Biomolecules | ISC Class 11 Biology Notes

This note covers biomolecules, chemical analysis of living tissues, carbohydrate classification and functions, amino acids, protein structure and functions, fats and oils, enzyme action and classification, factors affecting enzyme activity, competitive inhibition, cofactors and secondary metabolites.

What are biomolecules, and how are they studied?

Biomolecules are the carbon compounds obtained from living tissues. Living organisms also contain inorganic substances, including water and mineral ions. An ion is an atom or group of atoms carrying an electrical charge.

Living matter and the earth's crust contain similar kinds of elements, but their relative abundance differs. Carbon and hydrogen are relatively more abundant in living organisms. Identifying the elements present is different from identifying the compounds in which they occur.

How does chemical analysis separate tissue components?

A fraction is a separated portion of a mixture. Grinding tissue in trichloroacetic acid, an acid used for this extraction, produces a slurry. Straining it separates dissolved substances from material retained by the filter.

1. Grind living tissue, such as a piece of liver or vegetable tissue, in trichloroacetic acid using a mortar and pestle.
2. Pass the thick slurry through cheesecloth or cotton to separate the liquid from the retained material.
3. Collect the filtrate, the liquid that passes through, as the acid-soluble pool. It contains many small organic compounds and inorganic substances.
4. Collect the retentate, the material left behind, as the acid-insoluble fraction containing proteins, polysaccharides, nucleic acids and membrane-associated lipids.

Proteins are chains of amino acids, organic compounds with amino and carboxyl groups; polysaccharides are long chains of sugar units. Nucleic acids are chains of nucleotides, each containing a sugar, a nitrogenous base, a nitrogen-containing ring compound, and a phosphate group derived from phosphoric acid. Lipids include fats and oils and are generally water insoluble.

Why are lipids an important exception?

A monomer is a building unit; a polymer contains many linked building units. Proteins, polysaccharides and nucleic acids are true biological macromolecules, meaning large molecules. A homopolymer repeats one type of monomer; a heteropolymer contains different types.

Lipids are not strictly macromolecules. Grinding breaks membranes into fragments that form vesicles, small membrane-bound structures. These water-insoluble fragments separate with the acid-insoluble fraction. Their presence there does not establish that individual lipid molecules are large polymers.

Note: The acid-soluble pool represents roughly the cytoplasmic composition. The cytoplasm is the material within a cell outside its nucleus, when a nucleus is present. The insoluble fraction also includes macromolecules from the cytoplasm and cell structures.

How are carbohydrates classified, and what do simple sugars do?

Carbohydrates, also called saccharides, include sugars and polysaccharides. They provide energy, store food and form structural materials. Their classification depends on hydrolysis, the splitting of a compound by reaction with water, and the simpler sugar units produced.

What distinguishes the main classes?

A monosaccharide cannot be hydrolysed into a simpler carbohydrate unit. An oligosaccharide yields two to ten monosaccharide units on hydrolysis. A disaccharide is an oligosaccharide that yields two units. A polysaccharide yields a large number of monosaccharide units.

A pentose is a monosaccharide with five carbon atoms; a hexose has six. Glucose is a hexose, while ribose and deoxyribose are pentoses. Size and biological role are separate aspects of classification: a small sugar can be part of a much larger molecule.

Galactose and fructose are monosaccharides. A glycosidic linkage joins sugar units through an oxygen atom. Hydrolysis breaks this linkage. Thus, maltose releases two glucose molecules, whereas lactose and sucrose each release two different kinds of monosaccharide.

How do reducing and non-reducing sugars differ?

A reducing sugar can reduce Fehling's solution or Tollens' reagent, chemical reagents used to test reducing behaviour. In this context, reduction means gain of electrons by the reagent. Maltose and lactose are reducing sugars; sucrose is non-reducing because its reducing groups participate in the linkage.

Do not equate the word carbohydrate with sweetness. Polysaccharides are not sweet in taste and are also called non-sugars. The same broad chemical group therefore includes sweet dietary sugars, storage materials and the supporting material of plant cell walls.

How do the major polysaccharides differ in structure and function?

The functions of polysaccharides depend on their sugar units and the way those units are joined. Starch, glycogen and cellulose contain glucose, yet they have different structures and biological roles. Knowing the monomer alone does not fully identify a polysaccharide.

How are storage and structural polysaccharides distinguished?

Amylose is the unbranched component of starch; amylopectin is its branched component. Glycogen resembles amylopectin but is more highly branched. Enzymes break down glycogen to glucose when the body needs glucose. Starch and glycogen therefore provide carbohydrate reserves.

Cellulose forms straight chains and contributes to plant cell walls. Cotton fibre and paper made from plant pulp are cellulosic. Chitin occurs in the exoskeleton, the external supporting covering, of arthropods, animals with jointed appendages. These complex polysaccharides are mostly homopolymers.

Starch forms helical structures, meaning coiled arrangements, that can hold iodine molecules. The starch-iodine complex is blue. Cellulose does not contain complex helices and hence cannot hold iodine in this way. This distinction relates a visible test result to molecular organisation.

The cartoon shows a branching chain, with highlighted regions enlarged below it. The enlarged drawings show sugar rings linked through oxygen atoms and illustrate a branch as well as a chain segment.

See Fig. 9.2 in your NCERT textbook

A branched drawing should therefore be read at two levels: the overall pattern shows the polymer's organisation, while the enlarged portion shows linked sugar units. It is a diagrammatic representation of part of glycogen, not a photograph of an entire molecule.

What is the structure of an amino acid, and how are amino acids classified?

An amino acid contains an amino group and a carboxyl group. In the amino acids of proteins, these groups attach to the same carbon atom, called the alpha-carbon. Alpha, written α, identifies this carbon next to the carboxyl group.

In the chemical formulas below, C means carbon, H hydrogen, O oxygen, N nitrogen and S sulphur. A subscript gives the number of atoms. NH₂ is the amino group, COOH the carboxyl group, and R the variable side chain. Short lines between groups represent bonds.

The general uncharged structure is H₂N-CH(R)-COOH. The alpha-carbon is attached to the amino group, carboxyl group, hydrogen and R group. Changing R changes the amino acid: R is H in glycine, the methyl group CH₃ in alanine and the hydroxymethyl group CH₂OH in serine.

The amino-acid drawings show a central carbon with COOH above, H to the left and NH₂ to the right. The boxed groups below are H, CH₃ and CH₂OH respectively, showing the different side chains.

See Fig. 9.1 in your NCERT textbook

What does a zwitter-ion contain?

A proton is a positively charged hydrogen ion. In aqueous solution, the carboxyl group can lose a proton and the amino group can accept it. The resulting zwitter-ion, ⁺H₃N-CH(R)-COO⁻, has both positive and negative charges but is electrically neutral overall.

The superscripts ⁺ and ⁻ indicate positive and negative charges. pH is a measure of how acidic or alkaline a solution is. Amino-acid forms change with pH. In the zwitterionic form, amino acids are amphoteric, meaning that they react with both acids and bases.

These categories use different criteria and can overlap. Methionine, for example, is both sulphur-containing and essential. Non-essential does not mean unnecessary: it describes the body's ability to synthesise the amino acid rather than a lack of biological importance.

How do amino acids form proteins with different levels of structure?

A peptide bond is the linkage formed between the carboxyl group of one amino acid and the amino group of another, with removal of water. A polypeptide is a chain of amino acids joined by peptide bonds. Proteins are polypeptides.

How is a peptide linkage formed?

1. Identify the carboxyl group of one amino acid and the amino group of the amino acid that will join it.
2. Combine these groups in a reaction that eliminates a molecule of water, whose formula is H₂O.
3. Recognise the new CO-NH linkage as a peptide bond. Two joined amino acids form a dipeptide.
4. Add a third amino acid through another peptide linkage to form a tripeptide; further linking produces a longer peptide chain.

Proteins are heteropolymers because their chains contain different kinds of amino-acid units. Their biological identity depends on both the order of the units and the resulting structure. An extended sequence alone does not describe the complete folded protein.

An alpha-helix is a coiled secondary structure; a beta-pleated sheet is an arrangement of extended chain portions alongside one another. Beta is written β. Hydrogen bonds, attractions involving hydrogen and electronegative atoms, help stabilise these secondary structures.

Only some portions of a protein thread are arranged as a helix. Further folding gives the tertiary structure, which is necessary for many biological activities. Adult human haemoglobin, the oxygen-carrying blood protein, has four subunits: two of the alpha type and two of the beta type.

The figure progresses from a polypeptide chain to an alpha-helix and beta-pleated sheet, a folded tertiary structure, and a quaternary assembly. Hydrogen bonds and disulphide bonds, links between sulphur atoms, are labelled in the tertiary drawing.

See Fig. 9.3 in your NCERT textbook

What functions do proteins perform, and why does their shape matter?

Proteins perform structural, transport, protective, regulatory and catalytic functions. A catalyst speeds up a chemical reaction without being consumed by it. Many proteins act as enzymes, the biological catalysts that make chemical reactions proceed rapidly under the conditions within organisms.

Collagen is the most abundant protein in the animal world. Ribulose bisphosphate carboxylase-oxygenase, abbreviated RuBisCO, is the most abundant protein in the biosphere, the part of the earth where life occurs. These statements refer to different biological ranges.

What is denaturation?

A native protein has its characteristic three-dimensional structure and biological activity. Denaturation is the loss of that activity when changes such as heating or altered pH disturb the folded structure. Secondary and tertiary structures are disrupted while the primary structure remains intact.

Coagulation of egg white on boiling is an example of denaturation. It illustrates why a change in protein shape matters biologically. Loss of function does not require the entire amino-acid chain to be broken into its individual building units.

Protein shape also helps distinguish two broad forms. Fibrous proteins have fibre-like arrangements and are generally insoluble in water. Globular proteins have chains coiled into roughly spherical forms and are usually soluble in water. Insulin is a globular protein.

How are lipids classified, and what are fats and oils made of?

Lipids are generally water insoluble. They include fats, oils and phospholipids. A fatty acid has a carboxyl group attached to a hydrocarbon group, a group made of carbon and hydrogen. Glycerol is a three-carbon alcohol with three hydroxyl groups, written OH.

What distinguishes simple, compound and derived lipids?

Simple lipids are esters of fatty acids with alcohols; fats and oils are examples. Compound lipids contain additional groups, as in phospholipids. Derived lipids include substances obtained from lipid breakdown, such as fatty acids and glycerol.

An ester linkage forms when the carboxyl group of an acid joins an alcohol group with removal of water. Joining fatty acids to glycerol is called esterification. A monoglyceride has one fatty acid attached, a diglyceride has two and a triglyceride has three.

The glycerol drawing shows a three-carbon chain with a hydroxyl group attached to each carbon. The triglyceride drawing shows three fatty-acid portions attached through ester linkages to the glycerol backbone.

See Fig. 9.1 in your NCERT textbook

Saturated fatty acids have no carbon-carbon double bonds, whereas unsaturated fatty acids have one or more. A double bond joins two atoms through two shared pairs of electrons. Palmitic acid has sixteen carbon atoms, including its carboxyl carbon; arachidonic acid has twenty.

How do fats and oils differ?

Fats and oils are distinguished by melting behaviour. Oils have lower melting points; gingelly oil, for example, remains an oil in winter. Fats are generally solid and oils liquid at ordinary room temperature. Saturation is a fatty-acid classification, whereas fat or oil describes physical behaviour.

Fats and oils serve as energy reserves. Stored fat also provides thermal insulation, reducing heat loss, and cushioning around organs. These functions should be distinguished from the membrane role of phospholipids, lipids containing phosphate and an additional organic group.

Lecithin is a phospholipid found in cell membranes. The lipid category therefore contains compounds with different structures and functions. A triglyceride is not simply another name for a phospholipid, and a membrane fragment is not an individual lipid polymer.

How do enzymes accelerate reactions through a catalytic cycle?

Enzymes are biological catalysts. Almost all enzymes are proteins; some nucleic acids also have catalytic activity and are called ribozymes. Enzymes are highly specific for their substrates and reactions and are needed in small quantities.

A substrate is the substance an enzyme converts into a product, the substance formed. The enzyme's active site is a pocket or crevice where the substrate binds. Its three-dimensional arrangement allows the enzyme and substrate to interact.

What happens during one catalytic cycle?

In the reaction scheme below, E represents enzyme, S substrate and P product; here S denotes substrate rather than the element sulphur. ES denotes the enzyme-substrate complex; EP denotes the enzyme-product complex. A complex is a temporary associated state. The arrow → means proceeds to; ⇌ indicates reversible association.

E + S ⇌ ES → EP → E + P

1. The substrate binds to the enzyme's active site, producing a short-lived enzyme-substrate complex.
2. Binding induces a change in the enzyme's shape, so that it fits more tightly around the substrate.
3. The active site acts on the substrate's chemical bonds, leading to formation of an enzyme-product complex.
4. The products leave the active site. The unchanged enzyme can bind another substrate molecule and repeat the catalytic cycle.

Why does lowering activation energy matter?

The transition state is a high-energy, unstable arrangement through which the substrate passes while becoming product. Activation energy is the energy difference between the substrate and this transition state. Enzymes lower this energy barrier, making conversion easier.

Potential energy, the energy associated with a molecular state, is plotted vertically against reaction progress horizontally. Two curves connect the substrate and product levels. The enzyme-assisted curve has the lower peak; arrows mark the two activation energies.

See Fig. 9.4 in your NCERT textbook

The horizontal axis shows progress of the reaction, not elapsed time. The drawing's product lies below its substrate, but the enzyme's catalytic role is identified by the smaller barrier. Lowering that barrier should not be confused with changing the identity of the reaction's product.

How do temperature, pH, substrate concentration and inhibitors affect enzymes?

Enzyme activity measures the rate at which an enzyme catalyses a reaction. Temperature, pH and substrate concentration affect this rate. Chemicals that bind to an enzyme can also alter its activity. Conditions that change protein structure can change the functioning of the active site.

What are optimum temperature and optimum pH?

Enzymes generally function within a narrow range of temperature and pH. An optimum is the value at which activity is highest. Activity declines on either side of that value. Optimum conditions belong to a particular enzyme; they are not one universal value for all enzymes.

Low temperature preserves the enzyme in a temporarily inactive state. High temperature can destroy activity through protein denaturation. Enzymes of organisms adapted to extremely hot environments can retain catalytic activity at high temperatures, so heat sensitivity must not be treated as identical for all enzymes.

Why does increasing substrate eventually stop increasing the rate?

With increasing substrate concentration, the amount of substrate per unit volume, reaction velocity initially rises. It eventually reaches Vmax, the maximum velocity. At saturation, the available enzyme molecules are occupied, leaving no free enzyme molecules to bind additional substrate.

Increasing substrate concentration beyond this point does not raise the rate further under the same conditions. A rising curve that approaches a plateau therefore differs from an optimum-temperature curve, where activity falls after the peak because conditions become damaging.

How does a competitive inhibitor act?

Inhibition is a reduction or shutting off of enzyme activity caused by an inhibitor, a chemical that binds to the enzyme. A competitive inhibitor closely resembles the substrate and competes for its binding site, preventing substrate binding and reducing enzyme action.

Malonate inhibits succinic dehydrogenase, an enzyme acting on the substrate succinate, because malonate resembles succinate in structure. This example connects structural similarity with competition at the binding site. Such competitive inhibitors are often used in controlling bacterial pathogens, bacteria that cause disease.

Note: Low-temperature inactivity, high-temperature denaturation and competitive inhibition describe different causes of reduced activity. Explain the relevant cause before interpreting a change in reaction rate.

How are enzymes named and classified, and why do some need cofactors?

Enzyme nomenclature means the system of naming enzymes. Names generally relate to the substrate or the reaction and use the ending -ase. Maltase catalyses the hydrolysis of maltose into glucose. Names such as trypsin are also used.

Classification groups enzymes by the type of reaction catalysed. The following six classes distinguish electron transfer, group transfer, breakdown with water, other group removal, internal rearrangement and joining of compounds. Naming a substrate alone does not identify the reaction class.

What are the three kinds of cofactor?

A cofactor is a non-protein component required for the catalytic activity of certain enzymes. In these cases, the protein portion is the apoenzyme. Removal of the required cofactor causes loss of catalytic activity.

• Prosthetic groups are organic cofactors tightly bound to the apoenzyme. Haem is the prosthetic group of peroxidase and catalase, enzymes that break down hydrogen peroxide into water and oxygen.
• Coenzymes are organic cofactors whose association with the apoenzyme is transient, usually during catalysis. Many contain vitamins, organic nutrients required in small amounts. Nicotinamide adenine dinucleotide, abbreviated NAD, contains the vitamin niacin.
• Metal ions assist enzyme activity through coordination bonds, bonds involving donation of an electron pair, with the active site and substrate. Zinc is a cofactor of carboxypeptidase, a protein-cleaving enzyme.

The distinction between prosthetic groups and coenzymes concerns how they associate with the enzyme, not whether they are organic: both are organic. Metal ions form a separate cofactor category. Cofactors should also be distinguished from substrates, which are transformed into products.

What are secondary metabolites, and how do they differ from primary metabolites?

Metabolism is the collection of chemical reactions occurring in living organisms. Metabolites are compounds involved in these reactions. Primary metabolites, such as amino acids and sugars, have identifiable functions in normal physiological processes, the activities that sustain an organism.

Secondary metabolites are additional compounds found in plant, fungal and microbial cells beyond the familiar primary-metabolite categories. They include pigments, alkaloids, essential oils, toxins, drugs and polymeric substances. These names classify chemically and functionally diverse compounds, not one single molecular structure.

Does secondary mean unimportant?

The roles of all secondary metabolites in their host organisms are not fully understood. Some have ecological importance, meaning roles in interactions with the environment and other organisms. This does not justify claiming either that all are useless or that every function is known.

Many secondary metabolites are useful to human welfare, including rubber, drugs, spices, scents and pigments. Human usefulness and function within the producing organism are different questions. A compound can have a recognised human use even when its complete role in the organism remains unclear.

When distinguishing the two groups, connect primary metabolites with identifiable roles in normal physiology and secondary metabolites with the additional diverse compounds. Preserve the qualification that some have ecological importance. Do not turn this into a claim about every secondary metabolite.

Glossary

• Biomolecule — A carbon compound obtained from living tissue and considered as part of its chemical composition.
• Monosaccharide — A carbohydrate that cannot be hydrolysed further into simpler carbohydrate units.
• Polysaccharide — A carbohydrate polymer containing many linked sugar units, serving storage or structural functions.
• Glycosidic linkage — A bond joining sugar units through an oxygen atom in a larger carbohydrate.
• Zwitter-ion — An electrically neutral ion carrying both positive and negative charges within the same molecule.
• Essential amino acid — An amino acid that must be obtained through food because the body cannot synthesise it.
• Peptide bond — A linkage formed between amino and carboxyl groups of amino acids with removal of water.
• Denaturation — Loss of a protein's biological activity following disruption of its characteristic folded structure.
• Triglyceride — A lipid containing three fatty acids esterified to one molecule of glycerol.
• Active site — The pocket or crevice of an enzyme where its substrate binds during catalysis.
• Activation energy — The energy difference between the substrate and the transition state through which the reaction proceeds.
• Competitive inhibitor — A molecule resembling a substrate that competes for its binding site and reduces enzyme action.
• Cofactor — A non-protein component required by certain enzymes for their catalytic activity.
• Secondary metabolite — An additional compound found in plant, fungal or microbial cells beyond familiar primary-metabolite categories.

Common errors and misconceptions

• Misconception: Anything in the acid-insoluble fraction must be a large polymer. Correct: Lipids occur there because insoluble membrane fragments separate with that fraction; lipids are not strictly macromolecules.
• Misconception: Starch, cellulose and glycogen have identical functions because all contain glucose. Correct: Their structures differ; starch and glycogen store carbohydrate, whereas cellulose supports plant cell walls.
• Misconception: A zwitter-ion has no charged groups. Correct: It carries both positive and negative charges, although it is neutral overall.
• Misconception: Non-essential amino acids are unnecessary. Correct: They are required but can be made by the body, unlike essential amino acids supplied through diet.
• Misconception: Quaternary structure means extra folding of one chain. Correct: It describes the arrangement of multiple polypeptide subunits relative to one another.
• Misconception: All enzymes are proteins, and cooling destroys them. Correct: Ribozymes are catalytic nucleic acids; low temperature can preserve enzymes in a temporarily inactive state.
• Misconception: Increasing substrate concentration raises enzyme activity without limit. Correct: Saturation of available enzyme molecules produces a maximum reaction velocity.
• Misconception: Secondary metabolites have no biological importance. Correct: Some have ecological importance, and the roles of all secondary metabolites in host organisms are not fully understood.

Exam-style questions with model answers

Q1. What is a zwitter-ion, and why is the zwitterionic form of an amino acid neutral overall? [2 marks]

1. A zwitter-ion carries both a positively charged amino group and a negatively charged carboxylate group within the same molecule.
2. The opposite charges balance, giving no overall electrical charge despite the presence of charged groups.

Q2. State the monosaccharides obtained by hydrolysis of maltose, lactose and sucrose. [3 marks]

1. Maltose yields two molecules of glucose on hydrolysis. Its two linked carbohydrate units are therefore the same kind of monosaccharide.
2. Lactose yields glucose and galactose on hydrolysis. This milk sugar contains two different monosaccharide units joined within one disaccharide molecule.
3. Sucrose yields glucose and fructose on hydrolysis. It also contains two different monosaccharides, but its second component differs from that of lactose.

Q3. Describe the four levels of protein structure, using adult human haemoglobin as the example of quaternary structure. [4 marks]

1. Primary structure is the particular sequence of amino acids in a polypeptide, specifying their positions along the chain.
2. Secondary structure is regular local folding of the chain, including alpha-helices and beta-pleated sheets stabilised by hydrogen bonding.
3. Tertiary structure is the overall three-dimensional folding of a polypeptide, which is necessary for many biological activities of proteins.
4. Quaternary structure describes the arrangement of multiple polypeptide subunits. Adult human haemoglobin contains two alpha and two beta subunits.

Q4. Describe a triglyceride and distinguish saturated fatty acids from unsaturated fatty acids. [3 marks]

1. A triglyceride contains a glycerol molecule esterified with three fatty acids. Glycerol provides three hydroxyl groups through which these linkages can form.
2. A saturated fatty acid contains no carbon-carbon double bond in its hydrocarbon portion. Saturation therefore describes a structural feature of the fatty acid.
3. An unsaturated fatty acid contains one or more carbon-carbon double bonds. The distinction depends on these bonds, rather than simply on the number of carbon atoms.

Q5. Explain enzyme catalysis in five points: substrate binding, induced shape change, product formation, product release and activation energy. [5 marks]

1. The substrate binds at the enzyme's active site, a pocket or crevice suited to its interaction, forming a short-lived enzyme-substrate complex.
2. Substrate binding induces the enzyme to change its shape. The active site then fits more tightly around the substrate than before binding.
3. The active site acts on chemical bonds within the bound substrate, converting it into product and forming an intermediate enzyme-product complex.
4. The product is released and the enzyme remains unchanged overall. It can bind another substrate molecule and pass through the catalytic cycle again.
5. The enzyme lowers activation energy, the energy barrier between substrate and transition state, so that the conversion into product occurs more readily.

Q6. An enzyme reaction is tested at fixed enzyme concentration, temperature and pH. Its rate first rises as substrate concentration increases, then reaches a plateau. Explain both phases. Malonate resembles succinate and inhibits succinic dehydrogenase by competing for its substrate-binding site. Identify and explain this inhibition. [4 marks]

1. Initially, increasing substrate concentration raises reaction velocity because additional substrate can bind to available enzyme molecules and undergo conversion to product.
2. At the plateau, enzyme molecules are saturated. There are no free enzyme molecules for the additional substrate, so the maximum velocity is not exceeded.
3. Malonate acts as a competitive inhibitor because its structural resemblance to succinate allows it to compete for the same binding site.
4. When inhibitor occupies the site, substrate cannot bind there, reducing enzyme action. This explains the inhibition without requiring destruction of the enzyme.

Q7. Distinguish prosthetic groups, coenzymes and metal-ion cofactors, giving one example of each. [3 marks]

1. Prosthetic groups are organic components tightly bound to the apoenzyme. Haem is the prosthetic group in catalase and peroxidase and contributes to their active sites.
2. Coenzymes are organic components that associate transiently with the apoenzyme, usually during catalysis. Nicotinamide adenine dinucleotide is an example containing the vitamin niacin.
3. Metal ions assist enzyme activity through coordination bonds with the active site and substrate. Zinc is a cofactor required by the protein-cleaving enzyme carboxypeptidase.

Q8. Distinguish primary and secondary metabolites in two points, including examples and a qualification about the roles of secondary metabolites. [2 marks]

1. Primary metabolites, including amino acids and sugars, have identifiable functions in normal physiological processes.
2. Secondary metabolites include rubber, pigments and alkaloids. Some have ecological importance, but the roles of all secondary metabolites in their host organisms are not fully understood.

Key takeaways

• Biomolecules include small compounds and large polymers; membrane-associated lipids enter the insoluble fraction without being true macromolecules.
• Carbohydrate classification depends on hydrolysis products, while molecular structure helps explain storage and structural functions.
• Amino acids share amino and carboxyl groups but differ in side chains; their ionic forms change with pH.
• Protein sequence, local folding, overall folding and subunit arrangement describe primary, secondary, tertiary and quaternary structures respectively.
• Fats and oils contain fatty acids esterified with glycerol; oils have lower melting points than fats.
• Almost all enzymes are proteins, but ribozymes are exceptions; enzymes lower activation energy through interactions at their active sites.
• Temperature, pH, substrate concentration and inhibitors affect enzyme activity through different mechanisms that must be distinguished.
• Secondary metabolites include useful and ecologically important compounds, although the functions of all are not fully understood.

Test yourself

Why can lipids enter the acid-insoluble fraction?

Membranes break into water-insoluble fragments during grinding. Their associated lipids therefore separate with the insoluble fraction.

Which sugars form lactose, and which form sucrose?

Lactose contains glucose and galactose, whereas sucrose contains glucose and fructose.

How do the side chains of glycine, alanine and serine differ?

Glycine has hydrogen as its side chain, alanine has a methyl group and serine has a hydroxymethyl group.

Why is methionine placed in more than one amino-acid category?

It is sulphur-containing on a structural basis and essential on the basis of dietary requirement.

What remains intact during protein denaturation?

The primary structure remains intact, while secondary and tertiary structures are disrupted and biological activity is lost.

Why does a saturated enzyme system stop responding to additional substrate?

All available enzyme molecules are occupied, leaving no free enzyme molecules to bind the additional substrate.

What makes malonate a competitive inhibitor of succinic dehydrogenase?

Malonate resembles succinate and competes with it for the enzyme's substrate-binding site.

How does a prosthetic group differ from a coenzyme?

A prosthetic group is tightly bound to the apoenzyme; a coenzyme associates transiently, usually during catalysis.