Biomolecules Notes
Study Notes
Topics
6π 1. Chapter Overview
Overview
Biomolecules are the chemical substances present in living cells that build structures, store energy, carry genetic information and catalyse reactions. NCERT classifies them by chemical nature, molecular size and biological role. In this chapter, you study inorganic elements, micromolecules like sugars, amino acids and nucleotides, and macromolecules like polysaccharides, proteins and nucleic acids. Lipids are special because they are acid-insoluble but not true polymers. Enzymes are mostly proteins that lower activation energy and make life possible at body temperature. Metabolism connects all biomolecules through anabolic and catabolic pathways, maintaining the living state as a dynamic, non-equilibrium condition.
- 1NCERT emphasizes that living organisms are made of the same elements as non-living matter, but arranged in highly organized biomolecules.
- 2Amino acids, sugars, nitrogen bases, nucleotides and fatty acids are important low molecular weight compounds.
- 3Proteins, nucleic acids and polysaccharides are true macromolecular polymers; lipids are grouped with macromolecules because they appear in the acid-insoluble fraction.
- 4Enzymes are central NEET targets: nature, active site, activation energy, cofactors and factors affecting activity.
- 5The living state is maintained by metabolism and constant flow of energy; isolated metabolic reactions are non-living.
Chapter sequence mnemonic
Remember the flow as βCCP-NEMβ: Chemical Composition β Carbohydrates/Lipids β Proteins β Nucleic acids β Enzymes β Metabolism.
Major elements mnemonic
βCHONPSβ sounds like βchompsβ: life chomps mainly Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus and Sulphur.
Daily life biomolecules
Rice gives starch, oil gives lipids, pulses give proteins, fruits give simple sugars and every cell of food contains DNA/RNA.
Body-level integration
After a meal, carbohydrates may be used for ATP, amino acids for proteins, fatty acids for storage and nucleotides for DNA/RNA synthesis.
Calling lipids true polymers
Lipids are not true polymers like proteins or nucleic acids. They are grouped in the acid-insoluble macromolecular fraction because of solubility behavior.
Thinking enzymes change equilibrium
Enzymes speed up the rate of a reaction by lowering activation energy but do not change the final equilibrium of the reaction.
Many simple carbohydrates approximately follow this empirical formula, although not all carbohydrates strictly obey it.
Variables
C=Carbon atom
HβO=Hydrogen and oxygen in a 2:1 ratio
n=Number of repeating carbon-water units
βοΈ 2. Chemical Composition
Overview
Chemical analysis of living tissue shows that cells contain inorganic ions, water, minerals and organic compounds. The major elements in living organisms are carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur, with many trace elements also required. Biomolecules are broadly divided into micromolecules and macromolecules based on molecular size and solubility in trichloroacetic acid. Primary metabolites are directly involved in growth, development and reproduction, while secondary metabolites such as alkaloids, pigments and toxins often help in defense, attraction or ecological interactions. NCERT highlights that living and non-living matter contain the same elements, but living systems show complex organization and metabolism.
- 1Elements in living organisms are not unique to life; life depends on their arrangement into functional molecules.
- 2Chemical analysis often separates filtrate as acid-soluble pool and retentate as acid-insoluble fraction.
- 3Molecular weight below about 1000 Da is commonly associated with micromolecules in NCERT context.
- 4Secondary metabolites include alkaloids, flavonoids, rubber, essential oils, antibiotics and pigments.
- 5NEET often asks classification examples, not just definitions.
Primary metabolites
Primary = βPrime for lifeβ: sugars, amino acids, fatty acids and nucleotides are directly needed for basic life processes.
Secondary metabolites
Secondary = βSpecial servicesβ: pigments attract, toxins defend, alkaloids affect animals and rubber protects wounds.
CHONPS
Use CHONPS for the six life-heavy elements: Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Sulphur.
Primary metabolite example
Glucose is a primary metabolite because it participates directly in respiration and energy production.
Secondary metabolite example
Morphine, an alkaloid from plants, is not essential for basic plant respiration but acts as a specialized compound.
Chemical analysis example
When liver tissue is homogenized and treated with TCA, amino acids and sugars enter filtrate while proteins and nucleic acids remain in the retentate.
Confusing acid-soluble with water-soluble
NCERT separation is based on trichloroacetic acid treatment, not simply solubility in water.
Putting lipids with true polymers
Lipids are macromolecular fraction components but not true polymers made of repeating monomers.
Ignoring trace elements
Elements like Fe, Zn, Cu, I and Mg may be present in small amounts but are biologically essential.
Chemical composition is often studied after removing water because water forms a major part of living tissue.
Variables
Dry mass=Mass of tissue after water removal
Fresh mass=Mass of living tissue before drying
Water mass=Mass contributed by cellular water
π 3. Carbohydrates & Lipids
Overview
Carbohydrates are polyhydroxy aldehydes or ketones and their derivatives. Monosaccharides such as glucose and fructose are single sugar units; disaccharides such as sucrose, lactose and maltose contain two monosaccharides joined by glycosidic bonds; polysaccharides such as starch, glycogen and cellulose are large storage or structural molecules. Lipids are generally water-insoluble substances soluble in organic solvents. Simple lipids include fats and oils made of fatty acids and glycerol, while compound lipids such as phospholipids contain additional groups. Carbohydrates provide quick energy and structural support, while lipids store concentrated energy, form membranes, provide insulation and act as signaling molecules.
- 1Glycosidic bond is the key bond of carbohydrates and is formed by dehydration.
- 2Sucrose is glucose + fructose; lactose is glucose + galactose; maltose is glucose + glucose.
- 3Starch is plant storage polysaccharide; glycogen is animal storage polysaccharide; cellulose is plant cell wall structural polysaccharide.
- 4Phospholipids are amphipathic: hydrophilic head and hydrophobic tails, enabling membrane bilayer formation.
- 5Saturated fatty acids have no C=C double bonds; unsaturated fatty acids have one or more C=C bonds.
- 6NEET often tests examples, functions and differences among starch, glycogen and cellulose.
Disaccharide monomers
βSu-Fru, La-Gal, Mal-Gluβ: Sucrose has fructose, Lactose has galactose, Maltose has glucose plus glucose.
Storage polysaccharides
βPlants Store Starch, Animals Gather Glycogenβ: match plant storage with starch and animal storage with glycogen.
Lipid membrane clue
Phospholipid = βP head + fatty tailsβ; P reminds you of polar phosphate head.
Carbohydrate in food
Potato and rice are rich in starch, which is digested to glucose for respiration.
Lipid in membranes
Every cell membrane has a phospholipid bilayer that forms a selective barrier around the cell.
PYQ-style concept
If a question asks for the animal storage carbohydrate, choose glycogen; if it asks plant cell wall carbohydrate, choose cellulose.
Assuming all carbohydrates are sweet
Polysaccharides like starch and cellulose are carbohydrates but are not sweet like simple sugars.
Confusing cellulose and glycogen
Both are glucose polymers, but cellulose is structural in plants and glycogen is branched storage in animals.
Calling phospholipids fully hydrophobic
Phospholipids are amphipathic: their head is hydrophilic and tails are hydrophobic.
Many monosaccharides such as glucose follow this general formula.
Variables
n=Number of carbon atoms
C, H, O=Carbon, hydrogen and oxygen atoms
Two hexose monosaccharides form a disaccharide by losing one water molecule.
Variables
CβHββOβ=Hexose monosaccharide such as glucose
CββHββOββ=Disaccharide
HβO=Water released during condensation
𧬠4. Proteins
Overview
Proteins are polymers of amino acids joined by peptide bonds. Each amino acid has an amino group, carboxyl group, hydrogen atom and variable R group attached to a central carbon. The R group determines chemical nature, making amino acids acidic, basic, neutral, polar or non-polar. Protein structure is studied at four levels: primary sequence, secondary folding such as alpha helix and beta sheet, tertiary three-dimensional shape and quaternary arrangement of multiple polypeptide chains. Proteins perform diverse roles as enzymes, hormones, transporters, receptors, antibodies, structural fibers and contractile elements. For NEET, peptide bond formation, amino acid structure and protein levels are especially important.
- 1The sequence of amino acids determines higher structure and biological function.
- 2Peptide bond is a covalent amide linkage: -CO-NH-.
- 3Fibrous proteins are usually structural and elongated; globular proteins are compact and often functional.
- 4Examples of protein functions include collagen for support, haemoglobin for transport, insulin as hormone and enzymes for catalysis.
- 5Sickle-cell anaemia is a classic example of how one amino acid change can alter protein function.
- 6NEET commonly tests the order: primary β secondary β tertiary β quaternary.
Protein levels order
βPlease Stop Talking Quietlyβ: Primary, Secondary, Tertiary, Quaternary.
Amino acid structure
Amino acid is βA-CARβ: Amino group, Carboxyl group, Alpha carbon and R group.
Fibrous vs globular
Fibrous = fiber-like for support; Globular = globe-like for active functions.
Haemoglobin
Haemoglobin is a quaternary protein with multiple subunits and transports oxygen in blood.
Keratin
Keratin is a fibrous structural protein found in hair and nails.
One amino acid change
A single amino acid substitution in haemoglobin can cause sickle-cell anaemia, showing the importance of primary structure.
Saying denaturation always breaks peptide bonds
Denaturation usually disrupts secondary, tertiary or quaternary structure; primary peptide-bond sequence often remains intact.
Mixing tertiary and quaternary structure
Tertiary is one polypeptideβs 3D shape; quaternary requires multiple polypeptide subunits.
Forgetting R group importance
The R group decides the amino acidβs chemical behavior and strongly influences protein folding.
Amino acids have a central alpha carbon attached to amino group, carboxyl group, hydrogen and variable R group.
Variables
NHβ=Amino group
COOH=Carboxyl group
R=Variable side chain determining amino acid properties
Condensation reaction forms a peptide bond between two amino acids.
Variables
Amino acidβ=First amino acid donating carboxyl group
Amino acidβ=Second amino acid donating amino group
HβO=Water released during bond formation
𧬠5. Nucleic Acids
Overview
Nucleic acids are polymers of nucleotides. Each nucleotide contains a nitrogenous base, pentose sugar and phosphate group. DNA usually contains deoxyribose sugar and bases adenine, guanine, cytosine and thymine, while RNA contains ribose and uracil instead of thymine. DNA is generally double-stranded and stores hereditary information, whereas RNA is usually single-stranded and helps express genetic information. The backbone is formed by phosphodiester bonds between sugar and phosphate groups, while complementary bases pair through hydrogen bonds. Major types of RNA include mRNA, tRNA and rRNA. NEET frequently asks nucleotide components, DNA-RNA differences, base pairing and RNA functions.
- 1A nucleotide has three parts: base, sugar and phosphate.
- 2DNA strands are antiparallel and complementary.
- 3Hydrogen bonds between bases stabilize double-stranded nucleic acids, but phosphodiester bonds form the covalent backbone.
- 4Purines have two rings; pyrimidines have one ring.
- 5ATP is also a nucleotide and functions as cellular energy currency.
- 6NEET commonly traps students by asking nucleoside vs nucleotide.
Purines
βPure As Goldβ: Purines are Adenine and Guanine.
Pyrimidines
βCUT the PYβ: Cytosine, Uracil and Thymine are pyrimidines.
RNA types
mRNA = message, tRNA = transport, rRNA = ribosome.
Nucleoside vs nucleotide
Nucleotide has βtideβ like a wave carrying phosphate; nucleoside has no phosphate.
ATP as nucleotide
ATP is a nucleotide derivative with adenine, ribose and three phosphate groups; it acts as energy currency.
mRNA in protein synthesis
A gene is transcribed into mRNA, which is read by ribosomes to form a polypeptide.
NEET-style base pairing
If a DNA segment has 20% adenine, it has 20% thymine; the remaining 60% is guanine plus cytosine.
Confusing nucleoside and nucleotide
Nucleoside has base + sugar only; nucleotide has base + sugar + phosphate.
Replacing thymine in DNA with uracil
Thymine is in DNA, uracil is in RNA. Adenine pairs with thymine in DNA and uracil in RNA.
Thinking hydrogen bonds form the backbone
Hydrogen bonds join complementary bases; phosphodiester bonds form the sugar-phosphate backbone.
A nucleoside lacks phosphate and is not the complete monomer of nucleic acid.
Variables
Nitrogenous base=Purine or pyrimidine base
Pentose sugar=Ribose in RNA or deoxyribose in DNA
Nucleotides polymerize to form DNA and RNA.
Variables
Base=A, G, C, T or U
Sugar=Pentose sugar
Phosphate=Phosphate group forming backbone links
β‘ 6. Enzymes
Overview
Enzymes are biological catalysts that accelerate biochemical reactions without being consumed. Most enzymes are proteins, though some RNA molecules act as ribozymes. Enzymes possess active sites where substrates bind to form an enzyme-substrate complex. They lower activation energy, making reactions fast enough to sustain life at normal temperatures. Enzyme activity is influenced by temperature, pH, substrate concentration, inhibitors and cofactors. Cofactors may be metal ions, organic coenzymes or tightly bound prosthetic groups. Enzymes are classified into oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases. For NEET, enzyme mechanism, active site, cofactors and factor graphs are high-yield.
- 1Specificity arises from the active site shape and chemical environment.
- 2Induced-fit model says substrate binding causes active site adjustment for better catalysis.
- 3Substrate concentration increases rate initially, but rate plateaus when all active sites are saturated.
- 4Competitive inhibitors compete with substrate at active site; non-competitive inhibitors bind elsewhere.
- 5Coenzymes are organic cofactors often derived from vitamins; metal ions may activate enzymes.
- 6Ligases join molecules using energy, hydrolases break bonds using water.
Six enzyme classes
βOver The Hill, Lovely Islands Lieβ: Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases.
Cofactor vocabulary
Apoenzyme is alone and inactive; Holoenzyme is whole and active.
Enzyme graph clue
Substrate graph is a plateau; temperature/pH graph is a peak.
Digestive enzyme
Amylase breaks starch into smaller sugars, showing substrate specificity.
pH optimum
Pepsin works best in the acidic stomach, while many intestinal enzymes work best in alkaline conditions.
PYQ-style inhibitor concept
If an inhibitor resembles the substrate and binds the active site, it is a competitive inhibitor.
Saying enzymes are consumed
Enzymes participate in reactions but are regenerated at the end and can catalyse again.
Thinking higher temperature always increases activity
Activity increases only up to optimum; beyond that, protein enzymes denature.
Confusing coenzyme and prosthetic group
Coenzymes are loosely bound organic cofactors, while prosthetic groups are tightly bound.
Assuming all enzymes are proteins
Most enzymes are proteins, but ribozymes are catalytic RNA molecules.
Enzyme binds substrate to form enzyme-substrate complex and releases product while enzyme remains unchanged.
Variables
E=Enzyme
S=Substrate
ES=Enzyme-substrate complex
P=Product
A complete active enzyme may require a protein part plus a non-protein cofactor.
Variables
Apoenzyme=Protein part of enzyme
Cofactor=Non-protein helper such as metal ion or coenzyme
Holoenzyme=Functional enzyme complex
π 7. Metabolism
Overview
Metabolism is the total sum of all biochemical reactions occurring in a living cell. These reactions are organized into metabolic pathways, where each step is catalysed by a specific enzyme. Catabolism breaks complex molecules into simpler ones and usually releases energy, as in respiration. Anabolism builds complex molecules from simpler units and requires energy, as in protein synthesis or glycogen formation. Metabolism maintains the living state, which is a dynamic, non-equilibrium condition with continuous energy flow. NCERT emphasizes that isolated metabolic reactions outside the body are not living; life arises from coordinated, regulated metabolism inside cellular organization.
- 1Every metabolic reaction is linked to other reactions in cellular networks.
- 2Catabolic pathways often generate ATP, reducing power and precursor molecules.
- 3Anabolic pathways use ATP and precursors to synthesize macromolecules.
- 4Enzymes regulate pathway rate and direction by controlling specific steps.
- 5The living state cannot be explained by one molecule alone; it depends on organization and metabolism.
- 6NEET often asks the difference between metabolism and a single chemical reaction.
Anabolism vs catabolism
Anabolism = βA for Assembleβ; Catabolism = βC for Cutβ.
Living state
Life is βFLOWβ: Food, Living reactions, Organized regulation, Work/energy flow.
Pathway idea
Think of metabolism as a railway: substrate starts, intermediates are stations, enzymes are station controllers and product is destination.
Catabolism example
Cellular respiration breaks glucose into carbon dioxide and water while releasing energy conserved as ATP.
Anabolism example
Amino acids join to form proteins during growth, repair and enzyme production.
Metabolic pathway example
Glycolysis converts glucose to pyruvate through multiple enzyme-catalysed steps.
Calling any chemical reaction metabolism
A reaction becomes metabolic only in the context of a living system and its regulated biochemical network.
Thinking anabolism always releases energy
Anabolism generally consumes energy to build complex molecules; catabolism generally releases energy.
Ignoring enzyme control
Metabolic pathways are not random; each step is catalysed and regulated by enzymes.
Equating living state with equilibrium
Living cells maintain a non-equilibrium steady state. Equilibrium usually means no net work and is incompatible with life.
The total metabolic activity includes both constructive and breakdown pathways.
Variables
Anabolism=Biosynthetic, energy-consuming reactions
Catabolism=Breakdown, energy-releasing reactions
A common catabolic example where glucose is oxidized and energy is released.
Variables
CβHββOβ=Glucose
Oβ=Oxygen
COβ=Carbon dioxide
Energy=Energy conserved mainly as ATP and heat
Formula Sheet
10Many simple carbohydrates approximately follow this empirical formula, although not all carbohydrates strictly obey it.
Variables
C=Carbon atom
HβO=Hydrogen and oxygen in a 2:1 ratio
n=Number of repeating carbon-water units
Many biomolecular bonds such as peptide and glycosidic bonds form by removal of water.
Variables
Monomer=Small building block such as amino acid or monosaccharide
HβO=Water molecule released during bond formation
Chemical composition is often studied after removing water because water forms a major part of living tissue.
Variables
Dry mass=Mass of tissue after water removal
Fresh mass=Mass of living tissue before drying
Water mass=Mass contributed by cellular water
Used conceptually to compare elemental composition of biological samples.
Variables
Mass of element=Amount of a particular element in the dry sample
Total dry mass=Total mass after removing water
Many monosaccharides such as glucose follow this general formula.
Variables
n=Number of carbon atoms
C, H, O=Carbon, hydrogen and oxygen atoms
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NEET PYQs β Biomolecules
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The following reaction depicts the activity of a particular class of enzymes: Identify the enzyme class 'E' from the following options:
Match List I with List II: Choose the correct answer from the options given below :
Which of the following statements are correct regarding amino acids? A. They are substituted methanes. B. Serine is an aromatic amino acid. C. Valine is a neutral amino acid. D. Lysine is an acidic amino acid. Choose the correct answer from the options given below:
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