Maharashtra Board Class 11 Biology Chapter 6 Biomolecules PDF Download

Biomolecules

Our planet is having a wide diversity of living organisms that are classified as unicellular (consisting of a single cell; including bacteria and yeast) or multicellular having many cells (e.g. plants and animals). You have also learnt that living organisms have cell as the basic structural and functional unit. The cells have protoplasm containing numerous chemical molecules, the biomolecules.

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 for 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.

Can you recall?

1. Which are different cell components?

2. What is the role of each component of cell?

Chemically all living organisms have basic three types of macromolecules, which are polymers of simple subunits called monomers. The polysaccharides (carbohydrates), polypeptides (proteins) and polynucleotides (nucleic acids) are the polymers of monosaccharides, amino acids and nucleotides respectively (Figure 6.1). 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 \((CH_2O)_n\). They contain hydrogen and oxygen in the same ratio as in water (2:1). Carbohydrates can be broken down (oxidized) to release energy.

Based on number of sugar units, carbohydrates are classified into three types namely, monosaccharides, disaccharides and polysaccharides (Table 6.2).

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 \((CH_2O)_n\), 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 as mentioned in the table 6.2.

Monosaccharides containing the aldehyde (-CHO) group are classified as aldoses e.g. glucose, xylose, and those with a ketone(-C=O) group are classified as ketoses. eg. ribulose, fructose.

All monosaccharides are reducing sugars due to presence of free aldehyde or ketone group. These sugars reduce the Benedict's reagent (Cu\(^{2+}\) 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 disaccharide 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.

Teacher's Note

Glucose is the sugar in our blood that gives us energy to run and play. Just like petrol makes a car run, glucose makes our body run.

Exam Trick

Remember: Glucose = blood sugar. Fructose = fruit sugar. Galactose = milk sugar. These three words will help you remember the monosaccharides.

Points to Remember

Monosaccharides are simple sugars with sweet taste.
They cannot be broken into smaller sugar molecules.
Glucose is most important for our body energy.
Fructose is found in fruits and sweet items.
All monosaccharides are reducing sugars.

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.

Sucrose, lactose and maltose are examples of disaccharides. 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.

\[C_{12}H_{22}O_{11} + H_2O \rightarrow C_6H_{12}O_6 + C_6H_{12}O_6\]

Disaccharide + Water monosaccharide + monosaccharide

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.

Teacher's Note

Disaccharides are made of two monosaccharides joined together. Sucrose is table sugar we use at home. Lactose is the sugar in milk we drink.

Exam Trick

Remember: Dis = two. So disaccharide = two sugar units. Sucrose = table sugar, Lactose = milk sugar, Maltose = malt sugar.

Points to Remember

Disaccharides are made from two monosaccharides.
They are held together by glycosidic bonds.
Sucrose is the sugar in our table and sweets.
Lactose is found in milk and dairy products.
Disaccharides cannot easily pass through cell membranes.

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: amylose and 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 to the arrangement of glucose units in the polymer chain 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.

Teacher's Note

Starch is the food stored in potato and rice. Glycogen is stored in our muscles and liver for quick energy when we run or play sports.

Exam Trick

Remember: Starch = plant storage. Glycogen = animal storage. Cellulose = plant cell wall. These three words help you remember the three polysaccharides.

Points to Remember

Polysaccharides are made of many monosaccharide units joined together.
Starch is the energy storage in plants like potatoes and rice.
Glycogen is the energy storage in animals like us.
Cellulose makes the cell wall strong in plants.
Carbohydrates give us energy to run, jump and play.

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