Robert Hooke discovered cells in 1665 using a microscope. He defined cells as the fundamental units of life. Later, Schleiden and Schwann defined cells as the basic units of plant and animal structure, respectively. Prokaryotic cells are smaller than eukaryotic cells, lack membrane-bound organelles, and have their DNA found in the cytoplasm rather than a nucleus. The endosymbiotic theory explains how eukaryotic cells evolved from large cells engulfing smaller cells that became organelles in a symbiotic relationship around 2 billion years ago.

1. Prokaryotic and Eukaryotic cell

2. • British scientist Robert Hooke discovered cells in 1665. He used a microscope
he designed himself to study a slice of cork and noticed a honeycomb-like
structure. He named these structures cells and defined them as the fundamental
structural and functional units of life. He also coined the term "cells“.
• Anton van Leeuwenhoek: Discovered protozoa, bacteria, sperm cells, blood
cells, microscopic nematodes, and rotifers.
• Matthias Jacob Schleiden: Defined the cell as the basic unit of plant structure in
1838
• Theodor Schwann: Defined the cell as the basic unit of animal structure in 1839
• George Emil Palade: Known as the father of cell biology, he first used the
electron microscope to discover ribosomes and secretory proteins
• Mycoplasma gallicepticum is the smallest cell with a size of 0.2–0.3 μm.
• The ostrich egg is the largest cell in the world, measuring about 15–18
centimeters long and wide, and weighing around 1.4 kilograms.

4. Prokaryotic cells have the following characteristics
• Size: Prokaryotic cells range from 0.1 to 10 micrometers in diameter.
• Structure: Prokaryotic cells are unicellular, and have a simpler structure than eukaryotic
cells.
• Organelles: Prokaryotic cells lack membrane-bound organelles like mitochondria, Golgi
apparatus, and chloroplasts.
• DNA: Prokaryotic cells have single circular DNA as genetic material, and this DNA is found
in the cytoplasm in a region called the nucleoid.
• Cell wall: Prokaryotic cells have a rigid cell wall made up of polysaccharides and amino
acids.
• Plasma membrane: Prokaryotic cells have a plasma membrane made of phospholipids,
which allows some molecules to enter or leave the cell.
• Prokaryotic cells have mesosomes for respiration.
• There are two types of prokaryotic cell such as: Archaea and bacteria.
• Prokaryotes often have appendages (protrusions) on their surface. Flagella and some pili
are used for locomotion, fimbriae help the cell stick to a surface, and sex pili are used for
DNA exchange.

6. Components of prokaryotic cell
Components of prokaryotic cell

8. Evolution of eukaryotes
The first eukaryotic cells probably evolved about 2 billion years ago. The
endosymbiotic theory explains their evolution. According to which, large cells
engulfed small cells but are not digested by them. Instead, the small cells lived within
them and evolved into organelles. The large and small cells formed a symbiotic
relationship where both cells benefited from each other.

12. • Cytoskeleton

21. Prokaryotic flagella