NCERT Solutions for Class 11 Biology Chapter 8: Cell: The Unit of Life

Concept. The history of the cell concept involves four key names: Antonie von Leeuwenhoek (first saw and described a live cell), Robert Brown (discovered the nucleus), Schleiden (1838) and Schwann (1839) (formulated the cell theory) and Rudolf Virchow (1855) (explained that new cells arise from pre-existing cells — Omnis cellula-e cellula).

Answer: statement (a) “Robert Brown discovered the cell” is NOT correct.

Extra depth. The cell theory as understood today has two parts: (i) all living organisms are composed of cells and products of cells; (ii) all cells arise from pre-existing cells.

Conclusion: statement (a) is incorrect: Robert Brown discovered the nucleus, not the cell; the other three statements are correct.

Concept. The cell theory’s second postulate, completed by Rudolf Virchow in 1855, states that all cells arise from pre-existing cells (Omnis cellula-e cellula) — by cell division. The earlier Schleiden–Schwann theory did not explain how new cells were formed.

Answer: (c) pre-existing cells.

Extra depth. Cell division and the stages of the cell cycle are studied in Chapter 10. The continuity of life through cells also explains growth, repair and reproduction in organisms.

Conclusion: new cells arise only from pre-existing cells by cell division (option c), as explained by Virchow.

Concept. Each term denotes a membranous structure of a particular organelle: mitochondria (cristae), Golgi apparatus (cisternae) and chloroplast (thylakoids).

Final answer: (a) – (ii); (b) – (iii); (c) – (i).

Extra depth. Function-wise: cristae carry the enzymes (ATP synthesis); cisternae modify and package proteins; thylakoid membranes contain chlorophyll for the light reactions (grana), while the stroma carries out the dark reactions.

Conclusion: cristae – infoldings in mitochondria; cisternae – disc-shaped sacs in the Golgi apparatus; thylakoids – flat membranous sacs in the stroma of chloroplasts.

Concept. Compare the options with the basic facts: prokaryotes lack a membrane-bound nucleus and organelles; the cell wall is present in plant and fungal cells but not in animal cells; cells arise only from pre-existing cells.

Answer: (c) In prokaryotes, there are no membrane-bound organelles.

Extra depth. In prokaryotes the membranous structure mesosome is only an infolding of the plasma membrane, not an organelle in the eukaryotic sense.

Conclusion: option (c) is correct: prokaryotic cells have no membrane-bound organelles (only ribosomes), whereas the other statements contradict the facts about nucleus, cell wall and the origin of cells.

Concept. A mesosome is a specialised differentiated form of the cell membrane that is characteristic of prokaryotes. It is essentially an infolding (extension) of the plasma membrane into the cell in the form of vesicles, tubules and lamellae (Fig. below).

Functions of the mesosome:

1. Cell wall formation. Mesosomes help in the synthesis and formation of the cell wall (especially the new septum during division).
2. DNA replication and distribution to daughter cells. The bacterial DNA is attached to the mesosome, which helps in its replication and in separating the replicated chromosomes into the two daughter cells.
3. Respiration. They carry respiratory enzymes and thus take part in respiration — functionally comparable to the mitochondria of eukaryotes.
4. Secretion processes. They help in secretion.
5. Increase the surface area of the plasma membrane and the enzymatic content — more membrane means more space for enzymes.

Extra depth. In some prokaryotes such as cyanobacteria there are other membranous extensions into the cytoplasm called chromatophores that contain pigments for photosynthesis.

Conclusion: a mesosome is a vesicular, tubular or lamellar infolding of the plasma membrane in prokaryotes that helps in cell wall formation, DNA replication and distribution, respiration and secretion, and increases the membrane surface area and enzyme content.

Concept. The plasma membrane is a selectively permeable phospholipid bilayer with embedded proteins (fluid mosaic model of Singer and Nicolson, 1972). One of its most important functions is the transport of molecules across it.

1. Neutral solutes. Neutral (non-polar) solutes move across the membrane by simple diffusion along the concentration gradient, i.e., from the region of higher concentration to the region of lower concentration. This is passive transport — no energy is required — and the molecules can pass directly through the lipid bilayer because it is non-polar. Water also moves from higher to lower concentration across the membrane; this movement of water by diffusion is called osmosis.

2. Polar molecules cannot move in the same way. Polar molecules (and ions) cannot pass through the non-polar (hydrophobic) lipid bilayer. They require a carrier protein of the membrane to facilitate their transport (facilitated diffusion, still passive and along the gradient).

3. Active transport. A few ions or molecules are transported against their concentration gradient, i.e., from the lower to the higher concentration. This is an energy-dependent process in which ATP is utilised, and is called active transport — for example the Na⁺/K⁺ pump.

Extra depth. The fluid nature of the membrane (lateral movement of proteins within the quasi-fluid lipid) is important for functions such as cell growth, formation of intercellular junctions, secretion, endocytosis and cell division.

Conclusion: neutral solutes cross the plasma membrane by simple diffusion down the concentration gradient without energy; polar molecules cannot cross the non-polar bilayer in this way and are transported with the help of carrier proteins, and substances moved against the gradient are transported by energy-dependent active transport (e.g., Na⁺/K⁺ pump).

Concept. Two organelles bound by a double membrane (an outer and an inner membrane) are the mitochondria and the chloroplasts. (The nucleus is also enclosed by a double-membrane envelope.) Both contain their own circular DNA and 70S ribosomes and so can make some of their own proteins; both divide independently and both are concerned with energy conversion.

1. Mitochondria (sing. mitochondrion)

• Shape and size: typically sausage-shaped or cylindrical, diameter 0.2–1.0 µm (average 0.5 µm), length 1.0–4.1 µm; the number per cell varies with physiological activity; not visible unless stained.
• Membranes: a double membrane divides the lumen into an outer compartment (inter-membrane space) and an inner compartment filled with a dense homogeneous matrix. The outer membrane forms the continuous limiting boundary; the inner membrane forms infoldings, the cristae, which increase the surface area. The two membranes have specific enzymes.
• Matrix: contains a single circular DNA molecule, a few RNA molecules, 70S ribosomes and the components for protein synthesis; mitochondria divide by fission.
• Function: sites of aerobic respiration; they produce cellular energy in the form of ATP and are therefore called the ‘power houses’ of the cell.

2. Chloroplasts

• Shape and size: lens-shaped, oval, spherical, discoid or even ribbon-like; length 5–10 µm, width 2–4 µm; number from one per cell in Chlamydomonas to 20–40 per mesophyll cell; found mostly in mesophyll cells of leaves.
• Membranes: double membrane (the inner chloroplast membrane is relatively less permeable). The space limited by the inner membrane is the stroma.
• Thylakoids and grana: the stroma contains organised flattened membranous sacs, the thylakoids, arranged in stacks like piles of coins called grana (intergranal thylakoids), joined by flat membranous tubules, the stroma lamellae; the thylakoid membrane encloses a lumen. Chlorophyll pigments are in the thylakoids.
• Stroma: contains enzymes for the synthesis of carbohydrates and proteins, small double-stranded circular DNA and 70S ribosomes (smaller than the 80S cytoplasmic ribosomes).
• Function: contain chlorophyll and carotenoids that trap light energy for photosynthesis; grana are the site of the light reactions and the stroma of the dark (carbon-fixing) reactions.

Extra depth. Because mitochondria and chloroplasts have their own DNA, 70S ribosomes and double membranes, they are called semi-autonomous organelles; neither belongs to the endomembrane system since their functions are not coordinated with it.

Conclusion: mitochondria and chloroplasts are double-membrane-bound organelles containing circular DNA and 70S ribosomes; mitochondria have cristae and a matrix and generate ATP by aerobic respiration, whereas chloroplasts have stroma and grana of thylakoids and carry out photosynthesis.

Concept. Prokaryotic cells are the simple, primitive cells of bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). Although they exhibit a wide variety of shapes and functions, their basic organisation is fundamentally similar (see the labelled bacterial cell in Question 5).

Characteristics (NCERT):

1. Size and multiplication. Generally smaller than eukaryotic cells (bacteria 3–5 µm; Mycoplasma 0.3 µm, the smallest cells) and they multiply more rapidly.
2. Shape. Vary greatly in shape and size; four basic shapes of bacteria: bacillus (rod), coccus (spherical), vibrio (comma-shaped) and spirillum (spiral).
3. Cell wall. All prokaryotes have a cell wall surrounding the cell membrane except mycoplasma; the wall determines the shape and gives strong structural support, preventing bursting or collapse.
4. Cell envelope. In most bacteria a chemically complex three-layered envelope: outer glycocalyx (slime layer or capsule), cell wall and plasma membrane; bacteria are Gram positive or Gram negative by their response to the Gram stain.
5. Cytoplasm. The semi-fluid matrix filling the cell.
6. No well-defined nucleus. The genetic material is naked — not enveloped by a nuclear membrane — a single circular genomic DNA (chromosome); many bacteria have small circular plasmid DNA outside the genomic DNA, conferring unique phenotypic characters such as antibiotic resistance.
7. No membrane-bound organelles. No organelles like those of eukaryotes except ribosomes (70S = 50S + 30S, associated with the plasma membrane, site of protein synthesis, forming polysomes).
8. Inclusion bodies. Reserve materials (phosphate, cyanophycean and glycogen granules; gas vacuoles) lie free in the cytoplasm, not membrane bound.
9. Mesosome. A specialised infolding of the cell membrane (vesicles, tubules, lamellae) characteristic of prokaryotes; chromatophores containing pigments in cyanobacteria.
10. Locomotory and surface structures. Motile bacteria have flagella (filament, hook, basal body); pili and fimbriae are surface structures that do not help motility (fimbriae help attachment to rocks and host tissues).

Conclusion: prokaryotic cells are small and rapidly dividing, lack a membrane-bound nucleus and organelles (except 70S ribosomes), have naked circular DNA (and plasmids), a cell wall (except mycoplasma), mesosomes and non-membrane-bound inclusion bodies.

Concept. In unicellular organisms all functions — digestion, respiration, reproduction — are performed by a single cell. In the complex body of multicellular animals and plants the same basic functions are carried out by different groups of cells in a well-organised manner. This sharing of work among different cells, tissues, organs and organ systems is called division of labour.

Explanation:

1. Cells. A multicellular body is made of many types of cells, each specialised in shape, structure and function: for example, RBCs carry oxygen, nerve cells conduct impulses, columnar epithelial cells line and absorb, mesophyll cells photosynthesise and tracheids conduct water (Fig. 8.1). Hydra has several cell types in thousands of cells, while the human body has billions of cells.
2. Tissues. A group of similar cells along with intercellular substances performing a specific function is a tissue; all complex animals consist of only four basic tissues — epithelial, connective, muscular and neural.
3. Organs. Tissues are organised in specific proportion and pattern to form an organ such as the stomach, lung, heart or kidney. Each organ is made of one or more tissues; the heart contains all four types.
4. Organ systems. When two or more organs perform a common function by their physical and/or chemical interaction they form an organ system, e.g., digestive system, respiratory system.
5. Result. Cells, tissues, organs and organ systems split up the work so that each performs a particular task more efficiently while the activities of all are coordinated — this ensures better coordinated and more efficient functioning of millions of cells and the survival of the body as a whole.

Extra depth — example from the frog. In the frog the digestive system digests food, the respiratory system exchanges gases, the circulatory system transports, the excretory system removes wastes, and the nervous and endocrine systems coordinate — none can do the work of the others, yet all depend on one another.

Conclusion: in multicellular organisms the work is divided among specialised cells, tissues, organs and organ systems, each performing a specific function in a coordinated way, which makes the organism more efficient and ensures survival of the body as a whole.

Concept. What makes an organism living, and what a non-living thing lacks, is the presence of the cell — the basic unit of life present in all living organisms.

1. All organisms are composed of cells. Some are made of a single cell (unicellular: bacteria, Amoeba, Paramoecium) and others of many cells (multicellular: plants, animals, fungi, humans).
2. Cell is the structural unit. The bodies of all plants and animals are composed of cells and products of cells (Schleiden and Schwann); in multicellular organisms cells form tissues, organs and organ systems.
3. Cell is the functional unit. Unicellular organisms are capable of independent existence and of performing the essential functions of life — nutrition, respiration, excretion, reproduction. Anything less than a complete structure of a cell does not ensure independent living. Metabolic reactions occur in the cytoplasm and organelles of every cell.
4. Cells arise from pre-existing cells. Growth, reproduction and continuity of life take place by cell division (Virchow, Omnis cellula-e cellula).
5. Unity in diversity. Despite enormous differences in shape, size and activity, all cells have a plasma membrane, cytoplasm and genetic material (DNA), showing the cellular organisation underlying all life forms. Cell theory emphasised this unity.
6. Cell is the unit of heredity. The nucleus (or nucleoid) contains DNA, which stores hereditary information and controls the activities of the cell.

Extra depth. Viruses, viroids and prions do not have a cell structure; they are not considered truly living by cell-based definitions, which again shows that the cell is the criterion of life.

Conclusion: the cell is the fundamental structural and functional unit of all living organisms: it can live independently in unicellular forms, builds the bodies of multicellular organisms, arises from pre-existing cells and carries the hereditary material, so it is the basic unit of life.

Concept. The nuclear envelope consists of two parallel membranes (outer and inner) separated by a perinuclear space of 10–50 nm. It forms a barrier between the materials inside the nucleus and the cytoplasm. The outer membrane is continuous with the endoplasmic reticulum and bears ribosomes (Fig. 8.11, shown in Question 13).

Nuclear pores. At a number of places the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes. These are the nuclear pores.

Function. Nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm:

• Out of the nucleus: RNAs (mRNA, tRNA and the ribosomal subunits made in the nucleolus) pass into the cytoplasm for protein synthesis.
• Into the nucleus: proteins needed in the nucleus (histones, enzymes for DNA replication and transcription) enter from the cytoplasm.
• They thus regulate the exchange between nucleus and cytoplasm (selective transport).

Conclusion: nuclear pores are minute openings in the nuclear envelope, formed by fusion of its two membranes, which allow two-way movement of RNA and protein molecules between the nucleus and the cytoplasm.

Concept. The endomembrane system consists of the membranous organelles whose functions are coordinated — the endoplasmic reticulum, Golgi complex, lysosomes and vacuoles (mitochondria, chloroplasts and peroxisomes are not included). Lysosomes and vacuoles are both bound by a single membrane and derived from this system, but they perform different jobs.

Lysosomes

• Membrane-bound vesicular structures formed by packaging in the Golgi apparatus.
• Very rich in almost all types of hydrolytic enzymes (hydrolases — lipases, proteases, carbohydrases), optimally active at acidic pH.
• Function: digestion — the enzymes can digest carbohydrates, proteins, lipids and nucleic acids; they digest food taken in by the cell, worn-out organelles and foreign material (‘digestive / suicide bags’ — standard fact).

Vacuoles

• The vacuole is a membrane-bound space in the cytoplasm, bound by a single membrane called the tonoplast; in plant cells it can occupy up to 90 per cent of the cell volume.
• Contents: water, sap, excretory products and other materials not useful for the cell.
• Function: storage and maintaining turgidity / osmotic balance; in plants the tonoplast transports ions and other materials against concentration gradients into the vacuole, so their concentration is significantly higher in the vacuole than in the cytoplasm. In Amoeba the contractile vacuole performs osmoregulation and excretion; in protists food vacuoles are formed by engulfing food particles.

Extra depth. The coordination within the endomembrane system is visible here: proteins made on the rough ER are modified and packaged in the Golgi apparatus; the Golgi buds off lysosomes; vacuoles collect stored and waste materials — but the end functions differ: digestion (lysosome) versus storage and osmoregulation (vacuole).

Conclusion: although both are single-membrane parts of the endomembrane system, lysosomes (formed by the Golgi, filled with acid hydrolases) are the sites of intracellular digestion, whereas vacuoles (bound by tonoplast, filled with sap, water and wastes) serve storage, ion concentration and osmoregulation/excretion.

(i) Nucleus

Concept. The nucleus is the largest and most prominent organelle of the eukaryotic cell that controls its activities and plays a major role in heredity. It was first described by Robert Brown (1831); the material stained by basic dyes was named chromatin by Flemming.

1. Nuclear envelope. Two parallel membranes with a space between them, the perinuclear space (10–50 nm), form a barrier between the nucleus and the cytoplasm. The outer membrane is usually continuous with the endoplasmic reticulum and bears ribosomes.
2. Nuclear pores. At places the envelope is interrupted by minute pores formed by the fusion of the two membranes; they allow movement of RNA and protein molecules in both directions between the nucleus and the cytoplasm.
3. Nucleoplasm (nuclear matrix). The matrix inside the envelope contains the nucleolus and chromatin.
4. Nucleolus. One or more spherical bodies in the nucleoplasm; not membrane-bound (continuous with the nucleoplasm); the site of active ribosomal RNA synthesis; larger and more numerous in cells actively synthesising proteins.
5. Chromatin. In the interphase nucleus a loose, indistinct network of nucleoprotein fibres containing DNA, basic proteins called histones, non-histone proteins and RNA; during cell division the chromatin appears as structured chromosomes. A single human cell has about two metres of DNA distributed among 46 chromosomes (23 pairs).
6. Number. Normally one nucleus per cell; some cells are multinucleate; some mature cells such as the erythrocytes of many mammals and sieve tube cells lack a nucleus.

(ii) Centrosome

Concept. The centrosome is a non-membrane-bound organelle found in animal cells (absent in almost all plant cells) that helps in cell division.

1. Components. A centrosome usually contains two cylindrical structures called centrioles, surrounded by amorphous pericentriolar material.
2. Arrangement. The two centrioles lie perpendicular to each other.
3. Structure of a centriole (cartwheel). Each centriole is made up of nine evenly spaced peripheral fibrils of tubulin protein; each peripheral fibril is a triplet and adjacent triplets are linked. The central part of the proximal region is proteinaceous and called the hub, connected to the tubules of the peripheral triplets by radial spokes made of protein.
4. Functions. The centrioles form the basal body of cilia and flagella, and the spindle fibres that give rise to the spindle apparatus during cell division in animal cells.

Conclusion: the nucleus has a double-membrane envelope with pores, nucleoplasm, nucleolus and chromatin, and controls the cell and heredity; the centrosome has two perpendicular centrioles, each a cartwheel of nine microtubule triplets with a hub and radial spokes, surrounded by pericentriolar material, and forms basal bodies and spindle fibres.

Concept — centromere. Every chromosome (visible only in dividing cells) essentially has a primary constriction called the centromere, on the sides of which disc-shaped structures called kinetochores are present (Fig. 8.12). The centromere holds the two chromatids of a chromosome together; the kinetochores are the sites to which the spindle fibres attach during cell division, so it determines the movement of chromosomes.

Classification based on the position of the centromere (Fig. 8.13). The centromere divides the chromosome into two arms; the position of the centromere determines the relative length of the arms, and hence there are four types:

1. Metacentric chromosome — the centromere is in the middle, forming two equal arms (V-shaped in anaphase).
2. Sub-metacentric chromosome — the centromere is slightly away from the middle, resulting in one shorter arm and one longer arm (L-shaped).
3. Acrocentric chromosome — the centromere is situated close to one end, forming one extremely short and one very long arm (J-shaped).
4. Telocentric chromosome — the centromere is at the terminal (end) position (rod-shaped, I-shaped).

Extra depth. Some chromosomes have non-staining secondary constrictions at a constant location, which give the appearance of a small fragment called the satellite. Chromatin contains DNA, histones, non-histone proteins and RNA; a human cell has 46 chromosomes with about two metres of DNA.

Conclusion: the centromere is the primary constriction that holds the two chromatids and carries the kinetochores; according to its position chromosomes are metacentric (middle), sub-metacentric (slightly off-centre), acrocentric (near one end) and telocentric (terminal).