Q: Q1. Which of the following is NOT an organelle?\n(a) Endoplasmic reticulum\n(b) Plastid\n(c) Plasmid\n(d) Mitochondria

The correct option is (c) Plasmid.\n\nExplanation:\n(a) Endoplasmic reticulum: It is a network of membranes found throughout the cytoplasm of eukaryotic cells, involved in protein and lipid synthesis. It is a membrane-bound organelle.\n(b) Plastid: These are double-membrane-bound organelles found in plant cells and some eukaryotic algae, involved in photosynthesis (e.g., chloroplasts) or storage (e.g., amyloplasts). They are organelles.\n(c) Plasmid: A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA. Plasmids naturally exist in bacterial cells, and also in some eukaryotes. They are not considered organelles; rather, they are extrachromosomal genetic elements.\n(d) Mitochondria: These are double-membrane-bound organelles found in most eukaryotic cells, often called the 'powerhouses' of the cell because they generate most of the cell's supply of adenosine triphosphate (ATP). They are organelles.\n\nTherefore, plasmid is NOT an organelle.

Q: Q2. Match the columns:\nColumn I | Column II\n---|---\n(a) Cristae | (i) Flat membranous sacs in stroma\n(b) Cisternae | (ii) Infoldings in mitochondria\n(c) Thylakoids | (iii) Disc-sh

The correct matching is as follows:\n\n(a) Cristae - (ii) Infoldings in mitochondria\n(b) Cisternae - (iii) Disc-shaped sacs in Golgi apparatus\n(c) Thylakoids - (i) Flat membranous sacs in stroma\n\nExplanation:\n* **Cristae**: These are the inner membrane folds of mitochondria. They increase the surface area for cellular respiration, specifically for the electron transport chain.\n* **Cisternae**: These are flattened, disc-shaped sacs or tubules that make up the Golgi apparatus. They are involved in modifying, sorting, and packaging proteins and lipids.\n* **Thylakoids**: These are flattened, sac-like structures within the chloroplasts. They are the site of the light-dependent reactions of photosynthesis. A stack of thylakoids is called a granum, and they are suspended in the stroma of the chloroplast.

Q: Q4. Fill in the blanks:\n(a) All organisms are made up of ______ and ______.\n(b) The prokaryotic cell has no well-defined ______.\n(c) Ribosomes are found in all ______ cells.\n(d) Th

The completed statements are:\n(a) All organisms are made up of **cells** and **cell products**.\n(b) The prokaryotic cell has no well-defined **nucleus**.\n(c) Ribosomes are found in all **living** cells.\n(d) The **DNA** is the genetic material in almost all organisms.

Q: Q5. State whether the following statements are true or false:\n(a) The prokaryotic cells have a true nucleus.\n(b) Unicellular organisms carry out their life activities within a single

Here are the true/false statements with explanations:\n\n(a) **The prokaryotic cells have a true nucleus.**\n **False**. Prokaryotic cells lack a true nucleus, meaning they do not have a nuclear membrane enclosing their genetic material. Their genetic material (nucleoid) is located in the cytoplasm.\n\n(b) **Unicellular organisms carry out their life activities within a single cell.**\n **True**. By definition, unicellular organisms consist of a single cell that performs all essential life functions, such as nutrition, respiration, excretion, and reproduction.\n\n(c) **The cell wall of fungi is made up of peptidoglycan.**\n **False**. The cell wall of fungi is primarily made up of chitin. Peptidoglycan is the main component of the bacterial cell wall.\n\n(d) **The plasma membrane is selectively permeable.**\n **True**. The plasma membrane (or cell membrane) is selectively permeable (or semi-permeable), meaning it regulates the passage of substances into and out of the cell, allowing some molecules to pass freely while restricting others.

Question 1

Q1. Which of the following is NOT an organelle?
(a) Endoplasmic reticulum
(b) Plastid
(c) Plasmid
(d) Mitochondria

Accepted Answer

The correct option is (c) Plasmid.

Explanation:
(a) Endoplasmic reticulum: It is a network of membranes found throughout the cytoplasm of eukaryotic cells, involved in protein and lipid synthesis. It is a membrane-bound organelle.
(b) Plastid: These are double-membrane-bound organelles found in plant cells and some eukaryotic algae, involved in photosynthesis (e.g., chloroplasts) or storage (e.g., amyloplasts). They are organelles.
(c) Plasmid: A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA. Plasmids naturally exist in bacterial cells, and also in some eukaryotes. They are not considered organelles; rather, they are extrachromosomal genetic elements.
(d) Mitochondria: These are double-membrane-bound organelles found in most eukaryotic cells, often called the 'powerhouses' of the cell because they generate most of the cell's supply of adenosine triphosphate (ATP). They are organelles.

Therefore, plasmid is NOT an organelle.

Question 2

Q2. Match the columns:
Column I | Column II
---|---
(a) Cristae | (i) Flat membranous sacs in stroma
(b) Cisternae | (ii) Infoldings in mitochondria
(c) Thylakoids | (iii) Disc-sh

Accepted Answer

The correct matching is as follows:

(a) Cristae - (ii) Infoldings in mitochondria
(b) Cisternae - (iii) Disc-shaped sacs in Golgi apparatus
(c) Thylakoids - (i) Flat membranous sacs in stroma

Explanation:
*   **Cristae**: These are the inner membrane folds of mitochondria. They increase the surface area for cellular respiration, specifically for the electron transport chain.
*   **Cisternae**: These are flattened, disc-shaped sacs or tubules that make up the Golgi apparatus. They are involved in modifying, sorting, and packaging proteins and lipids.
*   **Thylakoids**: These are flattened, sac-like structures within the chloroplasts. They are the site of the light-dependent reactions of photosynthesis. A stack of thylakoids is called a granum, and they are suspended in the stroma of the chloroplast.

Question 3

Q3. Identify the correct statement.
(a) Both bacteria and blue-green algae are eukaryotes.
(b) All organisms are made up of cells.
(c) The cell wall is present in fungi but absent i

Accepted Answer

The correct statement is (b) All organisms are made up of cells.

Explanation of each statement:
(a) **Both bacteria and blue-green algae are eukaryotes.** This statement is **incorrect**. Bacteria and blue-green algae (cyanobacteria) are both prokaryotes. They lack a true nucleus and other membrane-bound organelles.

(b) **All organisms are made up of cells.** This statement is **correct**. This is a fundamental principle of the Cell Theory, proposed by Schleiden and Schwann. It states that all living organisms are composed of one or more cells, and cells are the basic unit of life.

(c) **The cell wall is present in fungi but absent in plant cells.** This statement is **incorrect**. The cell wall is present in both fungi and plant cells. In fungi, the cell wall is primarily made of chitin, while in plant cells, it is primarily made of cellulose.

(d) **Lysosomes have double membrane.** This statement is **incorrect**. Lysosomes are single-membrane-bound organelles that contain hydrolytic enzymes. Mitochondria and chloroplasts are examples of double-membrane-bound organelles.

Question 4

Q4. Fill in the blanks:
(a) All organisms are made up of ______ and ______.
(b) The prokaryotic cell has no well-defined ______.
(c) Ribosomes are found in all ______ cells.
(d) Th

Accepted Answer

The completed statements are:
(a) All organisms are made up of **cells** and **cell products**.
(b) The prokaryotic cell has no well-defined **nucleus**.
(c) Ribosomes are found in all **living** cells.
(d) The **DNA** is the genetic material in almost all organisms.

Question 5

Q5. State whether the following statements are true or false:
(a) The prokaryotic cells have a true nucleus.
(b) Unicellular organisms carry out their life activities within a single

Accepted Answer

Here are the true/false statements with explanations:

(a) **The prokaryotic cells have a true nucleus.**
    **False**. Prokaryotic cells lack a true nucleus, meaning they do not have a nuclear membrane enclosing their genetic material. Their genetic material (nucleoid) is located in the cytoplasm.

(b) **Unicellular organisms carry out their life activities within a single cell.**
    **True**. By definition, unicellular organisms consist of a single cell that performs all essential life functions, such as nutrition, respiration, excretion, and reproduction.

(c) **The cell wall of fungi is made up of peptidoglycan.**
    **False**. The cell wall of fungi is primarily made up of chitin. Peptidoglycan is the main component of the bacterial cell wall.

(d) **The plasma membrane is selectively permeable.**
    **True**. The plasma membrane (or cell membrane) is selectively permeable (or semi-permeable), meaning it regulates the passage of substances into and out of the cell, allowing some molecules to pass freely while restricting others.

Question 6

Q6. Describe the structure of the following with the help of labelled diagrams.
(i) Centrosome
(ii) Plasma membrane

Accepted Answer

### (i) Centrosome

The centrosome is an organelle typically containing two cylindrical structures called centrioles. It is surrounded by amorphous pericentriolar material. Both centrioles in a centrosome lie perpendicular to each other, in which each has an organisation like the cartwheel. The centrosome is a non-membranous organelle found in animal cells and some lower plant cells, usually located near the nucleus.

**Structure of a Centriole:**
Each centriole is made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibril is a triplet. The adjacent triplets are linked. The central part of the proximal region of the centriole is also proteinaceous and called the hub, which is connected with the triplets of the peripheral fibrils by radial spokes made of protein. The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

### (ii) Plasma Membrane

The plasma membrane (also called cell membrane) is the outermost boundary of the animal cell and lies just inside the cell wall in plant cells. It is a selectively permeable living membrane that regulates the movement of substances into and out of the cell. The most accepted model for the structure of the plasma membrane is the 'Fluid Mosaic Model' proposed by Singer and Nicolson (1972).

**Fluid Mosaic Model:**
According to this model, the plasma membrane is a quasi-fluid structure of lipids and proteins. It is composed of a lipid bilayer in which proteins are embedded. The lipids are arranged in a bilayer with the polar heads facing outwards and the hydrophobic tails facing inwards. This ensures that the nonpolar tail is protected from the aqueous environment. The lipid component consists mainly of phosphoglycerides.

**Components of Plasma Membrane:**
1.  **Lipids:** Primarily phospholipids, arranged in a bilayer. The fluid nature of the membrane is due to the ability of lipids to move within the bilayer (lateral movement, flip-flop, rotation). This fluidity is crucial for functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, etc.
2.  **Proteins:** Proteins are embedded in the lipid bilayer. They are classified into two main types:
    *   **Integral (Intrinsic) Proteins:** These are partially or totally buried within the lipid bilayer. Some integral proteins are transmembrane proteins, extending across the entire membrane.
    *   **Peripheral (Extrinsic) Proteins:** These lie on the surface of the membrane, either on the outer or inner side, and are easily separated from the membrane.
    Proteins play vital roles as enzymes, transport proteins (channels, carriers), receptors, and structural components.
3.  **Carbohydrates:** Small amounts of carbohydrates are present, usually attached to proteins (glycoproteins) or lipids (glycolipids) on the outer surface of the membrane. They are involved in cell recognition and adhesion.

The quasi-fluid nature of the lipid allows for lateral movement of proteins within the overall bilayer. This ability to move is described as fluidity.

Question 7

Q7. Distinguish between:
(a) Prokaryotic cell and Eukaryotic cell
(b) Plant cell and Animal cell

Accepted Answer

### (a) Distinction between Prokaryotic Cell and Eukaryotic Cell

### (b) Distinction between Plant Cell and Animal Cell

Question 8

Q8. How are prokaryotic cells different from eukaryotic cells?

Accepted Answer

Prokaryotic cells and eukaryotic cells represent the two fundamental types of cells that constitute all living organisms. They differ significantly in their structural organization, particularly concerning the presence of a true nucleus and membrane-bound organelles. The key differences are summarized below:

### 1. Nuclear Organization:
*   **Prokaryotic Cells:** Lack a well-defined nucleus. Their genetic material (a single, circular chromosome) is located in a region called the nucleoid, which is not enclosed by a nuclear membrane. Plasmids (small, circular DNA molecules) are often present.
*   **Eukaryotic Cells:** Possess a true nucleus, which is well-defined and enclosed by a double-layered nuclear envelope. The genetic material (multiple, linear chromosomes) is complexed with histone proteins to form chromatin.

### 2. Membrane-bound Organelles:
*   **Prokaryotic Cells:** Do not have any membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, or chloroplasts.
*   **Eukaryotic Cells:** Characterized by the presence of numerous membrane-bound organelles that compartmentalize various cellular functions.

### 3. Ribosomes:
*   **Prokaryotic Cells:** Have 70S type ribosomes, which are smaller and free in the cytoplasm.
*   **Eukaryotic Cells:** Have 80S type ribosomes, which are larger and can be free in the cytoplasm or attached to the endoplasmic reticulum. Mitochondria and chloroplasts (if present) in eukaryotic cells also contain 70S ribosomes.

### 4. Cell Size:
*   **Prokaryotic Cells:** Generally much smaller, typically ranging from 0.1 to 5.0 micrometers (µm) in diameter.
*   **Eukaryotic Cells:** Generally larger, typically ranging from 5 to 100 micrometers (µm) in diameter.

### 5. Cell Wall:
*   **Prokaryotic Cells:** Most prokaryotes (like bacteria) have a rigid cell wall made of peptidoglycan (murein), which provides structural support and protection.
*   **Eukaryotic Cells:** Plant cells have a cell wall made of cellulose, fungal cells have a cell wall made of chitin, while animal cells completely lack a cell wall.

### 6. Genetic Material:
*   **Prokaryotic Cells:** Genetic material is typically a single, circular, double-stranded DNA molecule, not associated with histones. Plasmids may carry additional genes.
*   **Eukaryotic Cells:** Genetic material consists of multiple, linear, double-stranded DNA molecules organized into chromosomes, which are tightly associated with histone proteins.

### 7. Cell Division:
*   **Prokaryotic Cells:** Reproduce primarily by binary fission, a simpler process of cell division.
*   **Eukaryotic Cells:** Divide by more complex processes of mitosis (for somatic cell division) and meiosis (for germ cell formation).

### 8. Cytoskeleton:
*   **Prokaryotic Cells:** Lack a well-developed cytoskeleton.
*   **Eukaryotic Cells:** Possess a complex cytoskeleton composed of microtubules, microfilaments, and intermediate filaments, providing structural support, facilitating movement, and organizing organelles.

### 9. Respiration:
*   **Prokaryotic Cells:** Cellular respiration often occurs on the inner folds of the plasma membrane called mesosomes.
*   **Eukaryotic Cells:** Cellular respiration primarily occurs in mitochondria.

**In summary, the defining characteristic distinguishing eukaryotic cells from prokaryotic cells is the presence of a membrane-bound nucleus and other membrane-bound organelles in eukaryotes, which allows for compartmentalization and greater complexity.**

Question 9

Q9. What are the main differences between plant and animal cell?

Accepted Answer

Plant cells and animal cells are both eukaryotic cells, meaning they share many common features such as a true nucleus and membrane-bound organelles. However, they exhibit several distinct differences that reflect their specialized functions and modes of life. These differences are primarily related to their structural components and some specific organelles.

### 1. Cell Wall:
*   **Plant Cell:** Possesses a rigid, protective cell wall located outside the plasma membrane. It is primarily composed of cellulose, hemicellulose, and pectin, providing structural support and protection against mechanical stress and osmotic lysis.
*   **Animal Cell:** Lacks a cell wall. The plasma membrane forms the outermost boundary of the cell.

### 2. Chloroplasts:
*   **Plant Cell:** Contains chloroplasts, which are specialized plastids containing chlorophyll. Chloroplasts are the sites of photosynthesis, enabling plants to produce their own food.
*   **Animal Cell:** Lacks chloroplasts and is therefore heterotrophic, relying on external sources for food.

### 3. Vacuoles:
*   **Plant Cell:** Typically has a single, large central vacuole that can occupy up to 90% of the cell volume. This vacuole stores water, nutrients, waste products, and maintains turgor pressure against the cell wall.
*   **Animal Cell:** Usually has several small, temporary, and scattered vacuoles, or sometimes no vacuoles at all. If present, they are involved in storage or waste removal but do not exert turgor pressure.

### 4. Centrosome and Centrioles:
*   **Plant Cell:** Generally lacks centrosomes and centrioles (except in some lower plant forms like Chlamydomonas). Spindle fibers during cell division are organized from microtubule organizing centers (MTOCs) in the cytoplasm.
*   **Animal Cell:** Possesses a centrosome containing two centrioles, which play a crucial role in organizing the spindle fibers during cell division.

### 5. Shape:
*   **Plant Cell:** Due to the presence of a rigid cell wall, plant cells typically have a fixed, regular, and often rectangular or cuboidal shape.
*   **Animal Cell:** Lacks a cell wall, allowing for a more flexible and often irregular or rounded shape.

### 6. Plasmodesmata vs. Cell Junctions:
*   **Plant Cell:** Intercellular communication and transport occur through plasmodesmata, which are cytoplasmic channels passing through the cell walls of adjacent plant cells.
*   **Animal Cell:** Intercellular communication and adhesion are facilitated by various types of cell junctions like tight junctions, gap junctions, and desmosomes.

### 7. Lysosomes:
*   **Plant Cell:** Lysosomes are generally absent or rare; their functions are often performed by the central vacuole.
*   **Animal Cell:** Contains numerous lysosomes, which are membrane-bound organelles containing hydrolytic enzymes for intracellular digestion and waste breakdown.

### 8. Storage Material:
*   **Plant Cell:** Stores carbohydrates primarily in the form of starch granules.
*   **Animal Cell:** Stores carbohydrates primarily in the form of glycogen granules.

### 9. Cytokinesis:
*   **Plant Cell:** During cell division, cytokinesis occurs by cell plate formation, where a new cell wall is synthesized between the two daughter nuclei.
*   **Animal Cell:** During cell division, cytokinesis occurs by furrow formation (cleavage furrow), where the plasma membrane pinches inward to divide the cytoplasm.

These differences highlight the evolutionary adaptations of plants for an autotrophic, sessile lifestyle and animals for a heterotrophic, motile existence.

Question 10

Q10. Describe the structure of the following with the help of labelled diagrams.
(i) Centrosome
(ii) Nucleus

Accepted Answer

### (i) Centrosome
The centrosome is an organelle typically containing two cylindrical structures called centrioles. It is surrounded by amorphous pericentriolar material. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel.

**Structure of Centriole:**
Each centriole is made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibril is a triplet. The adjacent triplets are also linked. The central part of the proximal region of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein. The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

### (ii) Nucleus
The nucleus is a large, spherical, and usually the most prominent organelle in a eukaryotic cell. It was first described by Robert Brown as a 'little nut'. It is the control center of the cell, containing the cell's genetic material.

**Structure of Nucleus:**
1.  **Nuclear Envelope:** The nucleus is bounded by a double-layered membrane called the nuclear envelope. The outer membrane is usually continuous with the endoplasmic reticulum and also bears ribosomes on it. The inner membrane is smooth. The space between the two membranes is called the perinuclear space (10-50 nm), which forms a barrier between the materials present inside the nucleus and that in the cytoplasm. The nuclear envelope is interrupted by minute pores called nuclear pores, which are formed by the fusion of its two membranes. These pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.
2.  **Nucleoplasm (Nuclear Matrix):** The nuclear envelope encloses the nucleoplasm, also known as the nuclear matrix. It is a transparent, semi-fluid, colloidal substance rich in proteins, enzymes, and nucleic acids. It contains the chromatin and the nucleolus.
3.  **Chromatin:** The nucleoplasm contains a network of thread-like structures called chromatin. Chromatin is composed of DNA and some basic proteins called histones, non-histone proteins, and also RNA. During different stages of cell division, cells show structured chromosomes in place of the nucleus. Chromatin can be broadly classified into:
    *   **Euchromatin:** Loosely packed, transcriptionally active chromatin.
    *   **Heterochromatin:** Densely packed, transcriptionally inactive chromatin.
4.  **Nucleolus:** The nucleolus is a spherical structure present in the nucleoplasm. It is a non-membrane bound structure. It is the site for ribosomal RNA (rRNA) synthesis and processing, and for the formation of ribosomal subunits. Cells actively carrying out protein synthesis have larger and more numerous nucleoli.

Question 11

Q11. What are the various components of endomembrane system? Why are they considered together?

Accepted Answer

### Components of the Endomembrane System
The endomembrane system is a group of organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins. The various components of the endomembrane system include:
1.  **Endoplasmic Reticulum (ER):** A network of tiny tubular structures scattered in the cytoplasm. It is involved in the synthesis of lipids (Smooth ER) and proteins (Rough ER, due to ribosomes). It also plays a role in detoxification and calcium storage.
2.  **Golgi Apparatus (or Golgi Complex/Bodies):** Consists of flattened, disc-shaped sacs or cisternae. It is primarily involved in the modification, sorting, and packaging of proteins and lipids synthesized in the ER, and their transport to various intracellular and extracellular destinations.
3.  **Lysosomes:** Membrane-bound vesicular structures formed by the packaging process in the Golgi apparatus. They contain hydrolytic enzymes (lipases, proteases, carbohydrases) that are active at acidic pH, responsible for digesting waste materials, cellular debris, and foreign particles.
4.  **Vacuoles:** Membrane-bound sacs found in the cytoplasm. In plant cells, a large central vacuole stores water, nutrients, and waste products, and maintains turgor pressure. In animal cells, vacuoles are generally smaller and more numerous, involved in storage and transport.

### Why are they considered together?
These organelles are considered together as an endomembrane system because their functions are coordinated and interdependent. Although each component is distinct in terms of structure and function, they work in a sequential and integrated manner to perform specific cellular tasks, primarily involving the synthesis, modification, transport, and degradation of macromolecules. For example:
*   Proteins and lipids are synthesized in the Endoplasmic Reticulum.
*   These are then transported to the Golgi apparatus for further modification, sorting, and packaging.
*   From the Golgi, they can be directed to lysosomes (e.g., enzymes), vacuoles (e.g., storage proteins), or secreted out of the cell.
*   Lysosomes, formed from the Golgi, are involved in the degradation of cellular waste, which might include components processed by other parts of the system.

This functional coordination highlights their collective role in the cell's internal transport and processing system, making them a 'system' rather than isolated organelles.

Question 12

Q12. Distinguish between:
(a) Prokaryotic cell and Eukaryotic cell
(b) Plant cell and Animal cell

Accepted Answer

Question 13

Q13. How are prokaryotic cells different from eukaryotic cells?

Accepted Answer

Prokaryotic cells and eukaryotic cells represent the two fundamental types of cells, differing significantly in their structural organization, particularly concerning the nucleus and membrane-bound organelles. Here's how they differ:

1.  **Nuclear Organization:**
    *   **Prokaryotic Cells:** Lack a true, membrane-bound nucleus. Their genetic material (a single, circular chromosome) is located in a region called the nucleoid, which is not enclosed by a nuclear envelope. Plasmids (small, circular DNA molecules) are often present.
    *   **Eukaryotic Cells:** Possess a well-defined, membrane-bound nucleus that houses their genetic material (multiple, linear chromosomes associated with histone proteins). The nuclear envelope separates the genetic material from the cytoplasm.

2.  **Membrane-bound Organelles:**
    *   **Prokaryotic Cells:** Do not have any membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, or chloroplasts. Their cellular functions are carried out in the cytoplasm or by specialized invaginations of the plasma membrane (e.g., mesosomes).
    *   **Eukaryotic Cells:** Characterized by the presence of numerous membrane-bound organelles, each performing specialized functions. Examples include mitochondria (for cellular respiration), endoplasmic reticulum (for protein and lipid synthesis), Golgi apparatus (for modification and packaging), lysosomes (for digestion), and chloroplasts (for photosynthesis in plant cells).

3.  **Size:**
    *   **Prokaryotic Cells:** Generally much smaller, typically ranging from 0.1 to 5.0 micrometers (µm) in diameter.
    *   **Eukaryotic Cells:** Generally larger, ranging from 5 to 100 micrometers (µm) in diameter.

4.  **Genetic Material:**
    *   **Prokaryotic Cells:** Have a single, circular chromosome, not associated with histones (except in archaea). Plasmids are common.
    *   **Eukaryotic Cells:** Have multiple, linear chromosomes, tightly packed with histone proteins to form chromatin.

5.  **Ribosomes:**
    *   **Prokaryotic Cells:** Possess 70S type ribosomes, which are smaller and free in the cytoplasm.
    *   **Eukaryotic Cells:** Possess 80S type ribosomes in the cytoplasm (free or attached to ER) and 70S type ribosomes in mitochondria and chloroplasts (reflecting their prokaryotic ancestry).

6.  **Cell Wall:**
    *   **Prokaryotic Cells:** Most have a rigid cell wall, typically made of peptidoglycan (in bacteria), which provides structural support and protection.
    *   **Eukaryotic Cells:** A cell wall is present in plants (made of cellulose) and fungi (made of chitin), but it is absent in animal cells.

7.  **Cell Division:**
    *   **Prokaryotic Cells:** Reproduce primarily by binary fission, a simpler process of cell division.
    *   **Eukaryotic Cells:** Divide by more complex processes of mitosis (for somatic cell division) and meiosis (for germ cell formation), involving a spindle apparatus.

In essence, the key difference lies in the compartmentalization and complexity. Eukaryotic cells are highly compartmentalized with specialized organelles, allowing for efficient execution of diverse metabolic processes, whereas prokaryotic cells are simpler, with most functions occurring in the cytoplasm or at the plasma membrane.

Question 14

Q14. Describe the structure of a centrosome.

Accepted Answer

A centrosome is an organelle typically found in animal cells, usually located near the nucleus. It is a non-membranous organelle that plays a crucial role in cell division and the formation of cilia and flagella.

**Structure of a Centrosome:**
1.  **Components:** A centrosome consists of two cylindrical structures called **centrioles**, which lie perpendicular to each other. These centrioles are surrounded by amorphous, electron-dense material known as **pericentriolar material (PCM)**.
2.  **Centriole Structure:** Each centriole is made up of nine evenly spaced peripheral fibrils of tubulin protein. These fibrils are arranged in a specific pattern:
    *   **Triplet Arrangement:** Each peripheral fibril is a triplet, meaning it consists of three closely associated microtubules (A, B, C). The adjacent triplets are linked to each other.
    *   **Cartwheel Organisation:** The arrangement of the peripheral triplets gives the centriole a 'cartwheel' appearance. The central part of the centriole is non-proteinaceous and is called the **hub**.
    *   **Radial Spokes:** The hub is connected to the peripheral triplets by nine proteinaceous **radial spokes**. These spokes are made of protein and radiate outwards from the hub to the 'A' tubule of each peripheral triplet.
3.  **Orientation:** The two centrioles within a centrosome are oriented perpendicular to each other, meaning they are at right angles.

**Functions (Briefly):**
*   The centrioles form the basal body of cilia and flagella.
*   They give rise to spindle fibres during cell division in animal cells, which help in the movement of chromosomes.

Question 15

Q15. What are the characteristics of a prokaryotic cell?

Accepted Answer

Prokaryotic cells are the most primitive and simplest type of cells, lacking a true nucleus and other membrane-bound organelles. They represent the organisms belonging to the Kingdom Monera, such as bacteria, blue-green algae (cyanobacteria), mycoplasma, and PPLO (Pleuro Pneumonia-Like Organisms).

Here are the key characteristics of a prokaryotic cell:

### 1. Size and Simplicity
*   **Small Size:** Prokaryotic cells are generally much smaller (typically 0.1-5.0 µm in diameter) than eukaryotic cells.
*   **Simple Organisation:** They have a relatively simple internal organisation, lacking complex compartmentalisation.

### 2. Genetic Material
*   **Nucleoid:** The genetic material is a single, circular chromosome (double-stranded DNA) that is not enclosed by a nuclear membrane. It lies naked in the cytoplasm in a region called the **nucleoid**.
*   **Plasmids:** Many prokaryotic cells (especially bacteria) also possess small, circular, extra-chromosomal DNA molecules called **plasmids**. Plasmids confer unique phenotypic traits like antibiotic resistance.

### 3. Cell Boundary
*   **Cell Wall:** Most prokaryotes (except Mycoplasma) possess a rigid **cell wall** that provides structural support and protection. In bacteria, the cell wall is primarily composed of **peptidoglycan** (murein).
*   **Cell Membrane:** A selectively permeable **cell membrane** (plasma membrane) is present beneath the cell wall. It is composed of lipids and proteins.
*   **Mesosomes:** The plasma membrane often forms characteristic infoldings called **mesosomes**, which are involved in respiration, cell wall formation, DNA replication, and secretion processes.

### 4. Organelles
*   **Absence of Membrane-bound Organelles:** Prokaryotic cells lack all membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, chloroplasts, and vacuoles.
*   **Ribosomes:** The only organelle present is the **ribosome**, which is responsible for protein synthesis. Prokaryotic ribosomes are of the **70S type** (composed of 50S and 30S subunits), which are smaller than eukaryotic (80S) ribosomes.

### 5. Other Structures
*   **Flagella:** Many motile prokaryotes possess **flagella**, which are thin, filamentous extensions from the cell wall, helping in locomotion. They are structurally different from eukaryotic flagella.
*   **Pili and Fimbriae:** These are surface structures that are not involved in motility. **Pili** are elongated tubular structures involved in bacterial conjugation (transfer of genetic material). **Fimbriae** are small, bristle-like fibres that help bacteria attach to surfaces or host tissues.
*   **Inclusion Bodies:** Prokaryotic cells often store reserve material in the cytoplasm in the form of **inclusion bodies** (e.g., phosphate granules, cyanophycean granules, glycogen granules, gas vacuoles). These are not membrane-bound.

### 6. Reproduction
*   **Asexual Reproduction:** Prokaryotes primarily reproduce asexually, mainly by **binary fission**. Sexual reproduction (involving fusion of gametes) is absent, though genetic recombination can occur through processes like conjugation, transformation, and transduction.

In summary, prokaryotic cells are characterised by their small size, lack of a true nucleus and membrane-bound organelles, presence of a nucleoid, 70S ribosomes, and a peptidoglycan cell wall (in most cases).

Chapter 8: Cell: The Unit of Life