Structure of RNA

 Structure of RNA 

1. The acceptor arm : This arm is capped with a sequence CCA (5′to 3′). The

amino acid is attached to the acceptor arm.

2. The anticodon arm : This arm, with the three specific nucleotide bases

(anticodon), is responsible for the recognition of triplet codon of mRNA. The

codon and anticodon are complementary to each other.

3. The D arm : It is so named due to the presence of dihydrouridine.

4. The TψC arm : This arm contains a sequence of T, pseudouridine

(represented by psi, ψ) and C.

5. The variable arm : This arm is the most variable in tRNA. Based on this

variability, tRNAs are classified into 2 categories :

(a) Class I tRNAs : The most predominant (about 75%) form with 3–5 base

pairs length.

(b) Class II tRNAs : They contain 13–20 base pair long arm.

Classification of proteins on the basis of functional and chemical nature

Classification of Proteins

Proteins are complex macromolecules that perform a wide variety of biological functions. They can be classified based on function, chemical nature, and modern structural properties.

A. Functional Classification of Proteins

Proteins can be classified based on the role they play in organisms:

1.     Structural proteins – Provide mechanical support

o    Examples: Keratin (hair and nails), Collagen (bone)

2.     Enzymes (Catalytic proteins) – Speed up biochemical reactions

o    Examples: Hexokinase, Pepsin

3.     Transport proteins – Carry molecules across membranes or in blood

o    Examples: Hemoglobin, Serum albumin

4.     Hormonal proteins – Regulate physiological processes

o    Examples: Insulin, Growth hormone

5.     Contractile proteins – Facilitate movement

o    Examples: Actin, Myosin

6.     Storage proteins – Store nutrients and amino acids

o    Examples: Ovalbumin (egg), Glutelin (wheat)

7.     Genetic proteins – Associated with nucleic acids

o    Example: Nucleoproteins

8.     Defense proteins – Protect organisms from pathogens or toxins

o    Examples: Immunoglobulins, Snake venom proteins

9.     Receptor proteins – Bind hormones, viruses, or ligands

o    Example: Hormone receptors

B. Classification Based on Chemical Nature and Solubility

This system classifies proteins based on amino acid composition, structure, shape, and solubility.

1. Simple Proteins

Composed entirely of amino acids. They are further divided into:

(a) Globular Proteins – Spherical or oval, water-soluble, digestible

·         Albumins: Soluble in water and dilute salts, heat-coagulable (e.g., Serum albumin, Ovalbumin, Lactalbumin)

·         Globulins: Soluble in neutral or dilute salt solutions (e.g., Serum globulins, Vitelline)

·         Glutelins: Soluble in dilute acids/alkalis; mostly plant proteins (e.g., Glutelin, Oryzenin)

·         Prolamines: Soluble in 70% alcohol (e.g., Gliadin, Zein)

·         Histones: Strongly basic, soluble in water/dilute acids, insoluble in ammonium hydroxide (e.g., Thymus histones)

·         Globins: Not basic; soluble proteins associated with oxygen transport

·         Protamines: Strongly basic, smaller than histones, soluble in ammonium hydroxide (e.g., Sperm proteins)

·         Lectins: Carbohydrate-binding proteins; maintain tissue structure and used in affinity chromatography (e.g., Concanavalin A, Agglutinin)

(b) Fibrous Proteins – Insoluble, fiber-like, resistant to digestion

·         Collagens: Connective tissue proteins; gelatin obtained by boiling

·         Elastins: Found in elastic tissues like tendons and arteries

·         Keratins: Present in hair, nails, horns; high cysteine content (~14% in human hair)

2. Conjugated Proteins

Proteins containing a non-protein prosthetic group:

·         Nucleoproteins: DNA or RNA as prosthetic group (e.g., Nucleohistones, Nucleoprotamines)

·         Glycoproteins: Contain carbohydrates (<4%); mucoproteins if >4% (e.g., Mucin, Ovomucoid)

·         Lipoproteins: Proteins with lipid prosthetic groups (e.g., Serum lipoproteins)

·         Phosphoproteins: Contain phosphate groups (e.g., Casein, Vitelline)

·         Chromoproteins: Contain colored prosthetic groups (e.g., Hemoglobin, Cytochromes)

·         Metalloproteins: Contain metal ions (e.g., Ceruloplasmin-Cu, Carbonic anhydrase-Zn)

3. Derived Proteins

Derived proteins are denatured or hydrolyzed products of simple or conjugated proteins.

(a) Primary Derived Proteins – Initial denaturation or hydrolysis products

·         Coagulated Proteins: Denatured by heat, acids, or alkalies (e.g., Cooked proteins, Coagulated albumin)

·         Proteans: Early hydrolysis products, insoluble in water (e.g., Fibrin from fibrinogen)

·         Metaproteins: Second-stage hydrolysis products (e.g., Acid/alkali metaproteins)

(b) Secondary Derived Proteins – Progressive hydrolytic products

·         Include proteoses, peptones, polypeptides, and peptides formed during enzymatic or chemical breakdown of proteins

Summary Table (Optional for Quick Revision)

Classification	Subtype / Examples	Solubility / Property
Globular	Albumin, Globulin	Water / Salt soluble
Fibrous	Collagen, Keratin	Insoluble, fiber-like
Conjugated	Nucleoprotein, Glycoprotein, Metalloprotein	Depends on prosthetic group
Derived	Coagulated, Proteans, Peptones	Denatured or hydrolyzed

 

Elemental composition of proteins

Elemental Composition of Proteins

Proteins are complex macromolecules that play vital roles in the structure, function, and regulation of living organisms. They are primarily composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S), which together form the building blocks of amino acids, the monomers of proteins. Understanding the elemental composition of proteins provides insights into their chemical properties, structural diversity, and biological functions.

Major Elements in Proteins

1.     Carbon (C) – 50–55%
Carbon forms the backbone of all amino acids, linking the amino group (-NH₂), carboxyl group (-COOH), hydrogen, and variable side chains (R groups). The carbon content provides structural stability and contributes to the hydrophobic and hydrophilic properties of proteins, influencing folding and three-dimensional conformation.

2.     Hydrogen (H) – 6–7.3%
Hydrogen is present in the amino group, carboxyl group, and side chains. It plays a key role in hydrogen bonding, which stabilizes secondary structures like α-helices and β-sheets, and contributes to protein interactions and enzyme catalysis.

3.     Oxygen (O) – 19–24%
Oxygen is found in the carboxyl group, side chains of certain amino acids (e.g., serine, threonine), and peptide bonds. It participates in hydrogen bonding, contributes to the polar nature of proteins, and is involved in enzymatic activity and protein solubility.

4.     Nitrogen (N) – 13–19%
Nitrogen is a defining element of proteins, present in the amino groups of amino acids and in peptide bonds. Nitrogen content is crucial for the formation of polypeptide chains, structural integrity, and interactions with nucleic acids in cells.

5.     Sulfur (S) – 0–4%
Sulfur occurs in the side chains of amino acids such as cysteine and methionine. It is essential for forming disulfide bonds, which stabilize the tertiary and quaternary structures of proteins. Even though present in small amounts, sulfur significantly impacts protein folding, stability, and function.

Trace Elements in Proteins

In addition to the five major elements, proteins may incorporate trace elements such as phosphorus (P), iron (Fe), copper (Cu), iodine (I), magnesium (Mg), manganese (Mn), and zinc (Zn). These elements are often associated with prosthetic groups, coenzymes, or metalloproteins. Examples include:

• Hemoglobin and myoglobin – contain iron in the heme group
• Zinc finger proteins – contain Zn ions for DNA binding
• Copper proteins – involved in electron transport
• Phosphoproteins – have phosphate groups covalently linked to serine, threonine, or tyrosine

These additional elements are essential for catalytic activity, electron transport, structural stability, and regulatory functions of proteins.

Summary

Proteins are predominantly made up of C, H, O, N, and S, which together form the amino acid backbone and side chains responsible for the structural and functional diversity of proteins. Minor elements such as P, Fe, Zn, and Cu are present in specialized proteins and are critical for enzymatic and regulatory functions. The unique elemental composition of proteins underlies their versatility as catalysts, structural components, signaling molecules, and transporters in living organisms.

Function of lipids

Functions of lipids:-

Lipids perform several important functions

1. They are the concentrated fuel reserve of the body.

2. Lipids are the constituents of membrane structure and regulate the membrane

permeability (phospholipids and cholesterol).

3. They serve as a source of fat soluble vitamins (A, D, E and K).

4. Lipids are important as cellular metabolic regulators (steroid hormones and

prostaglandins).

5. Lipids protect the internal organs, serve as insulating materials and give shape

and smooth appearance to the body

Classification of lipids

Classification of Lipids

Lipids are a heterogeneous group of organic compounds that are insoluble in water but soluble in non-polar organic solvents such as ether, chloroform, and benzene. They play essential roles in energy storage, membrane structure, and cellular regulation. Based on chemical composition and structural complexity, lipids are broadly classified according to Bloor’s classification into simple lipids, complex (compound) lipids, derived lipids, and miscellaneous lipids.

1. Simple Lipids

Simple lipids are esters of fatty acids with alcohols and do not contain any additional functional groups. On hydrolysis, they yield only fatty acids and alcohols. They are mainly involved in energy storage and protection.

(a) Fats and Oils (Triacylglycerols)

Fats and oils, collectively known as triacylglycerols (TAGs), are esters of three fatty acid molecules with glycerol. They represent the major storage form of lipids in plants and animals.

The distinction between fats and oils is physical rather than chemical.

• Fats are solid at room temperature due to a higher content of saturated fatty acids.
• Oils are liquid at room temperature because of a higher proportion of unsaturated fatty acids.

Triacylglycerols are energy-dense molecules, providing about 9 kcal g⁻¹, and serve as insulation and mechanical protection for organs.

(b) Waxes

Waxes are esters of long-chain fatty acids with long-chain monohydric alcohols other than glycerol. The alcohol component may be aliphatic or alicyclic, with cetyl alcohol being commonly present.

Waxes are highly hydrophobic and possess high melting points, making them resistant to water and degradation. They serve protective functions, such as preventing water loss in plants and forming protective coatings in animals. Industrially, waxes are used in the manufacture of candles, polishes, cosmetics, ointments, lubricants, and waterproofing agents.

2. Complex (Compound) Lipids

Complex lipids are esters of fatty acids with alcohols that, in addition, contain other functional groups such as phosphate, carbohydrate, protein, or nitrogenous bases. They are essential structural components of biological membranes.

(a) Phospholipids

Phospholipids contain phosphoric acid and often a nitrogenous base, along with fatty acids and an alcohol. They are the major constituents of cell membranes.

(i) Glycerophospholipids

In these phospholipids, glycerol serves as the alcohol backbone. Examples include:

• Phosphatidylcholine (lecithin)
• Phosphatidylethanolamine (cephalin)

Glycerophospholipids play a crucial role in membrane structure, lipid signaling, and transport processes.

(ii) Sphingophospholipids

In this group, sphingosine replaces glycerol as the alcohol component.

• Sphingomyelin is the most important member and is abundant in myelin sheaths of nerve fibers, contributing to nerve insulation and signal transmission.

(b) Glycolipids

Glycolipids contain fatty acids, carbohydrates, and a nitrogenous base, with sphingosine as the alcohol. They lack glycerol and phosphate and are therefore also known as glycosphingolipids.

Examples include:

• Cerebrosides
• Gangliosides

Glycolipids are predominantly found in the outer surface of plasma membranes, especially in nervous tissue, where they are involved in cell recognition, receptor function, and immune responses.

(c) Lipoproteins

Lipoproteins are macromolecular complexes of lipids and proteins that function as transport vehicles for lipids in the bloodstream. Based on density, they are classified as:

• Chylomicrons
• VLDL
• LDL
• HDL

They play a vital role in lipid metabolism and cardiovascular health.

(d) Other Complex Lipids

This group includes:

• Sulfolipids
• Aminolipids
• Lipopolysaccharides (LPS)

Lipopolysaccharides are important components of the outer membrane of Gram-negative bacteria and act as endotoxins.

3. Derived Lipids

Derived lipids are substances obtained from the hydrolysis of simple and complex lipids and still retain lipid-like properties. They include:

• Fatty acids
• Glycerol and other alcohols
• Mono- and diacylglycerols
• Steroids and steroid hormones
• Fat-soluble vitamins (A, D, E, K)
• Ketone bodies
• Hydrocarbons

These molecules play crucial roles in metabolism, hormone action, and cell signaling.

4. Miscellaneous Lipids

Miscellaneous lipids include a wide range of lipid-like compounds that do not fit into the above categories but exhibit hydrophobic properties.

Examples include:

• Carotenoids (precursors of vitamin A)
• Squalene (precursor of cholesterol)
• Terpenes
• Hydrocarbons such as pentacosane found in beeswax

These compounds are involved in pigmentation, membrane stability, and biosynthesis of steroids.

Conclusion

In summary, lipids represent a chemically diverse group of biomolecules with essential biological functions. Their classification into simple, complex, derived, and miscellaneous lipids helps in understanding their structure, function, and metabolic roles. A clear knowledge of lipid classification is fundamental for advanced studies in biochemistry, cell biology, and biotechnology, and is particularly important for competitive examinations such as CSIR-NET and GATE.

 

Functions of carbohydrates

Carbohydrates are one of the most abundant and diverse classes of biomolecules in living organisms. Chemically, they are polyhydroxy aldehydes or ketones and their derivatives. Biologically, carbohydrates play essential roles in energy metabolism, structural organization, cellular communication, and regulation of physiological processes. Due to their versatility and abundance, carbohydrates are indispensable for the survival of all forms of life, from microorganisms to higher eukaryotes.

1. Carbohydrates as a Primary Source of Energy

The most fundamental function of carbohydrates is their role as the primary source of metabolic energy. Upon complete oxidation, carbohydrates yield approximately 4 kcal g⁻¹, making them an efficient and readily available energy source. Glucose is the central carbohydrate in energy metabolism and serves as a universal fuel for cells.

In aerobic organisms, glucose undergoes glycolysis, producing pyruvate, ATP, and reducing equivalents (NADH). Pyruvate is further oxidized through the tricarboxylic acid (TCA) cycle and oxidative phosphorylation, leading to maximum ATP generation. Certain tissues such as the brain, red blood cells, and renal medulla depend almost exclusively on glucose for energy. In plants, carbohydrates synthesized during photosynthesis act as the primary energy source for all heterotrophic organisms in the biosphere.

2. Role as Energy Storage Molecules

Carbohydrates also function as storage forms of energy, ensuring a continuous supply of fuel during periods of fasting or increased energy demand. In animals, excess glucose is stored as glycogen, primarily in the liver and skeletal muscles. Liver glycogen maintains blood glucose levels, while muscle glycogen provides energy for muscle contraction.

In plants, carbohydrates are stored mainly as starch, composed of amylose and amylopectin. Starch serves as a long-term energy reserve in seeds, tubers, and roots. Storage polysaccharides are osmotically inert and compact, allowing efficient energy storage without disrupting cellular homeostasis.

3. Structural Role of Carbohydrates

One of the most important functions of carbohydrates is their role as structural components of cells and tissues. Structural polysaccharides provide mechanical strength, rigidity, and protection.

•    Cellulose, a linear polymer of β-D-glucose, is the major structural component of plant cell walls and contributes to cell shape and resistance against mechanical stress.
•     Chitin, composed of N-acetylglucosamine units, forms the exoskeleton of arthropods and the cell walls of fungi.
•   In bacteria, carbohydrates are integral components of the cell wall, such as peptidoglycan, which maintains cell shape and prevents osmotic lysis.

Thus, carbohydrates play a vital role in maintaining the integrity and architecture of biological systems.

4. Precursors of Other Biomolecules

Carbohydrates serve as metabolic precursors for the synthesis of numerous biologically important molecules. Intermediates of carbohydrate metabolism are diverted into various biosynthetic pathways.

•   Fatty acids and lipids are synthesized from acetyl-CoA derived from carbohydrate catabolism.
•      Amino acids such as alanine, glycine, and serine are formed from glycolytic intermediates.
•     Ribose-5-phosphate, generated through the pentose phosphate pathway, is essential for the synthesis of nucleotides, nucleic acids, and coenzymes (NAD⁺, FAD, ATP).

Therefore, carbohydrates act as central hubs connecting catabolic and anabolic pathways.

5. Role in Cell Membrane Structure and Function

Carbohydrates are key components of the cell membrane, where they are present as glycoproteins and glycolipids. These carbohydrate-containing molecules are primarily located on the extracellular surface of the plasma membrane, forming the glycocalyx.

The glycocalyx plays critical roles in:

•     Cell–cell recognition and adhesion
•     Signal transduction
•     Immune response
•     ·Fertilization and development

For example, blood group antigens (ABO system) are determined by specific carbohydrate moieties present on red blood cell membranes. Similarly, pathogen recognition by host cells often involves carbohydrate–protein interactions.

6. Role in Cell Communication and Signaling

Carbohydrates participate in cellular communication by serving as recognition markers and ligands for receptors. Lectins, a class of carbohydrate-binding proteins, specifically recognize carbohydrate residues and mediate biological processes such as immune responses, inflammation, and cell migration.

Glycosylation of proteins influences their folding, stability, targeting, and function. Improper glycosylation is associated with several diseases, including congenital disorders of glycosylation and cancer.

7. Protective and Lubricating Functions

Certain carbohydrates and their derivatives have protective and lubricating roles. Mucopolysaccharides or glycosaminoglycans (e.g., hyaluronic acid, chondroitin sulfate) are major components of connective tissues, synovial fluid, and cartilage.

Hyaluronic acid acts as a lubricant in joints, while chondroitin sulfate provides tensile strength to cartilage. These molecules also play roles in wound healing and tissue repair.

8. Role in Osmoregulation and Detoxification

Carbohydrates help in maintaining osmotic balance within cells. Some sugars and sugar alcohols act as compatible solutes in microorganisms and plants, protecting cells from osmotic stress.

Additionally, carbohydrates participate in detoxification processes. Glucuronic acid conjugates with toxic substances, drugs, and bilirubin in the liver, facilitating their excretion from the body.

Conclusion

In conclusion, carbohydrates are multifunctional biomolecules essential for life. They serve as primary sources and storage forms of energy, provide structural support, act as precursors for biosynthesis, and play crucial roles in cell membrane architecture, communication, and protection. Their involvement in metabolic regulation, development, immunity, and homeostasis highlights their central importance in biological systems. A thorough understanding of carbohydrate functions is fundamental for advanced studies in biochemistry, molecular biology, and biotechnology, particularly for competitive examinations such as CSIR-NET and GATE.

 

CSIR-NET MCQs: Functions of Carbohydrates

1. Major energy-yielding carbohydrate in human metabolism is:

Fructose
Glucose
Lactose
Cellulose

Explanation: Glucose is the universal fuel molecule and primary substrate for glycolysis.

2. Structural polysaccharide in plant cell wall is:

Starch
Glycogen
Cellulose
Dextran

Explanation: Cellulose is a β-1,4-linked glucose polymer providing rigidity to plant cell walls.

3. Glycogen is mainly stored in:

Liver and muscle
Brain and kidney
Heart and lungs
Bone marrow

Explanation: Liver glycogen maintains blood glucose, while muscle glycogen supplies local energy.

4. Pentose phosphate pathway mainly provides:

ATP
Acetyl-CoA
Pyruvate
NADPH and ribose-5-phosphate

Explanation: PPP supplies NADPH for reductive biosynthesis and ribose for nucleotide synthesis.

5. Glycoproteins are mainly involved in:

Energy storage
Cell recognition and signaling
DNA replication
Lipid digestion

Explanation: Carbohydrate moieties on glycoproteins mediate cell–cell recognition and signaling.