Cell : The Unit of Life — NCERT Solutions

Characteristics of Prokaryotic Cells:

Prokaryotic cells are primitive cells that lack a membrane-bound nucleus and membrane-bound organelles. Their main characteristics are:

1. Size: Generally smaller in size, typically $1–10\ \mu m$.

2. Cell wall: Most prokaryotes have a rigid cell wall made up of peptidoglycan (murein). In bacteria, the cell wall composition determines Gram-positive or Gram-negative nature.

3. Nucleus: No well-defined, membrane-bound nucleus. The genetic material (circular DNA) is located in an irregular region of the cytoplasm called the nucleoid. No nuclear envelope is present.

4. DNA: Genetic material is a single, circular, naked DNA molecule (not associated with histone proteins). Plasmids (extra-chromosomal DNA) may also be present.

5. Membrane-bound organelles: Absent. No mitochondria, ER, Golgi body, lysosomes, etc.

6. Ribosomes: Present but are smaller — 70S type (50S + 30S subunits), found freely in the cytoplasm.

7. Mesosome: Infoldings of the plasma membrane called mesosomes are present; they help in respiration, cell wall formation, and DNA replication.

8. Cytoskeleton: Absent (no true cytoskeleton).

9. Flagella: If present, they are made of a single fibril of flagellin protein (not the 9+2 arrangement of eukaryotes).

10. Reproduction: Primarily by binary fission (asexual). Sexual reproduction involves conjugation, transformation, or transduction.

11. Capsule/Slime layer: Some prokaryotes have a capsule (glycocalyx) outside the cell wall for protection.

12. Examples: Bacteria (Escherichia coli, Staphylococcus), Cyanobacteria (blue-green algae), Mycoplasma.

Division of Labour in Multicellular Organisms:

Definition: Division of labour means that different cells, tissues, and organs are specialised to perform specific functions, rather than a single cell performing all functions.

Explanation:

In unicellular organisms (e.g., Amoeba, Paramecium), a single cell performs all life functions — nutrition, respiration, excretion, reproduction, locomotion, etc. This limits the efficiency and complexity of functions.

In multicellular organisms (e.g., humans, plants), the body is composed of millions of cells. These cells are not all alike; they become specialised during development (differentiation) to perform particular functions:

• Cells → Tissues → Organs → Organ Systems

Examples of Division of Labour:

Advantages:

1. Each cell/organ becomes highly efficient at its specific task.
2. Complex functions (e.g., thinking, immunity) become possible.
3. The organism can grow larger and more complex.
4. Survival and adaptability are enhanced.

Conclusion: Thus, in multicellular organisms, different cells are specialised for different functions, leading to greater efficiency — this is the division of labour at the cellular and organismal level.

Cell is the Basic Unit of Life:

The cell theory, proposed by Schleiden (1838) and Schwann (1839) and later modified by Virchow (1855), states:

1. All living organisms are composed of cells and products of cells.
2. All cells arise from pre-existing cells.

The cell is considered the basic structural and functional unit of life for the following reasons:

1. Structural Unit:

• Every living organism — whether unicellular (e.g., Amoeba, bacteria) or multicellular (e.g., humans, plants) — is made up of cells.
• In unicellular organisms, a single cell constitutes the entire organism.
• In multicellular organisms, cells are organised into tissues, organs, and organ systems.

2. Functional Unit:

• All life processes (metabolism, growth, reproduction, response to stimuli, etc.) are carried out at the cellular level.
• Each organelle within the cell performs a specific function:
• Mitochondria → Energy production (ATP)
• Ribosomes → Protein synthesis
• Nucleus → Control of cell activities and heredity
• Chloroplasts → Photosynthesis (in plants)

3. Unit of Heredity:

• The nucleus of the cell contains DNA (genetic material) that carries hereditary information from one generation to the next.

4. Unit of Development:

• All multicellular organisms develop from a single fertilised cell (zygote) through repeated cell division and differentiation.

Conclusion:
Since all living organisms are made of cells, all life functions are performed by cells, and all new cells arise from pre-existing cells, the cell is rightly called the basic structural and functional unit of life.

Nuclear Pores:

Definition: Nuclear pores are small, circular openings or perforations present in the nuclear envelope (the double membrane surrounding the nucleus). They are formed at sites where the outer and inner nuclear membranes fuse together.

Structure:

• Each nuclear pore is approximately 100 nm in diameter.
• They are not simple holes; each pore is associated with a complex protein structure called the nuclear pore complex (NPC).
• The number of pores varies with the metabolic activity of the cell — more active cells have more nuclear pores.

Functions of Nuclear Pores:

1. Transport of RNA: Nuclear pores allow the transport of mRNA (messenger RNA), tRNA, and rRNA from the nucleus to the cytoplasm, where protein synthesis occurs.

2. Transport of proteins: Proteins synthesised in the cytoplasm (e.g., histones, DNA/RNA polymerases, transcription factors) are transported into the nucleus through nuclear pores.

3. Exchange of materials: They facilitate the bidirectional exchange of materials between the nucleoplasm and the cytoplasm.

4. Selective permeability: The nuclear pore complex acts as a selective gate, allowing only specific molecules to pass through, thus regulating nuclear-cytoplasmic communication.

5. Ribosome export: Ribosomal subunits assembled in the nucleus are exported to the cytoplasm through nuclear pores.

Lysosomes and Vacuoles — Both are Endomembrane Structures:

Both lysosomes and vacuoles are single membrane-bound structures that are part of the endomembrane system of the cell. However, they differ significantly in their functions:

Lysosomes:

• Lysosomes are membrane-bound vesicles containing hydrolytic (digestive) enzymes (acid hydrolases such as proteases, lipases, nucleases, carbohydrases).
• These enzymes work best at an acidic pH (~5), maintained by proton pumps.
• They are formed by the Golgi apparatus.

Functions of Lysosomes:

1. Intracellular digestion: Digest food particles taken in by phagocytosis or pinocytosis (heterophagy).
2. Autophagy: Digest worn-out, damaged, or non-functional organelles within the cell.
3. Autolysis: During metamorphosis or programmed cell death, lysosomes release their enzymes to digest the entire cell. Hence called 'suicidal bags of the cell' (de Duve).
4. Defence: In white blood cells, lysosomes help destroy bacteria and foreign particles.

Vacuoles:

• Vacuoles are membrane-bound sacs filled with cell sap (water, salts, sugars, pigments, waste products).
• The membrane surrounding the vacuole is called the tonoplast.
• In plant cells, the central vacuole is large and occupies up to 90% of the cell volume.
• In animal cells, vacuoles are small and temporary.

Functions of Vacuoles:

1. Osmoregulation: Maintain the osmotic pressure and water balance of the cell (turgor pressure in plants).
2. Storage: Store nutrients (sugars, amino acids), pigments (anthocyanins), and waste products.
3. Structural support: The central vacuole in plants provides rigidity and support to the cell.
4. In Amoeba: Contractile vacuoles expel excess water; food vacuoles help in digestion.
5. Detoxification: Store toxic metabolic by-products.

Summary of Differences:

Conclusion: Although both are endomembrane structures, lysosomes function primarily as digestive organelles, while vacuoles function primarily in storage and osmoregulation.

(i) Structure of the Nucleus

Introduction:
The nucleus is the most prominent organelle in a eukaryotic cell. It was discovered by Robert Brown (1831). It is the control centre of the cell and the repository of genetic information.

Structure of the Nucleus:

The nucleus consists of the following components:

1. Nuclear Envelope:

• A double membrane structure surrounding the nucleus.
• The outer membrane is continuous with the endoplasmic reticulum (ER).
• The space between the two membranes is called the perinuclear space.
• The nuclear envelope has nuclear pores — small openings that allow exchange of materials between the nucleus and cytoplasm.

2. Nucleoplasm (Nuclear Matrix):

• The fluid-filled interior of the nucleus enclosed by the inner nuclear membrane.
• Contains nucleotides, enzymes, and other molecules required for DNA replication and transcription.

3. Chromatin:

• The genetic material present in the nucleus as a network of thin, thread-like structures called chromatin fibres.
• Chromatin is composed of DNA + histone proteins.
• During cell division, chromatin condenses to form distinct chromosomes.
• Two types: Euchromatin (loosely packed, transcriptionally active) and Heterochromatin (densely packed, transcriptionally inactive).

4. Nucleolus:

• A dense, spherical body present within the nucleoplasm.
• Not membrane-bound.
• Site of rRNA synthesis and ribosome assembly.
• A nucleus may have one or more nucleoli.
• Disappears during cell division and reappears after division.

Labelled Diagram of Nucleus:
(Draw a large oval structure with the following labels:)

• Nuclear envelope (outer membrane + inner membrane)
• Nuclear pore
• Perinuclear space
• Nucleoplasm
• Chromatin network (euchromatin and heterochromatin)
• Nucleolus
• Endoplasmic reticulum (connected to outer nuclear membrane)

Functions of Nucleus:

1. Controls all metabolic activities of the cell.
2. Stores and transmits genetic information (DNA).
3. Site of DNA replication and transcription.
4. Nucleolus synthesises rRNA and ribosomes.
5. Plays a central role in heredity and cell division.

(ii) Structure of the Centrosome

Introduction:
The centrosome is an organelle found in animal cells and some lower plant cells. It was discovered by Van Beneden and described by Boveri.

Structure of the Centrosome:

The centrosome consists of:

1. Centrioles (usually two):

• Each centrosome contains two centrioles arranged perpendicular to each other, forming a structure called a diplosome.
• Each centriole is a cylindrical structure made up of 9 sets of triplet microtubules arranged in a ring (9 × 3 arrangement), with no central microtubules — described as '9 + 0' arrangement.
• Each triplet consists of three microtubules: A, B, and C tubules.
• The centrioles are made of tubulin protein.
• The two centrioles in a centrosome are: the mother centriole (older, with appendages) and the daughter centriole (younger).

2. Pericentriolar Material (PCM):

• An amorphous, electron-dense material surrounding the centrioles.
• It nucleates the formation of microtubules and is the actual microtubule-organising centre (MTOC).
• Contains proteins like $\gamma$-tubulin, pericentrin, etc.

Labelled Diagram of Centrosome:
(Draw two cylindrical centrioles arranged at right angles to each other, with the following labels:)

• Centriole 1 (mother centriole)
• Centriole 2 (daughter centriole) — perpendicular to centriole 1
• Triplet microtubules (A, B, C tubules) — 9 sets
• Pericentriolar material (PCM)
• Cross-section of centriole showing 9 × 3 arrangement

Functions of Centrosome:

1. Spindle formation: During cell division (mitosis/meiosis), centrioles give rise to the spindle fibres (aster) that help in the separation of chromosomes.
2. Basal body: Centrioles form the basal bodies of cilia and flagella, which are involved in locomotion.
3. Cell polarity: Help in establishing the polarity of the cell during division.
4. Microtubule organisation: The PCM organises the cytoplasmic microtubule network.

Centromere:

Definition: The centromere is a specialised, constricted region on the chromosome where the two sister chromatids are held together. It is also called the primary constriction. The centromere is the region where the kinetochore (a protein complex) is assembled, which attaches to spindle fibres during cell division.

Role of Centromere:

• Holds sister chromatids together until anaphase.
• Provides the attachment point for spindle microtubules via the kinetochore.
• Ensures accurate segregation of chromosomes to daughter cells during mitosis and meiosis.

Classification of Chromosomes Based on Position of Centromere:

The position of the centromere divides the chromosome into two arms: the shorter arm (called p arm) and the longer arm (called q arm). Based on the centromere position, chromosomes are classified into four types:

1. Metacentric Chromosome:

• Centromere is located at the middle of the chromosome.
• Both arms (p and q) are equal in length.
• The chromosome appears V-shaped during anaphase.
• Arm ratio = 1:1

2. Sub-metacentric Chromosome:

• Centromere is located slightly away from the middle.
• One arm is slightly shorter than the other.
• The chromosome appears L-shaped during anaphase.
• Arm ratio slightly > 1

3. Acrocentric Chromosome:

• Centromere is located close to one end of the chromosome.
• One arm is very short and the other is very long.
• The chromosome appears J-shaped during anaphase.
• Arm ratio is very large.

4. Telocentric Chromosome:

• Centromere is located at the terminal end of the chromosome.
• Only one arm is visible (the other is extremely short or absent).
• The chromosome appears rod-shaped (I-shaped) during anaphase.
• Arm ratio = ∞ (theoretically)

Labelled Diagram:

(Draw four chromosomes side by side with the following labels for each:)

  Metacentric    Sub-metacentric    Acrocentric    Telocentric

     p  p            p  p              p               |
     |  |             |  |             |               |
    [C]              [C]             [C]             [C]
     |  |             |  |             |               |
     q  q            q  q              q

   (V-shape)       (L-shape)        (J-shape)      (Rod/I-shape)

• C = Position of centromere (primary constriction)
• p = Short arm
• q = Long arm

Summary Table: