6. Biomolecules - part 01 - Carbohydrates

 

6. Biomolecules - part 01 - Carbohydrates

Introduction :

• Our planet is having a wide diversity of living organisms that are classified as -

1. unicellular (consisting of a single cell; including bacteria and yeast) or 
2. multicellular having many cells (e.g. plants and animals). 

• living organisms have cell as the basic structural and functional unit. 
• The cells have protoplasm containing numerous chemical molecules, the biomolecules.

Biochemistry  :

• Biochemistry is biological chemistry that provides us the idea of the chemistry of living organisms and molecular basis for changes taking place in plants, animals and microbial cells. 
• It develops the foundation or understanding all biological processes and communication within and between cells as well as chemical basis of inheritance and diseases in animals and plants.
• Chemical analysis of all living organisms indicates presence of the most common elements like carbon, hydrogen, nitrogen, oxygen, sulphur, calcium, phosphorus, magnesium and others with their respective content per unit mass of a living tissue.
• Chemically all living organisms have basic three types of macromolecules, which are polymers of simple subunits called monomers. 

1. The polysaccharides (carbohydrates) - polymers of monosaccharides
2. polypeptides (proteins) - polymers of monosaccharides, amino acids
3. polynucleotides (nucleic acids) -  polymers of  nucleotides .

•  Lipids are water insoluble and small molecular weight compounds as compared to macromolecules.
  

Biomolecules in the cell
A. Carbohydrates :

• The word carbohydrates means ‘hydrates of carbon’. They are also called saccharides. 
• They are biomolecules made from just three elements: carbon, hydrogen and oxygen with the general formula (CH2O)n.
• They contain hydrogen and oxygen in the same ratio as in water (2:1).
• Carbohydrates can be broken down (oxidized) to release energy.

Types of carbohydrates [Based on number of sugar units.] :

1. monosaccharides
2. disaccharides and
3. polysaccharides
   

1. Monosaccharides : 

• These are the simplest sugars having crystalline structure, sweet taste and soluble in water. 
• They cannot be further hydrolysed into smaller molecules.
• They are the building blocks or monomers of complex carbohydrates. 
• They have the general molecular formula (CH2O)n, where n can be 3, 4, 5, 6 and 7. 
•  They can be classified as triose, tetrose, pentose, etc. according to the number of carbon atoms in a molecule.
• Monosaccharides  are classified as -
  
   

1. aldoses - e.g. glucose, xylose, [containing the aldehyde (-CHO) group]
2.  ketoses.- eg. ribulose, fructose. [ containing a ketone(-C=O) group ]

• All monosaccharides are reducing sugars due to presence of free aldehyde or ketone group. 
• These sugars reduce the Benedict's reagent (Cu2+ to Cu+) since they are capable of transferring hydrogens (electrons) to other compounds, a process called reduction.

a. Glucose :

• It is the most important fuel in living cells. 
• Its concentration in the human blood is about 90mg per 100ml of blood. 
• The small size and solubility in water of glucose molecules allows them to pass through the cell membrane into the cell. 
• Energy is released when the molecules are metabolised by cellular respiration.

b. Galactose : 

• It looks very similar to glucose molecules. 
• They can also exist in α and β forms. 
• Galactose react with glucose to form the dissacharide lactose. 
• However, glucose and galactose cannot be easily converted into one another. 
• Galactose cannot play the same role in respiration as glucose.

c. Fructose : 

• It is the fruit sugar and chemically it is ketohexose but it has a five-atom ring rather than a six-atom ring. 
• Fructose reacts with glucose to form the sucrose, a disaccharide.

2. Disaccharides :

• Monosaccharides are rare in nature. 
• Most sugars found in nature are disaccharides. 
  
• Disaccharide is formed when two monosaccharide react by condensation reaction releasing a water molecule. This process requires energy. 
• A glycosidic bond forms and holds the two monosaccharide units together.
• Examples of disaccharides - Sucrose, lactose and maltose.
• Sucrose is a non reducing sugar since it lacks free aldehyde or ketone group. 
• Lactose and maltose are reducing sugars. 
• Lactose also exists in beta form, which is made from β-galactose and β-glucose.
• Disaccharides are soluble in water, but they are too big to pass through the cell membrane by diffusion. 
• They are broken down in the small intestine during digestion. Thus formed monosaccharides then pass into the blood and through cell membranes into the cells.
• Monosaccharides are used very quickly by cells but if a cell is not in need of all the energy released immediately then it may get stored. 
• Monosaccharides are converted into disaccharides in the cell by condensation reactions, which result in the formation of polysaccharides as macromolecules. These are too big to escape from the cell.

3. Polysaccharides :

• Monosaccharides can undergo a series of condensation reactions, adding one unit after the other to the chain till a very large molecule (polysaccharide) is formed. This is called polymerization. 
• Polysaccharides are broken down by hydrolysis into monosaccharides. 
• The properties of a polysaccharide molecule depend on its length, branching, folding and coiling.

a. Starch : 

• Starch is a stored food in the plants. It exists in two forms: 

1. amylose and 
2. amylopectin.

• Both are made from α-glucose. 
• Amylose is an unbranched polymer of α-glucose. 
• The molecules coil into a helical structure. 
• It forms a colloidal suspension in hot water. 
• Amylopectin is a branched polymer of α-glucose. 
• It is completely insoluble in water.

b. Glycogen : 

• It is amylopectin with very short distances between the branching side-chains.
• Glycogen is stored in animal body particularly in liver and muscles from where it is hydrolysed as per need to produce glucose.

c. Cellulose : 

• It is a polymer made from β-glucose molecules and the polymer molecules are 'straight'. 
• Cellulose serves to form the cell walls in plant cells. 
• These are much tougher than cell membranes. 
• This toughness is due tothe arrangement of glucose units in the polymerchain and the hydrogen-bonding between neighbouring chains.

Biological significance of Carbohydrates:

• It supplies energy for metabolism.
• Glucose is the main substrate for ATP synthesis. 
• Lactose, a disaccharide is present in milk provides energy to lactating babies.
• Polysaccharide serves as structural component of cell membrane, cell wall and reserved food as starch and glycogen.