NEET Biomolecules Notes by PW

Biomolecules is a chapter where small details decide marks. A question may ask which sugar is non-reducing, which bond joins nucleotides, which base belongs only to RNA or what a low Km indicates. The options are usually close, so you need clear comparisons, not just general understanding.

Physics Wallah’s notes follow the chapter in the order you will use it: the chemical makeup of living tissue, metabolism, then carbohydrates, proteins, lipids, nucleic acids and enzymes. Use the memory tips to lock in sequences, compare similar terms side by side and revisit the bonds and classifications that NEET questions return to most often.

Biomolecules: Basic Concepts

Biomolecules are carbon-containing compounds found in living cells. Living and non-living matter share many elements, such as carbon, hydrogen, oxygen, nitrogen and sulfur, but their relative abundance differs. Carbon, hydrogen and oxygen are comparatively abundant in living cells, while silicon is almost negligible in the human body.

Water forms about 70-90% of cellular mass. Among organic components, the relative order is:

Proteins → Nucleic acids → Carbohydrates → Lipids → Ions

Memory tip: Remember the order as Proteins, Nucleic acids, Carbohydrates, Lipids, Ions.

Chemical Analysis of Living Tissues

A living tissue sample is ground in trichloroacetic acid using a mortar and pestle. After filtration through cheesecloth, two fractions are obtained.

• Filtrate (acid-soluble pool): passes through the filter, usually below 1,000 daltons. It contains monomers, inorganic compounds, amino acids, simple sugars, nucleotides and nitrogenous bases.

• Retentate (acid-insoluble pool): remains on the filter, usually above 10,000 daltons. It mainly contains proteins, complex carbohydrates, nucleic acids and lipid aggregates.

Memory tip: Filtrate is the filtered material that passes through. Retentate is the material retained behind.

Individual lipids are generally below 800 daltons and are not strictly biomacromolecules. When membranes break during tissue disruption, however, lipids form vesicles and aggregates, which remain in the acid-insoluble pool.

Metabolism and Metabolites

Metabolism is the sum total of catabolism and anabolism.

• Catabolism: breakdown reactions.

• Anabolism: formation or building reactions.

The substances formed during metabolic reactions are called metabolites.

Primary and Secondary Metabolites

Primary metabolites directly control physiological processes and are essential for normal functioning. Examples include glucose in animals, amino acids and chlorophyll in plants.

Secondary metabolites are found in certain plants, microorganisms and fungi. They may not directly control basic physiological processes, but they have important economic and ecological value. They may act as medicines, attract pollinators or support defence.

• Pigments: carotenoids, anthocyanins

• Alkaloids: morphine, codeine

• Oils: lemongrass oil

• Toxins: abrin, ricin

• Drugs: vinblastine, curcumin

• Polymers: rubber, gum

Carbohydrates

Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones. An aldose has a terminal aldehyde group, while a ketose has an intermediate ketone group.

• The aldehyde group is always terminal.

• The ketone group is always intermediate.

• A carbohydrate contains at least three carbon atoms.

Classification by Number of Sugar Units

• Monosaccharide: one unit

• Oligosaccharide: about two to nine units

• Polysaccharide: more than ten units

Memory tip: Imagine sugar units as beads. One bead is mono, a few beads are oligo and many beads are poly.

Classification of Monosaccharides by Carbon Number

• 3 carbons: triose

• 4 carbons: tetrose

• 5 carbons: pentose

• 6 carbons: hexose

• 7 carbons: heptose

Important monosaccharides:

• Glucose: a hexose aldose sugar that occurs in a pyranose ring.

• Ribose: a pentose aldose sugar found in RNA that occurs in a furanose ring.

• Deoxyribose: found in DNA, formed from ribose by removal of oxygen from carbon 2.

Memory tip: Deoxy means removal of oxygen. Ribose becomes deoxyribose when oxygen is removed from carbon 2.

Glycosidic Bonds and Polysaccharides

Sugar units join through glycosidic bonds with the loss of water. Two units form a disaccharide, while many units form a polysaccharide.

• Glycogen: alpha-glucose units; storage polysaccharide in animals

• Starch: alpha-glucose units; storage polysaccharide in plants

• Inulin: beta-fructose units; storage polysaccharide in plants

• Cellulose: beta-glucose units; plant cell wall

• Chitin: N-acetylglucosamine units; insect exoskeleton and fungi

Key facts:

• Glycogen is branched. Straight portions have alpha 1-4 glycosidic linkages, while branch points have alpha 1-6 linkages.

• Starch contains about 30% amylose and 70% amylopectin. Amylose forms a helix that traps iodine, producing a blue-black colour in the iodine test.

• Cellulose is the most abundant organic substance on Earth.

• Chitin is the second most abundant organic substance on Earth.

Reducing and Non-Reducing Sugars

A reducing sugar has a free functional carbon. Monosaccharides, maltose and lactose are reducing sugars. Sucrose is non-reducing because the functional carbons of its component sugars take part in the glycosidic bond.

Proteins and Amino Acids

Proteins are polymers of amino acids. Each amino acid has an amino group, a carboxylic group, a hydrogen and a variable R group attached to the alpha carbon. The R group determines the identity and many properties of the amino acid.

Classification of Amino Acids by R Group

• Acidic: aspartic acid, glutamic acid

• Basic: arginine, lysine, histidine

• Neutral: glycine, alanine, valine

• Hydroxyl-containing: serine, threonine

• Sulfur-containing: methionine, cysteine

• Aromatic: phenylalanine, tryptophan, tyrosine

• Heterocyclic: proline, hydroxyproline

Essential amino acids are not synthesised by the body in sufficient amounts and must come from the diet. Examples: tryptophan, phenylalanine, methionine, threonine, leucine, isoleucine, lysine and valine.

Peptide Bonds

Amino acids join through peptide bonds with the loss of water. The first end of a protein chain is the N-terminal, with a free amino group. The last end is the C-terminal, with a free carboxyl group.

Zwitterionic Form

• The amino group becomes NH₃⁺.

• The carboxylic group becomes COO⁻.

• The molecule carries both charges but is overall neutral.

The pH at which an amino acid exists as a zwitterion is its isoelectric pH.

Levels of Protein Structure

• Primary: linear amino acid sequence

• Secondary: alpha helix or beta-pleated sheet

• Tertiary: three-dimensional folding of one chain

• Quaternary: association of multiple polypeptide chains

The alpha helix is right-handed. Beta-pleated sheets may be parallel or antiparallel. Tertiary folding creates active sites, pockets and crevices. Quaternary structure occurs in proteins such as haemoglobin, which has two alpha and two beta subunits.

Functions of Proteins

Proteins perform structural, hormonal, enzymatic, defensive, receptor and transport functions.

• Collagen: a major structural protein

• Insulin: a protein hormone

• RuBisCO: the most abundant protein in the biosphere

Lipids

Lipids are generally waxy, greasy or oily, and insoluble in water. They are commonly esters of fatty acids. Lipids are never polymers.

• Simple lipids: glycerol and fatty acids, e.g. triglycerides

• Conjugated lipids: lipid plus a non-lipid component, e.g. lecithin

• Derived lipids: ring-containing lipid derivatives, e.g. cholesterol

Glycerides

Glycerol has three hydroxyl groups and can combine with one, two or three fatty acids.

• Monoglyceride: 1 fatty acid, 1 water molecule lost

• Diglyceride: 2 fatty acids, 2 water molecules lost

• Triglyceride: 3 fatty acids, 3 water molecules lost

Fatty Acids

• Saturated fatty acids contain only single bonds.

• Unsaturated fatty acids contain one or more double bonds.

Important examples: palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid and arachidonic acid.

Fats are solid at room temperature because of their high melting point, while oils are liquid because of their lower melting point.

Nucleic Acids

Nucleic acids are acidic because they contain phosphate groups. DNA and RNA function as genetic material.

• Nucleoside: sugar + nitrogenous base

• Nucleotide: sugar + nitrogenous base + phosphate group

Nitrogenous Bases

• Purines: adenine, guanine

• Pyrimidines: cytosine, thymine, uracil

• DNA: adenine, guanine, cytosine, thymine

• RNA: adenine, guanine, cytosine, uracil

Memory tip: Adenine, guanine and cytosine occur in both. Uracil belongs to RNA, while thymine belongs to DNA.

Bonds in Nucleic Acids

• Sugar and phosphate within a nucleotide are joined by a phosphoester bond.

• Nucleotides join through phosphodiester bonds to form nucleic acids.

Enzymes and Biocatalysis

Enzymes are biological catalysts that increase reaction rate without being consumed. Most enzymes are proteins, although ribozymes and ribonuclease P show catalytic activity as RNA-based molecules.

Enzymes lower the activation energy required for a substrate to reach the transition state.

• Lock-and-key theory: the enzyme and substrate have a fixed fit.

• Induced-fit theory: the active site changes shape after the substrate binds.

Factors Affecting Enzyme Activity

• Enzyme activity depends on temperature, pH and substrate concentration.

• At high temperatures, enzymes may denature. At low temperatures, they become inactive.

• When all active sites are occupied, reaction velocity reaches Vmax.

• The substrate concentration at which velocity equals Vmax/2 is the Km. A lower Km indicates greater enzyme efficiency.

Major Enzyme Classes

• Oxidoreductases: catalyse oxidation-reduction reactions

• Transferases: transfer groups other than hydrogen

• Hydrolases: break bonds using water

• Lyases: break bonds without using water

• Isomerases: convert one isomer into another

• Ligases: form new bonds

Memory tip: Remember the classes as O-T-H-L-I-L.

Cofactors and Holoenzymes

Some enzymes need non-protein components called cofactors.

• Apoenzyme: the protein part of an enzyme, inactive without its cofactor

• Holoenzyme: the complete, active enzyme containing the apoenzyme and cofactor

Types of cofactors:

• Coenzymes: organic and loosely attached, e.g. NAD and NADP

• Prosthetic groups: organic and tightly bound, e.g. heme

• Metal ions: inorganic cofactors, e.g. zinc and chloride

Removing a required cofactor makes the enzyme catalytically inactive.

Biomolecules is easiest to score in when you revise it as a set of linked comparisons: which bond joins which units, which base belongs to which nucleic acid, which sugar is reducing and which enzyme class does what. Go through the memory tips again, rewrite the classification lists from memory and practise questions that test close options. PW’s Biology notes can support your NEET preparation with clear, topic-wise revision. 

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