Chapter-Cell: The Building Block of Life Science Exploration class 9 in english Medium CBSE Notes

Chapter 2. Cell: The Building Block of Life

This section explains the internal organisation of the cell. It covers the cytoplasm, cell organelles, prokaryotic and eukaryotic cells, and the structure and functions of the nucleus. These components work together to keep the cell alive and perform all life activities.

13. Cytoplasm

After crossing the cell membrane, the entire interior of the cell is filled with a jelly-like substance called the cytoplasm. It provides a medium in which all cell organelles remain suspended and perform their functions.

Definition

Cytoplasm is the semi-fluid, jelly-like substance present between the cell membrane and the nucleus.

Characteristics

• Jelly-like Nature – Mostly made up of water along with dissolved substances.
• Contains Organelles – Cell organelles remain suspended in the cytoplasm.
• Living Material – It actively participates in various cellular activities.
• Continuously Moving – Cytoplasm remains in constant motion inside living cells.

Functions

• Supports Organelles – Holds all cell organelles in place.
• Site of Chemical Reactions – Most metabolic activities occur in the cytoplasm.
• Transport of Materials – Helps move nutrients and waste materials within the cell.
• Maintains Cell Shape – Supports the internal structure of the cell.

14. Cell Organelles

Different parts of a cell perform different jobs. These specialised structures are known as cell organelles. Each organelle has a specific function that helps the cell survive and function efficiently.

Definition

Cell organelles are specialised structures present inside the cytoplasm that perform specific functions.

Major Cell Organelles

• Nucleus – Controls all activities of the cell.
• Mitochondria – Produce energy for the cell.
• Endoplasmic Reticulum (ER) – Transports materials within the cell.
• Golgi Apparatus – Modifies, packages and transports substances.
• Ribosomes – Synthesize proteins.
• Lysosomes – Digest waste materials and damaged cell parts.
• Vacuoles – Store food, water and waste materials.
• Plastids – Present mainly in plant cells and involved in food production and storage.

Importance

• Division of Work – Each organelle performs a specialised function.
• Efficient Functioning – Organelles work together to keep the cell alive.
• Coordination – Proper functioning of all organelles maintains healthy cells.

15. Prokaryotic and Eukaryotic Cells

Cells are broadly divided into two major types based on the presence or absence of a true nucleus and membrane-bound organelles.

Classification

Cells are classified into Prokaryotic Cells and Eukaryotic Cells.

Prokaryotic Cells

• Definition – Cells that do not possess a true nucleus.
• Nucleus – Genetic material is not enclosed by a nuclear membrane.
• Organelles – Membrane-bound organelles are absent.
• Examples – Bacteria and Cyanobacteria.

Eukaryotic Cells

• Definition – Cells that possess a well-defined nucleus enclosed by a nuclear membrane.
• Nucleus – Clearly organised and membrane-bound.
• Organelles – Membrane-bound organelles are present.
• Examples – Plants, animals, fungi and protists.

Difference

Prokaryotic Cell	Eukaryotic Cell
No true nucleus.	True nucleus present.
Membrane-bound organelles absent.	Membrane-bound organelles present.
Smaller in size.	Larger in size.
Simple structure.	Complex structure.
Example: Bacteria.	Example: Plant and Animal Cells.

16. Nucleus – The Control Centre of the Cell

The nucleus is the most important organelle of a eukaryotic cell. It controls all cellular activities and stores hereditary information that is passed from parents to offspring.

Definition

The nucleus is a membrane-bound organelle that controls the activities of the cell and contains the genetic material.

Main Parts of the Nucleus

• Nuclear Membrane – Protects the nucleus and regulates the movement of materials.
• Nucleoplasm – Jelly-like substance present inside the nucleus.
• Nucleolus – Helps in the formation of ribosomes.
• Chromatin – Thread-like material that forms chromosomes during cell division.

Functions

• Controls Cell Activities – Regulates growth, metabolism and reproduction.
• Stores Genetic Information – Contains DNA and genes.
• Cell Division – Plays an important role during cell division.
• Inheritance – Transfers hereditary characters from parents to offspring.

17. DNA, Chromosomes and Genes

The nucleus contains the hereditary material of the cell. DNA forms chromosomes, and chromosomes contain genes that determine the characteristics of an organism.

DNA (Deoxyribonucleic Acid)

DNA is the hereditary material responsible for storing genetic information.

Chromosomes

Chromosomes are thread-like structures made of DNA and proteins that become clearly visible during cell division.

Genes

Genes are small segments of DNA that control inherited characteristics such as eye colour, height and blood group.

Relationship

• DNA – Forms chromosomes.
• Chromosomes – Contain thousands of genes.
• Genes – Carry hereditary information from one generation to the next.

18. ATP Education Quick Concept

Cell Part	Main Function
Cell Membrane	Controls the movement of substances.
Cell Wall	Provides strength and protection.
Cytoplasm	Site of most metabolic activities.
Nucleus	Controls all activities of the cell.
DNA	Stores hereditary information.
Chromosome	Contains genes.
Gene	Controls inherited characters.

ATP Education Tip

Remember this sequence for CBSE examinations: Cell → Nucleus → Chromosome → DNA → Gene. Questions based on this relationship are frequently asked in objective, competency-based and case-study sections.

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