CHAPTER TWO: Carbohydrates summary

Composed of carbon, hydrogen, and oxygen


  • Acting as the main source of energy for the cell
  • Acting as the metabolite for organic compounds eg. fats
  • Participating in the structure and functionality of the cell e.g cell membrane liquidity
  • Structural components e.g receptors
  • Long-term storage of energy for the cell

Classes of carbohydrates

  • Monosaccharide
  • Oligosaccharide
  • Polysaccharides

Both monosaccharide and oligosaccharide are soluble in water

  • Aldoses – These have aldehyde functional group e.g glucose
  • Ketoses – These have keto group eg. fructose

Other classification in terms of the number of carbon atoms

  • Trioses – 3 carbon atoms
  • Tetroses – 4 carbon atoms
  • Pentoses – 5 carbon atoms
  • Hexoses – 6 carbon atoms
  • Heptoses – 7 carbon atoms

These are disaccharide and trisaccharides

  • These are tasteless and insoluble in water
  • They include homopolysaccharides and heteropolysaccharides

Common terms

Two types of isomers

  • D and L isomers
  • D isomer has an OH group on the right side of the molecule
  • L isomer has an OH group on the left side of the molecule
Optical activity of sugars

This is a property exhibited by molecules when a beam, of polarised right, is rotated either to the right or left (dextrorotatory + or levorotatory -) respectively.

Racemic mixture

  • A solution that has an equal concentration of both D and L isomers
  • The dextro- and levorotatory activities cancel each other out hence no optical activity in a racemic mixture
Other definitions
  • Epimers – This is when two monosaccharides differ from each other in terms of their configuration around a single carbon atom
  • Enantiomers – These are stereoisomers which are mirror images of each other
  • Diastereoisomers – These are stereoisomers which are no mirror images of each other


what are carbohydrates?
cellulose, sugars, starch

what are the functions of carbohydrates?
provide energy through oxidation; supply carbon for synthesis of cell components; store chemical energy; form parts of cell and tissue structures

carbohydrates have a large number of what kind of functional groups?
hydroxy- aldehydes/ketones (aka, polyhodroxy aldehydes and polyhydroxy ketones)

what is a monosaccharide?
single polyhydroxy aldehyde/ ketone unit

what is a disaccharide?
two monosaccharide units linked together by the acetal or ketal linkages

what is an oligosaccharide?
3-10 monosaccharide units (less common)

what is a polysaccharide?
very large chain of linked monosaccharide units;
polymer consisting entirely of D-glucose units

what are the different types of monosaccharides?
glucose, fructose, galactose

describe the similar structure of D-aldoses
at the top, H-C=O. at the bottom, CH2OH. Differences are in carbons rungs in between.

what is the smallest monosaccharide?

what geometric shape does an L- or D- Glyceraldehyde molecule take?

what is an enantiomer?
each of a pair of molecules that are mirror images of each other.

what is a chiral carbon?
a carbon atom with four different groups attached

what makes a compound chiral?
when two compounds have the same molecular and structural formulas but cannot be superimposed on each other

previous review: generally describe a carbonyl group with R functional groups

where is the carbonyl carbon placed in carbohydrates?
at or near the top

when drawing the D-aldoses (example, d-glucose), how do you determine which side of the polyhydroxide rungs will cycle to the above side of the cyclic formation ring?
the oxygen of the carbonyl group at the top will point to the side which goes above.

what represents a chiral carbon?
The intersection of two lines

There are two kinds of enantiomers; how do we distinguish between the two?
a prefix label of:
L- or “levo” meaning “left”
D- or “dextro” meaning “right”
(for example: D-glucose and L-glucose)

what does the “L-” and “D-” label apply to on the enantiomer itself?
the “L-” and “D-” label applies to the side of the molecule which contains the -OH farthest from the first chiral carbon.

what are sugars called when they contain 3, 4, 5, and 6 carbons?
trioses, tetroses, pentoses, and hexoses
(note, sugars have the “-ose” suffix)

how do we indicate the presence of an aldehyde or ketone group in a sugar?
aldehydes in a sugar contain prefix “aldo-“
ketones in a sugar contain prefix “keto-“
(for example: glucose, a six carbon sugar which contains an R-CHO, or aldehyde, group, is an “aldohexose”; ribulose, a five carbon sugar contains an R1-C=O
R2, a ketone, is a “ketopentose”)

are most monosaccharides aldoses or ketoses? which series do most natural monosaccharides belong to (D- or L-)?
most monosaccharides are aldoses. most monosaccharides belong to the D- series.

Are monosaccharides soluble or insoluble in water? What makes them this way? Why is it important in living organisms?
monosaccharides are extremely soluble in water; this is because of the multiple -OH groups; it is important in living organisms because their solubility allows them to be transported rapidly in the circulatory system.

What do aldehydes and ketones with alcohols react to form?
hemiacetals and hemiketals

what is the general structure of a hemiacetal?
R– C –OH

how does a straight monosaccharide cyclize into a ring (which carbons are active)?
carbon 5 adds to the aldehyde group of carbon 1; predominantly cyclic hemiacetals/hemiketals

what is the general name of the ring formed during monosaccharide cyclization?
a “Pyranose” (pi-ran-ose) ring (a cyclic hemiacetal or hemiketal)

What main change occurs during the cyclization of a monosaccharide into a Pyranose ring?
only the Carbons at positions 1 and 5 change, the rest of the molecule is simply along for the ride.

What are the steps to drawing monosaccharides forming Pyranose and Furanose rings (Halworth Structures)?
beginning with a properly drawn monosaccharide chain…

  1. draw the ring with its oxygen to the back
  2. put the anomeric carbon (C-1) on the right side
  3. the terminal -CH2OH group (C-6) is always shown above the ring for D-monosaccharides

What is the difference between an “α- hydroxyl” and a “β- hydroxyl” pyranose ring?
“α-” means that the -OH on C-1 of the ring is positioned below the ring, and a “β-” means that the -OH on C-1 of the ring is positioned above the ring.

What do we call monosaccharides which form 5-membered rings?
Furanose ring

what is an example of a furanose ring?

what is another name for a the C-1 carbon on a Pyranose or Furanose ring?
the “anomeric” carbon

what is an ester? what is its general structure?
a carbonyl adjacent to an ether linkage.
General formula:

what is benedict’s test?
test to indicate reducing sugars. reduces all monosaccharides and some disaccharides.

how does benedict’s test work?
aldehydes and ketones having adjacent -OH groups are readily oxidized; open chain and cyclic forms of monosaccharides are readily oxidized

what do hemiacetals and hemiketals react with alcohol to form?
acetals and ketals

what is a glycoside?
a glycoside is an acetal or ketal formed from the reaction of cyclic monosaccharides with alcohols in the presence of acid

generally describe glycoside formation
hemiacetal sugar + alcohol —> acetal ring + H2O

what is a glycosidic bond?
when two monosaccharide rings are bonded using a C-O-C bond

what is the main difference between glucose and galactose?
differs in the orientation of -OH group attached to C-4

what does the hydrolysis of disaccharides in presence of dilute acid (or specific enzymes) yield?
hydrolysis of disaccharides yields monosaccharide units

describe maltose
two glucose units joined by a glycoside (C-O-C) linkage between C-1 on the first glucose and C-4 on the second unit; has an “α-” gylcosidic linkage between glucose units (linkage points below).

describe lactose
composed of one molecule of D-galactose and one of D-glucose;
linkage between units is “β-” (1—>4)

describe sucrose
common sugar;
composed of glucose and fructose joined by an “α-” linkage from C-1 of glucose and a “β-” linkage from C-2 of fructose

what type of sugar the major storage form of D-glucose in plants?

what color is the reaction between a polysaccharide and iodine?
deep blue

what are two starch fractions?
amylose and amylopectin

describe amylose
long unbraced chain of D-glucose units connected by an “α-” (1—>4) linkages

what is glycogen?
animal starch;
similar to amylopectin

what is cellulose?
the most important polysaccharide and most abundant compound in nature; provides rigidity and strength to plant cell walls; consists of D-glucose units linked by “β-” (1–>4) linkage

what is galactosemia?
genetic disease (autosomal recessive) in which an enzyme needed to metabolize galactose is deficient or absent; typically develops shortly after birth

what is lactose intolerance?
Congenital disorder consisting of an inability to digest milk and milk products; absence or deficiency of lactase results in an inability to hydrolyse lactose

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