Biomolecules

Grind a piece of liver or a spoonful of yeast paste in trichloroacetic acid and strain it through cheesecloth, and every living cell splits into two very different worlds. What passes through the cloth is the acid-soluble pool: thousands of small organic compounds — amino acids, sugars, fatty acids, nucleotides — each weighing less than about a thousand daltons and mixing freely with water. Left behind is the acid-insoluble fraction: giant molecules built by joining thousands of small units into single enormous chains — proteins, nucleic acids, polysaccharides — typically ten thousand daltons or heavier. This one filtration experiment is the doorway into biochemistry, and it is where this chapter begins. This chapter is the chemistry every living cell runs on. It opens with the fact that living tissue and the earth's crust share the same elements, yet carbon and hydrogen are far more abundant in life. It classifies the small molecules of the acid-soluble pool — amino acids with their acid-base "zwitterion" chemistry, fatty acids and glycerol, nitrogen bases, nucleosides and nucleotides — and separates primary metabolites, compounds with known jobs in every organism, from the chemically wild secondary metabolites of plants, fungi and microbes: alkaloids, pigments, rubber, antibiotics. Its heart is the macromolecules: how twenty amino acids become a protein folded through four levels of structure, how sugar units become storage and structural polysaccharides, and how five nitrogen bases become the informational polymers DNA and RNA. It closes with enzymes — proteins, and occasionally RNAs, that make these reactions happen fast enough for life to run, by binding substrate at an active site and cutting the activation energy of the reaction. By the end you should be able to place any biomolecule correctly: micromolecule or macromolecule, primary or secondary metabolite, and — for a protein — which of the four structural levels a given feature belongs to.

Key takeaways

Elemental composition of living tissue and the earth's crust is qualitatively similar but quantitatively different: carbon (18.5% versus 0.03%) and hydrogen (9.5% versus 0.14%) are far more abundant in the human body, while silicon dominates the crust and is negligible in the body.
Grinding tissue in trichloroacetic acid and filtering splits it into the acid-soluble pool (micromolecules, roughly under 1000 Da: amino acids, sugars, fatty acids, nucleotides) and the acid-insoluble fraction (macromolecules: proteins, nucleic acids, polysaccharides, plus lipid-rich membrane fragments).
Primary metabolites (amino acids, sugars, nucleotides) have known roles in every organism; secondary metabolites (alkaloids, flavonoids, rubber, essential oils, antibiotics, pigments) occur mainly in plants, fungi and microbes.
Proteins are heteropolymers of up to 20 amino acids joined by peptide bonds, described at four levels — primary (sequence), secondary (helix, sheet), tertiary (3-D fold) and quaternary (subunit assembly, e.g. haemoglobin's two alpha plus two beta chains).
Polysaccharides are sugar chains: cellulose and chitin are structural homopolymers (cell wall, exoskeleton); starch and glycogen are branched storage homopolymers of glucose with a reducing and a non-reducing end.
Nucleic acids (DNA, RNA) are polynucleotides; a nucleotide is a nitrogen base (purine: adenine, guanine; pyrimidine: cytosine, uracil, thymine) plus a pentose sugar plus phosphate.
Enzymes are almost all proteins (a few RNAs, ribozymes, also catalyse); they bind substrate at an active site, form a transient ES complex, and lower the activation energy needed to reach the transition state — carbonic anhydrase speeds its reaction about ten million times.
Enzyme activity needs an optimum temperature and pH, rises with substrate concentration up to a maximum velocity Vmax, and can be blocked by a competitive inhibitor mimicking the substrate's shape.
Enzymes fall into six classes by reaction type and many need non-protein cofactors — prosthetic groups, coenzymes, or metal ions — bound to the apoenzyme to work at all.
Key idea: living matter is built from a small alphabet of micromolecules that polymerise into the macromolecules proteins, polysaccharides and nucleic acids, whose folded structure and whose enzyme catalysts, binding substrate at an active site to lower activation energy, together make cellular chemistry both organised and fast.

The core insight

Life's chemistry rests on a small alphabet of micromolecules — about 20 amino acids, a handful of sugars and fatty acids, and 5 nitrogen bases — that polymerise into the macromolecules proteins, polysaccharides and nucleic acids, whose folded structure and whose enzyme catalysts, binding substrate at a specific active site to lower activation energy, together make the chemistry of every living cell both precisely organised and fast enough to sustain life.

What this chapter covers

The NCERT chapter climbs from small molecules to working catalysts.

How to analyse chemical composition (9.1): elemental versus compound analysis; the acid-soluble and acid-insoluble fractions; amino acids, fatty acids and glycerol, nitrogen bases, nucleosides and nucleotides.
Primary and secondary metabolites (9.2): universal-role compounds versus the diverse secondary metabolites of plants, fungi and microbes.
Biomacromolecules (9.3): why roughly 1000 Da separates micromolecules from macromolecules, and why lipids sit in the insoluble fraction despite being small.
Proteins (9.4) and Polysaccharides (9.5): heteropolymers of amino acids versus homopolymers of sugars.
Nucleic acids (9.6): the three-part nucleotide, and the DNA/RNA distinction.
Structure of proteins (9.7): the four hierarchical levels of protein architecture.
Enzymes (9.8): the active site and ES complex; activation energy; the catalytic cycle; temperature/pH/substrate effects; the six enzyme classes; and cofactors.

The core ideas

Elemental composition: how living matter differs from non-living matter

Elemental analysis of any living tissue gives a list of elements — carbon, hydrogen, oxygen, nitrogen, sulphur and others — also found in the earth's crust; no element is unique to life. What differs is relative abundance: carbon and hydrogen are far richer in living tissue than in the crust.

Element	% weight, earth's crust	% weight, human body
Hydrogen	0.14	9.5
Carbon	0.03	18.5
Oxygen	46.6	65.0
Nitrogen	very little	3.3
Silicon	27.7	negligible

If a tissue sample is weighed wet, then dried, the loss is water; fully burning the dried tissue oxidises all carbon compounds to $CO_2$ and water vapour, leaving an ash of inorganic elements (calcium, magnesium) and ions (Na+, K+, Ca++, Mg++, and compounds like NaCl, CaCO3, phosphate, sulphate) that also sit in the acid-soluble fraction. Every carbon compound obtained from living tissue is a biomolecule.

Amino acids: the alpha-carbon and the zwitterion

Grinding tissue in trichloroacetic acid ( $Cl_3CCOOH$ ) and straining it gives the filtrate (acid-soluble pool) and the retentate (acid-insoluble fraction). Biologically, the acid-soluble compounds are classed as amino acids, nucleotide bases, fatty acids and their relatives.

Amino acids carry both an amino group and a carboxyl group on the same carbon, the alpha-carbon — substituted methanes with four valency positions: H, –COOH, –NH2, and a variable R group. A hydrogen R group gives glycine, a methyl gives alanine, a hydroxymethyl gives serine. Only 20 types occur in proteins, out of many more known to chemistry. By R group, amino acids are acidic (glutamic acid), basic (lysine), neutral (valine), or aromatic (tyrosine, phenylalanine, tryptophan). Because –NH2 and –COOH are both ionisable, an amino acid's structure changes with pH; the doubly charged, net-neutral form is the zwitterion.

Figure

General structure of an amino acid centred on the alpha carbon bonded to an amino group a carboxyl group a hydrogen atom and a variable R group, shown next to its zwitterionic form and the distinct R groups of glycine alanine and serine

Certain amino acids are essential: the body cannot make them, so diet must supply them; non-essential ones the body synthesises itself — the labels describe diet-dependence, not importance to protein structure.

Fatty acids, lipids and the components of nucleotides

Lipids are generally water-insoluble. A fatty acid is a carboxyl group attached to an R group of 1 to 19 carbons; palmitic acid has 16 carbons (with the carboxyl carbon), arachidonic acid 20. Fatty acids are saturated (no C=C) or unsaturated (one or more C=C). Glycerol (trihydroxy propane) esterified with fatty acids gives mono-, di- and triglycerides — fats and oils, distinguished by melting point (oils, e.g. gingelly oil, stay liquid in winter). Phospholipids carry a phosphorylated group too, and form cell membranes; lecithin is an example. Living organisms also contain heterocyclic nitrogen bases: adenine, guanine, cytosine, uracil, thymine. A base joined to a sugar is a nucleoside (adenosine, guanosine, thymidine, uridine, cytidine); add a phosphate and it is a nucleotide (adenylic, thymidylic, guanylic, uridylic, cytidylic acid). Nucleic acids — DNA and RNA — are built entirely of nucleotides and carry genetic information, developed below.

Primary and secondary metabolites

Animal tissues contain essentially all the categories above — amino acids, sugars, fatty acids, nucleotides — as primary metabolites, with identifiable, universal roles. Plant, fungal and microbial cells additionally contain thousands of secondary metabolites, whose role in the host is often still unclear, though many are valuable to human welfare.

Category	Examples
Pigments	Carotenoids, Anthocyanins
Alkaloids	Morphine, Codeine
Essential oils	Lemon grass oil
Toxins	Abrin, Ricin
Drugs	Vinblastin, curcumin
Polymeric substances	Rubber, gums, cellulose

Micromolecules versus macromolecules

Acid-soluble compounds run about 18 to 800 daltons (Da). The acid-insoluble fraction holds only four organic types — proteins, nucleic acids, polysaccharides, lipids — and, lipids aside, these run ten thousand Da and above and are true polymers. Biomolecules therefore split into micromolecules (under about 1000 Da) and macromolecules. Lipids, at no more than about 800 Da, are the exception: grinding disrupts membranes, whose fragments reseal into water-insoluble vesicles that co-sediment with the true polymers on filtration. Lipids are small molecules operationally grouped with, but not chemically, macromolecules.

Component	% of total cellular mass
Water	70–90
Proteins	10–15
Nucleic acids	5–7
Carbohydrates	3
Lipids	2
Ions	1

Water is, by far, the most abundant chemical in living organisms.

Proteins: heteropolymers with many jobs

Proteins are polypeptides — chains of amino acids joined by peptide bonds. Since 20 amino acids can occupy any position, a protein is a heteropolymer, not a homopolymer (one repeating monomer). Proteins transport nutrients, fight infection, act as hormones, enzymes, receptors and structural material.

Protein	Function
Collagen	Intercellular ground substance
Trypsin	Enzyme
Insulin	Hormone
Antibody	Fights infectious agents
Receptor	Sensory reception
GLUT-4	Glucose transport into cells

Collagen is the most abundant protein in the animal world; RuBisCO (Ribulose bisphosphate Carboxylase-Oxygenase), which fixes carbon dioxide in photosynthesis, is the most abundant protein in the entire biosphere.

Polysaccharides: sugar chains for storage and structure

Polysaccharides are long chains of sugars. Cellulose, a homopolymer of glucose alone, builds plant cell walls and is the raw material of paper and cotton. Starch stores energy in plants; animals use the analogous glycogen. Inulin is a fructose polymer. A chain such as glycogen has branches, plus a reducing end and a non-reducing end. Starch forms helices that trap iodine, turning blue; cellulose lacks these helices and shows no colour change. Complex polysaccharides use modified sugars as building blocks; chitin, in arthropod exoskeletons, is one such homopolymer.

Nucleic acids: the informational polymers

Nucleic acids are polynucleotides, built from nucleotides — each a heterocyclic base, a pentose sugar, and a phosphate. Adenine and guanine are substituted purines; cytosine, uracil and thymine are substituted pyrimidines. The sugar is ribose or 2' deoxyribose: deoxyribose gives DNA, ribose gives RNA.

The four levels of protein structure

Biologists describe protein structure at four levels. The amino-acid sequence — first to last along the chain — is the primary structure; the first residue is the N-terminal amino acid, the last the C-terminal amino acid. Portions of the chain fold into a right-handed alpha helix (proteins never use left-handed helices) or a beta pleated sheet — the secondary structure. The chain then folds on itself, like a hollow woollen ball, into the tertiary structure, essential for biological activity. Some proteins assemble several folded subunits into a quaternary structure: human haemoglobin has four subunits, two identical alpha chains and two identical beta chains.

Figure

Four hierarchical levels of protein structure showing a primary linear sequence of amino acids folding into an alpha helix and a beta pleated sheet as secondary structure then further folding into a compact tertiary globule and finally the quaternary assembly of two alpha and two beta subunits into human haemoglobin

Enzymes: active site, ES complex and activation energy

Almost all enzymes are proteins; a small number of catalytic nucleic acids, ribozymes, are the exception. As an enzyme's tertiary structure folds, it forms crevices; one, the active site, is where the substrate binds and reaction is catalysed. A chemical reaction breaks and re-forms bonds; rate is $\text{rate} = \delta P/\delta t$ . Enzymes accelerate reactions enormously: carbonic anhydrase catalyses

CO_2 + H_2O \;\underset{\text{carbonic anhydrase}}{\rightleftharpoons}\; H_2CO_3

Uncatalysed, this forms about 200 molecules of $H_2CO_3$ per hour; with the enzyme, about 600,000 molecules per second — a roughly ten-million-fold acceleration (worked out in the first derivation). The chemical converted is the substrate; binding it at the active site gives the transient ES complex, which passes through a high-energy transition state. The energy difference between substrate and transition state is the activation energy; an enzyme lowers this barrier without being consumed itself. The catalytic cycle runs: substrate binds the active site; the enzyme tightens around it; bonds break and re-form, giving an enzyme-product complex; the product is released and the enzyme is free again —

E + S \;\rightleftharpoons\; ES \;\rightarrow\; EP \;\rightarrow\; E + P

A sequence of such reactions, each catalysed by the same or different enzymes, is a metabolic pathway. Glucose to pyruvic acid,

\text{Glucose} \rightarrow 2\ \text{Pyruvic acid}, \qquad C_6H_{12}O_6 + O_2 \rightarrow 2C_3H_4O_3 + 2H_2O

proceeds through ten enzyme-catalysed steps (Chapter 12). The same pathway, with an added step or two, gives pyruvic acid aerobically, lactic acid in anaerobic muscle, and ethanol in fermenting yeast.

Factors affecting enzyme activity, classification and cofactors

Activity depends on temperature, pH, substrate concentration, and regulatory chemicals — each enzyme has an optimum temperature and pH, with activity falling on either side. Heat above roughly $40^\circ C$ generally denatures an enzyme permanently (rate roughly doubles or halves per $10^\circ C$ change); thermophilic enzymes, from hot vents and sulphur springs, stay stable to $80$ – $90^\circ C$ . As substrate concentration rises, velocity rises then plateaus at a maximum velocity, Vmax, once every enzyme molecule is occupied; the substrate concentration giving half of Vmax is the Michaelis constant, Km (the third derivation builds this curve). A competitive inhibitor resembles the substrate closely enough to occupy the active site itself — malonate inhibiting succinic dehydrogenase, which normally acts on the similarly shaped succinate, is the standard example. Enzymes fall into six classes by reaction type, named by a four-digit number.

Class	Catalyses
Oxidoreductases	Oxidoreduction between two substrates
Transferases	Transfer of a group (other than hydrogen)
Hydrolases	Hydrolysis of ester, peptide, glycosidic and similar bonds
Lyases	Removal of groups other than by hydrolysis, leaving a double bond
Isomerases	Interconversion of isomers
Ligases	Linking two compounds together

Many enzymes need a non-protein cofactor bound to the protein (apoenzyme) to work. Prosthetic groups are organic and tightly, permanently bound — haem in catalase and peroxidase. Coenzymes bind only transiently, often built from vitamins — NAD and NADP, from niacin. Metal ions form the third kind — zinc in carboxypeptidase. Removing the cofactor abolishes activity.

Derivation AHow an enzyme lowers the activation energy: the catalytic cycle and the ten-million-fold rate boost

The goal is to connect the qualitative idea of an active site and an activation-energy barrier to an actual number, using carbonic anhydrase as the worked case, and to show the catalytic cycle E + S to ES to EP to E + P as the mechanism that achieves it — linking the four-step cycle and the measured rate numbers NCERT states side by side into one chain of reasoning.

Step 1. State the uncatalysed rate: about 200 molecules of $H_2CO_3$ per hour, or $200 \div 3600 \approx 0.056$ molecules per second.

Step 2. State the catalysed rate: about 600,000 molecules per second with carbonic anhydrase present.

Step 3. Explain why, via the cycle. $CO_2$ and $H_2O$ (substrate) bind the active site, giving the ES complex; binding tightens the enzyme around them; in this close-contact geometry the bonds needed to form $H_2CO_3$ break and re-form far more easily than in free solution, because the enzyme has already positioned the atoms and stabilised the transition state — exactly what "lowering activation energy" means physically. The EP complex then releases $H_2CO_3$ , freeing the enzyme to repeat the cycle at once.

Step 4. Compute the rate enhancement: $600{,}000 \div 0.056 \approx 1.08\times10^{7}$ , about ten million times, matching NCERT. Because the enzyme is unchanged after each cycle, it repeats this acceleration indefinitely, which is why a small amount of enzyme processes a large amount of substrate.

Go deeper · Derivation B · 1 minFrom primary sequence to quaternary architecture: building human haemoglobin

Derivation BFrom primary sequence to quaternary architecture: building human haemoglobin

The goal is to walk through all four levels of protein structure in the order each is actually built, landing on a concrete example — human haemoglobin, whose quaternary structure NCERT names explicitly as four subunits, two alpha and two beta.

Step 1. Fix the primary structure: an ordered sequence of amino acids joined by peptide bonds, N-terminal to C-terminal. This sequence of R groups decides which parts of the chain can later fold into which shapes.

Step 2. Fold local stretches into secondary structure: parts of the backbone twist into a right-handed alpha helix, held by internal hydrogen bonds, while other stretches pair as a beta pleated sheet. Not every residue takes part.

Step 3. Fold the whole chain into tertiary structure: the locally coiled chain folds on itself, like a hollow woollen ball, burying some regions and exposing others, creating the crevices — including, for enzymes, the active site — that give the protein its functional 3-D shape.

Step 4. Assemble multiple folded chains into quaternary structure: haemoglobin assembles four independently folded subunits, two identical alpha chains and two identical beta chains, into one functional molecule — architecture built on top of, not instead of, the first three levels.

Go deeper · Derivation C · 1 minSubstrate concentration and enzyme velocity: building the saturation curve to Vmax and Km

Derivation CSubstrate concentration and enzyme velocity: building the saturation curve to Vmax and Km

The goal is to explain, from the fixed number of enzyme molecules relative to substrate molecules, why velocity rises with substrate concentration at first but flattens at a fixed maximum, and to define Vmax and Km, the two numbers NCERT's own velocity-versus-substrate-concentration graph is built around.

Step 1. Enzyme amount is fixed and, typically, far smaller than the amount of substrate available; each enzyme molecule processes one ES complex at a time.

Step 2. At low substrate concentration, most active sites sit empty; adding substrate fills more of them, so velocity rises roughly in step with substrate concentration.

Step 3. At high substrate concentration, essentially every enzyme molecule is continuously occupied; adding more substrate cannot speed the reaction further, since no free active sites remain. Velocity approaches a ceiling, Vmax, set by how fast each occupied enzyme completes one cycle and how many enzyme molecules exist.

Step 4. Km is defined as the substrate concentration giving exactly half of Vmax: a small Km means near-maximal velocity even at low substrate concentration (a tightly binding enzyme), a large Km means saturation needs much more substrate. This rising-then-flattening shape is exactly the curve of velocity against substrate concentration NCERT plots.

Worked examples

Board level - classifying compounds as micromolecules or macromolecules, with reasons

Grinding a liver sample in trichloroacetic acid and filtering it gives an acid-soluble filtrate and an acid-insoluble retentate. Compound P, molecular weight about 180 Da, is a monosaccharide found in the filtrate. Compound Q, molecular weight about 68,000 Da, found in the retentate, is built from 20 kinds of subunit joined by peptide bonds. Compound R, molecular weight about 800 Da, also found in the retentate, is a triglyceride. Classify each, and explain R's classification.

Step 1. P: 180 Da is far below the roughly 1000 Da boundary, and it is in the acid-soluble filtrate. P is a micromolecule.

Step 2. Q: 68,000 Da, in the retentate, built by joining 20 kinds of subunit via peptide bonds — a true polymer. Q is a macromolecule (a protein).

Step 3. R: 800 Da sits in micromolecule range, yet it was recovered from the insoluble fraction. This is the lipid exception — grinding disrupted triglyceride-containing membranes, whose fragments resealed into water-insoluble vesicles that co-sediment with the insoluble pool, not because R is a polymer. Answer: P is a micromolecule (sugar); Q is a true macromolecule (protein); R is chemically a micromolecule only operationally recovered with the macromolecular fraction, since lipids are not strictly macromolecules.

Board level - computing the rate enhancement carbonic anhydrase gives to its reaction

Without enzyme, $CO_2 + H_2O \rightleftharpoons H_2CO_3$ forms about 200 molecules of $H_2CO_3$ per hour. With carbonic anhydrase, about 600,000 molecules form every second. Calculate the acceleration factor.

Step 1. Convert the uncatalysed rate to molecules per second: $200 \div 3600 \approx 0.056$ .

Step 2. Divide: $600{,}000 \div 0.056 \approx 1.08 \times 10^{7}$ .

Step 3. Interpret: an acceleration of about $1.08\times10^7$ is roughly ten million times, consistent with the enzyme lowering the activation-energy barrier. Answer: about a 10-million-fold (1.08 x 10^7) acceleration.

Competitive level (NEET) - counting amino acid residues, peptide bonds and water molecules in a protein

A globular protein has a molecular mass of 11,000 Da. Taking the average contribution of one amino acid residue in a chain (after loss of one water molecule per peptide bond) as 110 Da, estimate (a) the number of residues, (b) the number of peptide bonds, and (c) the number of water molecules released during synthesis.

Step 1. Residues: $n = 11{,}000 \div 110 = 100$ .

Step 2. Peptide bonds: a chain of $n$ residues has $n - 1$ bonds, so $100 - 1 = 99$ .

Step 3. Water released: one molecule per peptide bond formed, so also 99. Answer: about 100 residues, 99 peptide bonds, 99 water molecules released. Timing note: under two minutes. Mass divided by 110 gives residues; residues minus one gives peptide bonds; peptide bonds equals water released during synthesis (hydrolysis instead consumes one water per bond broken). The one trap is applying "minus one" to the residue count itself instead of to the bond count.

Common traps

Thinking "macromolecule" always means "large molecule." Lipids are the exception: up to about 800 Da, chemically micromolecules, grouped with the acid-insoluble fraction only because membrane fragments co-sediment with it.
Confusing a nucleoside with a nucleotide. A nucleoside is only base plus sugar; a nucleotide adds a phosphate on top.
Assuming every biological catalyst is a protein. Ribozymes are catalytically active RNA — proof catalysis is not exclusively a protein property.
Mixing up primary and secondary metabolites. Primary metabolites have known, universal roles; secondary metabolites are mostly restricted to plants, fungi and microbes, their role in the host often still unclear despite economic value.
Treating "essential amino acid" as "important for protein." It means diet-dependent; non-essential ones are equally needed for structure, the body simply makes them itself.
Confusing a coenzyme with a prosthetic group. Both are organic cofactors, but a prosthetic group is tightly, permanently bound (haem); a coenzyme associates only transiently (NAD).

Common trapLipids sit in the acid-insoluble macromolecular fraction, but they are not macromolecules or polymers at all

The single biggest misconception here is assuming anything in the acid-insoluble fraction must be a true polymeric macromolecule, and applying that to lipids. Lipids are small molecules, no larger than about 800 Da — inside the micromolecule range that otherwise defines the acid-soluble pool. They land in the acid-insoluble fraction for a purely physical reason: grinding disrupts the cell membrane, and the resulting lipid fragments reseal into small, water-insoluble vesicles that separate along with the true polymers during filtration. NCERT is explicit that lipids are not strictly macromolecules. Whenever classifying a biomolecule, check molecular weight and polymeric structure first, and treat "found in the acid-insoluble fraction" as a clue about a lipid's physical behaviour on grinding, not proof of its chemical classification.

You should be able to…

Distinguish the acid-soluble pool (micromolecules) from the acid-insoluble fraction (macromolecules), and explain why lipids are the exception.
Draw the general structure of an amino acid, and explain what makes glycine, alanine and serine different.
Explain the difference between a primary metabolite and a secondary metabolite, with two examples of each.
Name the four levels of protein structure in order, and state what haemoglobin's quaternary structure consists of.
Distinguish a nucleoside from a nucleotide, and a purine base from a pyrimidine base.
Explain, using the ES complex and activation energy, why an enzyme speeds a reaction without being consumed.
State how temperature, pH, substrate concentration (Vmax, Km) and a competitive inhibitor each affect enzyme activity.
List the six classes of enzymes, with one example each.
Distinguish a prosthetic group, a coenzyme and a metal-ion cofactor, and define apoenzyme.

Ask the tutor: Want to work through a Vmax-and-Km enzyme-kinetics curve, or check a tricky macromolecule-classification question from your own textbook together?

If lipids are not polymers and are not even that large, why does this chapter still group them with the macromolecules?

The split comes from a physical procedure — grinding tissue in trichloroacetic acid and filtering — not from a chemical definition of "polymer." Proteins, nucleic acids and polysaccharides land in the insoluble retentate because they genuinely are large, water-insoluble polymers. Lipids land there too, only because grinding breaks membranes into small, resealed, water-insoluble vesicles that behave like the insoluble fraction on filtration, even though each lipid molecule is small and non-polymeric. NCERT flags this directly: lipids are not strictly macromolecules, only operationally recovered with them.

What is the actual difference between a nucleoside and a nucleotide?

A nucleoside is a nitrogen base joined to a pentose sugar and nothing else — adenosine (adenine plus ribose), thymidine (thymine plus deoxyribose). A nucleotide is a nucleoside with a phosphate also esterified onto the sugar — adenylic, thymidylic, guanylic, uridylic and cytidylic acid are the five built this way. Every nucleotide contains a nucleoside, but a bare nucleoside carries no phosphate. DNA and RNA are built entirely from nucleotides, never from bare nucleosides.

Why are only 20 amino acids used to build proteins when chemistry knows many more?

Chemistry recognises far more than 20 amino acids, but only 20 are incorporated into proteins, because these are specified by the genetic code that directs protein synthesis. The others either play different metabolic roles or are simply not read out by the translation machinery. Within those 20, R groups vary enormously — from a single hydrogen in glycine to a bulky aromatic ring in tryptophan — more than enough variety to build every protein shape a cell needs.

Why does heat destroy most enzymes, but enzymes from hot springs survive at 80 to 90 degrees Celsius?

An enzyme's activity depends on its precise tertiary structure, especially its active site's shape. Heat above roughly 40 degrees disrupts the weak bonds holding that fold together; once the shape is lost, the enzyme is denatured and usually permanently inactive. Enzymes from thermophilic organisms, adapted to hot vents and sulphur springs, have evolved sturdier folded structures that resist this disruption, staying active at 80 to 90 degrees, well past what would destroy an ordinary enzyme.

A coenzyme and a prosthetic group are both organic cofactors — what actually distinguishes them?

How tightly, and for how long, each stays attached to the apoenzyme. A prosthetic group is bound tightly and essentially permanently, often part of the active site itself — haem in catalase and peroxidase. A coenzyme associates only transiently, typically during catalysis, and the same coenzyme molecule can shuttle between many apoenzymes — NAD and NADP, built around niacin, are classic examples. Metal ions form a third, separate category of cofactor.