Category: Biology 🧫
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(Set-2) New NCERT Ch.2 Cell , Practice Questions
Cell Biology Exercises & Exercises Bank Cell Biology Exercises
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Chapter 2: Cell — The Building Block of Life
PAUSE AND PONDER (In-chapter Questions)
Q1. What argument would you give for the necessity of a cell wall in plants (fixed) vs animals (moving)?
Plants are stationary and face mechanical stresses like wind, rain, and gravity. The rigid cell wall provides structural support to withstand these forces and maintain upright posture. Animals move actively, so their cells need flexibility to change shape — a rigid wall would hinder movement of tissues like muscles. Thus, a cell wall is essential for plants but would be a disadvantage for animals.
Q2. What consequences if a plant cell’s cell wall became as flexible as the cell membrane?
- The cell would lose its fixed shape and rigidity.
- Plants would become unable to stand upright and would wilt permanently.
- Leaves and flowers would collapse.
- When placed in hypotonic solution, the cell could burst (like animal cells) since the wall wouldn’t resist excessive water entry.
- Overall structural integrity of the plant body would be lost.
Q3. Why is it important to cut potato pieces in roughly equal size and measure initial weight?
This ensures a fair and controlled experiment. If sizes differ, the surface area exposed to solution differs, affecting the rate of osmosis. Measuring initial weight allows accurate calculation of water gain or loss. Without these controls, results cannot be reliably compared between the two beakers.
Q4. Do white flowers contain any pigment? Give reasons.
Yes, white flowers do contain pigments, but they reflect all wavelengths of visible light rather than absorbing any specific colour, which makes them appear white. Some white flowers contain flavonoids or other colourless/UV-absorbing pigments. The whiteness may also result from air spaces in petals that scatter light. So white does not mean absence of all pigments.
Q5. Draw a well-labelled schematic diagram of a plant or animal cell.
(This is a drawing-based question — draw referring to Fig. 2.10 in the textbook.)
For a plant cell, include: cell wall, cell membrane, nucleus (dark round body), cytoplasm, ER (network extending from nuclear envelope), mitochondria (rod-shaped), chloroplasts (rod-shaped, green), vacuole (large, central), Golgi apparatus, ribosomes.
For an animal cell, include: cell membrane, nucleus, cytoplasm, ER, mitochondria (rod-shaped), Golgi apparatus, lysosomes, ribosomes, small vacuoles.
Q6. Why does a cell not have one giant mitochondrion instead of many small ones? (Surface area concept)
Many small mitochondria provide a much greater total surface area than one large mitochondrion of the same combined volume. The inner membrane (with cristae) is where ATP production occurs. More surface area means more space for chemical reactions, leading to greater and more efficient energy production. A single giant mitochondrion would have far less surface-area-to-volume ratio and would be metabolically inefficient.
Q7. If skin cells divide by meiosis instead of mitosis, what would happen to a skin cut?
Meiosis produces cells with half the chromosome number. If skin cells used meiosis for repair:
- Daughter cells would have only half the genetic information needed.
- They would be genetically different from the parent skin cells.
- Proper healing would not occur since the new cells would be abnormal and non-functional.
- The wound would not heal correctly, and skin integrity would be permanently compromised.
REVISE, REFLECT, REFINE (End-chapter Questions)
Q1. Differentiate between the following:
(i) Cell membrane vs Cell wall (permeability)
Cell Membrane Cell Wall Selectively permeable — allows only certain substances Fully permeable — water and dissolved minerals pass freely Present in all cells Present only in plant, fungal, and bacterial cells Made of lipids and proteins Made primarily of cellulose (in plants) Thin, flexible Thick, rigid (ii) RER vs SER (structure)
RER (Rough ER) SER (Smooth ER) Has ribosomes attached on its surface No ribosomes on surface Appears rough under electron microscope Appears smooth under electron microscope Involved in protein synthesis and secretion Involved in synthesis and storage of fats and hormones (iii) Chloroplasts vs Chromoplasts (pigments)
Chloroplasts Chromoplasts Contain green pigment chlorophyll Contain yellow, orange, or red pigments (not chlorophyll) Found in green parts of plants Found in flower petals and fruits Responsible for photosynthesis Responsible for bright colours attracting pollinators/animals
Q2. Cell X in pure water swells; Cell Y in concentrated salt solution shrinks. Which statement is correct?
Answer: (iii) — Water moved into Cell X and moved out of Cell Y through the cell membrane.
Explanation: The cell membrane is selectively permeable and allows water (not salt/sugar molecules) to move by osmosis. Pure water (hypotonic outside) causes water to enter Cell X → it swells. Concentrated salt solution (hypertonic outside) causes water to leave Cell Y → it shrinks. Option (i) is wrong because salt molecules don’t move in. Option (ii) is partially wrong. Option (iv) is wrong because osmosis involves water movement, not solute movement.
Q3. Identify parts (a) to (g) in Fig. 2.20 (plant cell diagram) and match functions:
Based on a standard plant cell diagram with labels (a) through (g):
Label Organelle Function (a) Chloroplast (vii) Helps in manufacturing food (b) Nucleus (i) Controlling all activities of a cell (c) Endoplasmic Reticulum (transport network) (d) Cell Wall (v) Provides structural rigidity (e) Cell Membrane (iv) Separates cell contents from surroundings (f) Golgi Apparatus (vi) Packs and stores materials received from ER (g) Vacuole (iii) Storage organelle that also provides rigidity Mitochondria Mitochondria (ii) Site of cellular respiration (Exact matching depends on the actual figure; match accordingly using the functions above.)
Q4. Which option correctly pairs cell organelles present in plant cells / absent in animal cells?
Answer: (i) — Leucoplast (present in plant cells) and Cell wall (absent in animal cells).
Explanation: Leucoplasts are plastids found only in plants. Cell wall is present in plants but absent in animals. Option (ii) is wrong — both mitochondria and ribosomes are present in animal cells. Option (iii) is wrong — Golgi apparatus is present in animal cells. Option (iv) is wrong — both lysosomes and ER are present in animal cells.
Q5. Renu says plastids are in all plant parts including roots. Rohit says roots have no plastids. Who is correct?
Rohit is partially correct, but Renu is more accurate overall.
Plastids are indeed present in root cells, but they are leucoplasts (colourless, storage plastids), not chloroplasts. Roots do not receive sunlight, so chloroplasts are absent. However, saying plastids are entirely absent in roots is incorrect. Leucoplasts in root cells store starch and other food materials. So Renu is correct that plastids are present in roots, but they are non-photosynthetic leucoplasts rather than chloroplasts.
Q6. Compare mitochondria and chloroplasts — similarities and differences:
Similarities:
- Both are double-membrane-bound organelles.
- Both contain their own DNA and ribosomes, so they can synthesise some proteins independently.
- Both are involved in energy-related processes.
- Both are rod-shaped.
- Both show evolutionary relationship with ancient bacteria (endosymbiotic theory).
Differences:
Mitochondria Chloroplasts Found in both plant and animal cells Found only in plant cells Inner membrane folded into cristae Has stroma with disc-shaped thylakoid membranes containing chlorophyll Carries out cellular respiration — breaks down glucose to release ATP Carries out photosynthesis — converts light energy into glucose Uses oxygen, releases CO₂ Uses CO₂, releases oxygen Active in all living cells 24 hours Active only in the presence of light
Q7. Which pair of cell organelles contains DNA?
Answer: (ii) Mitochondria and Nucleus
Explanation: The nucleus contains chromosomal DNA carrying the cell’s genetic information. Mitochondria have their own circular DNA (inherited maternally). Chloroplasts also have DNA, but they are not listed here. Ribosomes, Golgi bodies, and lysosomes do not contain DNA.
Q8. Researcher’s carrot experiment (plain water vs concentrated salt solution):
(i) Hypothesis she wants to test:
The hypothesis is: Cell membranes are selectively permeable, and water moves by osmosis from a region of lower solute concentration to higher solute concentration. Specifically: a carrot in plain water will remain firm (water entry) while a carrot in salt solution will become limp (water loss).(ii) Suggestions for improvement:
- Use carrots of identical size, shape, and weight to ensure a controlled experiment.
- Record initial and final weights to quantify water gain/loss.
- Specify exact concentration of salt solution for reproducibility.
- Use multiple trials and calculate average results.
- Keep temperature constant throughout the experiment.
(iii) Why does carrot in plain water stay stiff but carrot in salt solution become rubbery?
In plain water (hypotonic solution), water enters the carrot cells by osmosis (water concentration outside > inside), making cells turgid → carrot stays stiff and crunchy. In concentrated salt solution (hypertonic), water moves out of carrot cells by osmosis (solute concentration outside > inside), cells lose turgor pressure → carrot becomes flaccid, rubbery, and limp.
Q9. Presence/absence of structures in bacterial and animal cells:
Structure Bacterial Cell Animal Cell Chromosome Present (circular, in nucleoid, no membrane) Present (linear, enclosed in nucleus) Nucleus Absent (nucleoid present instead) Present (well-defined, membrane-bound) Mitochondria Absent Present Golgi complex Absent Present Chromoplasts Absent Absent
Q10. Potato cup experiment (Cups A, B, C, D):
(i) Why does water gather in Cup B (sugar) and Cup C (salt)?
Sugar and salt create a concentrated solution inside the potato cup. The surrounding potato cells have a lower solute concentration inside. By osmosis, water moves from the potato cells (higher water concentration) into the hollow cup (lower water concentration / higher solute concentration). This is why water accumulates in the hollow of Cup B and Cup C.(ii) Why is Cup A (empty) necessary?
Cup A is the control. It shows that water does not enter an empty potato cup by itself, confirming that the presence of solute (sugar/salt) is what drives water movement. Without a control, we cannot attribute the water gathering to osmosis specifically.(iii) Why does water not gather in Cup A and Cup D?
- Cup A (empty): No solute inside the cup, so no concentration gradient exists. No osmosis occurs; water has no driving force to enter.
- Cup D (boiled potato with sugar): Boiling kills the cells and denatures the cell membrane proteins, destroying selective permeability. The cell membranes are no longer functional, so osmosis cannot occur even though sugar is present. Therefore, water does not accumulate.
Q11. Identify the incorrectly matched pair:
Answer: (ii) SER — Lipid and cellulose synthesis
Explanation: SER is involved in synthesis of lipids and hormones — not cellulose. Cellulose synthesis occurs at the cell wall level involving Golgi apparatus. Options (i) and (iii) are correct: ribosomes do synthesise proteins, and lysosomes do digest foreign agents and waste materials.
Q12. What if all mitochondria are removed from a eukaryotic cell?
- Mitochondria are the powerhouses that produce ATP through cellular respiration.
- Without mitochondria, ATP production would stop almost entirely.
- The cell would have no energy currency to carry out any activity — protein synthesis, active transport, movement, division, etc.
- The cell would quickly become non-functional and die.
- Some minimal anaerobic energy production (glycolysis in cytoplasm) might occur briefly, but it would be insufficient to sustain the cell.
Q13. Which phenomenon inhibits tumour formation in humans? Can plants develop tumours?
Phenomenon: Contact Inhibition. When normal animal cells come in contact with neighbouring cells, they stop dividing. This prevents uncontrolled growth and tumour formation. Cancer cells lose this property and keep dividing uncontrollably.
Can plants develop tumours? Yes, plants can develop tumours, but through a different mechanism. Plant cells do not show contact inhibition because their rigid cell walls regulate growth differently. A well-known example is Crown Gall Disease, caused by the bacterium Agrobacterium tumefaciens, which inserts its DNA into plant cells causing uncontrolled cell proliferation and tumour formation. So plants can have tumours, but the regulatory mechanism differs from animals.
Q14. Which organelles help synthesise cell membrane? Trace the path of synthesis:
Cell membrane is made of proteins and lipids.
Organelles involved:
- Ribosomes (on RER) — synthesise proteins
- SER — synthesises lipids
- Golgi apparatus — modifies, packages, and transports both
Path:
- Proteins are synthesised on ribosomes attached to RER.
- Lipids are synthesised in SER.
- Both are transported to the Golgi apparatus via vesicles.
- Golgi apparatus modifies, sorts, and packages them into vesicles.
- These vesicles move toward and fuse with the cell membrane, adding new lipid and protein components to it.
(Draw a labelled diagram showing: Ribosome → RER → Vesicle → Golgi → Vesicle → Cell membrane, with SER also contributing lipids to Golgi.)
Q15. What would happen if gametes are formed by mitotic divisions?
Normally, gametes are formed by meiosis, which halves the chromosome number. If gametes were formed by mitosis instead:
- Each gamete would have the full (diploid) chromosome number instead of half (haploid).
- Upon fertilisation, the zygote would have double the normal chromosome number (tetraploid).
- With each successive generation, chromosome numbers would keep doubling.
- This would cause severe genetic abnormalities, developmental disorders, and would disrupt the normal chromosome balance essential for proper gene function.
- Genetic diversity (normally created by meiosis through crossing over and random segregation) would also be eliminated.
Q16. Farmer Deepa’s amla and lemon preservation:
(i) Which scientific concept has she applied?
She has applied the concept of osmosis and hypertonic solutions. Adding high concentrations of salt, sugar, or jaggery creates a hypertonic environment. By osmosis, water is drawn out of the cells of microorganisms (bacteria, fungi) that cause spoilage, dehydrating and killing them, thereby preserving the produce.(ii) How does high salt/sugar prevent microbial growth?
High concentrations of salt or sugar create a hypertonic external environment around microbial cells. Water moves out of the microbial cells by osmosis (plasmolysis), causing them to shrink and dehydrate. Without adequate water, enzymes of spoilage microorganisms cannot function, their metabolic processes stop, and they cannot grow or reproduce. This is called osmotic preservation.(iii) Suggest a healthy preservation recipe:
Raw Mango Preserve (Aam ki launji): Take raw mango pieces, add jaggery (natural sweetener, rich in iron) and a small amount of salt. Cook gently until jaggery dissolves and coats the mango. Store in a clean glass jar. The high sugar/jaggery concentration preserves the fruit while providing nutritional value. Alternatively, lemon in salt brine (lemon pieces immersed in concentrated salt water) preserves lemons for months and serves as a healthy condiment.(iv) Scientific values addressed:
- Observation and inquiry — noticing post-harvest loss and seeking solutions.
- Application of scientific knowledge — using osmosis practically.
- Sustainability — reducing food waste and environmental impact.
- Economic thinking — converting waste into value-added products.
- Traditional knowledge integration — combining indigenous methods with scientific understanding.
- Food security — ensuring produce reaches consumers rather than being wasted.
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Chapter 2: Cell 🔬Class-9th
Chapter 2: Cell – The Building Block of Life | Complete Master Notes © ABHISHEK.io.inChapter 2: Cell 🔬
The Building Block of Life – Master Academic Notes1. The Origin of Life & Introduction
It is widely accepted amongst the scientific community that life originated in water. Some researchers believe it began in small water pools with changing environmental conditions rather than vast oceans.
- Puga Valley, Ladakh: The hot springs here maintain near-boiling temperatures even in a cold climate, mimicking the early Earth (about 3.5 billion years ago).
- Thermophiles: Heat-loving, mostly unicellular bacteria found in these hot springs.
- Birbal Sahni Institute of Palaeosciences (Lucknow): Scientists studied these springs and found rapid calcium carbonate deposition. These deposits likely protected early organic molecules from harmful radiation, aiding the formation of the first protective membrane (the barrier defining a cell).
Hierarchical Organisation: Cells → Tissues → Organs → Organ Systems (e.g., nasal pores, cavity, trachea, and lungs form the respiratory system). The cell remains the fundamental unit of structure and function.
2. How to Study Cells? (Microscopy) 🔍
The limit of resolution of the human eye is 0.1 mm (when viewed from the near point of 25 cm). Objects closer or smaller appear as a single point. To study cells, microscopes are essential:
Types of Microscopes
1. Light Microscope: Uses visible light and a combination of lenses (objective and eyepiece). Magnification is the product of both lenses (e.g., 10X eyepiece × 10X objective = 100X total magnification). It improves resolution, contrast, and magnification.
2. Electron Microscope: Uses a powerful beam of electrons instead of light to produce highly magnified images, revealing fine details at the nanometre scale (1 nm = one-billionth of a metre).
🧪 Activity 2.1: Estimating the Size of a Cell
Procedure: Focus a transparent millimetre ruler under a light microscope. Measure the diameter of the circular field of view in mm and convert to micrometres (1 mm = 1000 µm). Replace the ruler with an onion peel slide. Count the number of cells across the diameter.
Calculation Formula:
Size of one cell =Diameter of visible field (in µm) Number of cells along the diameterExample: If the diameter is 5000 µm and you see 25 cells…
Size = 5000 / 25 = 200 µm per cell!👨🔬 Historical Milestone
Robert Hooke (1665) observed a thin slice of cork under a self-designed microscope (200-300X magnification). He saw box-like compartments and coined the term “cells”.3. Structure of a Cell: The Boundary
a. Cell Membrane (Plasma Membrane)
The universal, extremely thin (7 to 10 nm thick) boundary that protects cellular contents and defines the cell’s individuality.
- It is selectively permeable: allows some substances to pass while blocking others.
- Fluid-Mosaic Model: Explains its structure. It consists of a lipid bilayer (fats with water-attracting heads facing outwards and water-repelling tails inwards) with embedded proteins.
- It is “fluid” because molecules can move sideways, flip, and rotate. The proteins act as gatekeepers facilitating the passage of specific substances.
Diffusion vs. Osmosis
Diffusion: The net movement of particles from a region of higher concentration to lower concentration (concentration gradient). *E.g., O₂ and CO₂ exchange in alveolar cells.*
Osmosis: The diffusion of water across a selectively permeable membrane from a dilute solution (more water, less solute) to a concentrated solution (less water, more solute).
Extracellular Solution Type Condition Result on Cell Isotonic Outside concentration = Inside concentration No net movement. Cell stays the same. Hypotonic Outside is dilute (less solute, more water) Water enters cell. Cell swells. Hypertonic Outside is concentrated (more solute, less water) Water leaves cell. Cell shrinks. 🧪 Activity 2.2: Potato Osmometer
Two roughly equal potato pieces are weighed. Piece A is placed in plain water (Hypotonic); Piece B is placed in a 20% salt/sugar solution (Hypertonic). After an hour, Piece A swells (gains weight due to endosmosis), while Piece B shrinks (loses weight due to exosmosis).
b. Cell Wall (Outer Covering)
Plants, fungi, and bacteria possess an additional covering outside the cell membrane called the cell wall. Plants require this rigid structure to withstand environmental stresses (wind, rain) since they are immotile.
- In plants, it is primarily composed of Cellulose (a carbohydrate of linked glucose units that acts as roughage in human digestion).
- It helps leaves/flowers remain firm, maintains shape, and keeps plants upright.
- Unlike the membrane, the cell wall is permeable, allowing water and dissolved minerals to pass through.
🧪 Activity 2.3: Onion/Rhoeo vs. Human Cheek Cells
Observations: Onion peel or Rhoeo (Cradle lily) leaf cells are box-shaped and regularly arranged. Human cheek cells are irregularly arranged because animal cells lack a rigid cell wall. When 20% sugar solution is added to Rhoeo cells, the inner contents shrink away from the rigid cell wall (plasmolysis), but the overall boundary size remains the same.
4. Prokaryotic vs. Eukaryotic Cells 🦠
All cells possess a plasma membrane, cytoplasm (semi-fluid, jelly-like substance), and genetic material. Based on nuclear organisation, cells are classified as:
Characteristics Prokaryotic Cell (e.g., Bacteria) Eukaryotic Cell (e.g., Plants, Animals) Nucleus Primitive. Genetic material (single circular DNA) lies in an undefined region called the Nucleoid. True, well-defined nucleus enclosed by a double nuclear membrane. Size Typically 1 to 10 µm. Typically 10 to 100 µm. Organelles Lacks membrane-bound organelles. Cellular activities occur directly in cytoplasm. Contains specialised membrane-bound organelles (Mitochondria, ER, etc.). Organism Type Usually unicellular. Can be unicellular or multicellular. 🦠 Acellular Infectious Agents (Exceptions)
Viruses: Genetic material enclosed in a protein coat.
Viroids: Infectious RNA lacking a protein coat.
Prions: Infectious, misfolded proteins lacking genetic material.5. The Cell Interior: Organelles ⚙️
Eukaryotic cells are like tiny living factories. They use a network of fine fibres called the Cytoskeleton for structural support, shape maintenance, internal transport, and movement. Cells may also contain Cell Inclusions (stored inactive materials like starch, calcium oxalate, or silica crystals in plants).
🧠 1. Nucleus: House of Coded Instructions
Surrounded by a double-layered nuclear membrane with pores to allow material transfer to the cytoplasm. It contains a dense round body, the Nucleolus, which is the site of ribosomal subunit synthesis.
- Contains DNA (Deoxyribonucleic acid) which holds genetic information. Functional segments of DNA are called genes.
- In a non-dividing cell, DNA is an entangled mass called Chromatin.
- When dividing, chromatin organises into rod-shaped Chromosomes (composed of DNA and specific proteins).
Threads of Curiosity: Mature Red Blood Cells (RBCs) in humans are enucleate (lack a nucleus). This provides more space for haemoglobin to transport oxygen. Because they lack a nucleus, they cannot repair or divide, giving them a short lifespan of about 120 days.🏭 2. Endoplasmic Reticulum (ER) & 🔵 3. Ribosomes
Ribosomes are tiny structures (free in cytoplasm or attached to ER) responsible for protein synthesis.
The Endoplasmic Reticulum is a large continuous network of tubes connected to the outer nuclear envelope. It transports and synthesises materials.
- Rough ER (RER): Appears rough under an electron microscope due to attached ribosomes. Synthesises and secretes proteins (e.g., in pancreatic gland cells).
- Smooth ER (SER): Lacks ribosomes. Involved in the synthesis and storage of fats (lipids) and certain hormones.
📦 4. Golgi Apparatus
Observed first by Camillo Golgi (1898) in the nerve cells of a barn owl using special staining techniques. It consists of stacks of flattened, sac-like structures. It acts as the cell’s post office: it modifies, sorts, and packages proteins and lipids into vesicles for transport, secretion, or lysosome formation.
🗑️ 5. Lysosomes: The Clean-up System
Single membrane-bound sacs filled with powerful digestive enzymes. They break down unwanted proteins, carbohydrates, fats, worn-out organelles, and damaged parts, keeping the cell clean.
*Fun Fact: Human sperm cells contain lysosomal enzymes to break down the outer layer of the egg, allowing fertilisation!⚡ 6. Mitochondria: Powerhouse of the Cell
Responsible for energy supply via cellular respiration (breaking down glucose to release energy). The energy is stored as ATP (Adenosine Triphosphate), the energy currency of the cell.
- Surrounded by two membranes. The outer is smooth and porous.
- The inner membrane is folded into finger-like projections called Cristae, which drastically increase the surface area for energy-producing chemical reactions.
- Unique feature: They have their own DNA and ribosomes!
🌿 7. Plastids (Only in Plant Cells)
Used for food synthesis and storage. Like mitochondria, plastids possess their own DNA and ribosomes, suggesting a shared evolutionary history with bacteria.
- Chloroplasts: Double-membrane bound. Contain a semi-fluid stroma and disc-shaped structures bearing green chlorophyll. They absorb light energy for photosynthesis. Sugars synthesised are stored in the stroma as starch granules.
- Chromoplasts: Contain pigments (yellow, orange, red) providing bright colours to flower petals and fruits to attract pollinators and seed-dispersing animals.
- Leucoplasts: Colourless plastids that store food materials like starch, oils, or proteins (e.g., found in potato and Colocasia/taro).
💧 8. Vacuoles: Storage and Support
In mature plant cells, there is usually one large central vacuole surrounded by a single selectively permeable membrane. It is filled with a watery fluid called cell sap.
- Stores water, minerals, sugars, and waste.
- By storing large amounts of water, it maintains internal pressure, keeping the plant cell firm (turgid). Loss of water causes the plant to wilt.
- In animal cells, vacuoles are smaller and used only for temporary storage.
🧬 Synthetic Biology (J. Craig Venter, 2010)
His team sequenced the DNA of a bacterium (*Mycoplasma mycoides*). They chemically synthesised an exact copy of this DNA in a lab and inserted it into a closely related cell (whose own DNA was removed, keeping the cytoplasm and membrane intact). The cell grew and divided following instructions from the synthetic DNA! This proved that DNA masterminds the structure and activities of the cell.6. How do Normal Cells Grow and Divide? 🧬
Every day, an estimated hundreds of billions of cells in our body are replaced (almost 1% of total cells). Cells cannot grow indefinitely in size; growth occurs through cell division. In eukaryotes, this is an orderly process called the Cell Cycle.
🧪 Activity 2.5: Onion Root Tip Experiment (Studying Cell Division)
1. Grow an onion bulb over a water jar.
2. Cut the freshly grown root tips (2-3 cm) and place them in freshly prepared aceto-alcohol (glacial acetic acid : ethanol :: 1:3) for 24 hours. Transfer to 70% ethanol for preservation.
3. Soften the tissue with dilute Hydrochloric acid (HCl) for 10-15 mins.
4. Add 2-3 drops of aceto-carmine stain, warm gently over a spirit lamp.
5. Squash the coverslip to spread the cells and observe. The cells of a growing tip divide continuously, displaying various structural stages of cell division.A. Mitosis
Purpose: Normal growth, repair of damaged tissues, maintenance, and asexual reproduction.
- One parent cell divides once to form two genetically identical daughter cells.
- Each new cell gets the same DNA and the same number of chromosomes as the parent cell.
B. Meiosis
Purpose: Occurs only in reproductive organs to produce gametes for sexual reproduction (creates genetic diversity!).
- In animals: occurs in male testes (produces sperm) and female ovaries (produces eggs).
- In plants: occurs in anthers (pollen/sperm) and ovaries (eggs).
- It is a two-step division resulting in four daughter cells.
- The number of chromosomes in each gamete is reduced to half. When gametes fuse during fertilisation, the original number is restored.
👨🔬 Meet a Scientist
Arun Kumar Sharma was a famous Indian botanist renowned for his work on chromosomes, plant taxonomy, and evolution. He invented useful lab methods to study plant chromosomes and received the Shanti Swarup Bhatnagar award and Padma Bhushan.7. Cell Theory & Regulatory Mechanisms ⚖️
The Classical Cell Theory
Formulated by combining the findings of three prominent scientists:
- Matthias Schleiden (1838, Botanist): All plants are made of cells.
- Theodor Schwann (1839, Zoologist): All animals are made of cells.
- Rudolf Virchow (1855): Proposed that all cells arise from pre-existing cells.
Totipotency (Gottlieb Haberlandt, 1902): Proposed that any living plant cell (even a mature one) can develop into a complete plant if provided suitable nutrients and conditions. This is the foundation of modern Plant Tissue Culture Technology.
Do cells grow and reproduce forever?
No. Cells grow in a controlled way, carry out functions, and die when no longer needed. They have a definite lifespan.
- Contact Inhibition: In many animal cells, cell division stops when they physically touch neighbouring cells. Plant cells grow differently due to rigid walls and do not show contact inhibition.
- Tumours & Cancer: When normal regulatory systems fail (often due to errors in mitosis), cells lose contact inhibition and divide uncontrollably, forming tumours (benign or malignant). Cancerous cells can invade nearby tissues and spread.
- Programmed Cell Death (PCD): A genetically regulated, highly organised process of selective cell destruction. Essential for quality control, immune function, and normal development (e.g., PCD eliminates cells between digits during embryonic development to prevent webbed hands).
- Errors in Meiosis: Faulty meiosis can lead to abnormal chromosome numbers, causing genetic disorders, distinctive physical features, early pregnancy loss, or reduced fertility.
🔬 Bridging Science and Society: Cell Culture
Scientists have developed methods to grow plant and animal cells outside the body in special conditions, known as cell culture. Cells are placed in a nutrient-rich medium maintaining specific temperature, pH, moisture, and sterile conditions. It is crucial for producing biochemicals, food, medicines, and vaccines.