Cell: The Building Block of Life Notes Class 9 Science | New NCERT Chapter Notes

 

 

🌟 Chapter Notes: Cell – The Building Block of Life

📘 Class 9 Science Notes |

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🌍 1. Introduction – Where Did Life Begin?

🧬 (जीवन की शुरुआत कहाँ से हुई?)

Scientists believe that life first began in water 💧.
Some researchers think that life started in small water pools with changing environmental conditions instead of oceans.

♨️ Hot Springs – A Clue to Early Life

Hot springs are considered similar to conditions on early Earth 🌋.

🇮🇳 Example: Puga Valley, Ladakh

• Located in Ladakh, India
• Water remains extremely hot 🔥 even in cold weather ❄️
• Temperature is nearly equal to the boiling point of water

Scientists think these conditions were similar to Earth about 3.5 billion years ago 🌍.

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🦠 Thermophiles – Heat Loving Organisms

The organisms found in hot springs are mostly:

✅ Thermophiles
👉 Heat-loving bacteria
👉 Usually unicellular (single-celled)

🔬 Important Discovery

Scientists from the Birbal Sahni Institute of Palaeosciences, Lucknow discovered that:

🪨 Calcium carbonate deposits formed rapidly around these springs.

These deposits may have helped in:

📌 Functions of Calcium Carbonate Deposits

• 🛡️ Protected early organic molecules from harmful radiation
• 🧫 Helped form the first protective membrane (cell boundary)

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🧫 2. What is a Cell?

📖 Definition of Cell

The cell is the basic structural and functional unit of life.

👉 Every living organism is made up of cells.

🌳 Examples

• Tiny bacterium 🦠
• Giant tree 🌳
• Human beings 👨‍👩‍👧

All are composed of cells.

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🔍 Types of Organisms Based on Number of Cells

🏷️ Type	📖 Meaning	🌟 Examples
Unicellular	Made up of only one cell	Bacteria, Amoeba, Yeast
Multicellular	Made up of many cells working together	Plants, Fish, Birds, Humans

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🏗️ Organisation in Multicellular Organisms

Multicellular organisms have a proper arrangement of cells.

📚 Order of Organisation

Cells → Tissues → Organs → Organ Systems

🫁 Example: Respiratory System

• Nasal pores
• Nasal cavity
• Trachea
• Lungs

Together form the Respiratory System 🌬️

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⭐ Important Point

Even though cells form tissues and organs, the cell remains the basic unit of structure and function.

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🔬 3. How to Study Cells?

Cells are extremely small 👀 and cannot be seen with naked eyes.

👁️ Limit of Human Eye

The human eye can clearly distinguish objects only up to:

0.1\ \text{mm}

👉 Two points closer than this distance appear as one.

Therefore, scientists use microscopes 🔬 to study cells.

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✨ Quick Revision Points

📝 Key Terms

• Cell → Basic unit of life
• Unicellular → One-celled organism
• Multicellular → Many-celled organism
• Thermophiles → Heat-loving bacteria
• Membrane → Protective covering of cell

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🎯 One-Line Summary

👉 Life possibly began in hot water environments, and all living organisms are made up of tiny units called cells.

 

Size of the objects and its visibility through unaided to aided eye

Robert Hooke was the first person to observe a cell in 1665 using a self-designed microscope (200-300X magnification). He observed a thin slice of cork and saw small box-like compartments - he named them 'cells'.

Types of microscopes:

Microscope	What it does
Light Microscope	Uses visible light; used in school labs; magnifies using objective lens (10X, 40X) and eyepiece
Electron Microscope	Uses a beam of electrons instead of light; shows cell structure at the nanometre scale (1 nm = 0.000001 mm); much more powerful than a light microscope

Light Microscope

 🔬 Electron Microscope & Cell Structure

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🔭 Electron Microscope

🧪 (इलेक्ट्रॉन सूक्ष्मदर्शी)

An Electron Microscope is a very powerful microscope 🔬 used to study extremely tiny structures like cells and cell organelles.

It can show objects much more clearly than an ordinary microscope.

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✨ Features Improved Over Time

Scientists improved microscopes in three important ways:

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1️⃣ Resolution 🖼️

📖 Meaning:

Resolution means the clarity of an image.

👉 It tells how clearly two very close points can be seen separately.

⭐ Higher Resolution Means:

• Clearer image
• More details visible

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2️⃣ Contrast ⚫⚪

📖 Meaning:

Contrast is the difference in brightness between different parts of an object.

⭐ Importance:

Good contrast helps scientists identify different cell parts easily.

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3️⃣ Magnification 🔍

📖 Meaning:

Magnification tells how many times larger an object appears.

Example:
If magnification is 100X, the object appears 100 times bigger.

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🧮 How to Estimate the Size of a Cell?

📝 Activity 2.1

📌 Formula

 

📌 Formula

 

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✏️ Example Calculation

Suppose:

• Diameter of visible field = 5 mm
• Number of cells = 25

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✏️ Example Calculation

Suppose:

• Diameter of visible field = 5 mm
• Number of cells = 25

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🔄 Unit Conversion

So,

5 mm = 5000 µm

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🧪 Calculation

✅ Size of one cell = 200 µm

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🔍 Total Magnification

If:

• Eyepiece lens = 10X
• Objective lens = 10X

Then,

👉 The cell appears 100 times larger.

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🧠 Multiple Choice Question (MCQ)

❓ Who was the first person to observe a cell?

✅ Answer:

Robert Hooke

He observed cells in a thin slice of cork in 1665 🔬.

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🧫 4. Structure of a Cell

Every cell has a protective boundary called the cell membrane.

It separates the cell from its surroundings 🌍.

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🔄 Functions of Cell Membrane

The cell membrane allows exchange of materials between:

✅ Cell
⬅️➡️
✅ External environment

Even single-celled organisms exchange food, gases, and waste through the membrane.

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🌟 4.1 Cell Membrane – The Universal Feature of a Cell

📖 Definition

The cell membrane (also called plasma membrane) is a thin protective covering around the cell.

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🛡️ Functions of Cell Membrane

✅ 1. Surrounds and Protects the Cell

Acts like a protective shield 🛡️

✅ 2. Gives Identity to the Cell

Defines the individuality of each cell.

✅ 3. Selectively Permeable 🚪

Allows only certain substances to enter or leave the cell.

👉 Useful substances enter
👉 Harmful/unwanted substances are blocked

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🧬 Structure – Fluid Mosaic Model

The cell membrane is:

• Extremely thin
• About:

 

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🧪 Composition of Cell Membrane

It is made up of:

• 🧈 Lipids (fats)
• 🧩 Proteins

These molecules move freely, giving the membrane a fluid nature.

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✨ Quick Revision Points

📝 Important Terms

📘 Term	📖 Meaning
Resolution	Clarity of image
Contrast	Difference in brightness
Magnification	Enlargement of object
Plasma Membrane	Protective cell boundary
Selectively Permeable	Allows only some substances

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🎯 One-Line Summary

👉 The electron microscope helps scientists study tiny cells clearly, while the cell membrane protects the cell and controls movement of substances.

 

🧫 Structure of Cell Membrane

📘 Class 9 Science Notes | Easy & Beautified Version

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🌟 Structure of Cell Membrane

The cell membrane is made up of lipids and proteins arranged in a special pattern called the:

🧬 Fluid Mosaic Model

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🧈 Lipid Bilayer

The membrane has:

✨ Two Layers of Lipids (Fats)

This is called a:

\text{Lipid Bilayer}

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🔍 Arrangement of Lipids

💧 Water-Attracting Heads

• Face outward
• Attract water
• Called hydrophilic heads

🚫💧 Water-Repelling Tails

• Face inward
• Repel water
• Called hydrophobic tails

👉 This arrangement protects the inner part of the membrane.

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🧩 Proteins in Cell Membrane

Proteins are embedded inside the lipid bilayer.

🚪 Function of Proteins

They act like gatekeepers and control:

• What enters the cell
• What exits the cell

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🌊 Why is it Called “Fluid”?

The molecules in the membrane can:

• Move sideways ↔️
• Flip 🔄
• Rotate 🔃

Because of this movement, the membrane behaves like a fluid.

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🧱 Why is it Called “Mosaic”?

The arrangement of proteins and lipids looks like tiles of a mosaic pattern 🧩.

So the membrane is called the:

✨ Fluid Mosaic Model

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💨 4.2 Osmosis and Diffusion

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🌬️ Diffusion

📖 Definition

Diffusion is the movement of particles from:

🔺 Higher concentration

➡️

🔻 Lower concentration

until evenly distributed.

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⭐ Important Point

Diffusion can happen:

✅ Without a membrane

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🌍 Example

Smell of perfume spreading in a room 🌸

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💧 Osmosis

📖 Definition

Osmosis is the movement of:

💦 Water molecules

through a selectively permeable membrane from:

Dilute solution (more water)

➡️

Concentrated solution (less water)

until balance is achieved.

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🧠 Easy Trick to Remember

✨ In Osmosis:

💧 Water moves from:

Dilute Solution

➡️

Concentrated Solution

through a membrane.

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🧪 Types of Solutions & Their Effects on Cells

🌟 Solution Type	📖 Meaning	🧫 Effect on Cell
Isotonic	Solute concentration outside = inside	✅ Cell remains normal
Hypotonic	Solute concentration outside is less	💧 Water enters → Cell swells
Hypertonic	Solute concentration outside is more	💦 Water leaves → Cell shrinks

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🔍 Understanding with Simple Examples

1️⃣ Isotonic Solution ⚖️

• Water movement is equal in both directions
• Cell size remains same

👉 Example: Normal saline solution

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2️⃣ Hypotonic Solution 💧

• Outside solution has more water
• Water enters the cell

👉 Cell becomes swollen

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3️⃣ Hypertonic Solution 🍇

• Outside solution has less water
• Water moves out of the cell

👉 Cell shrinks

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✨ Quick Revision Points

📝 Key Terms

📘 Term	📖 Meaning
Lipid Bilayer	Double layer of fats
Hydrophilic	Water-attracting
Hydrophobic	Water-repelling
Diffusion	Movement from high → low concentration
Osmosis	Movement of water through membrane

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🎯 One-Line Summary

👉 The cell membrane follows the Fluid Mosaic Model and controls movement of substances through processes like diffusion and osmosis.

Example from Activity 2.2 (Potato experiment):

 

Experimental set-up, and initial and final states of potato pieces in (a) plain water, and (b) 20 per cent salt solution

• Potato in plain water (hypotonic) → swells (water enters by osmosis)
• Potato in 20% salt solution (hypertonic) → shrinks (water leaves by osmosis)

In plants, water from soil enters root cells by the process of osmosis.

🌿 4.3 Cell Wall – The Outer Covering of Cells

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🌱 Why Do Plant Cells Need a Cell Wall?

Plants cannot move from one place to another 🚶❌
So, they need extra:

✅ Strength
✅ Rigidity (कठोरता)
✅ Support

Therefore, plant cells have an extra covering outside the cell membrane called the:

🧱 Cell Wall

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✨ Key Features of Cell Wall

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🌟 1️⃣ Present in Some Organisms

Cell wall is found in:

✅ Plant cells 🌿
✅ Fungi 🍄
✅ Bacteria 🦠

❌ Absent in animal cells

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🌟 2️⃣ Made of Cellulose

The cell wall is mainly made of:

\text{Cellulose}

👉 Cellulose is a carbohydrate made of many glucose molecules.

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🌟 3️⃣ Rigid but Permeable

📖 Meaning:

• It is strong and hard 💪
• But allows water and minerals to pass through 💧

So, the cell wall is:

✅ Permeable

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🌟 4️⃣ Gives Shape and Support

Cell wall helps:

• Leaves stay firm 🍃
• Flowers remain fresh 🌸
• Plants stand upright 🌳
• Maintain shape of the cell

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💧 What Happens When Plant Cells Lose Water?

When a plant cell is placed in a concentrated sugar solution 🍬:

🔄 Process

• Water moves out of the cell due to osmosis
• Inner contents shrink

BUT…

✅ The cell wall remains unchanged and keeps the shape fixed.

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🧫 Plasmolysis

During water loss:

• Cell membrane pulls away from the cell wall

This process is called:

✨ Plasmolysis

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🐾 What Happens in Animal Cells?

Animal cells have:

❌ No cell wall

So when placed in concentrated solution:

• Water moves out
• Cell simply shrinks 😟

because there is no rigid wall for support.

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⭐ Importance of No Cell Wall in Animals

Because animal cells are flexible:

✅ Animals can move freely
✅ Tissues can bend and flex easily

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🧠 Quick Concept

🌿 Plant Cell	🐾 Animal Cell
Cell wall present	Cell wall absent
Fixed shape	Flexible shape
Undergo plasmolysis	Simply shrink

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🏭 5. The Cell Interior – A Coordinated Working System

A cell works like a tiny factory 🏭 where different parts perform different jobs together.

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🧩 Three Basic Parts of Every Cell

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1️⃣ Cell Membrane

• Outer covering
• Selectively permeable

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2️⃣ Cytoplasm

📖 Definition

A jelly-like semi-fluid substance inside the cell.

👉 It fills the cell and contains organelles.

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3️⃣ Nucleus 🧠

The control centre of the cell.

👉 Controls all activities of the cell.

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🧬 Cell Organelles

In eukaryotic cells, cytoplasm contains small specialised structures called:

✨ Organelles

Each organelle performs a specific function.

👉 Just like different departments in a factory.

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🔬 5.1 Prokaryotic vs Eukaryotic Cells

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🧠 Meaning of Terms

📖 Prokaryotic

• Pro = Primitive
• Karyon = Nucleus

👉 Primitive nucleus

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📖 Eukaryotic

• Eu = True
• Karyon = Nucleus

👉 True nucleus

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📊 Difference Between Prokaryotic & Eukaryotic Cells

🌟 Characteristics	🦠 Prokaryotic Cell	🧫 Eukaryotic Cell
Nucleus	No well-defined nucleus	Well-defined nucleus
Nuclear Membrane	Absent	Present
Size	1–10 µm	10–100 µm
Membrane-bound Organelles	Absent	Present
Examples	Bacteria	Plant cells, animal cells, fungi

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🦠 Prokaryotic Cells

In prokaryotic cells:

✅ Most activities occur in cytoplasm
✅ DNA is present in a region called:

✨ Nucleoid

❌ DNA is not enclosed by a membrane.

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✨ Quick Revision Points

📘 Term	📖 Meaning
Cell Wall	Rigid outer covering
Cellulose	Main material of cell wall
Plasmolysis	Shrinking of cell membrane away from wall
Cytoplasm	Jelly-like substance
Nucleus	Control centre of cell
Nucleoid	DNA region in prokaryotes

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🎯 One-Line Summary

👉 The cell wall provides strength and support to plant cells, while the cell interior contains organelles that work together like a tiny factory.

5.2 Cell Organelles in Detail

A. Nucleus - House of Coded Instructions

The nucleus is the control centre of the cell. It contains the genetic instructions for all cell activities.

Structure:

• Surrounded by a double-layered nuclear membrane with nuclear pores - allow transfer of material between nucleus and cytoplasm
• Contains the nucleolus - a dense round body inside the nucleus where ribosomal subunits are made
• Contains chromosomes - visible as rod-shaped structures only when the cell is about to divide
• Chromosomes are made of DNA + proteins
• DNA contains the genetic information. Functional segments of DNA are called genes
• When the cell is not dividing, DNA exists as chromatin - an entangled mass of thread-like material
• When the cell is about to divide, chromatin organises into chromosomes

Interesting fact: Mature Red Blood Cells (RBCs) in humans have no nucleus. The absence of a nucleus provides more space for haemoglobin, allowing more oxygen to be transported. Because they have no nucleus, RBCs cannot repair or divide themselves - their lifespan is only about 120 days.

In prokaryotic cells, DNA is present as a single circular molecule in the nucleoid region - not enclosed by any membrane.

B. Ribosomes - The Protein Factories

• Tiny structures found either freely in the cytoplasm or attached to the endoplasmic reticulum
• Function: Site of protein synthesis - they build proteins using instructions from DNA
• Present in both prokaryotic and eukaryotic cells

C. Endoplasmic Reticulum (ER) - Manufacturing Factory

The ER is a large organelle that spreads like a network through the cytoplasm. It is continuous with the outer nuclear membrane.

Function: Synthesis and transport of proteins, fats (lipids), and some hormones

Type	Appearance	Function
Rough ER (RER)	Rough - has ribosomes on its surface	Protein synthesis and secretion (e.g., in gland cells like pancreatic cells)
Smooth ER (SER)	Smooth - no ribosomes	Synthesis and storage of fats (lipids) and hormones

Two types:

D. Golgi Apparatus - The Packaging and Shipping Centre

• Made of stacks of flattened, sac-like structures
• First observed in 1898 by Italian scientist Camillo Golgi in nerve cells of a barn owl
• Functionally linked to the ER and cell membrane

Function: Modifies, sorts, and packages proteins and/or lipids received from the ER into vesicles - small membrane-bound packets - for transport, secretion, or lysosome formation

Think of it as the post office of the cell - it receives, packages, and ships materials to the right destinations.

E. Lysosomes - The Clean-Up System

• Single membrane-bound sacs filled with digestive enzymes
• Break down unwanted proteins, carbohydrates, fats, and even damaged parts of the cell
• Products of breakdown are released into the cytoplasm and reused in other cellular processes
• Keep the cell clean and healthy

Interesting fact: Human sperm cells contain lysosomal enzymes. When a sperm meets an egg, these enzymes help break down the outer layer of the egg, allowing fertilisation to happen.

MULTIPLE CHOICE QUESTION

Try yourself: What is the cell interior described as?

View Solution

F. Mitochondria - The Powerhouse of the Cell

Mitochondria supply the energy needed for most cellular activities.

Structure:

• Surrounded by two membranes
• Outer membrane - smooth and porous
• Inner membrane - folded into finger-like projections called cristae, which increase surface area for chemical reactions

Mitochondria

Function:

• Break down glucose and other molecules during cellular respiration
• Energy released is stored as ATP (Adenosine Triphosphate) - the energy currency of the cell
• ATP is used to power almost all cellular activities

Why many small mitochondria instead of one giant one? Many small mitochondria have a much greater total surface area than one large one - more surface area means more space for chemical reactions and faster energy production.

Special feature: Mitochondria have their own DNA and ribosomes - suggesting they were once free-living bacteria that got incorporated into larger cells during evolution.

G. Plastids - Centre for Food Synthesis in Plant Cells

Plastids are organelles found only in plant cells (and some algae). They are used for food synthesis and storage.

Type	Pigment/Content	Function	Example
Chloroplasts	Green pigment - chlorophyll	Photosynthesis - makes food using sunlight	Leaves
Chromoplasts	Yellow, orange, or red pigments	Give bright colours to flowers and fruits; attract pollinators and seed-dispersing animals	Flower petals, fruits
Leucoplasts	No pigment - colourless	Store food materials like starch, oils, or proteins	Potato (stores starch), taro/Colocasia

Three types of plastids:

 

Structure of Chloroplast:

• Double-membrane bound organelle (like mitochondria)
• Contains a semi-fluid substance called stroma
• Within stroma are disc-shaped membrane structures containing chlorophyll
• Light energy is absorbed during photosynthesis; sugars and starch are stored in stroma

Similarity with Mitochondria:Both mitochondria and plastids have their own DNA and ribosomes - this suggests they share an evolutionary history with ancient bacteria.

H. Vacuoles - The Organelles for Storage and Support

In plant cells:

• Usually one large central vacuole surrounded by a single selectively permeable membrane
• Filled with a watery fluid called cell sap
• Stores water, minerals, sugars, and waste materials
• By storing large amounts of water, the vacuole maintains pressure inside the cell → keeps the plant firm and upright
• When a plant does not get enough water → vacuole loses water → cells become less firm → plant wilts

In animal cells:

• Vacuoles are present but much smaller
• Help in temporary storage of materials

6. Prokaryotic vs Eukaryotic - Cell Organelle Comparison

Structure	Bacterial Cell (Prokaryotic)	Plant Cell (Eukaryotic)	Animal Cell (Eukaryotic)
Cell membrane	Present	Present	Present
Cell wall	Present	Present	Absent
Cytoplasm	Present	Present	Present
Well-defined nucleus	Absent	Present	Present
Nucleoid	Present	Absent	Absent
Membrane-bound organelles	Absent	Present	Present
Chloroplast	Absent	Present	Absent
Mitochondria	Absent	Present	Present
Golgi apparatus	Absent	Present	Present
Lysosomes	Absent	Rarely present	Present
Large central vacuole	Absent	Present	Absent (small vacuoles)

7. How Do Normal Cells Grow and Divide?

When you get a cut on your skin, it heals in a few days. When hair falls out, new hair grows. This happens because cells in our body can grow and divide to replace old, dead, or damaged cells.

Cells grow only up to a certain size and then divide to form new cells - this is how our body grows.

Cell division is the process by which new cells are formed from pre-existing cells. It allows organisms to:

• Grow
• Repair damaged tissues
• Reproduce

Both prokaryotic and eukaryotic cells divide, but eukaryotic cells divide in a more controlled and orderly manner through a process called the cell cycle.

Every day, an estimated hundreds of billions of cells in our body are replaced - about 1% of the total number of cells in the body.

7.1 Types of Cell Division

There are two major types of cell division:

Feature	Mitosis	Meiosis
Number of daughter cells produced	2	4
Chromosomes in daughter cells	Same as parent cell (full set)	Half the number of parent cell
Daughter cells identical to parent?	Yes - genetically identical	No - genetically different
Where it occurs	All body (somatic) cells	Reproductive organs only (testes, ovaries, anthers, ovaries in plants)
Purpose	Growth, repair, maintenance, asexual reproduction	Sexual reproduction; creates genetic diversity

Mitosis in Simple Terms:

• One parent cell → divides → two genetically identical daughter cells
• Each daughter cell gets the same DNA and the same number of chromosomes as the parent
• This ensures genetic information is maintained across all body cells
• Every human begins as a single fertilised egg, which then undergoes mitosis trillions of times to form all the cells of the body

Meiosis in Simple Terms:

• One parent cell → divides twice → four daughter cells, each with half the number of chromosomes
• Occurs only in reproductive organs
• In humans: meiosis in testes (males) produces sperm; meiosis in ovaries (females) produces eggs
• In plants: meiosis in anthers (male parts) forms pollen; in ovaries (female parts) forms egg cells
• When sperm and egg combine during fertilisation → original chromosome number is restored
• Because meiosis creates variation, children resemble but are not exactly like their parents

What happens if cell division goes wrong?

• Errors in mitosis → uncontrolled cell divisions → tumours and abnormal number of chromosomes
• Errors in meiosis → genetic disorders, developmental problems, reduced fertility

MULTIPLE CHOICE QUESTION

Try yourself: What is the main topic of the text?

View Solution

8. Cell Theory - The Unifying Principle of Biology 

History of Cell Theory:

Scientist	Year	Contribution
Robert Hooke	1665	First observed cells in cork
Matthias Schleiden (German botanist)	1838	All plants are made up of cells
Theodor Schwann (German zoologist)	1839	All animals are made up of cells
Rudolf Virchow (German scientist)	1855	New cells arise only from pre-existing cells

The Classical Cell Theory states:

1. All living organisms are made up of one or more cells

2. The cell is the basic unit of structure and function in living beings

3. All cells arise from pre-existing cells

This unifies all of biology - from bacteria to humans - and explains life's continuity through cell division.

Do Cells Grow and Reproduce Forever?

No. Cells grow and divide in a controlled way. They eventually die when they are no longer needed, and are replaced by new cells. Every cell has a definite lifespan.

Contact Inhibition: In many animal cells, cell division stops when cells come into contact with neighbouring cells. This is called contact inhibition - a natural "stop signal."

Cancer cells lose this control and keep dividing uncontrollably → formation of tumours.

• Benign tumours - stay in one place
• Malignant tumours - can invade nearby tissues and spread to other parts of the body

Programmed Cell Death (PCD): Cells also have natural ways of dying in a controlled, genetically regulated manner. This is essential for normal development. For example, when an embryo forms fingers, PCD eliminates the cells between the digits - without this, we would have webbed hands.

Plant cells do not show contact inhibition because of their rigid cell walls - they follow a different pattern of growth.

9. Scientists Spotlight

Camillo Golgi: Italian scientist who in 1898 first observed the Golgi apparatus in nerve cells of a barn owl. Early microscopes could not resolve it clearly and many doubted its existence. Electron microscopy confirmed it decades later. It was named the 'Golgi apparatus' in his honour.

Arun Kumar Sharma: Famous Indian scientist known for his work on chromosomes, plant taxonomy, evolution, and development. He invented useful lab methods to study chromosomes in plants and received the Shanti Swarup Bhatnagar award and Padma Bhushan.

Gottlieb Haberlandt (Austrian botanist, 1902) proposed that any living plant cell, even from a permanent tissue, can develop into a complete plant if given suitable nutrients and conditions. This ability is called totipotency. His idea laid the foundation for Plant Tissue Culture Technology.

Quick Revision - Key Terms Table

Term	What it Means
Cell	Basic structural and functional unit of all living organisms
Unicellular	Organism made of a single cell (e.g., bacteria, amoeba)
Multicellular	Organism made of many cells (e.g., humans, plants)
Limit of resolution	The minimum distance between two points that can be seen as separate - 0.1 mm for human eye
Cell membrane / Plasma membrane	Thin, selectively permeable outer boundary of all cells
Selectively permeable	Allows only certain substances to pass through
Osmosis	Movement of water through a selectively permeable membrane from dilute to concentrated solution
Diffusion	Movement of particles from higher to lower concentration (no membrane needed)
Hypotonic solution	Outside has less solute than inside the cell - water enters the cell
Hypertonic solution	Outside has more solute than inside the cell - water leaves the cell
Isotonic solution	Equal solute concentration on both sides - no net water movement
Cell wall	Rigid outer covering of plant, fungal, and bacterial cells - made of cellulose in plants
Fluid Mosaic Model	Model describing cell membrane as a flexible lipid bilayer with proteins embedded in it
Prokaryotic cell	Cell without a well-defined nucleus (e.g., bacteria)
Eukaryotic cell	Cell with a well-defined, membrane-bound nucleus (e.g., plant and animal cells)
Nucleus	Control centre of the cell - contains DNA and chromosomes
Nucleoid	Region in prokaryotic cells where circular DNA is located (no membrane around it)
Chromatin	Loosely arranged DNA + protein in non-dividing cells
Chromosomes	Condensed, rod-shaped structures of DNA + protein visible when cell is dividing
Genes	Functional segments of DNA that carry genetic information
Nucleolus	Dense body inside nucleus - site of ribosomal subunit synthesis
Ribosomes	Site of protein synthesis; present in both prokaryotic and eukaryotic cells
Endoplasmic Reticulum (ER)	Network organelle for synthesis and transport of proteins and lipids
RER	Rough ER - has ribosomes; makes proteins
SER	Smooth ER - no ribosomes; makes lipids and hormones
Golgi apparatus	Packages and ships proteins/lipids; post office of the cell
Lysosomes	Clean-up organelle - breaks down waste and damaged cell parts using enzymes
Mitochondria	Powerhouse of the cell - produces ATP through cellular respiration
Cristae	Finger-like folds of inner mitochondrial membrane - increase surface area
ATP	Adenosine Triphosphate - the energy currency of the cell
Plastids	Organelles found only in plant cells - for food synthesis and storage
Chloroplasts	Green plastids - perform photosynthesis
Chromoplasts	Coloured plastids (yellow/orange/red) - give colour to flowers and fruits
Leucoplasts	Colourless plastids - store food like starch, oils, proteins
Chlorophyll	Green pigment in chloroplasts that absorbs light for photosynthesis
Stroma	Semi-fluid matrix inside chloroplast where sugars are stored
Vacuole	Storage organelle - large in plant cells, small in animal cells
Cell sap	Watery fluid in plant cell vacuole
Turgid	Cell swollen with water (plants in hypotonic solution)
Flaccid / Wilted	Cell lacking water (plants in hypertonic solution or drought)
Cell division	Process by which new cells form from pre-existing cells
Mitosis	Cell division producing 2 identical daughter cells - for growth and repair
Meiosis	Cell division producing 4 daughter cells with half chromosomes - for sexual reproduction
Cell cycle	Controlled, orderly process of eukaryotic cell division
Contact inhibition	Normal cells stop dividing when they touch neighbouring cells
Tumour	Mass formed by uncontrolled cell division (cancer cells lack contact inhibition)
Programmed Cell Death (PCD)	Controlled, genetically regulated process of cell death - essential for development
Cell Theory	All organisms made of cells; cell is basic unit; new cells from pre-existing cells
Totipotency	Ability of a plant cell to develop into a complete plant - basis of tissue culture