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Cell: The Unit of Life

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

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Chapter Notes

Overview
This chapter establishes the cell as the fundamental structural and functional unit of all living organisms, tracing the historical development of cell theory and detailing the differences between prokaryotic and eukaryotic cells. It systematically describes the structure and function of all cellular components, from the plasma membrane and cell wall to organelles like mitochondria, plastids, and the nucleus. The one big idea a student must walk away with is that every living organism is composed of cells, and the intricate compartmentalisation of eukaryotic cells allows for specialised functions that collectively sustain life.
Key Concepts (31)
Cell
The fundamental structural and functional unit of all living organisms.
e.g. A single Amoeba cell can perform all life processes independently.
Cell Theory
All living organisms are composed of cells and products of cells; all cells arise from pre-existing cells.
e.g. A new skin cell forms only by division of an existing skin cell, not from non-living material.
Prokaryotic Cell
A cell that lacks a membrane-bound nucleus and other membrane-bound organelles.
e.g. Bacteria like Escherichia coli are prokaryotic cells.
Eukaryotic Cell
A cell that possesses a membrane-bound nucleus and other membrane-bound organelles.
e.g. Human cheek cells and onion peel cells are eukaryotic.
Plasma Membrane
A selectively permeable membrane composed of a phospholipid bilayer with embedded proteins that forms the outer boundary of the cell.
e.g. It allows oxygen to diffuse in but prevents large proteins from leaking out.
Cell Wall
A non-living rigid structure outside the plasma membrane in plant and fungal cells that provides shape and protection.
e.g. The tough outer layer of an onion peel is its cell wall.
Nucleus
A membrane-bound organelle containing the cell's genetic material (DNA) and controlling cellular activities.
e.g. The nucleus in a human cheek cell directs protein synthesis.
Endoplasmic Reticulum (ER)
A network of tubular structures in the cytoplasm that divides the intracellular space into luminal and extraluminal compartments.
e.g. Rough ER in pancreatic cells synthesises digestive enzymes.
Golgi Apparatus
A stack of flat, disc-shaped sacs (cisternae) that packages and modifies proteins and lipids for secretion or intracellular use.
e.g. It packages mucus in goblet cells of the respiratory tract.
Lysosome
A membrane-bound vesicle containing hydrolytic enzymes that digest macromolecules and worn-out organelles.
e.g. Lysosomes digest bacteria engulfed by white blood cells.
Mitochondrion
A double-membrane-bound organelle with inner folds (cristae) that is the site of aerobic respiration and ATP synthesis.
e.g. Muscle cells have many mitochondria to meet high energy demands.
Plastid
A pigment-containing, double-membrane-bound organelle found in plant cells and euglenoids, classified into chloroplasts, chromoplasts, and leucoplasts.
e.g. Chloroplasts in leaf mesophyll cells carry out photosynthesis.
Ribosome
A non-membrane-bound granular structure composed of RNA and proteins that is the site of protein synthesis.
e.g. Ribosomes on rough ER synthesise proteins for secretion.
Centrosome
An organelle in animal cells containing two centrioles perpendicular to each other, involved in spindle formation during cell division.
e.g. Centrosomes help organise microtubules during mitosis in human cells.
Cytoskeleton
An elaborate network of microtubules, microfilaments, and intermediate filaments that provides mechanical support and maintains cell shape.
e.g. Microtubules maintain the elongated shape of nerve cells.
Cilia and Flagella
Hair-like outgrowths of the cell membrane with a 9+2 array of microtubules; cilia are short and numerous, flagella are long and fewer.
e.g. Cilia line the respiratory tract to move mucus; a sperm cell uses its flagellum for swimming.
Chromatin
The nucleoprotein fibre complex of DNA, histones, and non-histone proteins found in the interphase nucleus.
e.g. During interphase, chromatin is extended and not visible as discrete chromosomes.
Centromere
A primary constriction on a chromosome that holds two chromatids together and has kinetochores for spindle attachment.
e.g. The centromere is the point where sister chromatids are attached.
Mesosome
A specialised infolding of the plasma membrane in prokaryotic cells involved in cell wall formation, DNA replication, and respiration.
e.g. Mesosomes increase the surface area of the plasma membrane in bacteria.
Plasmid
A small circular DNA molecule outside the genomic DNA in bacteria that confers unique phenotypic traits like antibiotic resistance.
e.g. The plasmid carrying the gene for ampicillin resistance is used in genetic engineering.
Inclusion Bodies
Reserve materials in prokaryotic cytoplasm that are not bound by a membrane, such as phosphate granules and glycogen granules.
e.g. Cyanophycean granules store nitrogen in blue-green algae.
Tonoplast
The single membrane that bounds the vacuole in plant cells.
e.g. The tonoplast facilitates ion transport into the vacuole against a concentration gradient.
Nucleolus
A spherical, non-membrane-bound structure in the nucleus that is the site of active ribosomal RNA synthesis.
e.g. Cells actively synthesising proteins have large and numerous nucleoli.
Nuclear Envelope
A double-membrane structure with perinuclear space and nuclear pores that separates the nucleus from the cytoplasm.
e.g. The outer nuclear membrane is continuous with the endoplasmic reticulum.
Glycocalyx
The outermost layer of the prokaryotic cell envelope, which may be a loose slime layer or a thick capsule.
e.g. The capsule of Streptococcus pneumoniae helps it evade the immune system.
Stroma
The fluid-filled space inside the inner chloroplast membrane that contains enzymes for carbohydrate and protein synthesis, DNA, and ribosomes.
e.g. The Calvin cycle of photosynthesis occurs in the stroma.
Thylakoid
A flattened membranous sac in the chloroplast that contains chlorophyll and is the site of the light reactions of photosynthesis.
e.g. Thylakoids are stacked into grana to increase surface area for light capture.
Crista
An infolding of the inner mitochondrial membrane that increases the surface area for ATP synthesis.
e.g. Cristae contain enzymes of the electron transport chain.
Kinetochore
A disc-shaped protein structure on the sides of the centromere that attaches to spindle fibres during cell division.
e.g. Kinetochores pull sister chromatids apart during anaphase.
Plasmodesmata
Cytoplasmic connections that traverse the cell wall and middle lamella, linking the cytoplasm of neighbouring plant cells.
e.g. Plasmodesmata allow ions and small molecules to move between adjacent plant cells.
Middle Lamella
A layer of calcium pectate that holds or glues neighbouring plant cells together.
e.g. The middle lamella is the outermost layer between two adjacent plant cells.
Formulas & Equations (4)
Omnis cellula-e cellula
Reads as: All cells from pre-existing cells
Variables: No variables; this is a Latin phrase stating the principle of cell division.
Use when: When explaining the third tenet of cell theory as modified by Rudolf Virchow.
⚠ Students often attribute this phrase to Schleiden and Schwann instead of Virchow.
9+2 array
Reads as: Nine plus two array
Variables: 9 = nine peripheral doublet microtubules; 2 = two central singlet microtubules.
Use when: Describing the internal structure of eukaryotic cilia and flagella (axoneme).
⚠ Students confuse the 9+2 array of eukaryotes with the 9+0 arrangement of centrioles or the different structure of prokaryotic flagella.
70S ribosome (prokaryotic) = 50S + 30S subunits
Reads as: 70 Svedberg ribosome equals 50 Svedberg large subunit plus 30 Svedberg small subunit
Variables: S = Svedberg unit, a measure of sedimentation coefficient (density and size).
Use when: Comparing prokaryotic and eukaryotic ribosome sizes and subunit composition.
⚠ Students incorrectly think S values are additive (50S + 30S = 80S); they are not additive because sedimentation coefficient depends on shape and density.
80S ribosome (eukaryotic) = 60S + 40S subunits
Reads as: 80 Svedberg ribosome equals 60 Svedberg large subunit plus 40 Svedberg small subunit
Variables: S = Svedberg unit.
Use when: Describing eukaryotic ribosome structure.
⚠ Students forget that 80S ribosomes are found in eukaryotic cytoplasm, while 70S ribosomes are in prokaryotes and also in mitochondria and chloroplasts.
How It Works
Fluid Mosaic Model of Plasma Membrane
  1. The plasma membrane consists of a phospholipid bilayer with polar heads facing outward and hydrophobic tails facing inward.
  2. Integral proteins are partially or totally buried in the bilayer; peripheral proteins lie on the surface.
  3. The quasi-fluid nature of the lipid bilayer allows lateral movement of proteins.
  4. Cholesterol is present in the membrane to modulate fluidity.
  5. Carbohydrates are attached to proteins or lipids on the outer surface.
💡 The fluid mosaic model was proposed by Singer and Nicolson in 1972; the 'fluid' refers to the lateral movement of components, and 'mosaic' refers to the patchwork of proteins in the lipid bilayer.
Transport Across Plasma Membrane
  1. Passive transport: Neutral solutes move by simple diffusion along the concentration gradient (high to low) without energy.
  2. Osmosis: Water moves by diffusion from higher to lower concentration across a selectively permeable membrane.
  3. Facilitated diffusion: Polar molecules require carrier proteins to cross the nonpolar lipid bilayer, still along the gradient without energy.
  4. Active transport: Ions or molecules move against their concentration gradient (low to high) using ATP energy, e.g., Na+/K+ pump.
💡 Simple diffusion does not require a carrier; facilitated diffusion requires a carrier but no energy; active transport requires both a carrier and energy (ATP).
Endomembrane System Coordination
  1. Proteins synthesised by ribosomes on RER enter the ER lumen.
  2. Vesicles bud off from the ER and fuse with the cis face of the Golgi apparatus.
  3. Proteins are modified (e.g., glycosylation) as they move through Golgi cisternae from cis to trans face.
  4. Modified proteins are packaged into vesicles at the trans face.
  5. Vesicles are delivered to intracellular targets or secreted outside the cell.
💡 The endomembrane system includes ER, Golgi apparatus, lysosomes, and vacuoles — but NOT mitochondria, chloroplasts, or peroxisomes.
Mitochondrial ATP Synthesis (Aerobic Respiration)
  1. The outer mitochondrial membrane is the continuous limiting boundary.
  2. The inner membrane forms cristae that increase surface area.
  3. Enzymes for electron transport and ATP synthesis are located on the inner membrane and in the matrix.
  4. The matrix contains circular DNA, 70S ribosomes, and enzymes for the Krebs cycle.
  5. ATP is produced and used as cellular energy currency.
💡 Mitochondria are called 'power houses of the cell' because they produce ATP; they have their own DNA and ribosomes, supporting the endosymbiotic theory.
Photosynthesis in Chloroplasts
  1. Light energy is trapped by chlorophyll pigments in the thylakoid membranes.
  2. Light reactions occur in the thylakoids, producing ATP and NADPH.
  3. Dark reactions (Calvin cycle) occur in the stroma, using ATP and NADPH to synthesise carbohydrates.
  4. Thylakoids are stacked into grana; stroma lamellae connect different grana.
💡 Chloroplasts have a double membrane, 70S ribosomes, and circular DNA; they are found in plant cells and euglenoids.
Chromosome Classification Based on Centromere Position
  1. Metacentric: Centromere in the middle, producing two equal arms.
  2. Sub-metacentric: Centromere slightly off-centre, producing one shorter and one longer arm.
  3. Acrocentric: Centromere near one end, producing one very short and one very long arm.
  4. Telocentric: Centromere at the terminal end.
💡 The mnemonic 'M-S-A-T' (Metacentric, Sub-metacentric, Acrocentric, Telocentric) helps recall the order from middle to terminal centromere.
Structure of Cilia and Flagella
  1. Cilia and flagella are covered by an extension of the plasma membrane.
  2. The core (axoneme) has a 9+2 array of microtubules: nine peripheral doublets and two central singlet microtubules.
  3. Central tubules are connected by bridges and enclosed by a central sheath.
  4. Radial spokes connect each peripheral doublet to the central sheath.
  5. Peripheral doublets are interconnected by linkers.
  6. Both structures emerge from a basal body (derived from centriole).
💡 The 9+2 arrangement is characteristic of eukaryotic cilia and flagella; prokaryotic flagella are structurally different (composed of filament, hook, and basal body).
Exam Traps — Don't Make These Mistakes
⚠ Confusing Cell Theory Contributors
✗ Schleiden and Schwann proposed that all cells arise from pre-existing cells.
→ Schleiden and Schwann formulated the cell theory that all organisms are composed of cells, but they did not explain how new cells form. Rudolf Virchow (1855) added the principle 'Omnis cellula-e cellula' (all cells arise from pre-existing cells).
✓ Schleiden and Schwann proposed that all living organisms are composed of cells. Rudolf Virchow later modified the theory to state that all cells arise from pre-existing cells.
⚠ Mixing Up 70S and 80S Ribosomes
✗ Prokaryotic ribosomes are 80S and eukaryotic ribosomes are 70S.
→ This is the exact opposite. Prokaryotic ribosomes are 70S (50S + 30S subunits), while eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits). However, mitochondria and chloroplasts have 70S ribosomes.
✓ Prokaryotic ribosomes are 70S (50S + 30S). Eukaryotic cytoplasmic ribosomes are 80S (60S + 40S), but mitochondria and chloroplasts contain 70S ribosomes.
⚠ Assuming All Cells Have a Nucleus
✗ All living cells have a well-defined nucleus.
→ Prokaryotic cells (bacteria, blue-green algae, mycoplasma) lack a membrane-bound nucleus. Also, some mature eukaryotic cells like mammalian erythrocytes and sieve tube cells of vascular plants lose their nucleus.
✓ Eukaryotic cells have a membrane-bound nucleus; prokaryotic cells lack a nucleus. Some mature eukaryotic cells (e.g., mammalian RBCs) are enucleated.
⚠ Calling the Cell Wall 'Living'
✗ The cell wall is a living structure that protects the cell.
→ The cell wall is explicitly described in NCERT as a 'non-living rigid structure'. It is secreted by the cell but is not metabolically active.
✓ The cell wall is a non-living rigid structure that provides shape, protection, and mechanical support to the cell.
⚠ Including Mitochondria in the Endomembrane System
✗ The endomembrane system includes the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and mitochondria.
→ NCERT explicitly states that mitochondria, chloroplasts, and peroxisomes are NOT part of the endomembrane system because their functions are not coordinated with the ER, Golgi, lysosomes, and vacuoles.
✓ The endomembrane system includes the endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles. Mitochondria, chloroplasts, and peroxisomes are not part of this system.
⚠ Confusing Prokaryotic and Eukaryotic Flagella
✗ Both prokaryotic and eukaryotic flagella have the same 9+2 microtubule arrangement.
→ Prokaryotic flagella are structurally different: they are composed of three parts (filament, hook, basal body) and do not have a 9+2 arrangement. The 9+2 array is unique to eukaryotic cilia and flagella.
✓ Eukaryotic flagella have a 9+2 microtubule arrangement in the axoneme. Prokaryotic flagella are composed of filament, hook, and basal body, and lack the 9+2 structure.
Compare & Contrast
Presence of membrane-bound nucleus and organelles
Prokaryotic Cell
Eukaryotic Cell
Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles; eukaryotic cells have a true nucleus and membrane-bound organelles like mitochondria, ER, and Golgi.
Presence of ribosomes and function
Rough Endoplasmic Reticulum (RER)
Smooth Endoplasmic Reticulum (SER)
RER has ribosomes attached to its outer surface and is involved in protein synthesis and secretion; SER lacks ribosomes and is the major site for lipid synthesis and steroid hormone production.
Function and occurrence
Mitochondrion
Chloroplast
Mitochondria are found in all eukaryotic cells and are the sites of aerobic respiration (ATP synthesis); chloroplasts are found only in plant cells and euglenoids and are the sites of photosynthesis.
Length, number, and function
Cilia
Flagella
Cilia are short, numerous, and work like oars to move the cell or surrounding fluid; flagella are long, few (usually 1-2 per cell), and are responsible for cell movement.
Exam Focus
Marks pattern: Typically 2 MCQs (1 mark each), 1 short answer of 2-3 marks, and 1 long answer of 5 marks. Total approximately 9-10 marks.
Most asked: Cell theory and its contributors (Schleiden, Schwann, Virchow) · Differences between prokaryotic and eukaryotic cells · Structure and functions of mitochondria and chloroplasts · Fluid mosaic model of plasma membrane · Endomembrane system (ER, Golgi, lysosomes, vacuoles) · Types of chromosomes based on centromere position
Don't skip: Draw a labelled diagram of a mitochondrion or chloroplast and explain its structure and functions (5 marks).; Describe the fluid mosaic model of the plasma membrane (3 marks).; Differentiate between prokaryotic and eukaryotic cells (3 marks).; Explain the endomembrane system with the functions of each component (5 marks).
One-Line Revision (read aloud before the exam)
  1. Cell is the fundamental structural and functional unit of all living organisms.
  2. Cell theory states that all living organisms are composed of cells and all cells arise from pre-existing cells.
  3. Prokaryotic cells lack a membrane-bound nucleus and organelles; eukaryotic cells have a true nucleus and membrane-bound organelles.
  4. The plasma membrane is a selectively permeable phospholipid bilayer described by the fluid mosaic model (Singer and Nicolson, 1972).
  5. Cell wall is a non-living rigid structure found in plants, fungi, and some prokaryotes; it is made of cellulose, hemicellulose, pectins, and proteins in plants.
  6. The endomembrane system includes the endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.
  7. Rough ER has ribosomes and synthesises proteins; smooth ER synthesises lipids and steroids.
  8. Mitochondria are double-membrane-bound organelles with cristae and are called the 'power houses of the cell'.
  9. Chloroplasts contain thylakoids (grana) and stroma; they are the sites of photosynthesis.
  10. Ribosomes are non-membrane-bound structures composed of RNA and proteins; prokaryotic ribosomes are 70S, eukaryotic ribosomes are 80S.
  11. Cilia and flagella have a 9+2 arrangement of microtubules in their axoneme.
  12. The nucleus contains chromatin, nucleolus, and is surrounded by a double nuclear envelope with pores.
  13. Chromosomes are classified as metacentric, sub-metacentric, acrocentric, or telocentric based on centromere position.
  14. Centrosome in animal cells contains two centrioles perpendicular to each other and helps in spindle formation.
  15. Prokaryotes have mesosomes (infoldings of plasma membrane) and inclusion bodies (reserve materials).
  16. Plasmids are small circular DNA in bacteria that confer traits like antibiotic resistance.

MCQs — 10 Questions with Answers

Q1. Which scientist first observed and described a living cell under the microscope?

  • A. Antonie Von Leeuwenhoek ✓
  • B. Robert Brown
  • C. Matthias Schleiden
  • D. Theodore Schwann

Answer: A — Antonie Von Leeuwenhoek was the first person to see and describe a live cell using the microscope.

Q2. The cell theory states that all cells arise from pre-existing cells. Who first proposed this concept?

  • A. Matthias Schleiden
  • B. Theodore Schwann
  • C. Rudolf Virchow ✓
  • D. Robert Brown

Answer: C — Rudolf Virchow in 1855 explained that new cells are formed only from pre-existing cells (Omnis cellula-e cellula).

Q3. Which of the following is the smallest known cell?

  • A. Bacterial cell
  • B. Mycoplasma ✓
  • C. Human cheek cell
  • D. Red blood cell

Answer: B — Mycoplasma cells are only 0.3 mm in length and are the smallest known cells, smaller than typical bacteria (3–5 µm).

Q4. Which statement correctly describes the difference between prokaryotic and eukaryotic cells?

  • A. Prokaryotic cells have a nucleus; eukaryotic cells do not
  • B. Prokaryotic cells lack membrane-bound organelles; eukaryotic cells possess them ✓
  • C. Both prokaryotic and eukaryotic cells have cell walls
  • D. Prokaryotic cells are larger than eukaryotic cells

Answer: B — Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, while eukaryotic cells have both.

Q5. Ribosomes are found in which of the following?

  • A. Only in eukaryotic cells
  • B. Only in prokaryotic cells
  • C. In both prokaryotic and eukaryotic cells ✓
  • D. Only in the nucleus of eukaryotic cells

Answer: C — Ribosomes are non-membrane-bound organelles present in all cells (both prokaryotic and eukaryotic) for protein synthesis.

Q6. Which of the following is NOT a membrane-bound organelle found in eukaryotic cells?

  • A. Mitochondria
  • B. Ribosome ✓
  • C. Golgi complex
  • D. Lysosome

Answer: B — Ribosomes are non-membrane-bound organelles; mitochondria, Golgi complex, and lysosomes are all membrane-bound structures.

Q7. A scientist observes a cell under an electron microscope and identifies a membrane-bound nucleus containing chromosomes. Which type of cell is it most likely to be?

  • A. Prokaryotic cell
  • B. Bacterial cell
  • C. Eukaryotic cell ✓
  • D. Mycoplasma cell

Answer: C — The presence of a membrane-bound nucleus containing chromosomes is the defining characteristic of eukaryotic cells.

Q8. The cell theory explains both the unity and diversity of living organisms. Which of the following best represents this concept? (i) All organisms are composed of cells. (ii) Cell shape and size vary based on function. (iii) All cells lack a nucleus.

  • A. Only (i) represents unity; (ii) represents diversity ✓
  • B. Only (ii) represents unity; (i) represents diversity
  • C. (i) and (iii) together explain cell theory
  • D. All three statements together explain cell theory

Answer: A — Statement (i) shows unity (all organisms have cells), statement (ii) shows diversity (cells vary in shape/size/function), and statement (iii) is incorrect (eukaryotic cells have nuclei).

Q9. If a unicellular organism is 0.5 mm in length and a bacterial cell is 4 µm in length, which cell is larger? (Hint: 1 mm = 1000 µm)

  • A. The unicellular organism at 0.5 mm ✓
  • B. The bacterial cell at 4 µm
  • C. Both are equal in size
  • D. Cannot be determined without additional information

Answer: A — 0.5 mm = 500 µm, which is 125 times larger than 4 µm, so the unicellular organism is significantly larger.

Q10. Consider the statement: 'The presence of a cell wall is a unique characteristic of all prokaryotic cells.' Evaluate this statement based on cell theory and observed cell structures.

  • A. True; all prokaryotic cells have cell walls
  • B. False; cell walls are found in plant cells and some prokaryotes, but not all prokaryotes have cell walls ✓
  • C. True; Theodore Schwann proved this experimentally
  • D. False; cell walls are found only in eukaryotic plant cells

Answer: B — While many prokaryotes have cell walls, not all prokaryotes possess them (e.g., some bacteria lack cell walls), and cell walls are also present in eukaryotic plant cells, making it a characteristic of some but not all prokaryotes.

Flashcards

What is the cell theory?

All living organisms are composed of cells and products of cells; all cells arise from pre-existing cells.

What is the fundamental structural and functional unit of life?

The cell, because unicellular organisms can survive independently and perform all life functions.

What is the key difference between prokaryotic and eukaryotic cells?

Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, while eukaryotic cells possess both.

Who first discovered the nucleus in a cell?

Robert Brown discovered the nucleus during his microscopic observations of plant cells.

What is cytoplasm and what happens there?

Cytoplasm is a semi-fluid matrix inside the cell where various chemical reactions occur to maintain the living state.

Name the smallest known cells.

Mycoplasmas, which are only 0.3 mm in length, are the smallest known cells.

What are ribosomes and which cells contain them?

Ribosomes are non-membrane-bound organelles found in all cells—both prokaryotic and eukaryotic—for protein synthesis.

Who proposed that cell wall is unique to plant cells?

Theodore Schwann identified and proposed that the cell wall is a distinguishing feature of plant cells only.

What is the relationship between cell shape and cell function?

Cell shape varies with the function it performs; for example, nerve cells are elongated for signal transmission.

What does Virchow's contribution 'Omnis cellula-e cellula' mean?

It means 'every cell comes from a cell,' explaining that new cells are formed only from pre-existing cells.

Important Board Questions

Define a cell and state two reasons why it is called the fundamental unit of life. [2 marks]

Cell is the smallest living unit; unicellular organisms can survive independently and perform all life functions, proving no smaller structure can sustain life alone.

Matthias Schleiden and Theodore Schwann separately studied plant and animal cells and formulated the cell theory. Explain how their observations led to the conclusion that 'all living organisms are composed of cells.' Also, state what Rudolf Virchow added to complete the cell theory. [5 marks]

Schleiden observed all plants have cells forming tissues; Schwann observed all animals have cells and identified the plasma membrane; together they proposed cellular composition. Virchow added that new cells arise only from pre-existing cells (Omnis cellula-e cellula), explaining cell reproduction.

With the help of a labeled diagram, describe the structure of a generalized eukaryotic cell and explain how the presence of membrane-bound organelles distinguishes it from a prokaryotic cell. Discuss the role of cytoplasm in maintaining the 'living state' of the cell and how the reductionist approach helps us understand cellular functions. [6 marks]

Draw a typical animal or plant cell showing nucleus, cell membrane, cytoplasm, and organelles (ER, Golgi, mitochondria, lysosomes); compare to prokaryotic structure lacking nucleus and organelles; explain cytoplasm as site of chemical reactions maintaining life; describe reductionist biology as applying physics/chemistry to understand molecular basis of physiological processes like digestion and respiration.

Assertion-Reason Questions for Cell: The Unit of Life

Q1. Assertion (A):

Prokaryotic cells are considered living organisms despite lacking a membrane-bound nucleus.

Reason (R):

Prokaryotic cells can independently exist and perform all essential functions of life, meeting the definition of a cell as the fundamental unit of life.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why prokaryotic cells are considered living units despite lacking a nucleus. The presence of a nucleus is not a requirement for life; the ability to independently exist and perform vital functions is the criterion.

Q2. Assertion (A):

Animal cells lack a cell wall because they do not require structural support like plant cells do.

Reason (R):

Theodore Schwann observed that plant cells have a distinct cell wall while animal cells have only a plasma membrane as their outer boundary.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both statements are true, but the reason does not explain why animal cells lack a cell wall. Schwann's observation is a descriptive statement of the structural difference, not an explanation of the functional or evolutionary reason for this difference. The assertion provides a functional reasoning that the reason does not support.

Q3. Assertion (A):

Ribosomes are considered membrane-bound organelles found only in eukaryotic cells.

Reason (R):

Ribosomes synthesize proteins and are present in the cytoplasm of eukaryotic cells but not in prokaryotic cells.

Answer: D — A is false but R is true — The assertion is false because ribosomes are non-membrane-bound organelles, and they are found in both eukaryotic and prokaryotic cells. The reason is also misleading because it incorrectly states that ribosomes are absent in prokaryotic cells. Both the assertion and reason are false.

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Case-Based Questions (CBQ) for Cell: The Unit of Life

Case Study:

Dr. Sharma, a microbiologist at a research institute in Delhi, is investigating antibiotic resistance in bacterial cells. She compares the cellular structures of two bacterial species: Species A (0.5 µm in length) and Species B (3 µm in length). Both species lack a membrane-bound nucleus but contain ribosomes in their cytoplasm. She notes that Species A has a very thin cell wall and minimal cytoplasmic content, while Species B has a more substantial cell wall and greater cytoplasmic volume. When exposed to the same antibiotic, Species A shows significantly higher survival rates than Species B, despite its smaller size.

Q1. Based on the cell structures described, which classification correctly identifies both species?

  • A. Both are eukaryotic cells because they contain ribosomes
  • B. Both are prokaryotic cells because they lack a membrane-bound nucleus ✓
  • C. Species A is prokaryotic and Species B is eukaryotic based on their size difference
  • D. Species A is eukaryotic due to its small size; Species B is prokaryotic

Answer: B

Q2. Why might Species A (the smaller bacterium) show higher antibiotic resistance despite its smaller cytoplasmic volume?

  • A. Smaller cells cannot be penetrated by antibiotic molecules
  • B. The thin cell wall of Species A allows efficient entry and rapid elimination of antibiotics
  • C. Species A likely has genetic mutations or structural adaptations that provide resistance, independent of its size ✓
  • D. Smaller cells have less genetic material to be damaged by antibiotics

Answer: C

Q3. Which statement correctly reflects the relationship between cell size and essential life functions in these bacterial species?

  • A. Species B can perform more essential functions because it is larger and has more cytoplasm
  • B. Both species can independently perform all essential life functions despite their size difference, as they meet the definition of cells ✓
  • C. Species A cannot perform photosynthesis due to its small size
  • D. Only Species B can divide and produce new cells

Answer: B

Q4. Dr. Sharma notes that in the cytoplasm of both species, different biochemical pathways occur in different regions. What cellular feature allows prokaryotic cells to perform complex, simultaneous metabolic processes without membrane-bound organelles?

  • A. The presence of ribosomes in strategic locations within the cytoplasm
  • B. Prokaryotic cells cannot perform simultaneous metabolic processes
  • C. The structural organization of the cell wall to separate metabolic processes
  • D. Organized regions within the cytoplasm where specific metabolic enzymes and pathways are concentrated ✓

Answer: D

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