🧫 Cell: The Fundamental Unit of Life — Detailed Notes (Class 9 Science)

🌟 Introduction

👉 Every living organism is made up of tiny units called cells.
👉 Cells perform all the basic functions necessary for life.

📌 Therefore, the cell is called the structural and functional unit of life.

🔎 Discovery of Cell

👨‍🔬 Robert Hooke (1665)

• Observed a thin slice of cork under a microscope.

• Saw many small box-like structures.

• Named them cells.

🔬 Antonie van Leeuwenhoek

• First scientist to observe living cells.

👨‍🏫 Matthias Schleiden and Theodor Schwann

• Proposed the Cell Theory.

👨‍⚕️ Rudolf Virchow

• Said: “Omnis cellula e cellula”
  ➡️ New cells arise from pre-existing cells.

📖 Cell Theory

Main Points:

✅ All living organisms are made of cells.
✅ Cell is the basic unit of life.
✅ All cells come from pre-existing cells.

🦠 Types of Organisms

1️⃣ Unicellular Organisms

➡️ Organisms made up of only one cell.

Examples:

• Amoeba

• Paramecium

• Bacteria

📌 One cell performs all life activities.

2️⃣ Multicellular Organisms

➡️ Organisms made up of many cells.

Examples:

• 🌳 Plants

• 🐘 Animals

• 👨 Humans

📌 Different cells perform different functions.

📏 Shape, Size and Number of Cells

🔹 Shape of Cells

Cells have different shapes according to their functions.

🔹 Size of Cells

📌 Most cells are microscopic.

• Smallest cell → Mycoplasma 🦠

• Largest cell → Ostrich egg 🥚

• Longest cell → Nerve cell ⚡

🧩 Structure of Cell

A cell mainly has:

1️⃣ Plasma membrane
2️⃣ Cytoplasm
3️⃣ Nucleus

1️⃣ Plasma Membrane 🫧

Also called cell membrane.

🌟 Features

✅ Thin and flexible covering
✅ Separates cell from surroundings
✅ Made of lipids and proteins

🌟 Functions

✅ Protects cell
✅ Gives shape
✅ Controls movement of substances

📌 It is selectively permeable.

💨 Diffusion

Movement of substances from an area of high concentration to low concentration.

Example:

Oxygen enters cells by diffusion.

💧 Osmosis

Movement of water through a selectively permeable membrane from high water concentration to low water concentration.

🌊 Osmosis in Different Solutions

1️⃣ Hypotonic Solution

➡️ Water enters the cell
➡️ Cell swells

2️⃣ Hypertonic Solution

➡️ Water leaves the cell
➡️ Cell shrinks

3️⃣ Isotonic Solution

➡️ No net movement of water

🌿 Cell Wall

Present only in plant cells.

🌟 Features

✅ Outer rigid covering
✅ Made of cellulose

🌟 Functions

✅ Gives strength and shape
✅ Protects cell
✅ Prevents bursting

🧃 Cytoplasm

➡️ Jelly-like fluid inside the cell membrane.

🌟 Functions

✅ Holds organelles
✅ Site of many chemical reactions

🧠 Nucleus

➡️ Most important part of the cell.

🌟 Structure of Nucleus

• Nuclear membrane

• Nucleoplasm

• Nucleolus

• Chromosomes

🌟 Functions of Nucleus

✅ Controls all activities of the cell
✅ Controls growth and reproduction
✅ Stores hereditary information

🧬 Chromosomes

➡️ Thread-like structures inside nucleus.

🌟 Functions

✅ Carry genes
✅ Transfer hereditary characters from parents to offspring

🧫 Types of Cells

🦠 Prokaryotic Cells

Features:

❌ No true nucleus
❌ No membrane-bound organelles

Example:

• Bacteria

🧬 Eukaryotic Cells

Features:

✅ True nucleus present
✅ Membrane-bound organelles present

Examples:

• Plants 🌿

• Animals 🐾

⚙️ Cell Organelles

🛣️ Endoplasmic Reticulum (ER)

🌟 Function

➡️ Transport system inside cell.

🔹 Types of ER

1️⃣ Rough ER

✅ Ribosomes present
✅ Helps in protein synthesis

2️⃣ Smooth ER

✅ Ribosomes absent
✅ Helps in fat synthesis

🏭 Ribosomes

➡️ Tiny structures attached to rough ER.

🌟 Function

✅ Site of protein synthesis

📌 Called “Protein factories”.

📦 Golgi Apparatus

🌟 Functions

✅ Packaging of materials
✅ Secretion of substances
✅ Formation of lysosomes

🗑️ Lysosomes

🌟 Features

➡️ Small sacs filled with digestive enzymes.

🌟 Functions

✅ Digest food particles
✅ Destroy foreign materials
✅ Remove worn-out organelles

📌 Called “Suicidal bags” because they can digest their own cell.

⚡ Mitochondria

🌟 Features

➡️ Double membrane structure.

🌟 Functions

✅ Site of cellular respiration
✅ Produces energy in form of ATP

📌 Known as “Powerhouse of the cell”.

🌿 Plastids

Present only in plant cells.

🌱 Chloroplast

✅ Contains chlorophyll
✅ Performs photosynthesis

🎨 Chromoplast

✅ Gives colour to fruits and flowers

🍚 Leucoplast

✅ Stores food materials

💧 Vacuoles

🌟 Functions

✅ Store food
✅ Store water
✅ Store waste materials

📌 Large and permanent in plant cells.
📌 Small in animal cells.

🌿 Plant Cell vs 🐾 Animal Cell

⭐ Important Definitions

🧫 Cell

Basic structural and functional unit of life.

🧵 Tissue

Group of similar cells performing same function.

❤️ Organ

Group of tissues working together.

🔗 Organ System

Group of organs performing a major function.

🚀 Important One-Liners

✅ Nucleus is the control center of the cell.
✅ Mitochondria produce energy.
✅ Ribosomes synthesize proteins.
✅ Chloroplast helps in photosynthesis.
✅ Lysosomes contain digestive enzymes.
✅ Plasma membrane is selectively permeable.
✅ Cell wall is made of cellulose.

📝 Frequently Asked Questions

❓Why are lysosomes called suicidal bags?

➡️ Because they contain digestive enzymes that can digest their own cell if they burst.

❓Why are mitochondria called powerhouse?

➡️ Because they produce energy (ATP) for all cell activities.

❓What is osmosis?

➡️ Movement of water through a selectively permeable membrane from high concentration to low concentration.

🎯 Exam Preparation Tips

📌 Learn Properly:

✅ Functions of organelles
✅ Differences between plant and animal cells
✅ Cell theory
✅ Osmosis and diffusion

📌 Practice Diagrams:

✏️ Plant cell
✏️ Animal cell
✏️ Nucleus

🧠 Super Quick Revision Table