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Compounds which have some structural formula but differ only in spatial configuration are known as stereoisomers.

No. of possible isomers depends on number of asymmetric carbon atoms and is equal to (2n), where n=no of asymmetric carbon atoms. 

  1. D& L Isomerism:
    D Glucose and L Glucose are mirror images and are known as enantiomers.

Figure. D- and L-isomerism of glycerose and glucose.(Ref. Harper, 28th edition Pg 114) 

  1. Optical activity:
    Presence of asymmetric carbon atom confers optical activity on the compound. When a beam of plane polarized light rotates to right, the compound is called dextrorotatory (d or + sign), when rotated to left, compound is called levorotary (l or - sign). When equal amount of dextrorotatory and laevo rotatory isomers are present the resulting mixture has not optical activity, such a mixture is called racemic mixture. 
  2. Anomers:
    The formations of ring structure results in C1 carbon becoming asymmetric carbon called anomeric carbon. Crystalline glucose is α – glucopyranose (38%) and β - glucopyranose (62%). Both Glucose and β Glucose are examples of anomers.


  1. Epimers:
    Isomers differing as a result of variation in configuration of the –OH and –H on carbon atoms 2, 3, 4 & 5 of glucose are known as epimers.  

Epimer pairs

with respect to carbon atom

Glucose & Mannose  


Glucose & Allose


Glucose & Galactose


Glucose & L- Idose   



Epimers of glucose. 


Note : carbon 6 being optically inactive does not lead to any stereo isomerism therefore does not show any epimer.


Reducing sugars

  1. Reducing sugars are sugars which have free aldehyde or ketonic group their in their structure. Because of presence of free aldehyde or ketonic group, they can reduce certain heavy metallic cations in an alkali medium and in the precess they themselves get oxidized to a mixture of sugar acids. All monosaccharides (e.g., glucose, fructose, galactose) have either free aldehyde or ketone groups. therefore, all monosaccharides are reducing sugars.
  2. In disaccharides, two monosaccharides are joined by glycosidic bond. If the carbon atom of monosaccharide, which has free aldehyde or ketonic group, is not involved in the formation of glycosidic bond, its reducing property is retained. On the other hand, if the carbon atom of monosaccharide, which has free aldehyde or ketonic group, is involved in formation of glycosidic bond, it loses its reducing property. Following example will help you to understand this: -
    1. Glucose and galactose have free aldehyde group at carbon-I , and fructose has free ketone group at carbon-2.
      Thus, reducing end of glucose and galactose is carbon-I and of fructose is carbon-2.
    2. Lactose (disaccharide of glucose and galactose) is formed due to formation of b-glycosidic bond between carbon-I of galactose and carbon-4 of glucose (Galactose - b1 à4- Glucose). Thus, reducing end of glucose (Carbon-I) is not involved in formation of glycosidic bond. Hence, lactose is a reducing disaccharide.
    3. Sucrose (disaccharide of glucose and fructose) is formed due to formation of a-glycosidic bond between carbon-I of glucose and carbon-2 of fructose (Glucose - a1 à 2 - Fructose). Thus, reducing end of both glucose (carbon-I) and fructose (carbon-2) are involved in glycosidic bond formation and therefore lost their reducing property. Hence, sucrose is a non-reducing disaccharide (PGIOS).
Reducing sugars                                  
  1. All monosaccharides (glucose, fructose, galactose)
  2. Diasaccharide: - Lactose, maltose, ellobiose, melibiose
Non-reducing sugars
Diasaccharide: - Sucrose, trehelose
Detection of reducing sugars
Reducing property of sugars in alkaline solution is utilized for both qualitative and quantitative determination of sugars. Reagent containing Cu++ ions are most commonly used. These are generally alkaline solution of cupric sulfate: -
  1. Benedict's quantitative reagent (CuSO4, Na2CO3, sodium citrate, potassium ferrocyanide, potassium thiocyanide) can detect any reducing sugar.
  2. Fehling solution contains CuSO4, Rochelle Salt (sodium potassium tartarate) and strong alkali (NaOH/KOH). It is not used now.
  1. Glucose oxidase method: - This method is specific for glucose. Peroxidase and oxidase enzymes are used for estimation of glucose. These enzymes are the basis of highly specific test strips used for detection of glucose in urine or blood.

Transport Of Glucose

  1. To get metabolized glucose must enter the cell. Two kinds of transport mechanisms are present in the cell membrane, which permit glucose to cross the membranous barrier and enter the cell. These are :
    1. the facilitated transport
    2. the secondary active transport. Both are carrier-mediated process, requiring transport protein (carrier).
  2. Facilitated diffusion (transport) occurs along a concentration gradient and is mediated via a family of at least five transport proteins located in the cell membrane. These are called glucose transporter (GLUT) and designated as GLUT-1, GLUT-2, GLUT-3, GLUT-4 and GLUT-5. Glucose transport proteins are tissue specific e.g., GLUT-4 is abundant in skeletal muscles, heart and adipose tissue. Insulin increases number and activity of GLUT-4 , thereby promoting entery of glucose in these tissues. Thus conditions which decrease insulin levels, e.g., fasting will decrease the glucose transporter (GLUT -4) in skeletal muscle, heart and adipose tissue adipocytes.
  3. Secondary active transport is the mechanism of sodium-glucose cotransport which is mediated by sodium-glucose transporter (SGLT). SGLT-1 helps in active uptake of glucose against concentration gradient in small intestine and kidney.
Major Glucose Transporters
Tissue location Functions
Facilitative bidirectional transporters
GLUT 1    
Brain, Kidney, colon, placenta, erythrocytes Glucose uptake
GLUT 2 Liver, pancreatic b cell, small intestine, kidney Rapid uptake or release of glucose
GLUT 3 Brain. kidney, placenta Glucose uptake
GLUT 4 Heart and skeletal muscle, adipose tissue Insulin-stimulated glucose uptake
GLUT 5 Small intestine Absorption of glucose
Sodium-dependent unidirectional transporter
SGLT 1 Small intestine and kidney Active uptake of glucose against a concentration gradient

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