Class 11 Biology NCERT Notes- Chapter 8: Cell – The Unit of Life

Detailed Study Notes – Chapter 8: Cell – The Unit of Life(Class 11 Biology, Notes, PDFs, Quizzes, MCQs)

1. The Fundamental Unit of Life

The cell is the foundational structural and functional unit of all living organisms. Its presence is the defining characteristic that distinguishes living things from non-living matter. A complete cell structure is the minimum requirement for independent existence and for performing the essential functions of life.

• Unicellular Organisms: Composed of a single cell, capable of independent existence and performing all life functions.
• Multicellular Organisms: Composed of many cells, which are organized into tissues and organs.

Key Historical Discoveries

• Antonie Von Leeuwenhoek: First to see and describe a live cell.
• Robert Brown (1831): Discovered the nucleus.
• Microscopy: The invention and improvement of the microscope, particularly the electron microscope, were crucial for revealing the detailed structure of the cell.

2. The Cell Theory

The Cell Theory provides a unifying framework for understanding the cellular basis of all life.

• Matthias Schleiden (1838): A German botanist who observed that all plants are composed of different kinds of cells that form plant tissues.
• Theodore Schwann (1839): A German zoologist who studied animal cells and noted they have a thin outer layer (plasma membrane). He also observed that plant cells have a unique cell wall. Schwann hypothesized that both animals and plants are composed of cells and their products.
• Rudolf Virchow (1855): Explained that new cells arise from the division of pre-existing cells, stating Omnis cellula-e cellula. This completed the theory.

Modern Tenets of Cell Theory

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

3. An Overview of Cell Structure and Types

Cells are broadly categorized into two types based on the presence or absence of a membrane-bound nucleus.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Lack a membrane-bound nucleus. Genetic material is “naked” in the cytoplasm.	Have a true, membrane-bound nucleus containing DNA.
Organelles	Lack membrane-bound organelles. Ribosomes are present.	Possess numerous membrane-bound organelles (ER, Golgi, etc.).
Examples	Bacteria, blue-green algae, Mycoplasma, PPLO.	Protists, fungi, plants, animals.
Size	Generally smaller (e.g., bacteria 3-5 µm).	Generally larger (e.g., typical eukaryotic cell 10-20 µm).

Common Cellular Components

• Cytoplasm: A semi-fluid matrix that occupies the volume of the cell. It is the main arena for cellular activities and chemical reactions that maintain the “living state.”
• Ribosomes: Non-membrane-bound organelles found in all cells, responsible for protein synthesis.
• Plasma Membrane: The outer boundary of animal cells and located just inside the cell wall of plant cells.

Cell Diversity in Size, Shape, and Activity

• Size: Varies from the smallest cells, Mycoplasma (0.3 µm), to the largest isolated single cell, the ostrich egg. Human red blood cells are about 7.0 µm in diameter.
• Shape: Diverse shapes include disc-like (RBCs), polygonal, columnar, cuboid, thread-like, branched (nerve cells), and irregular (white blood cells). Cell shape often correlates with its function.

4. Prokaryotic Cells

Prokaryotic cells have a fundamentally similar organization, although they exhibit a wide variety of shapes (bacillus, coccus, vibrio, spirillum) and functions. They are generally smaller and multiply more rapidly than eukaryotic cells.

4.1. Cell Envelope and Modifications

Most prokaryotes have a complex, three-layered cell envelope that acts as a single protective unit.

1. Glycocalyx (Outermost): Varies in composition and thickness. It can be a loose sheath called the slime layer or a thick, tough layer called the capsule.
2. Cell Wall: Surrounds the cell membrane (except in Mycoplasma). It determines the cell’s shape and provides structural support to prevent bursting or collapsing.
3. Plasma Membrane: The innermost layer, which is selectively permeable and structurally similar to the eukaryotic membrane.

• Gram Staining: A procedure developed by Gram to classify bacteria based on their cell envelope differences.
  • Gram-positive: Bacteria that take up the gram stain.
  • Gram-negative: Bacteria that do not take up the stain.
• Mesosomes: Specialized infoldings of the plasma membrane in the form of vesicles, tubules, and lamellae.
  • Functions: Cell wall formation, DNA replication and distribution, respiration, secretion processes, and increasing the surface area of the plasma membrane.
• Chromatophores: Membranous extensions into the cytoplasm found in some prokaryotes like cyanobacteria, which contain pigments.

4.2. Motility and Surface Structures

• Flagella: Thin, filamentous extensions from the cell wall responsible for motility. A bacterial flagellum consists of three parts: filament, hook, and basal body.
• Pili and Fimbriae: Surface structures that do not play a role in motility.
  • Pili: Elongated tubular structures made of a special protein.
  • Fimbriae: Small, bristle-like fibers that help bacteria attach to surfaces like rocks or host tissues.

4.3. Ribosomes and Inclusion Bodies

• Ribosomes: Associated with the plasma membrane, they are the site of protein synthesis.
  • Size: 70S, composed of 50S and 30S subunits.
  • Polysome (or Polyribosome): A chain of several ribosomes attached to a single mRNA molecule, which they translate into proteins.
• Inclusion Bodies: Reserve materials stored in the cytoplasm, not bound by any membrane.
  • Examples: Phosphate granules, cyanophycean granules, glycogen granules.
  • Gas Vacuoles: Found in blue-green, purple, and green photosynthetic bacteria.

4.4. Genetic Material

• Genomic DNA: The primary genetic material is a single, circular DNA molecule located in a region of the cytoplasm, but it is not enclosed by a nuclear membrane.
• Plasmids: Small, circular DNA molecules found outside the genomic DNA. They confer unique phenotypic characters, such as resistance to antibiotics.

5. Eukaryotic Cells

Eukaryotic cells are characterized by extensive compartmentalization of the cytoplasm through membrane-bound organelles and an organized nucleus with a nuclear envelope.

Key Differences Between Plant and Animal Cells

• Plant Cells: Possess cell walls, plastids, and a large central vacuole. Centrioles are absent in almost all plant cells.
• Animal Cells: Lack cell walls, plastids, and a large central vacuole. They contain centrioles.

5.1. Cell Membrane (Plasma Membrane)

The fluid mosaic model, proposed by Singer and Nicolson (1972), is the widely accepted model for the cell membrane structure.

• Composition:
  • Lipids: Primarily a phospholipid bilayer with polar heads facing outward and hydrophobic tails inward. Also contains cholesterol.
  • Proteins: Classified as integral (partially or totally buried in the membrane) or peripheral (lie on the surface).
  • Carbohydrates: Also present.
  • Example Ratio: Human red blood cell (erythrocyte) membranes are approximately 52% protein and 40% lipids.
• Fluidity: The quasi-fluid nature of the lipids allows for the lateral movement of proteins within the bilayer. This fluidity is important for functions like cell growth, secretion, endocytosis, and cell division.
• Transport Functions: The membrane is selectively permeable.
  • Passive Transport: Movement of molecules across the membrane without energy expenditure.
    • Simple Diffusion: Movement of neutral solutes along a concentration gradient (high to low).
    • Osmosis: The diffusion of water across the membrane.
  • Facilitated Diffusion: Polar molecules require a carrier protein to move across the membrane along the concentration gradient.
  • Active Transport: Movement of ions or molecules against a concentration gradient (low to high). This process requires energy (ATP). Example: Na+/K+ Pump.

5.2. Cell Wall

A non-living, rigid structure outside the plasma membrane in fungi and plants.

• Functions: Provides shape, protects from mechanical damage and infection, aids in cell-to-cell interaction, and acts as a barrier to undesirable macromolecules.
• Composition:
  • Algae: Cellulose, galactans, mannans, and minerals like calcium carbonate.
  • Other Plants: Cellulose, hemicellulose, pectins, and proteins.
• Structure:
  • Primary Wall: Found in young plant cells; capable of growth.
  • Secondary Wall: Formed on the inner side of the primary wall as the cell matures.
  • Middle Lamella: A layer mainly of calcium pectate that glues neighboring cells together.
  • Plasmodesmata: Channels that traverse the cell wall and middle lamella, connecting the cytoplasm of adjacent cells.

5.3. Endomembrane System

A group of membranous organelles whose functions are coordinated.

• Includes: Endoplasmic Reticulum (ER), Golgi complex, lysosomes, and vacuoles.
• Excludes: Mitochondria, chloroplasts, and peroxisomes, as their functions are not coordinated with this system.
• Endoplasmic Reticulum (ER): A network of tiny tubular structures scattered in the cytoplasm, dividing the intracellular space into luminal (inside ER) and extraluminal (cytoplasm) compartments.
  • Rough ER (RER): Surface bears ribosomes. It is actively involved in protein synthesis and secretion and is continuous with the outer nuclear membrane.
  • Smooth ER (SER): Lacks ribosomes. It is the major site for lipid synthesis. In animal cells, it synthesizes lipid-like steroidal hormones.
• Golgi Apparatus (or Golgi Complex): First observed by Camillo Golgi (1898). It consists of flat, disc-shaped sacs called cisternae (0.5µm to 1.0µm diameter) stacked parallel to each other.
  • Structure: Has a distinct convex cis (forming) face and a concave trans (maturing) face.
  • Function: Packages materials for delivery to intracellular targets or for secretion. Proteins from the RER are modified in the cisternae before release. It is the main site for the formation of glycoproteins and glycolipids.
• Lysosomes: Membrane-bound vesicular structures formed by packaging in the Golgi apparatus.
  • Content: Rich in hydrolytic enzymes (lipases, proteases, carbohydrases) that are optimally active at an acidic pH.
  • Function: Capable of digesting carbohydrates, proteins, lipids, and nucleic acids.
• Vacuoles: A membrane-bound space in the cytoplasm containing water, sap, and excretory products.
  • Tonoplast: The single membrane bounding the vacuole.
  • In Plant Cells: Can occupy up to 90% of the cell volume. The tonoplast transports ions against their concentration gradient into the vacuole.
  • In Amoeba: The contractile vacuole is important for osmoregulation and excretion.
  • In Protists: Food vacuoles are formed by engulfing food particles.

5.4. Mitochondria

Sausage-shaped or cylindrical organelles, known as the “power houses” of the cell.

• Structure: A double membrane-bound organelle.
  • Outer Membrane: Forms a continuous boundary.
  • Inner Membrane: Forms numerous infoldings called cristae, which increase the surface area.
  • Compartments: The lumen is divided into an outer compartment and an inner compartment filled with a dense substance called the matrix.
• Function: The sites of aerobic respiration, producing cellular energy in the form of ATP.
• Unique Features: The matrix contains a single circular DNA molecule, a few RNA molecules, and 70S ribosomes. Mitochondria divide by fission.

5.5. Plastids

Found in all plant cells and in euglenoids. They are classified based on the pigments they contain.

• Chloroplasts: Contain chlorophyll and carotenoid pigments. Responsible for trapping light energy for photosynthesis.
• Chromoplasts: Contain fat-soluble carotenoid pigments like carotene and xanthophylls, giving plants yellow, orange, or red colors.
• Leucoplasts: Colourless plastids that store nutrients.
  • Amyloplasts: Store carbohydrates (starch), e.g., potato.
  • Elaioplasts: Store oils and fats.
  • Aleuroplasts: Store proteins.
• Chloroplast Structure:
  • Membranes: Double membrane-bound; the inner membrane is less permeable.
  • Stroma: The space within the inner membrane, containing enzymes for carbohydrate and protein synthesis.
  • Thylakoids: Organized, flattened membranous sacs within the stroma. Chlorophyll pigments are located here. The membrane encloses a space called the lumen.
  • Grana (sing. Granum): Stacks of thylakoids, resembling piles of coins.
  • Stroma Lamellae: Flat membranous tubules connecting the thylakoids of different grana.
  • Unique Features: Contain small, double-stranded circular DNA and 70S ribosomes (smaller than the 80S cytoplasmic ribosomes).

5.6. Ribosomes

Granular, non-membranous structures composed of ribonucleic acid (RNA) and proteins. First observed by George Palade (1953).

• Function: Site of protein synthesis.
• Types and Subunits:
  • Eukaryotic: 80S (composed of 60S and 40S subunits).
  • Prokaryotic: 70S (composed of 50S and 30S subunits).
  • ‘S’ (Svedberg’s Unit): Stands for the sedimentation coefficient, an indirect measure of density and size.

5.7. Cytoskeleton

An elaborate network of filamentous proteinaceous structures in the cytoplasm.

• Components: Microtubules, microfilaments, and intermediate filaments.
• Functions: Mechanical support, motility, and maintenance of cell shape.

5.8. Cilia and Flagella

Hair-like outgrowths of the cell membrane involved in movement.

• Cilia: Small structures that work like oars to move the cell or surrounding fluid.
• Flagella: Comparatively longer structures responsible for cell movement. Eukaryotic flagella are structurally different from prokaryotic flagella.
• Internal Structure (Axoneme):
  • The core is called the axoneme, covered by the plasma membrane.
  • It possesses microtubules arranged in a 9+2 array: nine doublets of peripheral microtubules and a pair of central microtubules.
  • The central tubules are connected by bridges and enclosed by a central sheath.
  • Nine radial spokes connect the central sheath to the peripheral doublets.
• Basal Bodies: Cilia and flagella emerge from a centriole-like structure called the basal body.

5.9. Centrosome and Centrioles

An organelle found in animal cells, usually containing two cylindrical structures called centrioles.

• Structure: The two centrioles lie perpendicular to each other. Each has a “cartwheel” organization, made of nine evenly spaced peripheral fibrils of tubulin protein, where each fibril is a triplet.
• Function: Centrioles form the basal bodies of cilia and flagella. They also form the spindle fibers that give rise to the spindle apparatus during cell division in animal cells.

5.10. Nucleus

A large organelle first described by Robert Brown (1831). It contains the cell’s genetic material.

• Nuclear Envelope: A double membrane that separates the nucleus from the cytoplasm.
  • The space between the two membranes is the perinuclear space (10-50 nm).
  • The outer membrane is continuous with the ER and may have ribosomes.
  • Nuclear Pores: Interruptions in the envelope formed by the fusion of the two membranes. They regulate the movement of RNA and protein between the nucleus and cytoplasm.
• Nucleoplasm (Nuclear Matrix): The material filling the nucleus. Contains the nucleolus and chromatin.
• Nucleolus (pl. Nucleoli): A spherical, non-membrane-bound structure within the nucleoplasm. It is the site of active ribosomal RNA (rRNA) synthesis. Cells active in protein synthesis have larger and more numerous nucleoli.
• Chromatin: A loose network of nucleoprotein fibers seen in a non-dividing (interphase) nucleus.
  • Composition: DNA, basic proteins called histones, non-histone proteins, and RNA.
• Chromosomes: During cell division, chromatin condenses to form structured chromosomes.
  • A single human cell has about two meters of DNA distributed among 46 chromosomes.
  • Structure: Each chromosome has a primary constriction called the centromere, which holds two chromatids together. Disc-shaped structures called kinetochores are present on the sides of the centromere.
  • Classification by Centromere Position:
    • Metacentric: Centromere in the middle; two equal arms.
    • Sub-metacentric: Centromere slightly off-center; one short and one long arm.
    • Acrocentric: Centromere near one end; one extremely short and one very long arm.
    • Telocentric: Centromere at the terminal end.
  • Satellite: A small fragment-like appearance on some chromosomes due to a non-staining secondary constriction at a constant location.

5.11. Microbodies

Many membrane-bound, minute vesicles containing various enzymes. They are present in both plant and animal cells.

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Q & A

Short-Answer Questions (2-3 sentences each)

1. What are the two main tenets of the modern cell theory, and which scientist was responsible for the second tenet?
2. Differentiate between a unicellular and a multicellular organism based on their cellular composition and capabilities.
3. Describe the key structural difference between a prokaryotic and a eukaryotic cell regarding genetic material.
4. What is a mesosome, and what are three of its functions in a prokaryotic cell?
5. Explain the difference between a slime layer and a capsule in the bacterial cell envelope.
6. How are Gram-positive and Gram-negative bacteria distinguished from one another?
7. What is a plasmid, and what is a significant function it can confer upon a bacterium?
8. According to the fluid-mosaic model, what are the two main components of the cell membrane, and how are they arranged?
9. Explain the difference between active and passive transport across the plasma membrane, mentioning the role of energy.
10. What is osmosis, and in which direction does water move during this process?
11. Describe the composition and function of the middle lamella in plant tissues.
12. List the four organelles that comprise the endomembrane system and explain why they are considered a system.
13. Distinguish between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER) based on structure and primary function.
14. What is the primary function of the Golgi apparatus, and what are its cis and trans faces?
15. Why are lysosomes known for their digestive capabilities? What do they contain?
16. What is the tonoplast, and how does it contribute to the high concentration of ions inside a plant vacuole?
17. Why are mitochondria often referred to as the “power houses” of the cell? What process occurs there?
18. Name the three types of plastids and the primary function or substance stored by each.
19. Describe the internal structure of a chloroplast, mentioning the stroma, thylakoids, and grana.
20. Compare the ribosomes found in prokaryotes and eukaryotes in terms of their Svedberg units and subunits.
21. What is the cytoskeleton, and what are its three main functions in a cell?
22. Describe the “9+2 array” found in the axoneme of eukaryotic cilia and flagella.
23. What are centrioles, and what two major structures do they form in animal cells?
24. Explain the function of nuclear pores in the nuclear envelope.
25. Based on the position of the centromere, name and describe two of the four types of chromosomes.

Multiple-Choice Questions (MCQs)

1. Who first explained that new cells are formed from pre-existing cells (Omnis cellula-e cellula)? a) Robert Brown b) Matthias Schleiden c) Theodore Schwann d) Rudolf Virchow
2. Which of the following is a non-membrane bound organelle found in both prokaryotic and eukaryotic cells? a) Golgi complex b) Ribosome c) Mitochondrion d) Lysosome
3. The smallest known cells, only 0.3 µm in length, are: a) Viruses b) PPLO c) Mycoplasmas d) Bacteria
4. The cell envelope of a prokaryotic cell consists of how many layers acting as a single protective unit? a) One b) Two c) Three d) Four
5. In prokaryotes, reserve materials are stored in the cytoplasm in the form of: a) Mesosomes b) Vacuoles c) Inclusion bodies d) Chromatophores
6. In the human erythrocyte membrane, what is the approximate percentage of proteins and lipids? a) 40% protein, 52% lipids b) 52% protein, 40% lipids c) 50% protein, 50% lipids d) 70% protein, 30% lipids
7. The movement of water across a selectively permeable membrane by diffusion is called: a) Active transport b) Simple diffusion c) Osmosis d) Endocytosis
8. The cell wall of algae is made of: a) Cellulose, hemicellulose, and pectins b) Cellulose, galactans, mannans, and minerals c) Only calcium pectate d) Only proteins and lipids
9. Which organelle is the major site for the synthesis of lipids and steroidal hormones? a) Rough ER b) Smooth ER c) Golgi apparatus d) Lysosome
10. The formation of glycoproteins and glycolipids is an important function of the: a) Mitochondrion b) Vacuole c) Peroxisome d) Golgi apparatus
11. The infoldings of the inner mitochondrial membrane are called: a) Grana b) Cristae c) Thylakoids d) Cisternae
12. Which type of leucoplast is responsible for storing oils and fats? a) Amyloplasts b) Aleuroplasts c) Elaioplasts d) Chromoplasts
13. The ribosomes of a chloroplast are of which type? a) 80S b) 70S c) 60S d) 50S
14. The hair-like outgrowths of the cell membrane that work like oars are: a) Flagella b) Pili c) Cilia d) Fimbriae
15. The “cartwheel” organization is characteristic of which structure? a) Nucleolus b) Ribosome c) Centriole d) Axoneme
16. The site of active ribosomal RNA (rRNA) synthesis is the: a) Nucleoplasm b) Nuclear pore c) Nucleolus d) Cytoplasm
17. Chromatin is composed of DNA and basic proteins called: a) Albumins b) Globulins c) Histones d) Tubulins
18. A chromosome with a terminal centromere is classified as: a) Metacentric b) Sub-metacentric c) Acrocentric d) Telocentric
19. Gas vacuoles are found in: a) All protists and fungi b) Animal cells c) Blue-green and purple photosynthetic bacteria d) All plant cells
20. The two subunits of an 80S eukaryotic ribosome are: a) 50S and 30S b) 60S and 30S c) 50S and 40S d) 60S and 40S

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Answer Keys (Q&A)

Answer Key for Short-Answer Questions

1. The two main tenets are: (i) all living organisms are composed of cells and products of cells, and (ii) all cells arise from pre-existing cells. Rudolf Virchow (1855) was responsible for explaining the second tenet.
2. Unicellular organisms consist of a single cell that is capable of independent existence and performs all essential life functions. Multicellular organisms are composed of many cells, which are typically organized into tissues with a division of labor.
3. In a prokaryotic cell, the genetic material is “naked,” meaning it is a single circular DNA molecule not enclosed by a nuclear membrane. A eukaryotic cell has an organized, membrane-bound nucleus that contains its genetic material organized into chromosomes.
4. A mesosome is a specialized, differentiated form of cell membrane in prokaryotes, formed by infoldings into the cell. Its functions include cell wall formation, DNA replication and distribution to daughter cells, and helping in respiration and secretion processes.
5. Both are forms of the glycocalyx. The slime layer is a loose sheath, while the capsule is a thick and tough protective layer.
6. They are distinguished by their response to the Gram staining procedure, which is based on differences in their cell envelopes. Gram-positive bacteria take up the stain, while Gram-negative bacteria do not.
7. A plasmid is a small, circular DNA molecule found in many bacteria, separate from the main genomic DNA. A significant function is conferring unique phenotypic traits, such as resistance to antibiotics.
8. According to the fluid mosaic model, the cell membrane is composed of a phospholipid bilayer and proteins. The lipids are arranged with their polar heads towards the outer sides and hydrophobic tails towards the inner part, while proteins are embedded within or on the surface of this bilayer.
9. Passive transport does not require energy and involves the movement of substances along their concentration gradient (from high to low). Active transport requires energy (ATP) to move substances against their concentration gradient (from low to high).
10. Osmosis is the movement of water by diffusion across a selectively permeable membrane. Water moves from a region of its higher concentration to a region of its lower concentration.
11. The middle lamella is a layer found between the cell walls of adjacent plant cells. It is mainly composed of calcium pectate and its function is to hold or glue the neighboring cells together.
12. The endomembrane system includes the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles. They are considered a system because their functions are coordinated; for example, proteins synthesized in the ER are modified and packaged by the Golgi, which then forms lysosomes or transport vesicles.
13. RER has ribosomes attached to its surface and is primarily involved in protein synthesis and secretion. SER lacks ribosomes, appears smooth, and is the major site for the synthesis of lipids and steroidal hormones.
14. The primary function of the Golgi apparatus is to package materials for delivery within the cell or for secretion outside the cell. It has a convex cis or forming face that receives vesicles from the ER and a concave trans or maturing face from which modified materials are released.
15. Lysosomes are known for digestion because their vesicles contain a rich mixture of hydrolytic enzymes (hydrolases like lipases, proteases, carbohydrases). These enzymes are capable of breaking down almost all types of macromolecules.
16. The tonoplast is the single membrane that bounds the vacuole in plant cells. It facilitates the active transport of ions and other materials from the cytoplasm into the vacuole against their concentration gradients, leading to a higher concentration inside.
17. Mitochondria are called “power houses” because they are the sites of aerobic respiration. During this process, they produce cellular energy in the form of ATP (adenosine triphosphate).
18. The three types of plastids are: (1) Chloroplasts, which contain chlorophyll and are sites of photosynthesis; (2) Chromoplasts, which contain carotenoid pigments that give color to plants; and (3) Leucoplasts, which are colorless and store nutrients like starch (amyloplasts), oils (elaioplasts), or proteins (aleuroplasts).
19. A chloroplast’s inner membrane encloses a space called the stroma. Within the stroma are flattened membranous sacs called thylakoids, which are arranged in stacks known as grana.
20. Prokaryotic ribosomes are 70S, made of 50S and 30S subunits. Eukaryotic ribosomes are larger at 80S, made of 60S and 40S subunits.
21. The cytoskeleton is an elaborate network of filamentous proteinaceous structures. Its functions include providing mechanical support, enabling motility, and maintaining the shape of the cell.
22. The “9+2 array” is the arrangement of microtubules in the axoneme. It consists of nine doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules.
23. Centrioles are cylindrical structures found in the centrosome of animal cells. They form the basal bodies from which cilia and flagella emerge, and they also form the spindle fibers of the spindle apparatus during cell division.
24. Nuclear pores are passages in the nuclear envelope formed by the fusion of its two membranes. Their function is to regulate the movement of RNA and protein molecules in both directions between the nucleus and the cytoplasm.
25. Two types are: (1) Metacentric, where the centromere is in the middle, creating two equal arms. (2) Acrocentric, where the centromere is situated close to one end, resulting in one extremely short arm and one very long arm.

Answer Key for MCQs

1. (d) Rudolf Virchow
2. (b) Ribosome
3. (c) Mycoplasmas
4. (c) Three
5. (c) Inclusion bodies
6. (b) 52% protein, 40% lipids
7. (c) Osmosis
8. (b) Cellulose, galactans, mannans, and minerals
9. (b) Smooth ER
10. (d) Golgi apparatus
11. (b) Cristae
12. (c) Elaioplasts
13. (b) 70S
14. (c) Cilia
15. (c) Centriole
16. (c) Nucleolus
17. (c) Histones
18. (d) Telocentric
19. (c) Blue-green and purple photosynthetic bacteria
20. (d) 60S and 40S

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IV. Essay Questions and Model Answers

1. Discuss the development of the Cell Theory, highlighting the contributions of Schleiden, Schwann, and Virchow.

Answer: The Cell Theory is a cornerstone of modern biology, developed through the cumulative work of several scientists. In 1838, German botanist Matthias Schleiden examined numerous plants and concluded that all plants are composed of different types of cells, which form their tissues. A year later, in 1839, German zoologist Theodore Schwann studied animal cells and reported they had a thin outer layer, now known as the plasma membrane. He extended his research to plants and noted the unique presence of a cell wall. Based on these observations, Schwann proposed the hypothesis that the bodies of both animals and plants are composed of cells and their products. Together, Schleiden and Schwann formulated the initial cell theory. However, their theory did not explain how new cells were formed. This gap was filled in 1855 by Rudolf Virchow, who explained that cells divide and new cells are formed from pre-existing cells, a concept summarized by his statement Omnis cellula-e cellula. Virchow’s contribution modified the original hypothesis, giving the cell theory its final shape as understood today: all living organisms are composed of cells and their products, and all cells arise from pre-existing cells.

2. Compare and contrast the structure and function of prokaryotic and eukaryotic cells, focusing on at least five key differences.

Answer: Prokaryotic and eukaryotic cells represent the two fundamental types of cellular organization. The most significant difference is the nucleus: eukaryotic cells possess a true, membrane-bound nucleus that houses their organized chromosomes, whereas prokaryotic cells lack a nuclear membrane, and their single, circular DNA molecule lies naked in the cytoplasm. Secondly, eukaryotic cells feature extensive internal compartmentalization with numerous membrane-bound organelles like the endoplasmic reticulum, Golgi complex, and mitochondria, each performing specific functions. Prokaryotic cells lack such membrane-bound organelles. Thirdly, the ribosomes differ; eukaryotic cells have larger 80S ribosomes in their cytoplasm, while prokaryotes have smaller 70S ribosomes. Fourth, there is a difference in size and complexity: prokaryotic cells are generally smaller (1-5 µm) and simpler, allowing them to multiply more rapidly, while eukaryotic cells are larger (10-20 µm) and more complex. Finally, the cell wall composition differs; the bacterial cell wall is chemically complex, whereas the cell walls of eukaryotic plants and fungi are made of materials like cellulose and chitin, respectively. Animal cells, a type of eukaryotic cell, lack a cell wall entirely.

3. Describe the fluid mosaic model of the plasma membrane. Explain how its composition and structure relate to its function, including selective permeability and transport.

Answer: The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the plasma membrane as a dynamic and fluid structure. The model’s foundation is a phospholipid bilayer, which provides a quasi-fluid matrix. In this bilayer, the hydrophilic polar heads of the phospholipids face the aqueous environments on the outside and inside of the cell, while the hydrophobic nonpolar tails are shielded in the interior. Embedded within or associated with this lipid bilayer are proteins, which can be integral (partially or totally buried) or peripheral (on the surface). This arrangement creates a “mosaic” of proteins floating in a “fluid” lipid sea. The fluidity, which allows for the lateral movement of proteins, is crucial for functions like cell growth, secretion, and division.

This structure directly enables the membrane’s primary function of transport and selective permeability. The nonpolar lipid core prevents the free passage of polar molecules and ions. Passive transport occurs when small, neutral solutes move down their concentration gradient via simple diffusion. Facilitated diffusion is required for polar molecules, which use specific carrier proteins to cross the membrane. Active transport, such as the Na+/K+ pump, utilizes energy (ATP) and specific transport proteins to move molecules against their concentration gradient. Thus, the combination of a lipid barrier and specific protein channels and pumps allows the membrane to control precisely what enters and exits the cell.

4. Detail the journey of a protein destined for secretion from a eukaryotic cell, starting from its synthesis to its exit. Mention all the organelles involved in the endomembrane system.

Answer: The synthesis and secretion of a protein is a coordinated process involving the endomembrane system. The journey begins at the Rough Endoplasmic Reticulum (RER), where ribosomes attached to its surface synthesize the polypeptide chain, which enters the luminal space of the RER. Inside the RER, the protein may be folded and modified.

Next, the protein is packaged into small transport vesicles that bud off from the RER. These vesicles travel through the cytoplasm and fuse with the cis face (the forming face) of the Golgi apparatus. As the protein moves through the Golgi cisternae towards the trans face (the maturing face), it undergoes further modification, sorting, and packaging. For instance, sugars can be added to form a glycoprotein.

Finally, at the trans face of the Golgi, the finished protein is packaged into secretory vesicles. These vesicles move to the plasma membrane, fuse with it, and release their contents outside the cell in a process called exocytosis. Throughout this pathway, the interconnected functions of the ER and Golgi apparatus highlight the coordinated nature of the endomembrane system in processing and trafficking cellular materials.

5. Explain the structure of mitochondria and chloroplasts. What unique features do these organelles share, and how do these features support the theory of their evolutionary origin?

Answer: Mitochondria and chloroplasts are double membrane-bound organelles with distinct internal structures related to their functions. A mitochondrion has a smooth outer membrane and an inner membrane folded into cristae, which increase surface area. The inner membrane encloses the matrix, the site of many metabolic reactions. Mitochondria are the sites of aerobic respiration and ATP production. A chloroplast is also enclosed by a double membrane. Its inner membrane encloses the stroma, which contains flattened, interconnected sacs called thylakoids. Thylakoids are often stacked into grana and are the site of the light-dependent reactions of photosynthesis.

These two organelles share several remarkable features. Both possess their own single, circular DNA molecule, similar to that of prokaryotes. They also have their own 70S ribosomes, which are characteristic of prokaryotes, not the 80S ribosomes found in the eukaryotic cytoplasm. Furthermore, both organelles are capable of reproducing independently of the cell’s nuclear division through a process of fission. These shared characteristics strongly support the endosymbiotic theory, which proposes that mitochondria and chloroplasts originated from free-living prokaryotic cells that were engulfed by an ancestral eukaryotic host cell and formed a symbiotic relationship.

6. Describe the structure of a eukaryotic nucleus. What are its main components and their respective functions?

Answer: The nucleus is the control center of the eukaryotic cell, containing the genetic material. Its structure is highly organized. It is enclosed by a nuclear envelope, a double membrane with a perinuclear space between the two layers. The outer membrane is often continuous with the endoplasmic reticulum. This envelope is perforated by numerous nuclear pores, which are complex structures that regulate the transport of molecules like RNA and proteins between the nucleus and the cytoplasm.

Inside the envelope is the nucleoplasm or nuclear matrix, a semi-fluid substance that contains the other nuclear components. Suspended in the nucleoplasm is the nucleolus, a dense, non-membranous body that is the primary site of ribosomal RNA (rRNA) synthesis and ribosome assembly. The most prominent component of the nucleus is chromatin, a complex of DNA and proteins (primarily histones). In a non-dividing cell, chromatin appears as a diffuse, indistinct network. During cell division, chromatin condenses to form discrete, visible structures called chromosomes.

7. How are bacterial cells classified based on their shape and their response to Gram staining? Provide examples of shapes.

Answer: Bacterial cells are classified based on several criteria, including their shape and cell envelope structure. Morphologically, there are four basic shapes of bacteria: bacillus (rod-like), coccus (spherical), vibrio (comma-shaped), and spirillum (spiral). This shape is determined and maintained by the rigid cell wall.

A second major classification is based on the response to the Gram staining procedure, which reflects differences in the chemical complexity of their cell envelopes. Bacteria that take up the gram stain and appear purple are classified as Gram-positive. Those that do not retain the stain and are counterstained pink or red are classified as Gram-negative. This difference in staining is due to variations in the structure and composition of the cell wall and outer layers of the cell envelope.

8. What is the cytoskeleton? Describe its components and explain its importance in maintaining cell shape, motility, and providing mechanical support.

Answer: The cytoskeleton is an elaborate and dynamic network of filamentous proteinaceous structures found in the cytoplasm of eukaryotic cells. It is not a static scaffold but can be quickly disassembled and reassembled, allowing for changes in cell shape and movement. The cytoskeleton is composed of three main types of filaments: microtubules, microfilaments, and intermediate filaments.

The functions of the cytoskeleton are critical for the cell. It provides mechanical support, acting as an internal framework that helps the cell resist deformation. It is essential for maintaining the shape of the cell, giving animal cells, which lack a cell wall, their characteristic and often irregular shapes. Finally, the cytoskeleton is centrally involved in motility, including the movement of organelles within the cell, the crawling movement of cells like amoebas, and the beating of cilia and flagella, whose core axoneme is a highly organized arrangement of microtubules.

9. Explain the classification of chromosomes based on the position of the centromere, using diagrams to support the description.

Answer: Chromosomes in dividing eukaryotic cells are classified into four types based on the position of their primary constriction, the centromere. The position of the centromere determines the relative length of the chromosome arms.

1. Metacentric: The centromere is located in the middle, dividing the chromosome into two equal arms.
2. Sub-metacentric: The centromere is located slightly away from the middle, resulting in one arm being shorter than the other.
3. Acrocentric: The centromere is situated very close to one end, creating one extremely short arm and one very long arm.
4. Telocentric: The centromere is located at the terminal end of the chromosome, so there is effectively only one arm.

(Note: Diagrams will be uploaded soon.)

10. What is the role of vacuoles in plant cells and in unicellular organisms like Amoeba? How does the structure of the tonoplast relate to its function in plants?

Answer: Vacuoles are membrane-bound sacs that play different roles depending on the organism. In plant cells, the large central vacuole is a defining feature, often occupying up to 90% of the cell’s volume. Its primary functions include storing water, sap, nutrients, and waste products. It is also crucial for maintaining turgor pressure against the cell wall, which supports the plant’s structure.

In unicellular organisms like Amoeba, the contractile vacuole has a different primary role: osmoregulation. It collects excess water from the cytoplasm and expels it from the cell, preventing the cell from bursting in a hypotonic environment. In other protists, food vacuoles are formed by engulfing food particles for intracellular digestion.

The membrane surrounding the plant vacuole is called the tonoplast. This membrane is selectively permeable and contains transport proteins that actively pump ions and other materials from the cytoplasm into the vacuole, often against their concentration gradient. This active transport mechanism is why the concentration of solutes is significantly higher in the vacuolar sap than in the cytoplasm, which in turn facilitates the movement of water into the vacuole by osmosis and helps maintain turgor pressure.

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V. Glossary of Key Terms

Term	Definition
Acrocentric Chromosome	A chromosome where the centromere is situated close to its end, forming one extremely short and one very long arm.
Active Transport	The transport of ions or molecules across a membrane against their concentration gradient, a process that requires energy (ATP).
Aleuroplasts	Colourless plastids that store proteins.
Amyloplasts	Colourless plastids that store carbohydrates (starch).
Axoneme	The core of a cilium or flagellum, possessing a number of microtubules running parallel to the long axis, typically in a 9+2 array.
Basal Body	A centriole-like structure from which a cilium or flagellum emerges.
Centriole	A cylindrical structure found in the centrosome of animal cells, organized in a “cartwheel” pattern of microtubule triplets.
Centromere	The primary constriction on a chromosome that holds the two chromatids together.
Chloroplasts	Plastids containing chlorophyll and carotenoid pigments, responsible for trapping light energy for photosynthesis.
Chromatin	A network of nucleoprotein fibers (DNA, histones, and other proteins) found in the interphase nucleus.
Chromatophores	Membranous extensions into the cytoplasm of some prokaryotes (like cyanobacteria) that contain pigments.
Chromoplasts	Plastids containing fat-soluble carotenoid pigments like carotene and xanthophylls, giving parts of the plant a yellow, orange, or red color.
Cilia	Small, hair-like outgrowths of the cell membrane that work like oars to cause movement of the cell or the surrounding fluid.
Cisternae	Flat, disc-shaped sacs found in the Golgi apparatus and endoplasmic reticulum.
Cristae	The infoldings of the inner mitochondrial membrane, which increase its surface area.
Cytoplasm	The semi-fluid matrix that occupies the volume of the cell and is the main arena of cellular activities.
Cytoskeleton	An elaborate network of filamentous proteinaceous structures (microtubules, microfilaments, intermediate filaments) in the cytoplasm involved in support, motility, and shape maintenance.
Elaioplasts	Colourless plastids that store oils and fats.
Endomembrane System	A group of coordinated membranous organelles including the ER, Golgi complex, lysosomes, and vacuoles.
Eukaryotic Cell	A cell that has a membrane-bound nucleus and other membrane-bound organelles.
Flagella	Comparatively long, hair-like outgrowths of the cell membrane responsible for cell movement.
Fluid Mosaic Model	The widely accepted model of the cell membrane structure, describing it as a quasi-fluid lipid bilayer with proteins embedded in it.
Glycocalyx	The outermost layer of the prokaryotic cell envelope, which can be a loose slime layer or a tough capsule.
Golgi Apparatus	An organelle consisting of stacked, flat membranous sacs (cisternae) that packages and modifies materials for transport.
Grana (sing. Granum)	Stacks of thylakoids found in the stroma of chloroplasts.
Inclusion Bodies	Non-membranous structures in the cytoplasm of prokaryotes where reserve materials are stored.
Kinetochores	Disc-shaped structures present on the sides of the centromere of a chromosome.
Leucoplasts	Colourless plastids of varied shapes and sizes with stored nutrients.
Lysosomes	Membrane-bound vesicular structures containing hydrolytic enzymes capable of digesting macromolecules.
Mesosome	A specialized infolding of the plasma membrane in prokaryotes, involved in cell wall formation, DNA replication, and respiration.
Metacentric Chromosome	A chromosome with a centromere in the middle, forming two equal arms.
Microbodies	Many membrane-bound, minute vesicles that contain various enzymes, present in both plant and animal cells.
Middle Lamella	A layer mainly of calcium pectate that holds or glues neighboring plant cells together.
Mitochondria	Double membrane-bound organelles that are the sites of aerobic respiration and ATP synthesis; the “power houses” of the cell.
Nucleoid	The region in a prokaryotic cell where the naked, circular genetic material is located (not enclosed by a membrane).
Nucleolus	A spherical, non-membranous structure in the nucleoplasm that is the site for active ribosomal RNA (rRNA) synthesis.
Osmosis	The movement of water by diffusion across a selectively permeable membrane.
Passive Transport	The movement of molecules across a membrane without any requirement of energy, occurring along a concentration gradient.
Plasmids	Small, circular DNA molecules outside the genomic DNA in many bacteria, often conferring unique traits like antibiotic resistance.
Plasmodesmata	Cytoplasmic channels that traverse the cell walls of adjacent plant cells, connecting them.
Plastids	Pigment-containing organelles found in plant cells and euglenoides.
Polysome	A chain of several ribosomes attached to a single mRNA molecule, translating it into proteins.
Prokaryotic Cell	A cell that lacks a membrane-bound nucleus and other membrane-bound organelles.
Ribosomes	Non-membranous granular structures composed of RNA and protein that are the site of protein synthesis.
Svedberg’s Unit (S)	A unit for the sedimentation coefficient, used to characterize ribosome subunits and sizes; an indirect measure of density and size.
Sub-metacentric Chromosome	A chromosome where the centromere is slightly away from the middle, resulting in one shorter and one longer arm.
Telocentric Chromosome	A chromosome that has a terminal centromere.
Thylakoids	A number of organized, flattened membranous sacs present in the stroma of chloroplasts, containing chlorophyll.
Tonoplast	The single membrane that binds the vacuole in a plant cell.
Vacuole	A membrane-bound space found in the cytoplasm that contains water, sap, and other materials.