Cell the Unit of Life — Class 11 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 11 Biology chapter "Cell the Unit of Life" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 11 Biology chapter "Cell the Unit of Life" — 8 important questions with detailed answers for CBSE board ex…
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NCERT Solutions for Class 11 Biology — Cell the Unit of Life. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Cell the Unit of Life — All 8 Questions Solved
Q1. State the cell theory and explain its significance.
Cell theory has three main postulates:
1. All living organisms are composed of one or more cells.
2. The cell is the fundamental unit of life; it is the basic organizational level of living things.
3. All cells arise from pre-existing cells by cell division.
Significance:
1. Unifies all life forms; shows that cells are common to all organisms.
2. Establishes cell as the smallest unit capable of independent life and growth.
3. Explains reproduction and growth at the cellular level.
4. Forms the foundation of biology; allows study of life at the cellular level.
5. Supports evolution; cells divide and pass on genetic information.
Q2. Differentiate between prokaryotic and eukaryotic cells.
Prokaryotic cells (bacteria, cyanobacteria):
1. Nucleus: Absent; genetic material (DNA) in nucleoid region, not membrane-bound.
2. Organelles: No membrane-bound organelles; ribosomes (70S) present.
3. Size: Generally smaller (0.5-5 μm).
4. Cell wall: Present in most; composed of peptidoglycan.
5. Flagella: Simple, made of protein flagellin; rotate for movement.
6. Reproduction: By binary fission (asexual).
7. Ribosomes: 70S (smaller).
Eukaryotic cells (animals, plants, fungi, protists):
1. Nucleus: Present; genetic material (DNA) enclosed in nuclear membrane.
2. Organelles: Multiple membrane-bound organelles (mitochondria, ER, Golgi, lysosomes, chloroplasts).
3. Size: Generally larger (10-100 μm).
4. Cell wall: Present in plants (cellulose) and fungi (chitin); absent in animals.
5. Flagella: Complex, made of microtubules; beat in undulating motion.
6. Reproduction: By mitosis and meiosis (asexual and sexual).
7. Ribosomes: 80S (larger).
Q3. Describe the structure and function of the cell membrane (plasma membrane).
The cell membrane (plasma membrane) is a selectively permeable barrier separating the cell from its environment. Structure (Fluid Mosaic Model):
1. Phospholipid bilayer:
- Two layers of phospholipid molecules arranged tail-to-tail.
- Hydrophobic fatty acid tails face inward; hydrophilic phosphate heads face outward.
- Forms basic structural framework; provides barrier.
2. Proteins:
- Integral proteins: Span entire membrane; transport substances, receive signals.
- Peripheral proteins: Attached to outer or inner surface; provide structural support, enzyme activity.
3. Cholesterol:
- Fills gaps in bilayer; maintains fluidity and membrane integrity.
4. Carbohydrates:
- Form glycoproteins and glycolipids on outer surface.
- Involved in cell recognition, immune response.
Functions:
1. Protection: Encloses and protects cell contents.
2. Selective permeability: Controls entry and exit of substances.
3. Transport: Facilitates diffusion, osmosis, active transport, endocytosis, exocytosis.
4. Communication: Receives chemical signals from other cells.
5. Recognition: Identifies self and non-self cells.
6. Support: Provides structural integrity.
Q4. What is the nucleus? Describe its structure and functions.
The nucleus is a membrane-bound organelle containing genetic information (DNA). It controls cell activities and inheritance. Structure:
1. Nuclear envelope (nuclear membrane):
- Double membrane (outer and inner lipid bilayers).
- Perforated by nuclear pores (allow selective transport of large molecules and RNA).
- Connected to endoplasmic reticulum.
2. Nucleoplasm (nucleosol):
- Gel-like matrix inside nucleus.
- Contains DNA, proteins, and RNA.
3. Chromatin:
- Complex of DNA and histone proteins; appears as loosely coiled threads during interphase.
- Condenses into visible chromosomes during cell division.
- Contains genes coding for proteins and traits.
4. Nucleolus:
- Dense, darkly staining region inside nucleus.
- Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.
- Not membrane-bound.
Functions:
1. Controls cell activities by regulating gene expression (transcription and translation).
2. Stores and maintains genetic information (DNA).
3. Directs protein synthesis through mRNA production.
4. Enables cell division by replicating DNA.
5. Passes genetic information to daughter cells during reproduction.
Q5. Describe the structure and function of mitochondria.
Mitochondria are double-membrane bound organelles responsible for energy production. Structure:
1. Outer membrane:
- Smooth, permeable to most molecules.
- Surrounds entire organelle.
2. Intermembrane space:
- Region between outer and inner membrane.
- Contains enzymes and cytochrome c.
3. Inner membrane:
- Highly folded into cristae (singular: crista).
- Impermeable to many molecules; selectively permeable.
- Contains enzymes of electron transport chain.
4. Matrix (stroma):
- Inner compartment enclosed by inner membrane.
- Contains enzymes of citric acid cycle (Krebs cycle), pyruvate dehydrogenase, fatty acid oxidation enzymes.
- Contains mitochondrial DNA (mtDNA) and ribosomes (70S).
Functions:
1. ATP production: Site of oxidative phosphorylation and energy production (primary function).
2. Citric acid cycle: Matrix contains enzymes for Krebs cycle.
3. Fatty acid oxidation: β-oxidation of fatty acids occurs here.
4. Calcium regulation: Stores and releases calcium ions.
5. Heat production (in brown fat): Uncoupling protein allows heat generation.
6. Metabolism: Participates in amino acid metabolism, steroid synthesis.
Note: Mitochondrial DNA shows maternal inheritance; carries genes for mitochondrial proteins.
Q6. What is the endoplasmic reticulum? Distinguish between rough and smooth ER.
Endoplasmic reticulum (ER) is an extensive network of membranous tubules and sacs continuous with the nuclear envelope and cell membrane. It is the site of synthesis and transport of molecules.
Rough endoplasmic reticulum (RER):
1. Studded with ribosomes on outer (cytoplasmic) surface; gives rough appearance.
2. Functions:
- Synthesis of proteins destined for secretion (secretory proteins).
- Synthesis of membrane proteins and organellar proteins.
- Synthesis of enzymes for lysosomes.
3. Found in cells active in protein synthesis: pancreatic cells, plasma cells, hepatocytes.
Smooth endoplasmic reticulum (SER):
1. Lacks ribosomes; appears smooth.
2. Functions:
- Synthesis of lipids (phospholipids, steroids, cholesterol).
- Detoxification of harmful substances (in liver cells).
- Synthesis of carbohydrates.
- Storage of calcium ions (in muscle cells).
3. Found in cells active in lipid synthesis: liver cells, steroid-producing cells, muscle cells.
Common functions:
- Transport of newly synthesized proteins and lipids to other organelles.
- Modification of proteins (limited).
- Communication between nucleus and cytoplasm.
Q7. Describe the Golgi apparatus (Golgi body) structure and functions (5-mark answer).
The Golgi apparatus (Golgi body) is a membrane-bound organelle involved in modification, packaging, and secretion of molecules. Structure:
1. Morphology:
- Stack of flattened membranous sacs called cisternae (singular: cisterna).
- Usually 4-8 cisternae per stack.
- Multiple stacks in most cells (called dictyosomes in plants).
2. Regions:
a) Cis face (entry face): Receives products from ER; near nuclear envelope.
b) Trans face (exit face): Releases processed products; toward cell membrane.
c) Medial cisternae: Middle layers; site of main modifications.
3. Associated structures:
- Transport vesicles: Small membrane-bound sacs carry molecules between ER and Golgi and from Golgi to destinations.
- ERGIC (ER-Golgi intermediate compartment): Intermediate station between ER and Golgi.
Functions:
1. Modification of proteins:
- Glycosylation: Addition of carbohydrate groups to proteins (forming glycoproteins).
- Phosphorylation: Addition of phosphate groups.
- Sulfation: Addition of sulfate groups.
- Partial proteolysis: Breaking peptide bonds.
2. Synthesis and processing of polysaccharides:
- Synthesis of complex carbohydrates.
- Assembly of cellulose in plant cells.
- Production of pectin and mucilage.
3. Packaging and sorting:
- Packages modified molecules into vesicles for transport to final destinations.
- Directs proteins to lysosomes, cell membrane, or secretion.
4. Formation of lysosomes:
- Packages hydrolytic enzymes with mannose-6-phosphate markers into vesicles that become lysosomes.
5. Secretion:
- Packages secretory proteins into vesicles for exocytosis and release from cell.
Significance: Central hub for post-translational modification and protein trafficking; essential for cellular secretion and organization.
Q8. Define lysosomes and explain their role in cellular digestion and autophagy.
Lysosomes are small, membrane-bound organelles containing digestive enzymes. They are the cell's digestive/waste disposal system. Structure:
- Single-membrane bound sacs.
- Contain 40+ hydrolytic enzymes (acid hydrolases, proteases, lipases, nucleases, phosphatases).
- pH inside is acidic (4.8); maintains enzyme activity and denatures engulfed materials.
- Discovered by de Duve; called "suicide bags."
Functions:
1. Intracellular digestion:
- Digest materials brought into cell by endocytosis (bacteria, pathogens, food particles).
- Enzymes break down proteins, lipids, carbohydrates, nucleic acids.
2. Autophagy (cell cannibalism):
- Digestion of old or damaged cellular organelles.
- Damaged mitochondria, ER, ribosomes engulfed by autophagosomes (double-membrane vesicles).
- Autophagosome fuses with lysosome forming autolysosome.
- Contents are broken down; useful molecules recycled.
- Maintains cellular health; removes non-functional components.
3. Apoptosis (programmed cell death):
- Lysosomes release enzymes into cytoplasm.
- Enzymes break down nuclear DNA and cellular components.
- Cell dies in controlled manner without releasing harmful contents.
4. Phagocytosis:
- Lysosomes fuse with phagosomes (endocytic vesicles containing bacteria/pathogens).
- Enzymes kill and digest pathogens; protects against infection.
5. Bone resorption:
- Osteoclasts use lysosomes to dissolve bone matrix.
Significance: Essential for cellular cleanup, recycling, and maintaining cellular homeostasis. Without lysosomes, cells accumulate toxic byproducts and damaged organelles.
Cell the Unit of Life — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Who was the first scientist to observe living cells, which he termed 'animalcules'?
- A. Robert Hooke
- B. Anton von Leeuwenhoek
- C. Matthias Schleiden
- D. Theodor Schwann
Answer: B. Anton von Leeuwenhoek — Anton von Leeuwenhoek was the first to observe and describe living cells, including bacteria and protozoa, referring to them as 'animalcules'. Robert Hooke observed dead cork cells.
MCQ 2. Which of the following statements is a key postulate of the modern cell theory, as given by Rudolf Virchow?
- A. All cells contain a nucleus.
- B. All organisms are composed of cells and cell products.
- C. Cells arise from pre-existing cells.
- D. Cells are the basic unit of heredity.
Answer: C. Cells arise from pre-existing cells. — Rudolf Virchow proposed 'Omnis cellula-e cellula', which means all cells arise from pre-existing cells, a fundamental principle of modern cell theory.
MCQ 3. The cell wall of fungi is primarily composed of which complex polysaccharide?
- A. Cellulose
- B. Peptidoglycan
- C. Chitin
- D. Lignin
Answer: C. Chitin — The cell wall of fungi is primarily composed of chitin, while plant cell walls are made of cellulose and bacterial cell walls are made of peptidoglycan.
MCQ 4. Which of the following is NOT a part of the endomembrane system?
- A. Endoplasmic Reticulum
- B. Golgi apparatus
- C. Mitochondria
- D. Lysosomes
Answer: C. Mitochondria — The endomembrane system includes the Endoplasmic Reticulum, Golgi apparatus, lysosomes, and vacuoles, which function in a coordinated manner. Mitochondria operate independently and are not part of this system.
MCQ 5. In prokaryotic cells, the genetic material is concentrated in a region known as the:
- A. Nucleus
- B. Nucleoid
- C. Plasmid
- D. Cytosol
Answer: B. Nucleoid — Prokaryotic cells lack a true nucleus. Their genetic material (a single circular chromosome) is located in a region called the nucleoid, which is not membrane-bound.
MCQ 6. The 'fluid mosaic model' of the plasma membrane was proposed by:
- A. Schleiden and Schwann
- B. Robert Hooke
- C. Singer and Nicolson
- D. Rudolf Virchow
Answer: C. Singer and Nicolson — The fluid mosaic model, which describes the plasma membrane as a fluid structure with proteins embedded or associated with a lipid bilayer, was proposed by S.J. Singer and G.L. Nicolson in 1972.
MCQ 7. Which of the following organelles is responsible for the synthesis of lipids and detoxification of drugs?
- A. Rough Endoplasmic Reticulum
- B. Smooth Endoplasmic Reticulum
- C. Golgi apparatus
- D. Ribosomes
Answer: B. Smooth Endoplasmic Reticulum — The Smooth Endoplasmic Reticulum (SER) is involved in lipid synthesis, steroid hormone synthesis, and detoxification of drugs and poisons, as it lacks ribosomes.
MCQ 8. The stacks of flattened membrane-bound sacs inside chloroplasts are called:
- A. Cristae
- B. Cisternae
- C. Thylakoids
- D. Grana
Answer: D. Grana — Inside chloroplasts, the thylakoids are arranged in stacks called grana (singular: granum). Cisternae are found in the Golgi apparatus, and cristae are in mitochondria.
MCQ 9. Lysosomes are known as 'suicidal bags' due to the presence of:
- A. Oxidative enzymes
- B. Photosynthetic pigments
- C. Hydrolytic enzymes
- D. Synthesizing enzymes
Answer: C. Hydrolytic enzymes — Lysosomes contain a variety of powerful hydrolytic enzymes that can digest almost all types of organic matter. If a lysosome ruptures, these enzymes can digest the cell itself, hence the name 'suicidal bags'.
MCQ 10. Mycoplasma are unique prokaryotes because they:
- A. Lack a cell wall
- B. Are obligate anaerobes
- C. Are photosynthetic
- D. Have a nucleus
Answer: A. Lack a cell wall — Mycoplasma are the smallest known living cells and are unique among prokaryotes for completely lacking a cell wall, making them pleomorphic (variable in shape).
MCQ 11. The 9+2 arrangement of microtubules is characteristic of which structures?
- A. Centrioles
- B. Cilia and Flagella
- C. Microfilaments
- D. Ribosomes
Answer: B. Cilia and Flagella — Cilia and flagella, involved in cell motility, exhibit a characteristic 9+2 arrangement of microtubules in their axoneme. Centrioles have a 9+0 arrangement.
MCQ 12. Which of the following is true for both mitochondria and chloroplasts?
- A. They are part of the endomembrane system.
- B. They contain their own DNA and ribosomes.
- C. They are found in prokaryotic cells.
- D. They are involved in protein synthesis only.
Answer: B. They contain their own DNA and ribosomes. — Both mitochondria and chloroplasts are semi-autonomous organelles, meaning they possess their own circular DNA and 70S ribosomes, allowing them to synthesize some of their own proteins.
MCQ 13. The primary function of the nucleolus is:
- A. DNA replication
- B. Synthesis of ribosomal RNA (rRNA)
- C. Protein synthesis
- D. Storage of genetic material
Answer: B. Synthesis of ribosomal RNA (rRNA) — The nucleolus is a non-membrane-bound structure within the nucleus primarily involved in the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomal subunits.
MCQ 14. Which type of plastid is involved in storing oils and fats?
- A. Chromoplasts
- B. Chloroplasts
- C. Amyloplasts
- D. Elaioplasts
Answer: D. Elaioplasts — Leucoplasts are colourless plastids that store various nutrients. Amyloplasts store carbohydrates, aleuroplasts store proteins, and elaioplasts store oils and fats.
MCQ 15. The inner mitochondrial membrane is folded into structures called:
- A. Thylakoids
- B. Cisternae
- C. Cristae
- D. Grana
Answer: C. Cristae — The inner mitochondrial membrane folds into several infoldings called cristae. These cristae increase the surface area for the enzymatic reactions of cellular respiration.
MCQ 16. Which of the following is NOT a characteristic of prokaryotic cells?
- A. Presence of a cell wall
- B. Presence of membrane-bound organelles
- C. Lack of a true nucleus
- D. Presence of ribosomes
Answer: B. Presence of membrane-bound organelles — Prokaryotic cells are characterized by the absence of membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. They do have ribosomes.
MCQ 17. The main function of the Golgi apparatus is:
- A. Protein synthesis
- B. ATP production
- C. Packaging and modification of proteins and lipids
- D. Digestion of cellular waste
Answer: C. Packaging and modification of proteins and lipids — The Golgi apparatus is primarily involved in the packaging of materials to be delivered either to the intracellular targets or secreted outside the cell, along with modification of proteins and lipids.
MCQ 18. What is the primary component of the cell wall in most plants?
- A. Chitin
- B. Peptidoglycan
- C. Cellulose
- D. Pectin
Answer: C. Cellulose — The plant cell wall is a rigid structure outside the plasma membrane, primarily composed of cellulose, hemicellulose, pectins, and proteins.
MCQ 19. The smallest cell capable of independent existence and proliferation is:
- A. Human red blood cell
- B. Bacterium (E. coli)
- C. Mycoplasma
- D. Yeast cell
Answer: C. Mycoplasma — Mycoplasma are the smallest known living cells and are capable of independent existence, unlike viruses which are obligate intracellular parasites.
MCQ 20. Which type of ribosome is found in eukaryotic cytoplasm?
- A. 50S
- B. 70S
- C. 80S
- D. 30S
Answer: C. 80S — Eukaryotic ribosomes are 80S type, composed of 60S and 40S subunits. Prokaryotic ribosomes are 70S type (50S and 30S subunits).
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.