Cell Organelles | Chapter 5 | The Fundamental unit Of Life | Class 9 Science

Cell: The Fundamental Unit of Life — Structure and Function

The cell is the basic structural and functional unit of all living organisms. Every living thing — from the tiniest bacterium to the largest whale — is made up of cells. The cell was first discovered by Robert Hooke in 1665 when he observed thin slices of cork under a primitive microscope and saw small box-like compartments that he called "cells" (from the Latin word "cella" meaning small room). However, Hooke was observing the cell walls of dead plant cells. The first living cell was observed by Antonie van Leeuwenhoek, who described bacteria and protozoa as "animalcules." The Cell Theory, formulated by Matthias Schleiden (1838, for plants) and Theodor Schwann (1839, for animals), states that all living organisms are composed of cells, and the cell is the basic unit of life. Rudolf Virchow later added that all cells arise from pre-existing cells (Omnis cellula e cellula), completing the modern cell theory.

Cells are classified into two broad categories: prokaryotic and eukaryotic. Prokaryotic cells (found in bacteria and blue-green algae/cyanobacteria) lack a well-defined nucleus and membrane-bound organelles. Their genetic material exists as a naked circular DNA molecule in the nucleoid region, and they also contain small circular DNA called plasmids. Ribosomes in prokaryotes are smaller (70S) compared to those in eukaryotes (80S). Eukaryotic cells (found in plants, animals, fungi, and protists) have a well-defined nucleus enclosed by a nuclear membrane, and contain various membrane-bound organelles. The plasma membrane is a thin, flexible barrier that surrounds every cell and is composed of a phospholipid bilayer with embedded proteins — it is selectively permeable, regulating what enters and exits the cell. Transport across the membrane occurs through diffusion (movement from high to low concentration), osmosis (diffusion of water through a semi-permeable membrane), and active transport (movement against the concentration gradient, requiring energy from ATP).

The nucleus is the control centre of the cell, containing the genetic material (DNA) organised into chromosomes. Each chromosome is made of DNA and proteins (histones), and the functional segments of DNA are called genes. The nucleolus within the nucleus produces ribosomal RNA. The endoplasmic reticulum (ER) is a network of membrane-bound tubes and sheets: rough ER (with ribosomes) synthesises proteins, while smooth ER synthesises lipids and detoxifies poisons. The Golgi apparatus packages, modifies, and dispatches proteins and lipids to their targets inside or outside the cell. Lysosomes are membrane-bound sacs containing digestive enzymes — they break down waste materials, cellular debris, and foreign invaders, and are sometimes called "suicide bags" because they can digest the cell itself if they rupture. Mitochondria are the "powerhouses of the cell" where aerobic respiration occurs, producing ATP (adenosine triphosphate), the energy currency. They have their own DNA and ribosomes, supporting the endosymbiotic theory that mitochondria were once free-living prokaryotes. Plant cells have additional structures: the cell wall (made of cellulose, providing rigidity and protection), chloroplasts (containing chlorophyll for photosynthesis), and a large central vacuole (maintaining turgor pressure and storing substances). The process of cell division — mitosis (for growth and repair, producing two identical daughter cells) and meiosis (for producing gametes, reducing chromosome number by half) — ensures continuity of life.

• Cell theory: all organisms are composed of cells, cells are the basic unit of life, and all cells arise from pre-existing cells.
• Prokaryotic cells lack a nucleus and membrane-bound organelles; eukaryotic cells have both.
• Key organelles: nucleus (DNA control), ER (protein/lipid synthesis), Golgi (packaging), mitochondria (energy/ATP), lysosomes (digestion).
• Plant cells have a cellulose cell wall, chloroplasts, and a large central vacuole — structures absent in animal cells.
• Transport across the plasma membrane: diffusion, osmosis (passive), and active transport (requires ATP energy).