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Biomolecules

NCERT Class 11 · Biology Based on NCERT Class 11 Biology textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the chemical composition of living organisms, revealing that while the same elements found in non-living matter are present, their relative abundance, especially of carbon and hydrogen, is higher in living systems. It introduces the concept of biomolecules, classifying them into micromolecules and macromolecules, and delves into the structure and function of proteins, polysaccharides, nucleic acids, and enzymes. The central idea a student must grasp is that life is built from a limited set of small molecular building blocks (amino acids, nucleotides, monosaccharides, fatty acids) that polymerize into macromolecules with specific structures and functions, and that enzymes are biological catalysts that lower activation energy to drive metabolic reactions.
Key Concepts (43)
Biomolecules
All carbon compounds obtained from living tissues, along with inorganic elements and compounds present in them.
e.g. Glucose, amino acids, and DNA are biomolecules found in a plant leaf.
Acid-soluble pool
The filtrate obtained after grinding a living tissue in trichloroacetic acid and straining it, containing small molecular weight compounds (micromolecules).
e.g. Amino acids, sugars, and nucleotides are found in the acid-soluble pool of liver tissue.
Acid-insoluble fraction
The retentate obtained after straining the tissue slurry, containing macromolecules like proteins, nucleic acids, polysaccharides, and lipids.
e.g. Cellulose from plant cell walls and haemoglobin from red blood cells are in the acid-insoluble fraction.
Micromolecules
Biomolecules with molecular weights less than one thousand daltons (Da), also called small molecular weight compounds.
e.g. Water (18 Da), glucose (180 Da), and alanine (89 Da) are micromolecules.
Macromolecules (Biomacromolecules)
Biomolecules with molecular weights in the range of ten thousand daltons and above, found in the acid-insoluble fraction.
e.g. Proteins, nucleic acids, and polysaccharides are macromolecules.
Amino acids
Organic compounds containing an amino group and an acidic group as substituents on the same carbon (α-carbon), making them α-amino acids.
e.g. Glycine has a hydrogen as its R group, alanine has a methyl group, and serine has a hydroxymethyl group.
Zwitterionic form
The form of an amino acid in which the amino group is protonated (–NH3+) and the carboxyl group is deprotonated (–COO−), making it a neutral dipolar ion.
e.g. At a certain pH (isoelectric point), alanine exists as a zwitterion.
Lipids
Water-insoluble organic compounds that include simple fatty acids, glycerol, and their esters (fats and oils), as well as phospholipids.
e.g. Palmitic acid (16-carbon saturated fatty acid) and lecithin (a phospholipid) are lipids.
Fatty acids
Carboxylic acids with a long hydrocarbon R group, which can be saturated (no double bonds) or unsaturated (one or more C=C double bonds).
e.g. Palmitic acid is saturated; arachidonic acid (20 carbons) is unsaturated.
Nucleosides
Compounds formed when a nitrogenous base is attached to a sugar (ribose or deoxyribose).
e.g. Adenosine (adenine + ribose) and uridine (uracil + ribose) are nucleosides.
Nucleotides
Compounds formed when a phosphate group is esterified to the sugar of a nucleoside.
e.g. Adenylic acid (AMP) and thymidylic acid (TMP) are nucleotides.
Primary metabolites
Biomolecules that have identifiable functions and play known roles in normal physiological processes, found in all living organisms.
e.g. Amino acids, sugars, and nucleic acids are primary metabolites.
Secondary metabolites
Compounds found in plant, fungal, and microbial cells that do not have a clearly understood role in the host organism but are often useful for human welfare.
e.g. Alkaloids (morphine), antibiotics, rubber, and essential oils are secondary metabolites.
Proteins
Polypeptides that are linear chains of amino acids linked by peptide bonds; they are heteropolymers of 20 types of amino acids.
e.g. Collagen (structural protein), trypsin (enzyme), and insulin (hormone) are proteins.
Peptide bond
A covalent bond formed between the carboxyl group of one amino acid and the amino group of another amino acid, linking them in a polypeptide chain.
e.g. In a dipeptide like glycylalanine, the peptide bond connects glycine and alanine.
Polysaccharides
Long chains of sugars (monosaccharides) that can be homopolymers (one type of monomer) or heteropolymers.
e.g. Cellulose (homopolymer of glucose), starch (homopolymer of glucose), and inulin (polymer of fructose) are polysaccharides.
Homopolymer
A polymer made of only one type of monomer repeating 'n' number of times.
e.g. Cellulose and starch are homopolymers of glucose.
Heteropolymer
A polymer made of more than one type of monomer.
e.g. Proteins are heteropolymers of 20 different amino acids.
Nucleic acids
Polynucleotides that function as genetic material; DNA contains deoxyribose sugar, and RNA contains ribose sugar.
e.g. DNA in the nucleus and RNA in the cytoplasm are nucleic acids.
Primary structure of proteins
The sequence of amino acids in a polypeptide chain, starting with the N-terminal amino acid and ending with the C-terminal amino acid.
e.g. The primary structure of insulin is the specific order of its 51 amino acids.
Secondary structure of proteins
The folding of portions of the polypeptide chain into regular structures like α-helix (right-handed) or β-pleated sheet, stabilized by hydrogen bonds.
e.g. Keratin in hair has an α-helix secondary structure.
Tertiary structure of proteins
The three-dimensional folding of the entire polypeptide chain upon itself, giving a compact globular or fibrous shape, essential for biological activity.
e.g. Myoglobin has a tertiary structure that allows it to bind oxygen.
Quaternary structure of proteins
The arrangement of multiple folded polypeptide subunits (prosthetic groups) relative to each other in a multi-subunit protein.
e.g. Adult human haemoglobin has four subunits: two α-type and two β-type.
Enzymes
Proteins (with some exceptions called ribozymes) that catalyse biochemical reactions by lowering the activation energy.
e.g. Carbonic anhydrase catalyses the conversion of CO2 and H2O to carbonic acid.
Ribozymes
Nucleic acids that behave like enzymes, possessing catalytic activity.
e.g. Ribosomal RNA (rRNA) acts as a ribozyme in peptide bond formation.
Active site
A crevice or pocket in the tertiary structure of an enzyme into which the substrate fits and where catalysis occurs.
e.g. The active site of carbonic anhydrase binds CO2 and H2O.
Substrate
The chemical that is converted into a product by an enzyme.
e.g. Starch is the substrate for the enzyme amylase.
Enzyme-substrate complex (ES)
A transient complex formed when the substrate binds to the active site of the enzyme, essential for catalysis.
e.g. Carbonic anhydrase forms an ES complex with CO2 and H2O.
Activation energy
The difference in average energy content of the substrate from that of the transition state; the energy barrier that must be overcome for a reaction to proceed.
e.g. Enzymes lower the activation energy, making the reaction faster.
Competitive inhibitor
A chemical that closely resembles the substrate in molecular structure and competes for the substrate-binding site of the enzyme, inhibiting activity.
e.g. Malonate inhibits succinic dehydrogenase by competing with succinate.
Co-factors
Non-protein constituents bound to an enzyme that are required for catalytic activity; include prosthetic groups, co-enzymes, and metal ions.
e.g. Haem is a prosthetic group in catalase; NAD is a co-enzyme containing niacin; zinc is a metal ion cofactor for carboxypeptidase.
Apoenzyme
The protein portion of an enzyme that requires a co-factor to become catalytically active.
e.g. Carboxypeptidase without zinc is an apoenzyme.
Prosthetic group
An organic co-factor that is tightly bound to the apoenzyme, often as part of the active site.
e.g. Haem is the prosthetic group in peroxidase and catalase.
Co-enzymes
Organic co-factors that associate transiently with the apoenzyme during catalysis and often contain vitamins.
e.g. NAD and NADP are co-enzymes that contain the vitamin niacin.
Optimum temperature
The specific temperature at which an enzyme shows its highest activity; activity declines below and above this value.
e.g. Human enzymes typically have an optimum temperature around 37°C.
Optimum pH
The specific pH at which an enzyme shows its highest activity; activity declines below and above this value.
e.g. Pepsin has an optimum pH around 2, while trypsin has an optimum pH around 8.
Vmax
The maximum velocity of an enzymatic reaction, reached when all enzyme molecules are saturated with substrate.
e.g. In a graph of velocity vs. substrate concentration, Vmax is the plateau value.
Metabolic pathway
A multistep chemical reaction where each step is catalysed by a specific enzyme or enzyme complex.
e.g. Glycolysis converts glucose to pyruvic acid through ten enzyme-catalysed reactions.
Collagen
The most abundant protein in the animal world, functioning as an intercellular ground substance.
e.g. Collagen is found in skin, bones, and tendons.
RuBisCO
Ribulose bisphosphate Carboxylase-Oxygenase, the most abundant protein in the whole biosphere.
e.g. RuBisCO catalyses the first step of carbon fixation in photosynthesis.
Chitin
A complex polysaccharide found in the exoskeletons of arthropods, made of amino-sugars like N-acetylgalactosamine.
e.g. The hard outer shell of a cockroach contains chitin.
Essential amino acids
Amino acids that cannot be synthesized by the body and must be obtained through diet.
e.g. Lysine, tryptophan, and methionine are essential amino acids for humans.
Non-essential amino acids
Amino acids that the body can synthesize and do not need to be supplied through diet.
e.g. Alanine, glycine, and serine are non-essential amino acids.
Formulas & Equations (5)
Ba(OH)2 + H2SO4 → BaSO4 + 2H2O
Reads as: Barium hydroxide plus sulphuric acid yields barium sulphate plus two water molecules
Variables: Ba(OH)2: barium hydroxide; H2SO4: sulphuric acid; BaSO4: barium sulphate; H2O: water
Use when: When illustrating an example of an inorganic chemical reaction where bonds are broken and new bonds are formed
⚠ Students often forget to balance the equation, writing Ba(OH)2 + H2SO4 → BaSO4 + H2O, missing the coefficient 2 for water.
CO2 + H2O →(Carbonic anhydrase) H2CO3
Reads as: Carbon dioxide plus water, catalysed by carbonic anhydrase, yields carbonic acid
Variables: CO2: carbon dioxide; H2O: water; H2CO3: carbonic acid; Carbonic anhydrase: the enzyme catalyst
Use when: When explaining enzyme-catalysed reactions and the dramatic increase in reaction rate (200 molecules/hour without enzyme vs. 600,000 molecules/second with enzyme)
⚠ Students write the reaction as CO2 + H2O → H2CO3 without mentioning the enzyme, missing the point about catalysis.
C6H12O6 + O2 → 2C3H4O3 + 2H2O
Reads as: Glucose plus oxygen yields two molecules of pyruvic acid plus two molecules of water
Variables: C6H12O6: glucose; O2: oxygen; C3H4O3: pyruvic acid; H2O: water
Use when: When describing the metabolic pathway of glycolysis (aerobic conditions) and how it leads to different products under different conditions
⚠ Students often incorrectly balance this equation or forget that it represents a multi-step pathway, not a single reaction.
Rate = δP/δt
Reads as: Rate equals change in product concentration divided by change in time
Variables: δP: change in amount of product formed; δt: change in time; Rate: amount of product per unit time (e.g., mol/s)
Use when: When defining the rate of a physical or chemical process, and when explaining how enzyme activity is measured
⚠ Students confuse rate with velocity; rate is scalar, velocity has direction. Also, they often forget that δP refers to product, not substrate.
E + S → ES → EP → E + P
Reads as: Enzyme plus substrate forms enzyme-substrate complex, which forms enzyme-product complex, which dissociates into enzyme and product
Variables: E: enzyme; S: substrate; ES: enzyme-substrate complex; EP: enzyme-product complex; P: product
Use when: When describing the catalytic cycle of enzyme action, showing that the enzyme is unchanged at the end
⚠ Students omit the EP intermediate and write E + S → ES → E + P directly, which is incomplete and does not reflect the NCERT mechanism.
How It Works
Chemical Analysis of Living Tissue
  1. Grind a living tissue (e.g., vegetable or liver) in trichloroacetic acid (Cl3CCOOH) using a mortar and pestle to obtain a thick slurry.
  2. Strain the slurry through cheesecloth or cotton to separate two fractions: the filtrate (acid-soluble pool) and the retentate (acid-insoluble fraction).
  3. The acid-soluble pool contains small molecular weight compounds (micromolecules) like amino acids, sugars, and nucleotides.
  4. The acid-insoluble fraction contains macromolecules like proteins, nucleic acids, polysaccharides, and lipids (due to membrane fragments forming vesicles).
💡 Lipids are not true macromolecules (MW < 800 Da) but appear in the acid-insoluble fraction because they form membrane vesicles that are not water-soluble.
Elemental Analysis of Living vs. Non-living Matter
  1. Weigh a small amount of living tissue (wet weight) and dry it to remove water, obtaining dry weight.
  2. Burn the dry tissue fully; all carbon compounds are oxidised to gaseous CO2 and water vapour.
  3. The remaining material is called 'ash', containing inorganic elements like calcium and magnesium.
  4. Compare the elemental composition with that of a non-living sample (e.g., earth's crust).
  5. Observe that all elements present in earth's crust are also present in living tissue, but the relative abundance of carbon and hydrogen is higher in living organisms.
💡 The key difference is not the presence or absence of elements, but their relative abundance — carbon and hydrogen are much more abundant in living matter.
Catalytic Cycle of Enzyme Action
  1. The substrate (S) binds to the active site of the enzyme (E), forming the enzyme-substrate complex (ES).
  2. The binding of the substrate induces the enzyme to alter its shape, fitting more tightly around the substrate (induced fit).
  3. The active site, now in close proximity to the substrate, breaks chemical bonds and forms new bonds, creating the enzyme-product complex (EP).
  4. The product (P) is released from the active site, and the free enzyme is ready to bind another substrate molecule and repeat the cycle.
💡 The enzyme is not consumed in the reaction; it is regenerated unchanged at the end of the cycle. The ES complex is transient and essential for catalysis.
How Enzymes Lower Activation Energy
  1. The substrate (S) must pass through a high-energy transition state to be converted into product (P).
  2. The difference in average energy content of S and the transition state is called activation energy.
  3. Enzymes bind the substrate and stabilise the transition state, thereby lowering the activation energy barrier.
  4. With a lower activation energy, a larger fraction of substrate molecules can be converted to product per unit time, dramatically increasing the reaction rate.
💡 Enzymes do not change the energy levels of S or P; they only lower the activation energy, making the reaction faster. The reaction is exothermic if P is at a lower energy level than S.
Effect of Substrate Concentration on Enzyme Activity
  1. At low substrate concentrations, the velocity of the reaction increases linearly with increasing substrate concentration because many enzyme active sites are free.
  2. As substrate concentration continues to increase, the velocity increases more slowly because enzyme molecules become increasingly saturated.
  3. At very high substrate concentrations, the reaction reaches a maximum velocity (Vmax), where all enzyme active sites are occupied.
  4. Further increase in substrate concentration does not increase the velocity because there are no free enzyme molecules to bind additional substrate.
💡 Vmax is reached when the enzyme is saturated with substrate. The Michaelis constant (Km) is the substrate concentration at which the velocity is half of Vmax — a measure of enzyme affinity for substrate.
Competitive Inhibition
  1. An inhibitor that closely resembles the substrate in molecular structure competes for the substrate-binding site on the enzyme.
  2. The inhibitor binds to the active site, blocking the substrate from binding.
  3. Because the inhibitor is structurally similar, it fits into the active site but cannot be converted to product.
  4. The enzyme action declines because fewer enzyme molecules are available to bind the actual substrate.
  5. Competitive inhibition can be overcome by increasing the substrate concentration.
💡 The classic CBSE example is malonate inhibiting succinic dehydrogenase — malonate resembles succinate but has two carbons instead of four.
Exam Traps — Don't Make These Mistakes
⚠ Confusing micromolecules and macromolecules based on solubility
✗ Lipids are macromolecules because they are found in the acid-insoluble fraction.
→ Lipids have molecular weights below 800 Da, which is the range of micromolecules. They appear in the acid-insoluble fraction not because they are macromolecules, but because they form membrane vesicles that are not water-soluble.
✓ Lipids are not strictly macromolecules; they are small molecular weight compounds that appear in the acid-insoluble fraction due to their association with membrane fragments.
⚠ Mistaking homopolymer for heteropolymer in proteins
✗ Proteins are homopolymers because they are made of amino acids.
→ A homopolymer has only one type of monomer repeating many times. Proteins are made of 20 different types of amino acids, so they are heteropolymers.
✓ Proteins are heteropolymers because they are composed of 20 different types of amino acids, not just one repeating monomer.
⚠ Confusing nucleoside and nucleotide
✗ A nucleotide is a nitrogenous base attached to a sugar.
→ That is the definition of a nucleoside. A nucleotide includes a phosphate group esterified to the sugar in addition to the nitrogenous base.
✓ A nucleoside is a nitrogenous base attached to a sugar. A nucleotide is a nucleoside with a phosphate group esterified to the sugar.
⚠ Thinking all enzymes are proteins
✗ All enzymes are proteins.
→ While almost all enzymes are proteins, there are some nucleic acids that behave like enzymes, called ribozymes.
✓ Almost all enzymes are proteins, but some nucleic acids (ribozymes) also have catalytic activity.
⚠ Confusing primary and secondary metabolites
✗ Secondary metabolites have known functions in the host organism.
→ Secondary metabolites do not have clearly understood roles in the host organism, unlike primary metabolites which have identifiable functions in normal physiological processes.
✓ Primary metabolites have identifiable functions in normal physiological processes, while the roles of many secondary metabolites in the host organism are not yet understood.
⚠ Misinterpreting the effect of temperature on enzymes
✗ Enzymes work best at high temperatures because reactions are faster.
→ Enzymes are proteins that get denatured at high temperatures (above about 40°C for most human enzymes), losing their tertiary structure and activity. Low temperature preserves them in a temporarily inactive state.
✓ Enzymes have an optimum temperature; activity declines above and below this value. High temperature denatures the enzyme, destroying its catalytic activity.
Compare & Contrast
Chemical composition
Nucleoside
Nucleotide
A nucleoside consists of a nitrogenous base attached to a sugar (ribose or deoxyribose), while a nucleotide has an additional phosphate group esterified to the sugar.
Role in the organism
Primary metabolites
Secondary metabolites
Primary metabolites have identifiable functions and play known roles in normal physiological processes (e.g., amino acids, sugars), whereas secondary metabolites (e.g., alkaloids, rubber) do not have clearly understood roles in the host organism but are often useful for human welfare.
Sugar component
DNA
RNA
DNA contains deoxyribose sugar, while RNA contains ribose sugar.
Iodine test and structure
Starch
Cellulose
Starch forms helical secondary structures that can hold I2 molecules, giving a blue colour, while cellulose does not contain complex helices and cannot hold I2.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (2-3 marks), 1 Long Answer (5 marks) — total approximately 10-12 marks
Most asked: Structure and classification of amino acids (zwitterion, essential vs. non-essential) · Levels of protein structure (primary, secondary, tertiary, quaternary) · Enzyme action mechanism (ES complex, activation energy, factors affecting activity) · Competitive inhibition (malonate-succinate example) · Difference between nucleoside and nucleotide · Classification of enzymes (six classes with examples) · Co-factors (prosthetic groups, co-enzymes, metal ions)
Don't skip: Draw and explain the four levels of protein structure with a diagram (5 marks); Describe the mechanism of enzyme action, including the formation of ES complex and the concept of activation energy (5 marks); Explain competitive inhibition with the example of succinic dehydrogenase and malonate (3 marks); Differentiate between nucleoside and nucleotide, and between DNA and RNA (3 marks)
One-Line Revision (read aloud before the exam)
  1. Biomolecules are carbon compounds from living tissues, including both organic and inorganic constituents.
  2. The acid-soluble pool contains micromolecules (MW < 1000 Da), while the acid-insoluble fraction contains macromolecules like proteins, nucleic acids, and polysaccharides.
  3. Lipids are not true macromolecules but appear in the acid-insoluble fraction due to membrane vesicle formation.
  4. Amino acids are α-amino acids with an amino group and a carboxyl group on the same carbon, and they exist as zwitterions at certain pH.
  5. There are 20 types of proteinaceous amino acids, which can be essential (dietary) or non-essential (synthesised by the body).
  6. Fatty acids can be saturated (no double bonds) or unsaturated (one or more C=C bonds); triglycerides are esters of glycerol and fatty acids.
  7. A nucleoside is a nitrogenous base + sugar; a nucleotide is a nucleoside + phosphate group.
  8. Proteins are heteropolymers of amino acids linked by peptide bonds, with primary, secondary, tertiary, and quaternary structures.
  9. Collagen is the most abundant animal protein; RuBisCO is the most abundant protein in the biosphere.
  10. Polysaccharides like cellulose, starch, and glycogen are homopolymers of glucose; chitin is a complex polysaccharide in arthropod exoskeletons.
  11. Enzymes are mostly proteins that catalyse reactions by lowering activation energy; ribozymes are catalytic nucleic acids.
  12. The active site of an enzyme is a crevice where the substrate binds, forming a transient enzyme-substrate (ES) complex.
  13. Enzymes have an optimum temperature and pH; high temperature denatures them, while low temperature preserves them temporarily.
  14. Competitive inhibitors resemble the substrate and compete for the active site; malonate inhibits succinic dehydrogenase.
  15. Co-factors include prosthetic groups (tightly bound), co-enzymes (transient, often vitamin-derived), and metal ions.
  16. Primary metabolites have known physiological roles; secondary metabolites (alkaloids, rubber, etc.) have unclear roles in the host but are useful to humans.
  17. The rate of an enzyme-catalysed reaction increases with substrate concentration until Vmax is reached when all enzyme molecules are saturated.
  18. Metabolic pathways are multistep reactions where each step is catalysed by a specific enzyme, e.g., glycolysis converts glucose to pyruvic acid.
  19. DNA contains deoxyribose and functions as genetic material; RNA contains ribose and also functions as genetic material in some viruses.
  20. The zwitterionic form of an amino acid has a protonated amino group and a deprotonated carboxyl group, making it a neutral dipolar ion.

MCQs — 10 Questions with Answers

Q1. Which element shows the largest relative increase in abundance from earth's crust to human body compared to other elements?

  • A. Carbon ✓
  • B. Hydrogen
  • C. Oxygen
  • D. Nitrogen

Answer: A — Carbon increases from 0.03% in earth's crust to 18.5% in human body, the highest relative increase among all elements listed in Table 9.1.

Q2. When tissue is ground in trichloroacetic acid and filtered through cheesecloth, the two fractions obtained are:

  • A. Ash and dry matter
  • B. Acid-soluble pool (filtrate) and acid-insoluble fraction (retentate) ✓
  • C. Wet weight and dry weight
  • D. Macromolecules and inorganic compounds only

Answer: B — Grinding in trichloroacetic acid and filtering produces the acid-soluble pool (containing small organic compounds) as filtrate and the acid-insoluble fraction (macromolecules) as retentate.

Q3. An amino acid with R = H is called:

  • A. Alanine
  • B. Glycine ✓
  • C. Serine
  • D. Valine

Answer: B — Glycine is the amino acid where the R group is simply hydrogen, making it the simplest amino acid found in proteins.

Q4. A fatty acid with 16 carbons (including carboxyl carbon) and no double bonds is classified as:

  • A. Unsaturated and is palmitic acid
  • B. Saturated and is palmitic acid ✓
  • C. Unsaturated and is arachidonic acid
  • D. Saturated and is arachidonic acid

Answer: B — Palmitic acid has 16 carbons and is saturated (no C=C double bonds); arachidonic acid has 20 carbons and is unsaturated.

Q5. At physiological pH, the zwitterionic form of an amino acid has the structure:

  • A. –NH₂ and –COOH both neutral
  • B. –NH₃⁺ (protonated) and –COO⁻ (deprotonated) ✓
  • C. –NH₂ and –COO⁻
  • D. –NH₃⁺ and –COOH

Answer: B — The zwitterionic form exists when the amino group is protonated (–NH₃⁺) and the carboxyl group is deprotonated (–COO⁻), which occurs at physiological pH.

Q6. Which of the following is NOT a correct pairing of compound and its component?

  • A. Nucleoside: nitrogenous base + sugar
  • B. Nucleotide: nitrogenous base + sugar + phosphate
  • C. Triglyceride: glycerol + one fatty acid ✓
  • D. Phospholipid: phosphorus-containing organic compound + glycerol + fatty acids

Answer: C — A triglyceride contains glycerol esterified with three fatty acids, not one; one fatty acid + glycerol would be a monoglyceride.

Q7. Phospholipids like lecithin are primarily found in which cellular component?

  • A. Cell wall
  • B. Mitochondrial matrix
  • C. Cell membrane ✓
  • D. Vacuole

Answer: C — Phospholipids are the major structural component of cell membranes, forming the lipid bilayer that surrounds all cells.

Q8. In an experiment, tissue of mass 100 g (wet weight) is dried completely, yielding 20 g dry weight. If the dry tissue is then completely burned, 2 g of ash remains. What is the mass of organic matter in the tissue?

  • A. 80 g
  • B. 20 g
  • C. 18 g ✓
  • D. 2 g

Answer: C — Dry weight = 20 g; ash (inorganic matter) = 2 g; organic matter = dry weight − ash = 20 − 2 = 18 g.

Q9. Both statements: (1) Amino acids are called α-amino acids because the amino and carboxyl groups are attached to the α-carbon. (2) All amino acids in proteins have identical R groups.

  • A. Both statements are correct
  • B. Statement 1 is correct; statement 2 is incorrect ✓
  • C. Statement 1 is incorrect; statement 2 is correct
  • D. Both statements are incorrect

Answer: B — Statement 1 is correct: α-amino acids have both groups on the same carbon. Statement 2 is incorrect: the 20 amino acids in proteins differ precisely in their R groups.

Q10. Consider the chemical composition data: Element X has 0.14% in earth's crust but 9.5% in human body; Element Y has 46.6% in earth's crust but 65% in human body. Based on this, which inference about living organisms is most accurate?

  • A. Living organisms concentrate both C and O equally because they need both for energy
  • B. Living organisms selectively concentrate certain elements (C and H) far above their crustal levels due to biochemical requirements ✓
  • C. All elements in living tissues occur in the same ratio as in earth's crust
  • D. Oxygen is accumulated more than hydrogen because oxygen is more abundant in water

Answer: B — The data shows C (0.03%→18.5%) and H (0.14%→9.5%) increase dramatically in living matter, indicating selective bioaccumulation of carbon and hydrogen for organic molecule synthesis, distinct from their crustal availability.

Flashcards

What is the acid-soluble pool obtained after grinding tissue in trichloroacetic acid?

The filtrate containing thousands of small organic compounds extracted from living tissue.

Define an α-amino acid.

An organic compound with amino (–NH₂) and carboxyl (–COOH) groups attached to the same carbon atom, along with hydrogen and a variable R group.

How many types of amino acids are found in proteins?

Twenty types of amino acids occur naturally in proteins, differing in their R group structure.

What is the difference between a nucleoside and a nucleotide?

A nucleoside is a nitrogenous base bonded to a sugar; a nucleotide adds a phosphate group esterified to the sugar.

Name three inorganic constituents found in living tissues.

Sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), phosphate (PO₄³⁻), and sulphate (SO₄²⁻) are examples.

What does 'dry weight' mean in tissue analysis?

The mass of tissue remaining after all water has been evaporated.

Distinguish between saturated and unsaturated fatty acids.

Saturated fatty acids lack carbon-carbon double bonds; unsaturated fatty acids contain one or more C=C double bonds.

What is a triglyceride?

A lipid formed when three fatty acids are esterified with one glycerol molecule.

Why are phospholipids important in cells?

Phospholipids are the major structural component of cell membranes, forming the lipid bilayer.

What is the zwitterionic form of an amino acid?

The form where the amino group is protonated (–NH₃⁺) and the carboxyl group is deprotonated (–COO⁻), occurring at physiological pH.

Important Board Questions

Define a biomolecule. Give two examples each of organic and inorganic biomolecules found in living tissues. [2 marks]

Biomolecule = all carbon compounds and inorganic elements/compounds in living tissue. Organic examples: glucose, amino acids, fatty acids, nucleotides. Inorganic examples: water, phosphate, calcium ions, sodium ions.

Explain the structure of an amino acid. Why are amino acids called α-amino acids? How does the structure change at different pH levels, and what is this form called? [5 marks]

Amino acid has 4 groups on one carbon: H, –NH₂, –COOH, and R group. Called α-amino acids because both amino and carboxyl groups attach to the same (α) carbon. At physiological pH, –NH₂ becomes –NH₃⁺ and –COOH becomes –COO⁻, forming the zwitterionic form. pH change causes ionization of these groups.

With the help of a diagram, explain the structural organization and composition of nucleotides. Describe how nucleotides link together to form nucleic acids (DNA/RNA) and explain their biological significance in living organisms. [6 marks]

Nucleotide structure: nitrogenous base + pentose sugar + phosphate group. Show base + sugar = nucleoside; nucleoside + phosphate = nucleotide. Nucleotides polymerize via phosphodiester bonds between sugar of one nucleotide and phosphate of next, forming DNA (deoxyribose + T) and RNA (ribose + U). Significance: DNA stores genetic information; RNA functions in protein synthesis and regulation. Both are essential for heredity and life processes.

Assertion-Reason Questions for Biomolecules

Q1. Assertion (A):

All elements present in Earth's crust are also found in living tissues, but their relative abundances differ significantly.

Reason (R):

Carbon and hydrogen are essential components of organic molecules, which are the basis of all living matter.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. The reason correctly explains why relative abundances differ—organic molecules require C and H in much higher proportions than found in Earth's crust. The assertion and reason are logically connected.

Q2. Assertion (A):

Nucleotides are more complex than nucleosides because they contain an additional phosphate group.

Reason (R):

Nucleosides consist only of nitrogen bases and are not functional in genetic material storage.

Answer: C — A is true but R is false — The assertion is true—nucleotides do contain an additional phosphate group esterified to the sugar. However, the reason is false. While nucleotides are the structural units of DNA and RNA, nucleosides like adenosine and uridine exist as free compounds in cells with various biological roles. The reason does not correctly explain why nucleotides are more complex.

Q3. Assertion (A):

When a tissue is completely burnt, the remaining ash contains all the inorganic elements that were present in the original tissue.

Reason (R):

Inorganic elements cannot be oxidized to gaseous form during burning, unlike carbon compounds.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true and logically connected. Ash is the non-combustible residue containing inorganic elements (Ca, Mg, Na, etc.) because only organic carbon compounds are oxidized to CO2 and water vapor during burning. The reason correctly explains the assertion.

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Case-Based Questions (CBQ) for Biomolecules

Case Study:

A biochemistry laboratory in Delhi conducted an experiment on fresh spinach leaves to determine their chemical composition. They first measured the wet weight (350g), then dried the leaves completely to get dry weight (70g). The dried leaves were then completely burnt in a furnace, leaving ash (7g). The filtrate from acid extraction contained thousands of dissolved organic compounds. The researchers found that approximately 60% of the organic matter was carbohydrates, 25% was proteins, 10% was lipids, and 5% was other organic compounds.

Q1. What percentage of the original wet weight of spinach leaves was water?

  • A. 70%
  • B. 80% ✓
  • C. 90%
  • D. 60%

Answer: B

Q2. How much protein (in grams) was present in the original spinach sample?

  • A. 13g
  • B. 15.75g ✓
  • C. 21g
  • D. 17.5g

Answer: B

Q3. What fraction of the dry weight was inorganic matter (ash)?

  • A. 1/5
  • B. 1/7
  • C. 1/8
  • D. 1/10 ✓

Answer: D

Q4. Which biomolecule class constituted the majority of the organic matter in these spinach leaves?

  • A. Proteins
  • B. Lipids
  • C. Carbohydrates ✓
  • D. Nucleic acids

Answer: C

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