified by # of alkyl groups attached to alcoholic carbon
- –Primary (1o) alcohol: 1 alkyl group
- –Secondary (2o): 2 alkyl groups
- –Tertiary (3o): 3 alkyl groups
Monosaccharides contain 1o and 2o alcohols.
Carbonyl group with a hydrogen bonded to one or both sides of the carbon
- –Once used as a biological preservative
- –H on both sides
Carbonyl group with an alkyl or aromatic group on both sides of the carbonyl carbon
- –Once the main ingredient in fingernail polish remover
- –Methyl group on both sides
Aldehydes vs. Ketones
- The top is Ketone & the bottom is Aldehyde.
- M.S. w/ aldehyde = aldose
- M.S. w/ ketone = ketose
- M.S. w/ three C = triose
- Four = tetrose
- Five = pentose
- Six = hexose
- –Aldose w/ six carbons
- –Ketose w/ six carbons
Stereochemistry in Monosaccharides
•How many chiral carbons does glucose have?
4 chiral carbons means 16 (24) possible stereoisomers for glucose!
We need an easier way to draw monosaccharides.
Horizontal lines on a chiral center represent wedges.
Vertical lines on a chiral center represent dashes.
C atoms are understood at the intersections.
D-sugars & L-sugars
D-sugars have the –OH on the chiral carbon farthest from the carbonyl on the right side.
- Most carbs in nature are D-sugars.
- –The carbs we use for energy are D-sugars.
L-sugars have the –OH on the chiral carbon farthest from the carbonyl on the left side.
(Tip: L-sugar = Left side)
Remember: Enantiomers are mirror images: all chiral carbons are opposite.
There can only be 1 mirror image for any stereoisomer.
–What about the other 14 stereoisomers?
Diastereomers – stereoisomers that are not enantiomers
–Different names: galactose and talose are diastereomers of glucose
Several of the most common monosaccharides are hexoses.
–Produced and used only as D-isomers.
- –D-glucose is commonly called dextrose, blood sugar, and grape sugar
- –Found in fruits, vegetables, and corn syrup
–Broken down by the body during glycolysis to provide energy
–Monitored by diabetics
–Glucose doesn’t undergo glycolysis as easily
–Glucose may build up
–Blood becomes corrosive, damaging body systems
–Galactose + glucose = lactose (milk sugar)
–1 chiral center on C4, arranged opposite C4 on glucose
–Can be converted to glucose by the body
–Diastereomers that differ in just one chiral center are called epimers.
–Rare genetic disorder
–Body breaks lactose into glucose and galactose
–Body lacks enzymes needed to further metabolize galactose
–Toxic build-up of galactose-1-phosphate
–NOT the same as lactose intolerance
–Another epimer of glucose
–Not easily absorbed by the body
–Used to treat UTI
–Bacteria in the bladder attach to mannose and can be eliminated
–“Fruit sugar” or “levulose”
–Found in fruits, vegetables, and honey
–70% sweeter than glucose
–Same sweet taste, fewer calories!
DNA and RNA
DNA and RNA differ in their name based on the monosaccharide they contain.
Deoxyribonucleic acid (DNA) – 2-deoxyribose
Ribonucleic acid (RNA) – ribose
Only difference: Absence of an oxygen on C2 of deoxyribose
Section 5.3: Oxidation and Reduction Reactions
Oxidation – loss of e- from one species in a chemical rxn
Reduction – gain of e- by one species in a rxn
Oxidation and reduction always occur together in redox reactions.
The species that is oxidized is called the reducing agent.
–It causes reduction of the other species.
The species that is reduced is called the oxidizing agent.
–It causes oxidation of the other species.
- –Oxidation Is Loss (of e-)
- –Reduction Is Gain (of e-)
- In organic molecules…
- –Oxidation: gain of O or loss of H
- –Reduction: loss of O or gain of H
Monosaccharides and Redox
- In aldoses…
- –Oxidation and reduction occur at the C=O.
- –Can oxidize to a carboxylic acid or reduce to an alcohol
Oxidized molecules reduce other substances.
Benedict’s Test – addition of Cu2+ ions (blue) to a sugar
–Aldehyde is oxidized to a sugar acid by Cu2+.
–Cu2+ is reduced to Cu+ by the aldehyde.
–Cu+ forms Cu2O (brick red), indicating the presence of a “reducing sugar” (aldose)
Reducing sugar – a sugar capable of reducing another substance, like Cu2+
Benedict’s test can be used to monitor glucose levels in urine.
–Excess glucose in urine suggests high levels of glucose in the bloodstream, an indicator for diabetes.
Reduction of aldoses or ketoses produces sugar alcohols.
–Used as artificial sweeteners in sugar-free foods.
Produced from excess glucose in the blood.
–Glucose reduces to sobitol, which, at high concentrations, can contribute to cataracts.
–“Sugar cataracts” commonly seen in diabetics.
Section 5.4: Ring Formation – The Truth About Monosaccharide Structure
The Truth About Monosaccharide Structure
Carbonyl groups can react with hydroxyl groups.
When this happens, the product contains the hemiacetal functional group.
Hemiacetal – an –OH group and an –OR group attached to the same carbon
M.S. contain both a carbonyl and several hydroxyl groups.
–F.G. can react within the same molecule.
-OH on the 2nd carbon from the end curls around to react with the carbonyl carbon.
- –A ring containing an O is formed, with one carbon left outside the ring.
- M.S. exist in a ring form most of the time
Carbonyl carbon shape: trigonal planar
–-OH group can attach on either the top or the bottom.
–Produces two different stereoisomers, called anomers.
–In a ring, the carbon from the carbonyl carbon is called the anomeric carbon
– it is the only carbon directly bonded to two oxygen atoms.
Anomers are either alpha (α) or beta (β).
–α: if –OH on anomeric C is on the opposite side of the ring from the C that is outside the ring
–β: if –OH on anomeric C is on the same side of the ring as the C that is outside the ring
- Pyranose – any cyclic sugar containing a ring made of 5 C atoms and an O atom
- –Typically formed from an aldose
- Furanose – any cyclic sugar containing a ring made of 4 C atoms and an O atom
- –Typically formed from a ketose
For 6-C sugars, C-6 is always drawn on the top side of the ring.
Section 5.6: Polysaccharides
Storage polysaccharides – P.S. that are stored in cells as a glucose energy reserve
–α-glucose M.S. are connected into polysaccharides.
Storage Polysaccharides - Plants
- Starch is a mixture of two P.S.
As fruit ripens, the enzyme amylase hydrolyzes (breaks, using water) glycosidic bonds in starch, producing glucose and maltose, which taste sweet.
Storage Polysaccharides - Animals
Animals store glucose as the P.S., glycogen.
–Mostly found in liver and muscles
Hydrolyzed to glucose in the liver, sent into the bloodstream
Used to maintain glucose levels during fasting periods
Structural polysaccharides – P.S. whose function is to provide structure for an organism
β-glucose M.S. are connected into polysaccharides.
Cellulose – P.S. that provides plant structure
–Called “insoluble fiber” because we lack the enzyme, cellulase, required to digest it.
Obtained in our diet via whole-grain foods
–A component of “dietary fiber”
Assists with digestive movement in small and large intestines
Chitin – P.S. that makes up exoskeletons of insects and crustaceans and cell walls of some fungi
Present in surgical thread that biodegrades as a wound heals
Used in cosmetics and lotions to hold in moisture
Section 5.7: Carbohydrates and Blood
Carbohydrates and Blood
Blood types – A, B, AB, O – refer to carbohydrates.
Carbs act as chemical markers on red blood cells.
–They identify your cells as yours, and not as those of a pathogen.
ABO Blood Markers
Contain either three or four of the following M.S. units:
ABO Blood Markers
- All blood types include
Type O blood contains only these three carbs attached to the RBC surface via a glycosidic bond.
ABO Blood Markers
Types A and B contain these three carbs, plus one extra, bonded to the galactose unit.
- –Type A contains N-acetylglucosamine.
- –Type B contains a second galactose.
Type AB has both Type A and Type B carb sets on the RBC.
ABO Blood Markers
- –universal donor
- –Carb set for O is present in ALL blood markers
- –Body never sees Type O as an invader
- –universal acceptor
- –AB carb set contains all other types of blood marker
- –Body sees all types of blood as its own
Heparin – P.S. that prevents blood clotting (anticoagulant)
–Test tubes, tubing, and needles used for drawing blood are normally coated with heparin.
Belongs to a group of P.S. called glycosaminoglycans, which have highly charged repeating disaccharide units