Cell theory

Cells are the fundamental building blocks of all living organisms. Despite their diversity, all cells share key structural features and functions.

1. Cell Theory

Cell theory is a thoroughly tested scientific explanation for the characteristics and origin of cells, developed over centuries by multiple scientists.

1.1 Historical Context

  • Antony van Leeuwenhoek (1600s–1670s): Observed single-celled organisms ("animalcules") and discovered bacteria and protozoa using early microscopes.

  • Robert Hooke (1665): Coined the term "cell" in Micrographia after observing box-like structures in cork tissue.

  • Matthias Schleiden & Theodor Schwann (Late 1830s): Formulated the unified cell theory.

  • Rudolf Virchow: Expanded the theory by establishing that all cells originate from existing cells.

1.2 Core Principles of Cell Theory

  1. All organisms are made up of cells

    • Every living thing is composed of one or more cells.

  2. Cells are the smallest units of life

    • A cell is the basic unit of life; if an organism is not made of cells, it cannot be considered living.

  3. All cells arise from preexisting cells

    • New cells form exclusively through cell division rather than spontaneous generation.

2. Basic Characteristics of All Cells

Regardless of their size or complexity, all cells contain four fundamental components:

  • Plasma Membrane (Cell Membrane): Outer layer that encloses internal cell structures and regulates entry and exit of materials.

  • Cytoplasm: Jelly-like fluid residing between the plasma membrane and nucleus that supports cell components.

  • DNA: Genetic material containing instructions for cell function and reproduction.

  • Ribosomes: Organelles responsible for protein synthesis.

3. Eukaryotic vs. Prokaryotic Cells
3.1 Prokaryotic Cells

  • Organisms: Bacteria and Archaea.

  • Key Features:

    • DNA is not enclosed within a membrane-bound nucleus.

    • Lack membrane-bound organelles (contain non-membrane-bound ribosomes).

    • Structurally simpler and generally smaller in size.

3.2 Eukaryotic Cells

  • Organisms: Animals, plants, fungi, and protists.

  • Key Features:

    • Genetic material (DNA) is contained inside a membrane-bound nucleus.

    • Enclosed by a lipid bilayer plasma membrane composed of two layers of phospholipids.

    • Contain complex membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

    • Structurally complex and generally larger in size.