NCERT Solutions for Class 11 Biology Chapter 8: Cell – The Unit of Life (NCERT 2026–27)
These Class 11 Biology Chapter 8 solutions cover Cell: The Unit of Life from Unit 3 of the NCERT textbook (session 2026–27). The chapter establishes the cell as the fundamental structural and functional unit of all living organisms, traces the development of the cell theory, and explores the detailed structure and functions of prokaryotic and eukaryotic cells and their organelles. Below you will find every NCERT “Exercises” question reproduced word-for-word and solved in clear, exam-ready prose, plus extra practice, MCQs, assertion–reason items and FAQs.
Class: 11Subject: BiologyUnit: 3 — Cell: Structure and FunctionsChapter: 8Chapter Name: Cell: The Unit of LifeSession: 2026–27
Chapter 8, Cell: The Unit of Life, is the first chapter of Unit 3 and forms the foundation for cell biology in Grades 11 and 12. It begins by defining the cell as the basic unit of life and credits Antonie van Leeuwenhoek with first seeing a live cell and Robert Brown with discovering the nucleus. The cell theory — built by Schleiden, Schwann and finalised by Virchow (Omnis cellula-e cellula) — states that all organisms are made of cells and that all cells arise from pre-existing cells. The chapter then contrasts prokaryotic cells (bacteria, cyanobacteria, mycoplasma, PPLO — no membrane-bound nucleus or organelles, 70S ribosomes, cell envelope, mesosomes, plasmids) with eukaryotic cells, and describes each organelle: plasma membrane (fluid mosaic model), cell wall, the endomembrane system (ER, Golgi, lysosomes, vacuoles), mitochondria, plastids, ribosomes, cytoskeleton, cilia and flagella, centrosome and centrioles, and the nucleus with its chromatin and chromosomes. Its big idea is that the cell is both the structural and functional unit of life.
Key Concepts & Definitions
Cell: the fundamental structural and functional unit of all living organisms; unicellular organisms can live independently, while multicellular organisms are made of many cells.
Cell theory: (i) all living organisms are composed of cells and products of cells; (ii) all cells arise from pre-existing cells. Schleiden and Schwann proposed it; Virchow added cell division.
Prokaryotic cell: a cell lacking a membrane-bound nucleus and membrane-bound organelles (e.g., bacteria, cyanobacteria). Has 70S ribosomes, a nucleoid, often plasmids, a cell envelope and mesosomes.
Eukaryotic cell: a cell with an organised, membrane-bound nucleus and membrane-bound organelles, 80S cytoplasmic ribosomes and a cytoskeleton (protists, plants, animals, fungi).
Plasma membrane: selectively permeable bilayer of phospholipids and proteins; best described by the fluid mosaic model (Singer and Nicolson, 1972), allowing passive and active transport.
Endomembrane system: organelles whose functions are coordinated — endoplasmic reticulum, Golgi complex, lysosomes and vacuoles.
Mesosome: infoldings of the plasma membrane in prokaryotes that aid cell wall formation, DNA replication, respiration and secretion.
Centromere & kinetochore: the primary constriction of a chromosome bearing disc-shaped kinetochores; its position classifies chromosomes as metacentric, sub-metacentric, acrocentric or telocentric.
NCERT Exercises — Solutions
All questions below are reproduced verbatim from the NCERT textbook (Chapter 8, “Exercises”). Answers are original, exam-ready and figure questions are answered in words.
1. Which of the following is not correct?
(a) Robert Brown discovered the cell.(b) Schleiden and Schwann formulated the cell theory.(c) Virchow explained that cells are formed from pre-existing cells.(d) A unicellular organism carries out its life activities within a single cell.
ANSWER(a) Robert Brown discovered the cell — this statement is not correct.Robert Brown did not discover the cell; he discovered the nucleus (1831). The cell was first observed by Robert Hooke (in cork) and the first live cell was seen by Antonie van Leeuwenhoek. Statements (b), (c) and (d) are all correct.
2. New cells generate from
(a) bacterial fermentation(b) regeneration of old cells(c) pre-existing cells(d) abiotic materials
ANSWER(c) pre-existing cells.Rudolf Virchow (1855) established that new cells arise only by the division of pre-existing cells — expressed as Omnis cellula-e cellula. Cells are never formed de novo from non-living (abiotic) material.
3. Match the following
Column I — (a) Cristae (b) Cisternae (c) ThylakoidsColumn II — (i) Flat membranous sacs in stroma (ii) Infoldings in mitochondria (iii) Disc-shaped sacs in Golgi apparatus
ANSWER
Column I
Column II
(a) Cristae
(ii) Infoldings in mitochondria
(b) Cisternae
(iii) Disc-shaped sacs in Golgi apparatus
(c) Thylakoids
(i) Flat membranous sacs in stroma
Cristae are the infoldings of the inner mitochondrial membrane that increase surface area; cisternae are the flat, disc-shaped sacs of the Golgi apparatus; thylakoids are the flattened membranous sacs in the stroma of the chloroplast.
4. Which of the following is correct:
(a) Cells of all living organisms have a nucleus.(b) Both animal and plant cells have a well defined cell wall.(c) In prokaryotes, there are no membrane bound organelles.(d) Cells are formed de novo from abiotic materials.
ANSWER(c) In prokaryotes, there are no membrane bound organelles.Prokaryotic cells lack membrane-bound organelles (the only organelle present is the ribosome, which is non-membranous). Option (a) is wrong because prokaryotes lack a true nucleus and some mature eukaryotic cells (mammalian RBCs, sieve tube cells) lack a nucleus; (b) is wrong because animal cells have no cell wall; (d) is wrong because cells arise only from pre-existing cells.
5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
ANSWERA mesosome is a special membranous structure formed by the extensions (infoldings) of the plasma membrane into the cytoplasm of a prokaryotic cell. These extensions occur in the form of vesicles, tubules and lamellae.Functions of mesosomes: (i) they help in the formation of the cell wall; (ii) they assist in DNA replication and its distribution to daughter cells; (iii) they help in respiration; (iv) they help in secretion processes; and (v) they increase the surface area of the plasma membrane and its enzymatic content.
6. How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
ANSWERNeutral solutes move across the plasma membrane by the process of simple diffusion, travelling along the concentration gradient — from a region of higher concentration to a region of lower concentration — without any expenditure of energy. This is a form of passive transport.No, polar molecules cannot move in the same way. Because the membrane has a non-polar (hydrophobic) lipid bilayer interior, polar molecules cannot pass freely through it. They require a carrier (transport) protein of the membrane to facilitate their movement across it. When moved along the concentration gradient with the help of a carrier protein, it is facilitated transport (passive); when ions or molecules are moved against their concentration gradient, energy (ATP) is used and the process is called active transport (e.g., the Na+/K+ pump).
7. Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.
ANSWERThe two double-membrane-bound organelles are the mitochondrion and the chloroplast (plastid).Mitochondrion: sausage-shaped or cylindrical (0.2–1.0 µm diameter, 1.0–4.1 µm length). It has a smooth outer membrane and an inner membrane folded into cristae that project into a dense matrix; the two membranes divide it into an outer and an inner compartment. The matrix contains a single circular DNA molecule, 70S ribosomes and enzymes. Function: it is the site of aerobic respiration and produces energy as ATP, hence is called the “power house” of the cell; mitochondria divide by fission.Chloroplast: a plastid (5–10 µm long, 2–4 µm wide), double membrane bound, with the inner membrane enclosing the stroma. Within the stroma are stacks of flattened sacs called thylakoids arranged in piles (grana) joined by stroma lamellae; thylakoids contain chlorophyll. The stroma has enzymes, small circular DNA and 70S ribosomes. Function: chloroplasts trap light energy for photosynthesis — grana are sites of the light reactions and the stroma is the site of the dark reactions (carbohydrate synthesis).Diagram note (figures cannot be shown here): A labelled mitochondrion should show the outer membrane, inner membrane, cristae, matrix and inter-membrane space; a labelled chloroplast should show the outer and inner membranes, stroma, granum (stack of thylakoids), thylakoid and stroma lamellae.
8. What are the characteristics of prokaryotic cells?
ANSWERThe main characteristics of prokaryotic cells are: (i) they are represented by bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO; (ii) they are generally smaller and multiply more rapidly than eukaryotic cells; (iii) they lack a well-defined, membrane-bound nucleus — the genetic material is naked, circular DNA (a single chromosome) not enclosed by a nuclear membrane; (iv) many possess small circular DNA called plasmids apart from the genomic DNA; (v) they have no membrane-bound organelles, only non-membranous 70S ribosomes; (vi) most have a chemically complex three-layered cell envelope (glycocalyx, cell wall and plasma membrane); (vii) they possess specialised membrane infoldings called mesosomes; and (viii) reserve material is stored in non-membranous inclusion bodies. Motile forms bear flagella made of filament, hook and basal body.
9. Multicellular organisms have division of labour. Explain.
ANSWERDivision of labour means that different groups of cells in a multicellular organism are specialised to perform different specific functions, instead of every cell doing every task. As a result of this specialisation, no single cell carries out all life activities by itself; instead, particular cells become structurally and functionally suited to particular jobs.For example, in our bodies nerve cells conduct impulses, muscle cells bring about movement, red blood cells transport oxygen, and gland cells secrete substances. Specialised cells group together to form tissues, tissues form organs, and organs form organ systems. This organisation makes the body work efficiently as a whole, which is why multicellular organisms show a clear division of labour.
10. Cell is the basic unit of life. Discuss in brief.
ANSWERThe cell is called the basic structural and functional unit of life because every living organism is built from one or more cells and all the activities of life are carried out within cells. A unicellular organism is capable of independent existence and performs all the essential functions of life within a single cell; anything less than a complete cell cannot ensure independent living.According to the cell theory, all organisms are composed of cells and products of cells, and all cells arise from pre-existing cells. Inside each cell, the cytoplasm is the main arena where chemical reactions keep the cell in the “living state”, while organelles perform specialised functions and the nucleus controls them and carries hereditary information. Since no smaller part can independently show all the characteristics of life, the cell is rightly regarded as the basic unit of life.
11. What are nuclear pores? State their function.
ANSWERNuclear pores are minute pores present at a number of places on the nuclear envelope, formed by the fusion of the two parallel membranes (the inner and outer membranes) of the envelope.Function: nuclear pores act as passages that allow the two-way movement of large molecules — mainly RNA and proteins — between the nucleus and the cytoplasm. Thus they regulate the exchange of materials in both directions across the nuclear envelope.
12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
ANSWERAlthough both lysosomes and vacuoles are part of the endomembrane system and are bound by membrane, their functions are clearly different.Lysosomes are membrane-bound vesicles formed by the packaging activity of the Golgi apparatus. They are rich in hydrolytic enzymes (lipases, proteases, carbohydrases) that are optimally active at acidic pH and are capable of digesting carbohydrates, proteins, lipids and nucleic acids. Their main role is therefore intracellular digestion.Vacuoles are membrane-bound spaces (bound by a single membrane, the tonoplast) found in the cytoplasm. They store water, sap, excretory products and other materials, can occupy up to 90 per cent of a plant cell’s volume, and help maintain turgidity. In Amoeba the contractile vacuole performs osmoregulation and excretion, and food vacuoles digest engulfed food in protists. Thus lysosomes are mainly digestive while vacuoles are mainly storage and osmoregulatory structures.
13. Describe the structure of the following with the help of labelled diagrams.
(i) Nucleus(ii) Centrosome
ANSWER(i) Nucleus: The nucleus is bounded by a nuclear envelope made of two parallel membranes separated by a perinuclear space (10–50 nm). The outer membrane is often continuous with the endoplasmic reticulum and bears ribosomes. The envelope is interrupted at places by nuclear pores for the exchange of RNA and proteins. The inner space is filled with the nucleoplasm (nuclear matrix), which contains one or more spherical nucleoli (sites of ribosomal RNA synthesis) and the network of chromatin (DNA + histone and non-histone proteins + RNA). During cell division chromatin condenses into chromosomes. (A labelled diagram should show the nuclear envelope, nuclear pore, nucleoplasm, nucleolus and chromatin.)(ii) Centrosome: The centrosome is an organelle that usually contains two cylindrical structures called centrioles lying perpendicular to each other, surrounded by amorphous pericentriolar material. Each centriole has a cartwheel organisation made of nine evenly spaced peripheral fibrils of tubulin, with each fibril being a triplet; adjacent triplets are linked. The central proteinaceous part is the hub, connected to the peripheral triplets by radial spokes. Centrioles form the basal body of cilia and flagella and produce the spindle fibres during cell division in animal cells. (A labelled diagram should show the two perpendicular centrioles, peripheral triplet fibrils, hub and radial spokes.)
14. What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes.
ANSWERA centromere is the primary constriction of a chromosome on the sides of which lie disc-shaped structures called kinetochores. The centromere holds the two chromatids of a chromosome together. Based on the position of the centromere, chromosomes are classified into four types:
Type
Position of centromere
Arms
Metacentric
In the middle
Two equal arms
Sub-metacentric
Slightly away from the middle
One shorter arm and one longer arm
Acrocentric
Close to one end
One extremely short and one very long arm
Telocentric
At the terminal end
A single (terminal) arm
(A labelled diagram should show a metacentric chromosome with a central centromere, a sub-metacentric with a slightly displaced centromere, an acrocentric with a near-terminal centromere, and a telocentric with a terminal centromere.)
Extra Practice Questions
Short Answer Type Questions
Q1. Differentiate between 70S and 80S ribosomes.
ANSWER70S ribosomes are found in prokaryotes and in mitochondria and chloroplasts; they consist of 50S and 30S subunits. 80S ribosomes are found in the cytoplasm of eukaryotic cells and consist of 60S and 40S subunits. Here “S” (Svedberg unit) is the sedimentation coefficient, an indirect measure of density and size.
Q2. Why are mitochondria called the “power houses” of the cell?
ANSWERMitochondria are the sites of aerobic respiration, where the energy of food is released and stored in the form of ATP. Because this ATP powers nearly all cellular activities, the mitochondrion is called the power house of the cell.
Q3. What is the fluid mosaic model of the plasma membrane?
ANSWERProposed by Singer and Nicolson (1972), the fluid mosaic model describes the plasma membrane as a phospholipid bilayer in which proteins are embedded like a mosaic. The quasi-fluid nature of the lipid allows lateral movement of proteins within the bilayer, a property measured as membrane fluidity, which is important for growth, secretion, endocytosis and cell division.
Q4. What are plasmids and why are they important?
ANSWERPlasmids are small circular DNA molecules present in many bacteria outside the genomic DNA. They confer unique phenotypic characters such as resistance to antibiotics and are used to monitor bacterial transformation with foreign DNA.
Q5. Name the three parts of a bacterial flagellum and state which surface structures do not help in motility.
ANSWERA bacterial flagellum has three parts — the filament, the hook and the basal body. The filament is the longest part. Pili and fimbriae are also surface structures of bacteria but do not help in motility; fimbriae help the bacteria attach to surfaces and host tissues.
Long Answer Type Questions
Q1. Compare prokaryotic and eukaryotic cells.
ANSWERProkaryotic cells (bacteria, cyanobacteria, mycoplasma, PPLO) are generally small and lack a membrane-bound nucleus; their genetic material is a naked circular DNA (nucleoid), and many also have plasmids. They have no membrane-bound organelles — only 70S ribosomes — and possess special structures such as the cell envelope and mesosomes. Eukaryotic cells (protists, plants, animals, fungi) are usually larger and possess a true, membrane-bound nucleus with the genetic material organised into chromosomes. Their cytoplasm is extensively compartmentalised by membrane-bound organelles (ER, Golgi, lysosomes, mitochondria, plastids, etc.), the cytoplasmic ribosomes are 80S, and they have a cytoskeleton. Plant eukaryotic cells additionally have a cell wall, plastids and a large central vacuole, while animal cells have centrioles. Despite these differences, both types share a plasma membrane, cytoplasm, genetic material and ribosomes.
Q2. Describe the endomembrane system and the role of each of its components.
ANSWERThe endomembrane system is a group of membranous organelles whose functions are coordinated: the endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles. The ER is a network of tubules and cisternae; rough ER (RER), studded with ribosomes, synthesises and transports proteins, while smooth ER (SER) synthesises lipids and steroidal hormones. The Golgi apparatus consists of stacked cisternae with a convex cis (forming) face and a concave trans (maturing) face; it packages, modifies and dispatches materials and forms glycoproteins and glycolipids. Lysosomes are vesicles rich in acidic hydrolytic enzymes that digest carbohydrates, proteins, lipids and nucleic acids. Vacuoles are single-membrane (tonoplast) spaces that store water, sap and wastes and, in some cells, carry out osmoregulation. Because mitochondria, chloroplasts and peroxisomes do not have functions coordinated with these, they are not part of the endomembrane system.
Q3. Describe the structure and functions of the chloroplast.
ANSWERChloroplasts are green, pigment-containing plastids found mainly in the mesophyll cells of leaves; they are lens-shaped or oval, 5–10 µm long and 2–4 µm wide. They are double-membrane bound, with the inner membrane being relatively less permeable and enclosing a matrix called the stroma. Suspended in the stroma are flattened membranous sacs called thylakoids, stacked like piles of coins to form grana, with stroma lamellae connecting thylakoids of different grana. Thylakoid membranes contain chlorophyll and carotenoid pigments. The stroma also contains enzymes for the synthesis of carbohydrates and proteins, double-stranded circular DNA and 70S ribosomes. Functions: chloroplasts trap light energy for photosynthesis — the light reactions occur in the grana (thylakoids) and the dark reactions (carbohydrate synthesis) occur in the stroma — making the plant cell self-sufficient in food.
MCQs
1. The first person to observe a live cell was:
(a) Robert Hooke (b) Robert Brown (c) Antonie van Leeuwenhoek (d) Rudolf Virchow
2. “Omnis cellula-e cellula” was given by:
(a) Schleiden (b) Schwann (c) Virchow (d) Brown
3. Which of the following is a non-membrane-bound organelle present in both prokaryotes and eukaryotes?
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: Mitochondria are called the power houses of the cell.
Reason: They are the sites of aerobic respiration and produce ATP.
A-R 3. Assertion: Polar molecules cannot cross the plasma membrane by simple diffusion.
Reason: The interior of the lipid bilayer is non-polar (hydrophobic).
A-R 4. Assertion: The nucleolus is bounded by a double membrane.
Reason: The nucleolus is the site of ribosomal RNA synthesis.
A-R 5. Assertion: Mature mammalian red blood cells lack a nucleus.
Reason: All eukaryotic cells must always possess a nucleus throughout life.
Answer key: 1-(A), 2-(C), 3-(A), 4-(D), 5-(C).
Common Mistakes to Avoid
Watch out for these
Confusing the contributions of scientists — Robert Brown discovered the nucleus, not the cell.
Saying prokaryotes have “no organelles” — they have no membrane-bound organelles, but they do have (non-membranous) ribosomes.
Calling mitochondrial/chloroplast ribosomes 80S — they are 70S, the same as prokaryotes.
Mixing up cristae, cisternae and thylakoids — cristae = mitochondria, cisternae = Golgi, thylakoids = chloroplast.
Treating the nucleolus as membrane-bound — it is a non-membranous structure.
Forgetting that some mature eukaryotic cells (mammalian RBCs, sieve tubes) lack a nucleus.
Confusing the cis (forming) and trans (maturing) faces of the Golgi apparatus.
Exam Tips — how to score full marks
Learn the cell theory and the scientists in order (Leeuwenhoek → Brown → Schleiden → Schwann → Virchow). For organelle questions, always state the structure first and then the function, and label your diagrams clearly — mitochondria and chloroplasts are favourite diagram questions, so practise both. Use precise terms (mesosome, tonoplast, kinetochore, cristae, thylakoid) and quote sizes and ribosome types (70S/80S) where the marks reward detail. For transport questions, clearly distinguish passive transport (simple/facilitated diffusion, osmosis) from active transport (ATP-driven, e.g. the Na+/K+ pump).
Frequently Asked Questions
What is Class 11 Biology Chapter 8 about?
Chapter 8, Cell: The Unit of Life, explains that the cell is the basic structural and functional unit of all living organisms. It covers the cell theory, the differences between prokaryotic and eukaryotic cells, and the structure and functions of all the major cell organelles — the plasma membrane, cell wall, endomembrane system, mitochondria, plastids, ribosomes, cytoskeleton, cilia and flagella, centrosome and the nucleus.
Which two cell organelles are double membrane bound?
The mitochondrion and the chloroplast (plastid) are double-membrane-bound organelles. The mitochondrion is the site of aerobic respiration (ATP production) and the chloroplast traps light energy for photosynthesis. Both also contain their own circular DNA and 70S ribosomes.
What is the difference between a lysosome and a vacuole?
Lysosomes are membrane-bound vesicles full of acidic hydrolytic enzymes that digest carbohydrates, proteins, lipids and nucleic acids. Vacuoles are single-membrane (tonoplast) spaces that mainly store water, sap and wastes and help maintain turgidity and osmoregulation. Both are part of the endomembrane system but perform different functions.
Are these Class 11 Biology Chapter 8 solutions free?
Yes. All solutions are free and follow the official NCERT Biology textbook for the 2026–27 session.
