Chemical Constituents of Living Cells: Biomolecules –Structure

Biomolecules are organic compounds found in living organisms. They are primarily
composed of carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulphur. These
Elements combine in various ways to form four major classes of biomolecules:

1. Carbohydrates
2. Proteins
3. Lipids
4. Nucleic acids
   These biomolecules form the building blocks of life and are involved in various
   structural and functional roles, from forming cell membranes to carrying genetic
   information.
   Importance of Biomolecules in Living Organisms
   Every cell in our body depends on biomolecules. Here’s why:
    Carbohydrates provide quick energy and serve as structural components
   in cell walls and membranes.
    Proteins are versatile; they act as enzymes, hormones, antibodies, and
   structural units.
    Lipids are vital for storing energy, insulating the body, and forming the core
   structure of cell membranes.
    Nucleic acids, namely DNA and RNA, are responsible for storing and
   transmitting genetic information.
   Without these molecules, life wouldn’t just be difficult—it would be impossible.
   Carbohydrates – Structure and Function
   Carbohydrates are the body’s primary source of energy. They are made up of carbon
   (C), hydrogen (H), and oxygen (O), typically in a 1:2:1 ratio. These molecules can
   range from simple sugars to large polysaccharides.
   Definition and Classification of Carbohydrates
   Carbohydrates are classified based on the number of sugar units they contain:
5. Monosaccharides – single sugar units
6. Disaccharides – two sugar units
7. Polysaccharides – many sugar units
   Monosaccharides – Glucose, Fructose, Galactose
   These are the simplest carbohydrates and cannot be hydrolysed into smaller units.
    Glucose (C₆H₁₂O₆): Known as blood sugar, it’s the main energy
   source.
    Fructose: Found in fruits; it’s the sweetest naturally occurring sugar.
    Galactose: A component of lactose found in milk.

These sugars are absorbed directly into the bloodstream and provide instant energy.
Disaccharides – Sucrose, Lactose, Maltose
Formed by the condensation of two monosaccharides:
 Sucrose (Glucose + Fructose): Common table sugar.
 Lactose (Glucose + Galactose): Milk sugar.
 Maltose (Glucose + Glucose): Found in germinating grains.
These require enzymatic breakdown into monosaccharides before absorption.
Polysaccharides – Starch, Glycogen, Cellulose
These are complex carbohydrates:
 Starch: Storage form of glucose in plants.
 Glycogen: Storage form of glucose in animals, mainly in liver and muscles.
 Cellulose: Structural component of the plant cell wall; indigestible to
humans but crucial for dietary fibre.
Functions of Carbohydrates in the Body
Carbohydrates aren’t just energy providers; their functions include:
 Providing energy (4 kcal/gram)
 Serving as structural components (cellulose in plants)
 Participating in cell recognition and signalling (glycoproteins)
 Sparing protein use for energy, thus preserving muscle mass
Without carbohydrates, the brain would lack fuel, and cells would struggle to
function.

Proteins – Structure and Function
Proteins are the workhorses of the cell. Everything from muscle contractions to
immune defence depends on these vital biomolecules. Composed of amino acids,
proteins are involved in almost every cellular function.
Basic Building Blocks – Amino Acids
There are 20 different amino acids, each with a central carbon atom bonded to:
 An amino group (-NH₂)

 A carboxylic acid group (-COOH)
 A hydrogen atom
 A variable side chain (R-group)
Peptide bonds link amino acids to form long chains, which fold into functional
proteins.
Levels of Protein Structure
Proteins have a complex architecture:
 Primary structure: Linear sequence of amino acids.
 Secondary structure: Local folding into alpha-helices or beta-pleated sheets.
 Tertiary structure: 3D folding due to R-group interactions.
 Quaternary structure: Multiple polypeptide chains assembled into one
functional protein (e.g., haemoglobin).
Each level determines the protein’s shape and function.
Biological Functions of Proteins
Proteins are indispensable:
 Enzymatic role: Catalysts for biochemical reactions (e.g., amylase, lipase)
 Structural role: In skin, hair, nails (e.g., keratin, collagen)
 Transport role: Haemoglobin carries oxygen
 Defence: Antibodies fight pathogens
 Signalling: Hormones like insulin regulate physiology
A malfunction in protein structure often leads to diseases like sickle cell anaemia or
cystic fibrosis.
Lipids – Structure and Function
Lipids are another major class of biomolecules that are insoluble in water but
soluble in organic solvents like ether and chloroform. These hydrophobic molecules
play crucial roles in energy storage, cell membrane structure, and hormone
synthesis.
Classification of Lipids
Lipids can be broadly classified into three categories:

1. Simple lipids – fats and oils
2. Compound lipids – phospholipids, glycolipids
3. Derived lipids – steroids, fat-soluble vitamins
   Let’s break them down.
   Simple Lipids – Fats and Oils
   These are esters of fatty acids and glycerol:
    Fats are solid at room temperature (mostly animal origin).
    Oils are liquid at room temperature (mainly from plants).
   Fatty acids can be:
    Saturated (no double bonds, e.g., palmitic acid)
    Unsaturated (one or more double bonds, e.g., oleic acid)
   Triglycerides are the most common form of fat in the body and diet. They are crucial
   for long-term energy storage.
   Compound Lipids – Phospholipids, Glycolipids
   These contain additional functional groups:
    Phospholipids: Major component of the cell membrane. They have
   hydrophilic heads and hydrophobic tails, forming the lipid bilayer.
    Glycolipids: Found in the brain and nerve tissues; play a role in cell
   recognition.
   Their amphipathic nature (both hydrophilic and hydrophobic parts) allows them to
   form biological membranes, a fundamental aspect of cellular life.
   Derived Lipids – Steroids
   Steroids include cholesterol and hormones like estrogen, testosterone, and cortisol.
   Cholesterol is vital for:
    Maintaining membrane fluidity
    Precursor for steroid hormone synthesis
    Synthesis of vitamin D and bile salts
   Roles of Lipids in the Body
   Lipids serve multiple purposes:

 Energy storage: Provide 9 kcal/gram, more than twice that of carbs or
proteins
 Insulation and protection: Fat under the skin insulates against heat
loss
 Structural component: Essential in membrane formation
 Hormonal function: Steroid hormones regulate numerous physiological
processes
 Nerve function: Myelin sheath, rich in lipids, ensures rapid nerve impulse
conduction
Lipids are often misunderstood due to their link with obesity, but they are essential
for health when consumed in balance.
Nucleic Acids – Structure and Function
Nucleic acids are the blueprint of life, storing and transmitting genetic information
from one generation to the next. These macromolecules are made up of nucleotides
and are found in all living cells.
Types of Nucleic Acids – DNA and RNA
There are two main types:
 DNA (Deoxyribonucleic acid): Stores genetic information
 RNA (Ribonucleic acid): Assists in protein synthesis
While DNA is double-stranded and stable, RNA is single-stranded and more
reactive.
Components – Nucleotides and Nitrogenous Bases
Each nucleotide consists of:

1. A phosphate group
2. A pentose sugar (ribose in RNA, deoxyribose in DNA)
3. A nitrogenous base (Adenine, Guanine, Cytosine, Thymine in DNA; Uracil
   replaces Thymine in RNA)
   Structure of DNA and RNA
    DNA has a double-helix structure discovered by Watson and Crick. The
   strands run anti-parallel and are held together by hydrogen bonds between
   complementary bases (A-T and G-C).

 RNA is single-stranded and comes in various forms – mRNA, tRNA, and
rRNA – each with specific roles in protein synthesis.
Functions of Nucleic Acids in Heredity and Protein
Synthesis
 DNA is the genetic material in most organisms. It:
o Directs protein synthesis through transcription and translation
o Replicates itself during cell division
 RNA translates genetic code from DNA into proteins, acting as a messenger
and assistant in the synthesis process.
Disruptions in nucleic acid function can lead to genetic disorders like sickle cell
anaemia, Down syndrome, or cancer.
Integration of Biomolecules in Cell Function
Interaction and Metabolic Pathways
Biomolecules don’t act alone; they function in concert within the cell’s metabolic
pathways:
 Glycolysis uses carbohydrates to produce ATP.
 Proteins can be broken down to generate energy or synthesize new proteins.
 Lipids serve as a backup energy source and play a role in signalling
pathways.
 Nucleic acids direct all these processes through gene expression.
This interconnectedness ensures that cells adapt, grow, and respond to changes
efficiently.
Role in Enzyme Activity and Regulation
Many biomolecules, particularly proteins, serve as enzymes. These:
 Speed up biochemical reactions
 Are specific to substrates
 Are regulated by feedback mechanisms
Other biomolecules serve as coenzymes (like vitamins) or substrates, contributing
directly or indirectly to enzymatic action.
NEET Previous Year Questions (2015–2024) with Answers

Carbohydrates-based Questions
Q (NEET 2022): Which of the following is a reducing sugar?
A: Maltose (Answer: Maltose)
Q (NEET 2020): Which polysaccharide is found abundantly in plant cell walls?
A: Cellulose (Answer: Cellulose)
Proteins-based Questions
Q (NEET 2021): The structural level of protein affected in Sickle Cell Anaemia is:
A: Primary structure (Answer: Primary structure)
Q (NEET 2019): Which of the following has both structural and enzymatic proteins?
A: Muscle cells (Answer: Muscle cells)
Lipids-based Questions
Q (NEET 2023): Which lipid component is majorly found in the myelin sheath?
A: Sphingomyelin (Answer: Sphingomyelin)
Q (NEET 2017): Which of the following is not a lipid?
A: Collagen (Answer: Collagen)
Nucleic Acids-based Questions
Q (NEET 2024): The base sequence of one strand of DNA is AATCG. The
complementary base sequence of the other strand would be:
A: TTAGC (Answer: TTAGC)
Q (NEET 2016): DNA differs from RNA in having:
A: Thymine instead of Uracil (Answer: Thymine)
Conclusion
Biomolecules are the chemical foundation of life. From the food we eat to the
energy we use to think and move, everything boils down to the chemical interactions
of proteins, carbohydrates, lipids, and nucleic acids. Each of these macromolecules
has a unique structure that determines its function, and together, they coordinate the
incredibly complex symphony of life processes.
 Carbohydrates are our quick energy source and play a key role in cellular
structure and communication.
 Proteins are versatile molecules involved in structural integrity, enzymatic
functions, and immune responses.
 Lipids provide insulation, store energy efficiently, and build cell membranes.

 Nucleic acids store and transmit genetic information, enabling continuity of
life and the production of proteins.
These biomolecules are not isolated units. They constantly interact within the
metabolic web of a living cell. Disruption in any of these molecules can result in
metabolic disorders or diseases, emphasizing their importance in maintaining health
and homeostasis.
FAQs

1. What is the most abundant biomolecule in the cell?
   Answer: Water is the most abundant molecule in cells, but among organic
   biomolecules, proteins are the most abundant. They play structural and functional
   roles in nearly every cellular process.
2. What is the difference between DNA and RNA structurally and functionally?
   Answer: DNA is a double-stranded molecule with deoxyribose sugar and contains
   thymine, whereas RNA is single-stranded, contains ribose sugar, and has uracil
   instead of thymine. Functionally, DNA stores genetic information, while RNA helps in
   translating that information into proteins.
3. How do proteins perform so many different functions?
   Answer: Proteins are made from 20 different amino acids which can fold into
   complex 3D structures. Their diverse shapes and chemical properties enable
   them to act as enzymes, structural components, transporters, receptors, and more.
4. Why are lipids considered energy-rich molecules?
   Answer: Lipids provide about 9 kcal/gram of energy, which is more than double that
   of carbohydrates or proteins (both provide 4 kcal/gram). This high energy density is
   due to the large number of C-H bonds in fatty acid chains.
5. How are nucleotides linked together in DNA?
   Answer: Nucleotides are linked by phosphodiester bonds between the 3′ hydroxyl
   group of one sugar and the 5′ phosphate group of the next. This linkage forms the
   sugar-phosphate backbone of DNA.