Biomolecules — Long Questions

Biochemistry
Biochemistry is the study of the chemical processes that occur within living organisms (e.g., photosynthesis, cellular respiration).
Molecular Biology
Molecular biology is the study of the structure and function of the biomolecules (e.g., carbohydrates, proteins, nucleic acids.

Definition
The molecules produced by organisms are called biomolecules or biological molecules. They include carbohydrates, lipids, proteins, and nucleic acids (DNA and RNA). They are mostly large in size and are called macromolecules. The following table mentions important biomolecules, their location in the cell and main functions.

Table 6.1 Major Biomolecules and their Function.

Carbohydrates
Location in the cell:

• Cytoplasm
• Cell membrane Main functions:
• Act as source of energy
• Act as energy storage molecules

Proteins
Location in the cell:

• Cell membrane
• Cytoplasm
• Endoplasmic reticulum
• Golgi apparatus Main functions:
• Many proteins act as enzymes
• Some hormones are proteins
• Make membranes and many other structures in cell
• Control cellular traffic

Lipids
Location in the cell:

• Cell membrane
• Cytoplasm Main functions:
• Act as energy storage molecules
• Act as heat insulators
• Make structure of cell membrane

DNA (Deoxyribonucleic Acid)
Location in the cell:

• Nucleus (eukaryotes)
• Nucleoid region (Prokaryotes)
• Mitochondria
• Chloroplasts Main functions:
• Carries genetic information for the development, functioning, and characteristics of organism

RNA (Ribonucleic Acid)
Location in the cell:

• Nucleus
• Ribosomes
• Cytoplasm Main functions:
• Carries genetic information from DNA to ribosome for protein synthesis.

Percentage Composition of Biomolecules in Protoplasm

Biomolecules make the 93% of the dry mass of protoplasm. The remaining 7% of dry mass comprises vitamins and inorganic substances like carbon dioxide, acids, bases and salts.

Table 6.2 Percentage of Biomolecules in the Dry Mass of Protoplasm

Biomolecules %Dry Mass
Proteins: 50
Nucleic Acids: 18
Carbohydrates: 15
Lipids: 10

Metabolism
The sum of all the chemical reactions that occur in an organism are collectively called metabolism.

Types
i. Anabolism
Anabolism is the type of metabolism in which simpler substances are combined to form complex substances. Energy is used in these reactions. e.g. photosynthesis.
ii. Catabolism
It is the type of metabolism in which complex molecules are broken down into simpler ones. Energy is released in these reactions. e.g respiration.

Introduction
Carbohydrate means hydrated carbons. They are the organic compounds in which the ratio of H and O is 2:1 (same as in water). They are also known as "Saccharides" (meaning sugar).

General Formula
They have general formula (CH₂O)ₙ where n is the number of carbon atoms, ranges from 3 to many thousands.

Classes of Carbohydrates
There are three classes of carbohydrates: Monosaccharides, disaccharides, or polysaccharides.

1. Monosaccharides
Introduction
Monosaccharides (simple sugars) are made of single sugar molecule. They are easily soluble in water and have sweet taste. They may have 3 to 7 carbon atoms. Pentoses (5 C) and hexoses (6 C) are most common.
Examples

• Ribose (C₅H₁₀O₅) and deoxyribose (C₅H₁₀O₄) are pentoses.
• Glucose, fructose, and galactose are hexoses (C₆H₁₂O₆).

2. Disaccharides
Introduction
They are made of two monosaccharides units. They are less soluble in water and are less sweet in taste.

Examples

• Sucrose (table sugar) is made of two monosaccharides i.e. glucose and fructose.
• Maltose is made of two glucose molecules.

3. Polysaccharides
Introduction
Polysaccharides are large molecules composed of hundreds to thousands of monosaccharides units. They are insoluble in water and are tasteless. Polysaccharides are the most abundant carbohydrates in nature.
Examples

• Starch is a storage polysaccharide found in plants. It is composed of glucose units
• Glycogen is the animal starch mainly stored in liver and muscles. It is broken down into glucose when energy is needed.
• Cellulose is a polysaccharide that also consists of glucose units. It is found in the cell walls of plants.
• Chitin is a modified form of cellulose. It is found in the exoskeletons of crabs, lobsters and insects. It also makes the cell wall of fungi.

Sources and Functions of Carbohydrates

Sources

• Monosaccharides: Glucose, fructose and galactose are found, in fruits, vegetables, honey and cereals.
• Disaccharides: Sucrose is found in sugar beet, sugar cane and fruits. Lactose is found in milk and dairy products. Maltose is found in cereals.
• Polysaccharides: Starch is found in cereal crops, wheat, barley, maize, rice etc.

Functions

• Carbohydrates are the primary sources of energy. Glucose is used by cells to produce energy through cellular respiration.
• Dietary fibre contains undigestible carbohydrates e.g., cellulose. It helps to maintain the proper bowel movements.
• Pentoses (ribose and deoxyribose) are essential part of nucleic acids (RNA and DNA respectively).
• Plants convert their monosaccharides to disaccharides like sucrose to transport monosaccharides between body parts.
• Cellulose is the most abundant carbohydrate. It provides support to plant cells and ultimately to the whole plant.
• Chitin is a polysaccharide found in the exoskeleton of insects and in the cell walls of fungi. It provides strength and support to these organisms.

Introduction
Proteins are the most abundant biomolecules in cell. They are defined as the polymers of amino acids. Proteins are important for the structures of cells.

Structure of Proteins
a) Monomers of Proteins
Proteins are made up of monomers called amino acids. Different proteins contain different numbers of amino acids. For example, insulin protein has 51 amino acids and haemoglobin has 574 amino acids.
b) Nature of Amino Acids
Amino acids are the organic molecules that join in specific number and sequence to make proteins.
c) Types of Amino Acids
About 170 types of amino acids occur in organisms. However, 20 types of amino acids participate in making most of the proteins.
d) Essential Amino Acids
There are the 09 amino acids which cannot be synthesized by our body and are supplied by foods.
e) Non- essential Amino Acids
There are 11 amino acids that can be synthesized in our body.
f) Chemical Composition of Amino Acids
An amino acid is an organic molecule made of an amino group (NH₂), a carboxyl group (COOH), a hydrogen group (H) and a side group (R) are attached to a central carbon atom;

Amino acid - general structure

Different amino acids contain different R groups. For example, in amino acid glycine the R group in H and in amino acid alanine, the R group is CH₃.

Sources and Functions of Proteins

Sources

• Good sources of protein include meat (mutton, chicken), fish, eggs, milk, pulses, beans etc.

Functions
Proteins perform various functions in our bodies, including:

• Proteins are an important part of all cell membranes.
• Some proteins e.g. collagen and keratin make almost whole structures of cartilage, hair, and nails.
• Enzymes are proteins that catalyse all biochemical reactions occurring in organisms.
• Some proteins are hormones. They regulate body processes.
• Heamoglobin protein transports oxygen in the blood.
• Actin and myosin proteins are the main components of muscle cells. They are responsible for muscular contractions.
• Fibrin is a blood clotting protein that make blood clot to prevent the loss of blood after an injury.
• Some proteins called antibodies (part of our immune system) defend the body against harmful pathogens.

Introduction
Lipids are organic compounds that are insoluble in water but are soluble in organic solvents. (e.g., alcohol, ether and benzene).

Chemical Composition
They are composed of glycerol and fatty acids.
a) Glycerol is an alcohol having 3 carbon atoms. Each carbon has a hydroxyl group.

Glycerol structure shown

b) Fatty acids are long hydrocarbon chains with carboxyl group (COOH) at the end.

Types of Fatty Acids
There are two types of fatty acids.
1. Saturated Fatty Acids have internal carbon atoms bonded with maximum number of hydrogen atoms. They do not have double bonds between carbon atoms. Saturated fatty acids are solid at room temperature. e.g. fats.
2. Unsaturated Fatty Acids have one or more double bonds between carbon atoms. They are liquid at room temperature. e.g. oil.

Main Groups of Lipids
1. Fats and Oils
Fats and oils are the most familiar lipids. They contain one glycerol and three fatty acids. Fats contain saturated fatty acids and so are solid at room temperature e.g., animal fats. On the other hand, oils contain unsaturated fatty acids and so are liquid at room temperature e.g., plant oils such as olive oil, corn oil, and coconut oil.
2. Phospholipids
These lipids make the core of all membranes. A phospholipid molecule consists of one glycerol, two fatty acids and a phosphate group.

Sources and Functions of Lipids

Sources
Sources of lipids from animals are meat and dairy products, while the sources of lipids from plants are nuts, seeds, olive oil etc. Plants synthesize oils and store them in seeds, such as sunflower oil, coconut oil, and corn oil.

Functions

• Lipids are the most energy-rich biomolecules. They serve as a long-term energy reserve in the form of fats in adipose tissue. When the, body requires energy, these stored lipids are broken down to release fatty acids and glycerol, which can be used as fuel for energy.
• Lipids are essential components of cell membranes.
• Lipids act as insulator and protect vital organs. For example, adipose tissue surrounding organs, provides cushioning and heat insulation.
• Some lipids help in the synthesis of hormones. Steroid hormones are derived from a lipid i.e., cholesterol.
• Lipids help in the absorption of fat-soluble vitamins. (A,D,E and K) in the digestive system.

Introduction
Nucleic acids are the biomolecules that are composed of units called nucleotides. A nucleotide is made up of three components.
1. Pentose sugar (ribose or deoxyribose)
2. Nitrogenous base
3. Phosphate group (PO₄)

General Structure of Nucleotide shown

Types of Nucleic Acids
There are two types of nucleic acids.

1. Deoxyribonucleic Acid (DNA)
Introduction
DNA is made of Deoxyribonucleotides (de-oxy-ribo-nucleotides). In this nucleotide, the pentose sugar is deoxyribose while the nitrogenous base may be adenine (A), thymine (T), cytosine (C) , or guanine (G)

Double Helix Model of DNA / Explain double helix structure of DNA, and discuss the base pairing in this structure.
In 1953 US biologist James Watson and British biologist Francis Crick proposed the double helix model of DNA.

Main Points
According to this model:

• DNA is a double helix molecule. It is made of two strands of nucleotides.
• Both strands are coiled around each other.
• The nitrogenous base of one strand make hydrogen bonds with the nitrogenous bases of the opposite strand.
• The pairing of nitrogenous bases is specific i.e., adenine of one strand form a pair with thymine of opposing strand. Similarly, cytosine forms a pair with guanine.
• There are two hydrogen bonds between adenine and thymine and three hydrogen bond between cytosine and guanine.

Function of DNA
i. Hereditary Information
DNA contains the hereditary information. This information is in the form of a sequence of nucleotides. This sequence determines the order of amino acids during protein synthesis.
ii. Gene
The segment of DNA in which the sequence of nucleotides determines the synthesis of a proteins is called a gene. During reproduction, DNA is passed from one generation to the next. In this way DNA carries the heredity information to the next generation.

2. Ribonucleic Acid (RNA)
Introduction
RNA is single stranded molecule. Its strand consists of ribonucleotides. A ribonucleotide contains ribose sugar instead of deoxyribose. In a ribonucleotide, the nitrogenous base may be adenine (A) uracil (U) cytosine (C) or guanine (G).

Types of RNA
There are three types of RNA
a) Messenger RNA (mRNA)
Carries the genetic information from DNA to the ribosomes during protein synthesis.
b) Transfer RNA (tRNA)
Transfer specific amino acids to the ribosomes, ensuring the correct sequence during protein synthesis.
c) Ribosomal RNA (rRNA)
Constitutes the structural and functional components of ribosomes, the cellular machinery for protein synthesis.

a) Introduction
The DNA molecule in a chromosome consists of thousands of nucleotides. Along the length of DNA molecule, there are specific segments called genes. Each gene consists of specific sequence of nucleotides that carries information for the synthesis of a specific protein.

b) Gene Expression (Central Dogma)
i. Transcription
During the working of a gene, the specific sequence of DNA nucleotides is copied in the form of messenger RNA (mRNA). This process is called transcription.

ii. Translation
The mRNA carries the sequence of its nucleotides to the ribosome. The ribosome reads this sequence and joins specific amino acids, according to it, to form protein. This step is known as translation.