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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.