Class 8 Science – Chapter 8: Cell: Structure and Functions | Full NCERT Notes | NCERT Nation

---

🔹 Introduction

Every living organism — from the tiniest ant to the mighty elephant — is made up of small units called cells. These cells are the building blocks of life, just like bricks are the building blocks of a house. Without cells, life simply cannot exist.

The chapter “Cell: Structure and Functions” in the Class 8 NCERT Science book introduces you to this fascinating world of cells — their discovery, structure, types, and various functions. In this complete guide, we will explore every concept deeply, using simple explanations and examples.

By the end, you’ll understand not only what a cell is but also how these microscopic units together create and sustain life.

---

🔹 Discovery of the Cell

The story of cell discovery is quite interesting.

• In 1665, an English scientist Robert Hooke observed a thin slice of cork under a primitive microscope.

• He noticed tiny box-like structures and named them “cells” (from the Latin word cellula, meaning “small room”).

However, Hooke saw only dead cells.
It was later that other scientists like Anton van Leeuwenhoek used improved microscopes to observe living cells.

Then came Theodor Schwann and Matthias Schleiden, who proposed the Cell Theory, which states that:

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

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

3. All cells arise from pre-existing cells (added later by Rudolf Virchow).

---

🔹 What is a Cell?

A cell is the smallest structural and functional unit of life.
It performs all essential functions such as respiration, growth, reproduction, excretion, and response to stimuli.

Every organism — whether simple or complex — begins life as a single cell. For example:

• A bacterium has only one cell.

• A human starts life as one fertilized egg cell that divides again and again to form trillions of cells.

---

🔹 Types of Organisms Based on Number of Cells

There are two main types of organisms:

1. Unicellular Organisms – Made up of only one cell.

   • Example: Amoeba, Paramecium, Bacteria, Euglena.

   • A single cell performs all life functions.

2. Multicellular Organisms – Made up of many cells.

   • Example: Humans, Plants, Animals.

   • Cells are specialized to perform different functions.

For instance, in humans:

• Nerve cells carry messages.

• Muscle cells help in movement.

• Red blood cells carry oxygen.

---

🔹 Shape and Size of Cells

Cells come in various shapes and sizes, depending on their function.

Shape:

• Amoeba – irregular shape.

• Red Blood Cell (RBC) – round and biconcave.

• Nerve Cell – long and branched to carry signals.

• Muscle Cell – spindle-shaped for contraction.

Size:

• Smallest cell: Mycoplasma (~0.1 μm).

• Largest cell: Ostrich egg (~17 cm).

• Longest cell: Nerve cell in humans (up to 1 meter long).

---

🔹 Structure of a Cell

Cells have different parts that perform specific functions. These parts are called cell organelles.
Before exploring them, let’s first understand the basic structure common to most cells.

A cell has three main parts:

1. Cell Membrane (Plasma Membrane)

2. Cytoplasm

3. Nucleus

Let’s study each in detail.

---

🔹 1. Cell Membrane (Plasma Membrane)

The cell membrane is the outermost covering of the cell.

• It is thin, flexible, and semi-permeable (allows some substances to pass through but not others).

• It separates the contents of the cell from the external environment.

• It helps in exchange of materials like oxygen, carbon dioxide, and nutrients.

In plant cells, there is an additional layer called the cell wall outside the cell membrane.

---

🔹 2. Cell Wall

• Found only in plant cells, fungi, and some bacteria.

• It is rigid and strong, made of a substance called cellulose.

• Provides support and protection to the plant cell and gives it a definite shape.

When plant cells absorb water, the cell wall prevents them from bursting — this is why plants stay upright.

---

🔹 3. Cytoplasm

The cytoplasm is a jelly-like substance present between the cell membrane and the nucleus.

• It contains many cell organelles (tiny cell parts that perform specific functions).

• It allows movement of materials inside the cell.

The organelles floating in the cytoplasm include:
Mitochondria, Endoplasmic Reticulum, Golgi apparatus, Lysosomes, Ribosomes, Vacuoles, and Plastids.

---

🔹 4. Nucleus

The nucleus is the control center of the cell.
It regulates all cellular activities such as growth, metabolism, and reproduction.

Parts of a nucleus:

• Nuclear membrane: A double-layered covering separating the nucleus from the cytoplasm.

• Nucleolus: A small dense structure inside the nucleus that makes ribosomes.

• Chromosomes: Thread-like structures containing DNA (Deoxyribonucleic Acid) — the genetic material.

The DNA controls heredity and passes genetic information from parents to offspring.

---

🔹 Cell Organelles and Their Functions

Each organelle inside the cell has a specific function, just like organs in our body. Let’s explore them one by one.

Mitochondria – The Powerhouse of the Cell

• Produces energy through cellular respiration.

• Converts food (glucose) into energy (ATP – Adenosine Triphosphate).

• Found in both plant and animal cells.

Endoplasmic Reticulum (ER) – The Transport System

• A network of membranes that transports materials within the cell.

• Two types:

  • Rough ER: Has ribosomes; helps make proteins.

  • Smooth ER: Lacks ribosomes; helps make fats and steroids.

Golgi Apparatus – The Packaging Unit

• Modifies, stores, and packs materials (like proteins and lipids).

• Forms lysosomes and secretory vesicles.

Lysosomes – The Suicide Bags

• Contain digestive enzymes to break down waste or damaged cell parts.

• Help keep the cell clean.

Ribosomes – The Protein Factories

• Tiny structures either free in cytoplasm or attached to Rough ER.

• Help in protein synthesis.

Vacuoles – The Storage Units

• Membrane-bound sacs that store water, food, and waste.

• Large and central in plant cells; small in animal cells.

Plastids – The Colour Makers (Plant Cells Only)

• Found only in plant cells.

• Three types:

  1. Chloroplasts – green (contain chlorophyll; help in photosynthesis).

  2. Chromoplasts – colored (give flowers and fruits their colors).

  3. Leucoplasts – colorless (store starch, oils, and proteins).

---

🔹 Plant Cell vs Animal Cell

Feature	Plant Cell	Animal Cell
Cell Wall	Present	Absent
Shape	Fixed, rectangular	Irregular, round
Vacuole	One large central vacuole	Many small vacuoles
Plastids	Present	Absent
Centrioles	Absent	Present
Energy Storage	Starch	Glycogen

---

🔹 Functions of the Cell

Cells perform various essential functions that keep organisms alive. These include:

• Respiration – Energy production.

• Growth – Cells divide to increase number and size.

• Reproduction – Formation of new cells or organisms.

• Transport – Movement of nutrients and waste.

• Response – Reacting to changes in surroundings.

---

🔹 Levels of Organization

In multicellular organisms, cells are organized in a hierarchy:

Cell → Tissue → Organ → Organ System → Organism

Example (in humans):

• Muscle cells → form Muscle Tissue

• Muscle tissue → forms Heart (Organ)

• Heart → part of Circulatory System (Organ System)

• Organ systems together → form the Human Body (Organism)

---

🔹 Microscopy: Seeing the Cell

Modern microscopes allow us to see cells clearly.

• Light Microscope – uses visible light; can magnify up to 1000x.

• Electron Microscope – uses electrons; magnifies up to 1,000,000x, showing internal details.

These instruments have helped scientists study organelles and understand cell functions in detail.

---

🔹 Cell Division

Cells divide to grow, replace old cells, or reproduce.
There are two main types:

1. Mitosis:

   • Occurs in body (somatic) cells.

   • Produces two identical daughter cells.

   • Responsible for growth and repair.

2. Meiosis:

   • Occurs in reproductive cells.

   • Produces four daughter cells with half chromosomes.

   • Responsible for sexual reproduction.

---

🔹 Importance of Cells in Life

• Basis of life and structure.

• Perform all vital functions.

• Carry hereditary information.

• Maintain and repair tissues.

• Form specialized tissues for advanced functions.

Without cells, there would be no life — they are the true units of life.

---

🔹 Quick Summary

• All living beings are made of cells.

• Cell is the basic unit of life.

• Plant and animal cells differ in structure.

• Organelles perform specific tasks.

• Cells divide to form new cells.

• DNA carries genetic information.

---

🔹 

✅ Class 8 Science Chapter 8 Notes
✅ Cell: Structure and Functions
✅ Class 8 Science NCERT Notes
✅ Cell Organelles and Functions
✅ Difference between Plant and Animal Cells
✅ NCERT Nation Science Notes
✅ What is a Cell Class 8
✅ Mitochondria Function Class 8

---

🔹 Conclusion

Cells are the foundation of life — from a single-cell organism to complex human beings. Each cell functions in harmony to maintain life. Understanding their structure and functions gives us insight into how our bodies and nature work.

The study of cells has opened the door to countless scientific advancements — from genetic engineering to medicine and biotechnology. Truly, life begins and ends with the cell.