Structure of Prokaryotic and Eukaryotic Cells

Cells are the fundamental units of life, and all living organisms are composed of one of two basic cell types: prokaryotic cells or eukaryotic cells. The primary difference between these two cell types lies in the presence or absence of a membrane-bound nucleus and other organelles.

1.0Prokaryotic Cells

• Prokaryotic cells are the simplest and oldest forms of life on Earth. The term "prokaryote" comes from Greek words meaning "before nucleus." 
• These organisms are always unicellular, and they include all bacteria and archaea. 
• They lack a true nucleus and other membrane-bound organelles, giving them a relatively simple internal structure.

Structures of a Prokaryotic Cell

• Nucleoid: A region within the cytoplasm where the single, circular chromosome (DNA) is located. Unlike a nucleus, it is not enclosed by a membrane.
• Cell Wall: A rigid outer layer that provides structural support and protection to the cell. Its composition (e.g., peptidoglycan in bacteria) varies among different prokaryotes.
• Plasma Membrane: A semipermeable membrane that encloses the cytoplasm and regulates the movement of substances in and out of the cell.
• Cytoplasm: The jelly-like substance that fills the cell, containing ribosomes and other cellular components.
• Ribosomes: Tiny structures responsible for protein synthesis. They are smaller than those in eukaryotic cells.
• Flagella (optional): Long, whip-like appendages used for locomotion.
• Pili/Fimbriae (optional): Hair-like structures used for attachment to surfaces or for transferring genetic material (in the case of sex pili).
• Capsule (optional): A sticky, outermost layer that provides additional protection and helps in adhesion.
• Plasmids (optional): Small, circular rings of extra DNA that carry non-essential genes, such as those for antibiotic resistance.

2.0Eukaryotic Cells

• Eukaryotic cells are more complex and are believed to have evolved from prokaryotic cells. The term "eukaryote" means "true nucleus." 
• These cells are found in all plants, animals, fungi, and protists. 
• They can be either unicellular or multicellular. 
• The defining feature of a eukaryotic cell is the presence of a membrane-bound nucleus and numerous other membrane-bound organelles that compartmentalise cellular functions.

Structures of a Eukaryotic Cell

• Nucleus: The most prominent organelle, enclosed by a double membrane called the nuclear envelope. It houses the cell's genetic material as multiple linear chromosomes.
• Mitochondria: Often called the "powerhouses" of the cell, these organelles generate the majority of the cell's energy (ATP) through cellular respiration.
• Endoplasmic Reticulum (ER): A network of membranes involved in synthesising lipids (smooth ER) and proteins (rough ER, due to attached ribosomes).
• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.
• Lysosomes: Contain digestive enzymes that break down waste materials, cellular debris, and foreign substances.
• Chloroplasts (in plant cells): The site of photosynthesis, where light energy is converted into chemical energy.
• Vacuoles: Membrane-bound sacs that serve various functions, including storage of water, nutrients, and waste. Plant cells typically have a large central vacuole.
• Cytoskeleton: A network of protein filaments that provides structural support, helps maintain cell shape, and is involved in cell movement and transport.
• Cell Wall (in plants and fungi): A rigid outer layer for support and protection, distinct in composition from prokaryotic cell walls.

Also Read: Cell Structure and Function

3.0Comparison: Prokaryotic vs. Eukaryotic Cells

The following table summarises the key differences between prokaryotic and eukaryotic cells.