CBSE Class 9 Science Notes: The Fundamental Unit of Life

Definitions and Importance of Cells

The cell is the basic structural and functional unit of life. All living organisms are made up of cells. It’s the smallest unit that can carry out all the processes of life, including growth, reproduction, and metabolism.

Importance: Understanding cells is crucial because:

• It helps us understand how organisms function.
• It’s the foundation for studying diseases and developing treatments.
• It explains how life evolves and adapts.

Prokaryotic vs. Eukaryotic Cells

Cells are broadly classified into two types based on their structure:

• Prokaryotic Cells: These are simpler cells, typically smaller and lack a defined nucleus and membrane-bound organelles. The genetic material (DNA) is present as a nucleoid. Examples include bacteria.
• Eukaryotic Cells: These are more complex cells, larger in size, and possess a well-defined nucleus and membrane-bound organelles. The genetic material (DNA) is organized within the nucleus. Examples include plant and animal cells.

Here’s a table summarizing the key differences:

Unicellular and Multicellular Organisms

Organisms are classified based on the number of cells they are composed of:

• Unicellular Organisms: These organisms consist of a single cell. This single cell carries out all the life processes. Examples include amoeba, paramecium, and bacteria.
• Multicellular Organisms: These organisms are made up of many cells, which are specialized to perform different functions. The cells work together to maintain the organism’s life. Examples include plants, animals, and fungi.

Cell Membrane and Cell Wall

These are the outer boundaries of the cell that provide structure, protection, and regulate the movement of substances in and out.

• Cell Membrane (Plasma Membrane): Found in both prokaryotic and eukaryotic cells, it is a flexible, selectively permeable membrane made up primarily of lipids and proteins. It controls the entry and exit of substances.
• Cell Wall: Found only in plant cells, bacteria, and fungi. It’s a rigid outer covering that provides shape, support, and protection. In plant cells, it is mainly composed of cellulose.

Cell Organelles

Eukaryotic cells contain various specialized structures called organelles, each with a specific function.

• Mitochondria: The “powerhouses” of the cell; they generate energy (ATP) through cellular respiration.
• Chloroplasts: Found in plant cells, they perform photosynthesis, converting light energy into chemical energy (glucose).
• Nucleus: The control center of the cell, containing the genetic material (DNA).
• Vacuoles: Storage sacs for water, nutrients, and waste products. Larger in plant cells.
• Endoplasmic Reticulum (ER): A network of membranes involved in protein and lipid synthesis and transport. Rough ER has ribosomes attached; smooth ER does not.
• Golgi Apparatus: Processes, packages, and modifies proteins and lipids. It’s involved in the secretion of substances.

Chromosomes and Their Basic Structure

Chromosomes are thread-like structures found within the nucleus, made up of DNA and proteins. They carry the genetic information in the form of genes.

Structure:

• Chromosomes are composed of DNA tightly coiled around proteins called histones.
• Each chromosome contains a constricted region called the centromere, which is important during cell division.
• At the ends of the chromosomes are telomeres, which protect the DNA from degradation.

Humans have 46 chromosomes (23 pairs) in most of their cells.