APSC Scientific Officer: The cell theory, Prokaryotic and Eukaryotic cells (Plant cell and animal cell)

The Cell Theory

The cell theory is a fundamental concept in biology that describes the properties of cells. It was developed by Matthias Schleiden, Theodor Schwann, and later expanded by Rudolf Virchow. The main points of the cell theory are:

1. All living organisms are composed of one or more cells.
2. The cell is the basic unit of structure and function in living organisms.
3. All cells arise from pre-existing cells.

This theory emphasizes that cells are the building blocks of life and play a crucial role in the growth, development, and functioning of all living beings.

---

Prokaryotic and Eukaryotic Cells

Cells are categorized into two main types based on their structural differences: prokaryotic and eukaryotic cells.

1. Prokaryotic Cells:
   • Definition: Prokaryotic cells are simple, single-celled organisms without a nucleus or other membrane-bound organelles.
   • Examples: Bacteria and Archaea.
   • Key Features:
     • No nucleus; genetic material (DNA) is present in the nucleoid region.
     • Smaller in size (0.1-5.0 µm).
     • Lack membrane-bound organelles.
     • Have a cell wall (in most cases).
     • Ribosomes are smaller (70S type).
     • May have flagella or pili for movement and attachment.
2. Eukaryotic Cells:
   • Definition: Eukaryotic cells have a true nucleus enclosed by a nuclear membrane and contain membrane-bound organelles.
   • Examples: Plant cells, animal cells, fungi, and protists.
   • Key Features:
     • Nucleus containing DNA.
     • Larger in size (10-100 µm).
     • Presence of membrane-bound organelles like mitochondria, endoplasmic reticulum, Golgi apparatus, etc.
     • Ribosomes are larger (80S type).
     • Cell division occurs by mitosis or meiosis.

---

Plant Cell and Animal Cell

Though both plant and animal cells are eukaryotic, they have some differences in structure and function:

1. Plant Cell:
   • Cell Wall: Present, made of cellulose, providing structure and protection.
   • Chloroplasts: Present, containing chlorophyll for photosynthesis.
   • Vacuole: Large central vacuole for storage and maintaining cell turgidity.
   • Shape: Usually rectangular or square.
   • Centrioles: Absent in most plant cells.
   • Plasmodesmata: Present for cell-to-cell communication.
2. Animal Cell:
   • Cell Wall: Absent; only plasma membrane present.
   • Chloroplasts: Absent.
   • Vacuole: Small and temporary, mainly for storage.
   • Shape: Usually round or irregular.
   • Centrioles: Present, playing a role in cell division.
   • Lysosomes: More prominent and involved in digestion of cellular waste.

---

Key Differences between Plant and Animal Cells:

Feature	Plant Cell	Animal Cell
Cell Wall	Present (cellulose)	Absent
Chloroplasts	Present	Absent
Vacuole	Large central vacuole	Small and temporary
Shape	Rectangular or square	Round or irregular
Centrioles	Absent (except in lower plants)	Present
Lysosomes	Rare	Prominent

---

These concepts form the foundation for understanding cellular biology, allowing scientists to explore more complex biological processes.