Carbohydrates Summary

Structure of Carbohydrates

Carbohydrates are molecules made up of hydrogen, oxygen, and carbon

There are two main types of carbohydrates:
  • Monosaccharides– known as simple sugars eg glucose
  • Polysaccharides- they are complex carbohydrates


  • This is single sugars
  • They have a generic formula; CH2O
  • They are colorless and crystalline solids which are water-soluble and insoluble in nonpolar solvents
  • They have a sweet taste
  • Monosaccharides have the following common features
    • 3,4,5,6,7 carbon backbone
    • A hydroxyl group (OH) is attached to every group of carbon. This group makes these sugars polar
    • One double-bonded oxygen that is attached to the carbon backbone. This is called a carbonyl group.

Monosaccharide classes are ketoses and aldoses

  • Aldoses – carbonyl group is found at the carbon chain end
  • Ketoses – carbonyl group is found on any carbon group within the carbon chain
  • Simplest monosaccharides include; two carbon chains aldotriose and glyceraldehyde and three carbon chains ketotrioses and dihydroxyacetone


  • Polysaccharides are polymers of monosaccharides.
  • To generate polysaccharides, Monosaccharides are combined by the condensation process.
  • During condensation, a  molecule of water is removed and a bond is generated between atoms of the growing polysaccharide chain.
  • Homopolysaccharides contain a single monomer type
  • Heteropolysaccharides– contain two or more types of  monomers
Classification of polysaccharides
  • Disaccharides- these are chains of two monosaccharides. An example is a fructose and lactose
  • Oligosaccharides-this are short chains of monosaccharides. They are part of plasma membrane receptors.
  • Polysaccharides- are long chains of monosaccharides. Consist of starch and cellulose
    • Starch-contains two types of polymers of glucose, amylopectin, and amylose
    • Glycogen– it is the main polysaccharide store in animal cells
    • Glycogen is more branched than starch

Functions of carbohydrates

  • Carbohydrates are great energy producers in living cells.
  • They perform the following important functions in cells
    • Carbohydrates are an important energy source for cells. Glucose is widely used as a source of cellular energy.
    • Carbohydrates are used as important structural molecules of cells. Cell walls of plants are majorly made up of polysaccharides. Cellulose with residues of D- glucose provides rigidity and strength to cell walls
    • Carbohydrates are useful cellular identity markers. Various cellular surfaces are marked by glycoproteins.
    • Carbohydrates are useful extracellular molecules. The sticky matrix surrounding the cell is composed of polysaccharides.


  • Polysaccharides serve also as information carriers
  • The polysaccharides also help in blood clotting, migration of cells during migration and healing of wounds.
  • Glycoconjugate– a carbohydrate  that is covalently attached to a lipid or protein
  • Proteoglycans-are cell surface macromolecules where one or more chains of glycosaminoglycan are attached covalently to secreted proteins
  • Glycoproteins –have oligosaccharides of different complexity attached covalently to a protein
  • Glycolipids– are lipids in membrane whose hydrophilic head group is an oligosaccharide.

Glycolipids and lipopolysaccharides as membrane components

  • Many proteins on the cell surface are glycoproteins
  • Oligosaccharides attached facilitate folding of the proteins and stability
  • Lipopolysaccharides are the most common feature of the outer membrane surface of gram-negative bacteria like escherichia coli and salmonella Typhi 


  • They read the sugar code
  • They mediate several biological processes
  • Serve widely in cell signaling, adhesion, and cell-cell recognition
  • Selectins- a group of lectins of the plasma membrane that facilitate adhesion and cell-cell recognition
Structure of CarbohydratesCarbohydrates are molecules made up of hydrogen, oxygen, and carbon.
Types of CarbohydratesMonosaccharides – Single sugars with a generic formula of CH2O. Polysaccharides – Complex carbohydrates made up of monosaccharide units.
MonosaccharidesSingle sugars with a sweet taste, a carbon backbone, hydroxyl groups attached to each carbon, and a carbonyl group. Divided into aldoses and ketoses based on the location of the carbonyl group.
PolysaccharidesPolymers of monosaccharides formed by condensation. Homopolysaccharides contain a single monomer type, while heteropolysaccharides contain multiple monomer types.
Classification of PolysaccharidesDisaccharides – Chains of two monosaccharides. Oligosaccharides – Short chains of monosaccharides. Polysaccharides – Long chains of monosaccharides, such as starch and cellulose.
Functions of CarbohydratesEnergy production, carbon supply for cell component synthesis, chemical energy storage, and structural molecules in cells and tissues.
Functional Groups in CarbohydratesCarbohydrates have hydroxy-aldehydes/ketones (polyhydroxy aldehydes and polyhydroxy ketones) as functional groups.
MonosaccharideA single polyhydroxy aldehyde or ketone unit.
DisaccharideTwo monosaccharide units linked together by acetal or ketal linkages.
OligosaccharideShort chains of 3-10 monosaccharide units.
PolysaccharideVery large chains of linked monosaccharide units, often consisting entirely of D-glucose units.
Types of MonosaccharidesGlucose, fructose, and galactose.
Structure of D-AldosesAt the top, H-C=O. At the bottom, CH2OH. Differences in carbon rungs in between.
Smallest MonosaccharideD-glyceraldehyde.
Shape of L- or D-GlyceraldehydeTetrahedral.
EnantiomerA pair of molecules that are mirror images of each other.
Chiral CarbonA carbon atom with four different groups attached.
Chirality in CompoundsCompounds are chiral when they cannot be superimposed on their mirror image despite having the same molecular and structural formulas.
Placement of Carbonyl CarbonThe carbonyl carbon is placed at or near the top in carbohydrates.
Drawing D-AldosesThe oxygen of the carbonyl group at the top determines which side of the polyhydroxide rungs cycles to the above side of the cyclic formation ring.
Enantiomer Distinguishing LabelsL- (levo) for left and D- (dextro) for right.
L- and D- Labels in EnantiomersThe L- and D- labels apply to the side of the molecule that contains the -OH farthest from the first chiral carbon.
Sugars with 3, 4, 5, and 6 CarbonsTrioses, tetroses, pentoses, and hexoses, respectively.

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What is a monosaccharide?A simple sugar consisting of a single sugar unit.
What is the most common monosaccharide?Glucose.
What are the two types of monosaccharides based on the functional groups they have?Aldose (has an aldehyde group) and ketose (has a ketone group).
What are trioses, pentoses, and hexoses?Monosaccharides with three, five, and six carbon atoms, respectively.
Name two other common monosaccharides.Galactose and fructose.
What is the difference between glucose, galactose, and fructose?They have the same chemical formula (C6H12O6) but different structures due to the arrangement of functional groups.
What are disaccharides?Carbohydrates formed by the joining of two monosaccharides through a glycosidic bond.
Give examples of common disaccharides.Lactose, maltose, and sucrose.
What are the monomers of lactose?Glucose and galactose.
What is maltose composed of?Two glucose molecules.
What are the monomers of sucrose?Glucose and fructose.
What is a polysaccharide?A long chain of monosaccharides linked by glycosidic bonds.
Name examples of polysaccharides.Starch, glycogen, cellulose, chitin, and peptidoglycans.
What is the main function of starch in plants?It serves as the stored form of sugars in plants.
What is glycogen?The storage form of glucose in humans and other vertebrates.
Where is glycogen primarily stored in the body?Liver and muscle cells.
What is cellulose and its function?It is a major component of plant cell walls and provides structural support.
What is chitin and where is it found?It forms the exoskeleton of arthropods and is a major component of fungal cell walls.
What are peptidoglycans and their role?They are the primary component of bacterial cell walls, providing structural support.
What are the functions of carbohydrates in cells?Energy storage, structural support, and as building blocks for complex molecules.
What is the role of carbohydrates in photosynthesis?They are synthesized by plants, algae, and some bacteria using energy from the sun.
What are the functions of fiber in the diet?Promoting regular bowel movement, regulating blood glucose, and reducing cholesterol levels.
What is the primary source of energy in cellular respiration?Glucose, which is broken down to produce ATP.
Why is it important to consume carbohydrates in a well-balanced diet?They provide instant energy and are necessary for overall health and function.
What is a sensible approach to weight loss?A low-calorie diet rich in whole grains, fruits, vegetables, lean meat, exercise, and proper hydration.

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