Chapter 5: The Fundamental Unit of Life
This comprehensive study material is fully aligned with the KVS Class IX Science Student Support Material (2025-26). It details all foundational concepts of discovery of cell, cell theory, prokaryotic vs eukaryotic cells, plant vs animal cells, plasma membrane, cell wall, cell organelles, active and passive transport, and cell division (mitosis and meiosis), with complete solved questions from the official KVS bank.
1. Discovery of Cell & Cell Theory
The cell is the basic structural and functional unit of all living organisms. All life functions are carried out inside these microscopic structural units.
Key Historical Discoveries:
- Robert Hooke (1665): First discovered and introduced the term 'cell' after examining a thin slice of cork under a self-designed primitive microscope. He observed structures resembling little rooms or compartments.
- Anton van Leeuwenhoek (1674): With an improved microscope, discovered free-living cells in pond water for the first time.
- Robert Brown (1831): Discovered the nucleus in the cell.
- J. E. Purkinje (1839): Coined the term 'protoplasm' for the fluid substance of the cell.
- M. Schleiden (1838) & T. Schwann (1839): Proposed the Cell Theory, stating that all plants and animals are composed of cells and that the cell is the basic unit of life.
- Rudolf Virchow (1855): Further expanded the cell theory by suggesting: "Omnis cellula-e cellula" (all cells arise from pre-existing cells).
2. Types of Organisms & Cells
Based on cellular organization, organisms can be divided into two main categories:
- Unicellular Organisms: Single-celled organisms where a single cell carries out all vital life processes (e.g., Amoeba, Chlamydomonas, Paramecium, Bacteria).
- Multicellular Organisms: Organisms composed of a large number of cells cooperating together. These cells undergo division of labor to perform specialized functions (e.g., Fungi, Plants, Animals).
Difference Between Prokaryotic Cell and Eukaryotic Cell
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent (nuclear region lacks nuclear membrane and is called a nucleoid) | Present (well-defined nuclear region surrounded by a double-layered nuclear envelope) |
| Size | Generally small ($1 - 10\mathrm{~\mu m}$) where $1\mathrm{~\mu m} = 10^{-6}\mathrm{~m}$ | Generally large ($5 - 100\mathrm{~\mu m}$) |
| Chromosomes | Single circular chromosome | More than one chromosome |
| Membrane-bound Organelles | Absent | Present (e.g., mitochondria, plastids, endoplasmic reticulum, Golgi body) |
| Ribosomes | Smaller (70S) type | Larger (80S) type |
Difference Between Plant Cell and Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (made of cellulose, provides rigidity) | Absent |
| Plastids (Chloroplasts) | Present (helps in photosynthesis) | Absent |
| Vacuoles | Large, central, and permanent vacuole (takes up to 50–90% of cell volume) | Small, temporary, and multiple vacuoles |
| Centrioles / Centrosomes | Present only in a few lower plants | Present (helps in spindle formation during cell division) |
| Lysosomes | Rare | Present (highly abundant) |
3. Structure of a Cell & Membrane Transport
A typical eukaryotic cell is divided into three major structural regions: **Plasma Membrane**, **Nucleus**, and **Cytoplasm**.
A) Plasma Membrane (Cell Membrane)
- It is the outermost, flexible, semi-permeable membrane composed of lipids and proteins.
- It is called a selectively permeable membrane because it permits and regulates the entry and exit of only selective substances in and out of the cell, preventing other materials.
B) Cell Wall
- Found exclusively in plant cells, fungi, and bacteria. It is a rigid, non-living, outer protective covering situated outside the plasma membrane.
- Made up of cellulose in plants. Cellulose is a complex substance that provides structural strength and mechanical rigidity to plants.
- Plasmolysis: When a living plant cell loses water through osmosis, there is shrinkage or contraction of the contents of the cell away from the cell wall. This biological phenomenon is known as plasmolysis.
C) Movement of Substances In and Out of the Cell
- Diffusion: The spontaneous movement of particles (solute or gas) from a region of high concentration to a region of low concentration (e.g., exchange of $\mathrm{CO_2}$ or $\mathrm{O_2}$ across the cell membrane). It is a passive transport process requiring no metabolic energy.
- Osmosis: The movement of water (solvent) molecules from a region of higher water concentration to a region of lower water concentration through a semi-permeable membrane.
Types of Solutions & Their Effect on Living Cells
| Type of Solution | Description (Water Concentration) | Effect on Animal Cell | Effect on Plant Cell |
|---|---|---|---|
| Hypotonic Solution | External solution has a higher concentration of water than the cell cytoplasm (dilute solution). | Water enters the cell (endoosmosis). Cell swells up and may eventually burst. | Water enters the cell. Cell swells, creating turgor pressure against the rigid cell wall, becoming turgid. |
| Isotonic Solution | External solution has exactly the same water concentration as the cell cytoplasm. | No net movement of water. Cell size and shape remain constant. | No net movement of water. Cell remains flaccid but stable. |
| Hypertonic Solution | External solution has a lower concentration of water than the cell cytoplasm (concentrated solution). | Water leaves the cell (exoosmosis). Cell shrinks and dehydrates. | Water leaves the cell. The cytoplasm shrinks away from the cell wall (Plasmolysis). |
4. Specialized Cell Organelles
Suspended inside the jelly-like cytoplasm are various specialized membrane-bound structures called cell organelles. They cooperate to run cell metabolic activities.
| Cell Organelle | Structure & Composition | Primary Functions & Roles |
|---|---|---|
| Endoplasmic Reticulum (ER) | Network of tube-like structures. Consists of Rough ER (RER) containing ribosomes, and Smooth ER (SER). | • Forms the endoskeleton of the cell. • RER: Active site of protein synthesis. • SER: Synthesis of fats and lipids. Plays a crucial role in the **detoxification of drugs and poisons** in liver cells. • **Membrane Biogenesis:** Building cell membranes using synthesized lipids and proteins. |
| Golgi Apparatus | System of membrane-bound vesicles arranged parallel to each other in stacks called cisterns. Discovered by Camillo Golgi. | • Crucial for storage, modification, packaging, and dispatch of materials inside and outside the cell. • Synthesizes complex sugars from simple sugars. • Involved in the formation of **Lysosomes**. |
| Lysosomes | Single membrane-bound spherical waste disposal sacs containing powerful hydrolytic digestive enzymes. | • Cleans the cell by digesting foreign particles (like bacteria, viruses) and worn-out cell organelles. • **Suicide Bags:** If the cell gets damaged or metabolic systems fail, lysosomes can burst, allowing their hydrolytic enzymes to digest their own parent cell. |
| Mitochondria | Double membrane-bound rod-shaped organelles. Outer membrane is porous, and the inner membrane is deeply folded into finger-like cristae. | • Known as the **\"Powerhouse of the Cell\"**. • Generates cellular energy in the form of **ATP (Adenosine Triphosphate)** molecules through cellular respiration. • Highly autonomous: Contains its own **DNA** and **70S Ribosomes**, allowing it to synthesize its own structural proteins. |
| Plastids | Double membrane-bound organelles found exclusively in plant cells. Classified into **Chromoplasts** (colored) and **Leucoplasts** (white/colorless). | • **Chloroplasts** (containing green pigment chlorophyll) are sites of photosynthesis (\"Kitchen of the Cell\"). • Leucoplasts are involved in the storage of starch, oils, and protein granules. • Contains its own **DNA** and **70S Ribosomes** (autonomous). |
| Vacuoles | Single membrane-bound storage sacs. The membrane is called tonoplast. | • Stores solid or liquid content (water, amino acids, sugars, proteins). • Provides turgidity and rigidity to plant cells. • In amoeba, the food vacuole stores food particles captured by pseudopodia. |
| Ribosomes | Non-membrane bound granules made of RNA and proteins. | • Known as the **\"Protein Factory of the Cell\"**. • Essential site for translating genetic codes into structural and functional proteins. |
5. Cell Division
Cell division is the biological process by which new cells are produced from existing cells to facilitate organism growth, tissue repair, replacement of dead cells, and gamete formation for reproduction.
A) Mitosis
- Definition: The cell division process used primarily for vegetative growth, replacement, and tissue repair in body cells.
- Mechanism: A diploid mother cell ($2n$) divides once to produce two genetically identical diploid daughter cells ($2n$).
- The number of chromosomes in the daughter cells remains **exactly the same** as that of the parent cell.
B) Meiosis
- Definition: The specialized cell division process used to produce male and female reproductive gametes (sperm and ova).
- Mechanism: A diploid germ cell ($2n$) undergoes two successive rounds of division to produce four non-identical haploid daughter cells ($n$).
- The daughter cells contain **only half the number of chromosomes** compared to the parent cell. This ensures the restoration of the original chromosome count upon fertilization.
6. KVS Chapterwise Question Bank
Multiple Choice Questions (1 Mark Each)
MCQ 1 A student is studying the history of cell biology and learns about the discovery of "cells." Which scientist first observed and named "cells" after examining a thin section of cork under a microscope?
(a) Anton van Leeuwenhoek (b) Robert Hooke
(c) Matthias Schleiden (d) Theodor Schwann
MCQ 2 Based on the study material, what is the primary reason for the different shapes and sizes of different cells in the human body (like nerve cells, muscle cells, blood cells, fat cells)?
(a) To suit their function (b) As they are formed first or last in the body
(c) As they are all animal cells (d) As some are plant cells and some are animal cells
MCQ 3 What will likely happen if an animal cell and a plant cell are placed in a sugar solution that has a higher water concentration than that of the animal and plant cells?
(a) Both the animal and plant cell will burst.
(b) Both the animal and plant cell will swell.
(c) Animal cell will swell while the plant cell will burst.
(d) Animal cell will burst while the plant cell will swell.
MCQ 4 A student is observing different cell organelles under an electron microscope. She notes that some organelles are surrounded by a single membrane, while others have double membranes. Based on this observation, which of the following organelles is most likely to be surrounded by a single membrane?
(a) Mitochondria (b) Lysosome (c) Plastid (d) All of these
MCQ 5 What is one of the primary roles of the smooth endoplasmic reticulum (SER) in a cell?
(a) Expelling excess water and waste from the cell (b) Producing ATP
(c) Digesting small foreign particles (d) Detoxifying harmful drugs and poisons
MCQ 6 During summer, the leaves of a potted plant droop when the soil dries up. Which cell organelle is mainly responsible for this drooping due to water loss?
(a) Nucleus - as it stops making DNA (b) Cell wall - as it starts to shrink
(c) Lysosome - as it releases digestive enzymes (d) Vacuole - as it loses all the water
Assertion-Reason Based Questions (1 Mark Each)
Directions: Choose (a) if both A and R are true and R is correct explanation of A; (b) if both are true but R is not correct explanation; (c) if A is true, R is false; (d) if A is false, R is true.
A/R 7
Assertion: Lysosomes are called 'suicide bags' of the cell.
Reason: They digest foreign substances and worn-out cell organelles.
A/R 8
Assertion: Nucleus controls all cellular activities.
Reason: It is located outside the cytoplasm.
A/R 9
Assertion: Mitochondria are abundant in active cells like muscle cells.
Reason: Mitochondria provide energy required for cell activities.
A/R 10
Assertion (A): Plasma membrane is called a selectively permeable membrane.
Reason (R): It allows only water molecules to pass through it.
Very Short Answer Questions (2 Marks Each)
Q11 Why is the nucleus called the control center of the cell? Mention one function it performs.
Q12 What is the role of the plasma membrane in the functioning of a cell? Mention one property that allows this function.
Q13 A student learns that certain cell organelles help release energy for various functions of the body. Which organelle performs this function, and why is it commonly called the "powerhouse of the cell"?
Q14 A student observes that raisins swell when kept in water and explains it using a cellular transport process. Identify the process and explain how it differs from diffusion.
Q15 While observing cells under a microscope, a student notes that some cells have a well-defined nucleus while others do not. Classify these two types of cells and mention any two key differences between them.
Short Answer Questions (3 Marks Each)
Q16 While preparing a slide of onion peel, a student observes a distinct boundary layer and a central dense structure. Identify these structures and explain their functions.
Q17 A student notices that the skin on their fingers shrinks after washing clothes for a long time in soapy water. Explain this observation using the concept of osmosis.
Q18 Explain what will happen to a plant cell when placed in: (i) a hypotonic solution, (ii) a hypertonic solution, and (iii) an isotonic solution.
Q19 A student learns that growth and repair in the human body involve the formation of new cells. Identify the type of cell division involved and explain how it helps in these processes.
Long Answer Questions (5 Marks Each)
Q20 Analyze each of the following situations based on your understanding of cell biology. Explain what would happen in each case and why:
(a) Soaking dry apricots in plain water, and then transferring them to a concentrated sugar solution.
(b) Placing a red blood cell in a concentrated saline (salt) solution.
(c) Damaging or breaking the plasma membrane of a cell.
(d) Boiling a Rheo leaf before placing it in sugar syrup.
(e) Completely removing the Golgi apparatus from a cell.
Q21 (a) Why can't Amoeba use a complex digestive system like humans?
(b) Identify and explain the biological phenomenon responsible when a person vomits after consuming a highly concentrated salt solution.
Case Study Based Questions (4 Marks Each)
Case Study 22 Riya observed onion peel under a microscope during her biology lab. She noted that the cells were rectangular and arranged in rows. Each cell had a thick boundary and a dense, round structure in the center. Later, she examined cheek cells and found them irregular in shape but also had a prominent central structure.
Questions:
1. What is the dense, round structure Riya observed in both cells, and what is its primary function?
2. What is the difference between the outer boundary of onion cells and cheek cells, and why is there a difference?
3. Based on Riya's observation, compare plant and animal cells in terms of shape, outer boundary, and presence of specific cell structures.
Case Study 23 In a science project, Meena compared the activities of a cell to a city. She identified certain organelles that function like power stations, storage units, waste disposal systems, and factories. Her analogy helped her understand how the cell works efficiently like a well-organized city.
Questions:
1. Which organelle is referred to as the \"powerhouse\" of the cell, and why is it given this name?
2. Meena compared the Golgi apparatus to a courier service in the city. Justify this comparison by explaining its function.
3. Based on Meena's analogy, explain the role of lysosomes and vacuoles in a cell. How do they contribute to cell maintenance and survival?
No comments:
Post a Comment