Chapter 8: Cell: The Unit of Life Long Questions | biology Class 11 CBSE

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Chapter 8: Cell: The Unit of Life Long Questions | biology Class 11 CBSE | ByteNotes.in

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Question 1

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Compare the structural organisation of prokaryotic and eukaryotic cells, highlighting the key differences in their cellular components.

Prokaryotic cells lack a membrane-bound nucleus; their genetic material is naked circular DNA (nucleoid) lying free in the cytoplasm, whereas eukaryotic cells have a well-organised nucleus enclosed by a double-layered nuclear envelope.
Eukaryotic cells possess membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes; prokaryotic cells lack all such organelles.
Ribosomes in prokaryotes are 70S (50S + 30S), while in eukaryotes they are 80S (60S + 40S); both serve as sites of protein synthesis.
Prokaryotes often have additional small circular DNA called plasmids and unique structures called mesosomes (infoldings of plasma membrane), which are absent in eukaryotes.
Eukaryotic genetic material is organised into chromosomes associated with histone proteins; prokaryotic DNA is not associated with histones and is therefore called naked.
Prokaryotic cells are generally smaller (1–2 µm) and multiply more rapidly; eukaryotic cells are larger (10–20 µm) and have complex cytoskeletal structures and locomotory apparatus.

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Question 2

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Describe the structure and functions of the mitochondrion. Why is it considered a semi-autonomous organelle?

Mitochondria are double membrane-bound organelles, typically sausage-shaped or cylindrical, with a diameter of 0.2–1.0 µm and length 1.0–4.1 µm; their number per cell varies with physiological activity.
The outer membrane forms a smooth continuous boundary; the inner membrane folds inward to form cristae, increasing the surface area available for enzyme activity.
The two membranes divide the lumen into two aqueous compartments: the outer compartment (between the membranes) and the inner compartment filled with a dense matrix.
The matrix contains enzymes for aerobic respiration, and mitochondria produce ATP via oxidative phosphorylation — earning the name powerhouse of the cell.
Mitochondria possess their own single circular DNA molecule, a few RNA molecules, and 70S ribosomes capable of independent protein synthesis.
Their ability to divide by binary fission and synthesise some proteins using their own genetic machinery makes them semi-autonomous organelles.

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Question 3

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Explain the structure of the plasma membrane according to the fluid mosaic model and describe the various mechanisms by which molecules are transported across it.

The plasma membrane is mainly composed of a phospholipid bilayer with polar heads facing outward and hydrophobic tails facing inward, along with cholesterol, integral proteins, peripheral proteins, and carbohydrates.
Singer and Nicolson (1972) proposed the fluid mosaic model, in which the quasi-fluid lipid bilayer allows lateral movement of proteins; this mobility is measured as membrane fluidity.
Passive transport moves neutral solutes along their concentration gradient (higher to lower) without energy expenditure, and water moves by osmosis similarly.
Polar molecules cannot cross the nonpolar lipid bilayer by simple diffusion; they require specific carrier proteins for facilitated diffusion, still without ATP consumption.
Active transport moves molecules against their concentration gradient (lower to higher) and is energy-dependent, utilising ATP — the Na+/K+ pump is a classic example.
The fluid nature of the membrane is functionally vital for cell growth, formation of intercellular junctions, secretion, endocytosis, and cell division.

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Question 4

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Describe the structure of a chloroplast and explain how its internal organisation supports its role in photosynthesis.

Chloroplasts are double membrane-bound organelles found in plant mesophyll cells; they are lens-shaped, oval, or discoid with length 5–10 µm and width 2–4 µm, with 20–40 chloroplasts per mesophyll cell.
The inner membrane is less permeable and encloses the stroma, a fluid-filled space containing enzymes required for carbohydrate and protein synthesis.
Flattened membranous sacs called thylakoids are present in the stroma; they are stacked into grana (resembling piles of coins) and interconnected by stroma lamellae.
Chlorophyll and carotenoid pigments are located in the thylakoid membranes of grana, which serve as the site of light-dependent reactions of photosynthesis.
The stroma, containing the Calvin cycle enzymes, is the site of light-independent (dark) reactions where carbohydrates are synthesised.
Like mitochondria, chloroplasts contain their own circular double-stranded DNA and 70S ribosomes (smaller than cytoplasmic 80S ribosomes), confirming their semi-autonomous nature.

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Question 5

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Analyse the endomembrane system of a eukaryotic cell, describing each component and explaining how they function in a coordinated manner.

The endomembrane system includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vacuoles; mitochondria, chloroplasts, and peroxisomes are excluded as their functions are not coordinated with the system.
Rough ER (RER) bears ribosomes and is the site of protein synthesis; proteins synthesised here are packaged into vesicles for delivery. Smooth ER (SER) lacks ribosomes and synthesises lipids and steroidal hormones.
Vesicles from the ER fuse with the cis face of the Golgi apparatus, where proteins are modified (e.g., glycosylation to form glycoproteins/glycolipids) and then released from the trans face as secretory or intracellular vesicles.
Lysosomes are membrane-bound vesicles formed by Golgi packaging; they contain hydrolytic enzymes (lipases, proteases, carbohydrases) active at acidic pH, enabling digestion of all types of macromolecules.
Vacuoles are membrane-bound spaces bounded by the tonoplast; in plant cells they occupy up to 90% of cell volume, storing water, sap, and metabolic waste. Tonoplast facilitates active transport of ions into the vacuole.
The coordinated flow — ER synthesis, Golgi modification, lysosomal digestion, vacuolar storage — represents a cellular conveyor belt ensuring efficient synthesis, processing, transport, and disposal of cellular materials.

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Question 6

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Describe the structure of the nucleus in a non-dividing (interphase) cell. Explain how its structural components relate to its functions in the cell.

The nucleus is enclosed by a nuclear envelope consisting of two parallel membranes separated by a perinuclear space of 10–50 nm; the outer membrane is continuous with the ER and bears ribosomes.
Nuclear pores — formed by fusion of the two nuclear membranes — are passages for bidirectional transport of RNA (out) and proteins (in), linking nuclear gene expression to cytoplasmic protein synthesis.
The nucleoplasm contains chromatin, a network of nucleoprotein fibres composed of DNA, histone proteins, non-histone proteins, and RNA; chromatin condenses into chromosomes during cell division.
Nucleoli (singular: nucleolus) are non-membrane-bound spherical structures in the nucleoplasm; they are the site of active ribosomal RNA synthesis, and cells engaged in heavy protein synthesis have larger, more numerous nucleoli.
The nucleus acts as the genetic and regulatory centre: its DNA carries hereditary information and directs all cellular activities through mRNA synthesis (transcription) and subsequent protein synthesis.
A single human cell contains approximately 2 m of DNA distributed among 46 chromosomes, all compactly packaged as chromatin within the nucleus — highlighting the remarkable organisational role of histone proteins.

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Question 7

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Explain the classification of chromosomes based on the position of the centromere. How does the centromere contribute to chromosome function?

Each chromosome has a primary constriction called the centromere, on either side of which disc-shaped kinetochores are present; the centromere holds the two chromatids of a chromosome together.
Metacentric chromosomes have a centromere in the middle, producing two arms of equal length — forming a V-shape during movement.
Sub-metacentric chromosomes have a centromere slightly off-centre, producing one shorter arm and one longer arm — giving an L-shape.
Acrocentric chromosomes have a centromere very close to one end, creating one extremely short arm and one very long arm — nearly rod-like.
Telocentric chromosomes have a terminal centromere and thus possess essentially one arm only, appearing rod-like.
The kinetochores on the centromere serve as the attachment site for spindle fibres during cell division, ensuring accurate segregation of chromatids to daughter cells, making the centromere functionally indispensable for faithful chromosome distribution.

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Question 8

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Describe the cell envelope of a bacterial (prokaryotic) cell. How do Gram-positive and Gram-negative bacteria differ in this respect?

The bacterial cell envelope is a chemically complex, tightly bound three-layered structure: the outermost glycocalyx, the cell wall in the middle, and the plasma membrane innermost.
The glycocalyx varies in composition and thickness: it may be a loose slime layer or a thick, tough capsule; it protects the bacterium and aids in pathogenicity.
The cell wall gives the cell its shape and provides strong structural support, preventing the bacterium from bursting or collapsing due to osmotic changes.
The plasma membrane is selectively permeable, similar in basic structure to eukaryotic membranes, and interacts with the external environment.
Gram-positive bacteria retain the Gram stain due to their cell envelope composition, while Gram-negative bacteria do not, reflecting fundamental differences in their cell wall structure and chemical composition.
All three layers act together as a single protective unit, and the cell envelope as a whole serves in protection, shape maintenance, interaction with the environment, and permeability control.

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Question 9

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Compare and contrast the structure and function of cilia and flagella in eukaryotic cells. How do they differ from prokaryotic flagella?

Both cilia and flagella are hair-like outgrowths of the cell membrane, containing an axoneme with a 9+2 arrangement: nine peripheral doublets of microtubules and one central pair, all enclosed within the plasma membrane.
Cilia are shorter structures that function like oars, creating movement of either the cell or surrounding fluid; flagella are comparatively longer and primarily responsible for cell locomotion.
The central microtubules are connected by bridges and enclosed by a central sheath; each peripheral doublet is connected to the central sheath by a radial spoke, giving nine radial spokes total.
The peripheral doublets are interconnected by linkers; both cilia and flagella emerge from centriole-like basal bodies at the cell surface.
Prokaryotic flagella are structurally entirely different from eukaryotic flagella; bacterial flagella lack the 9+2 arrangement and are not covered by a plasma membrane — they are composed of flagellin protein.
The eukaryotic flagellum is built from tubulin and is membrane-bound, while the prokaryotic flagellum is a simpler rotating structure made of flagellin, representing a fundamental structural divergence between the two cell types.

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Question 10

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Describe the structure of ribosomes and explain the significance of polysomes in protein synthesis. How do ribosomes in chloroplasts and mitochondria differ from cytoplasmic ribosomes?

Ribosomes are non-membrane-bound granular structures composed of rRNA and proteins, first observed as dense particles by George Palade (1953) under the electron microscope.
Each ribosome has two subunits: eukaryotic 80S ribosomes have 60S (large) and 40S (small) subunits; prokaryotic 70S ribosomes have 50S (large) and 30S (small) subunits.
Ribosomes are found free in the cytoplasm, attached to the outer surface of the rough ER, and within organelles; in prokaryotes they are associated with the plasma membrane.
Multiple ribosomes simultaneously translate a single mRNA molecule, forming a chain called a polysome (polyribosome); this greatly amplifies protein production efficiency from a single mRNA.
Ribosomes within chloroplasts and mitochondria are 70S — smaller than the 80S cytoplasmic ribosomes — and resemble prokaryotic ribosomes, supporting the endosymbiotic theory of organelle evolution.
This functional and structural difference between organellar (70S) and cytoplasmic (80S) ribosomes is evidence for the semi-autonomous nature of mitochondria and chloroplasts.

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Chapter 8: Cell: The Unit of Life | Long Questions

Compare the structural organisation of prokaryotic and eukaryotic cells, highlighting the key differences in their cellular components.

Describe the structure and functions of the mitochondrion. Why is it considered a semi-autonomous organelle?

Explain the structure of the plasma membrane according to the fluid mosaic model and describe the various mechanisms by which molecules are transported across it.

Describe the structure of a chloroplast and explain how its internal organisation supports its role in photosynthesis.

Analyse the endomembrane system of a eukaryotic cell, describing each component and explaining how they function in a coordinated manner.

Describe the structure of the nucleus in a non-dividing (interphase) cell. Explain how its structural components relate to its functions in the cell.

Explain the classification of chromosomes based on the position of the centromere. How does the centromere contribute to chromosome function?

Describe the cell envelope of a bacterial (prokaryotic) cell. How do Gram-positive and Gram-negative bacteria differ in this respect?

Compare and contrast the structure and function of cilia and flagella in eukaryotic cells. How do they differ from prokaryotic flagella?

Describe the structure of ribosomes and explain the significance of polysomes in protein synthesis. How do ribosomes in chloroplasts and mitochondria differ from cytoplasmic ribosomes?

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