Prokaryotic and Eukaryotic Cells: Structure, Features and Differences
Complete Guide • Cell Biology • Prokaryotes • Eukaryotes • Cell Structure • NEET & Board Notes
Key Takeaways
- Understand the structure and characteristics of prokaryotic and eukaryotic cells.
- Learn the major differences between primitive and advanced cells.
- Study important bacterial cell structures such as capsule, cell wall, plasma membrane and nucleoid.
- Understand Gram-positive and Gram-negative bacteria.
- Useful for Class 11 Biology, NEET, CUET and university entrance examinations.
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| Structure of Prokaryotic and Eukaryotic Cells |
Introduction
Cells are the basic structural and functional units of life. Every living organism is composed of one or more cells that perform essential life processes such as growth, metabolism, reproduction and response to the environment. Based on their internal organization, cells are classified into two major types: prokaryotic cells and eukaryotic cells.
Prokaryotic cells are simple, primitive cells lacking a true nucleus and membrane-bound organelles, whereas eukaryotic cells possess a well-defined nucleus and numerous specialized organelles. Understanding the differences between these two cell types is fundamental to the study of biology.
Quick Fact
Prokaryotic cells appeared on Earth nearly 3.5 billion years ago and are considered the earliest forms of cellular life. Eukaryotic cells evolved later and gave rise to plants, animals, fungi and protists.
What is a Prokaryotic Cell?
A prokaryotic cell is a simple, unicellular organism that lacks a true nucleus and membrane-bound organelles. The genetic material is present in a region called the nucleoid, where the DNA is not enclosed by a nuclear membrane.
Prokaryotic organisms include bacteria, cyanobacteria and archaea. They reproduce rapidly by binary fission and possess a relatively simple internal organization compared to eukaryotic cells.
Definition
A prokaryotic cell is a primitive cell lacking a true nucleus and membrane-bound organelles, with genetic material organized within a nucleoid region.
General Features of Prokaryotic Cells
Prokaryotic cells possess several structural and functional characteristics that distinguish them from eukaryotic cells. These features enable rapid growth, adaptation and reproduction under diverse environmental conditions.
Major Characteristics
- Small and simple cellular organization.
- No true nucleus.
- No membrane-bound organelles.
- Single circular double-stranded DNA molecule.
- 70S ribosomes present.
- Reproduce by binary fission.
- Usually surrounded by a rigid cell wall.
- May possess capsule, pili and flagella.
Capsule (Glycocalyx)
Many bacteria possess an outermost protective covering called the capsule or glycocalyx. It surrounds the cell wall and is mainly composed of polysaccharides, although in some bacteria it consists of polypeptides.
The capsule protects bacterial cells from harsh environmental conditions, prevents dehydration and enables them to adhere to surfaces and host tissues. Pathogenic bacteria also use the capsule to evade the host's immune system.
Functions of Capsule
- Protects against desiccation.
- Prevents phagocytosis by immune cells.
- Helps bacteria adhere to surfaces.
- Provides protection from harmful chemicals.
- Increases bacterial virulence.
Cell Wall
The cell wall is a rigid protective layer that surrounds the plasma membrane of most prokaryotic cells. It provides shape, mechanical strength and protection against osmotic pressure.
The bacterial cell wall is primarily composed of peptidoglycan (murein), a complex polymer of sugars and amino acids that gives rigidity to the bacterial cell.
Functions of Cell Wall
- Maintains cell shape.
- Protects against osmotic bursting.
- Provides mechanical support.
- Protects the plasma membrane.
- Determines Gram staining characteristics.
Gram Positive vs Gram Negative Bacteria
Bacteria are classified into Gram-positive and Gram-negative based on their cell wall composition and their response to the Gram staining technique developed by Hans Christian Gram.
| Feature | Gram Positive | Gram Negative |
|---|---|---|
| Peptidoglycan Layer | Thick | Thin |
| Outer Membrane | Absent | Present |
| Gram Stain | Purple / Violet | Pink / Red |
| Teichoic Acid | Present | Absent |
| Examples | Lactobacillus, Clostridium | Escherichia coli, Rhizobium |
Plasma Membrane
The plasma membrane is a selectively permeable phospholipid bilayer located just beneath the cell wall. It encloses the cytoplasm and regulates the movement of substances into and out of the cell.
Since prokaryotic cells lack membrane-bound organelles, many important metabolic activities occur on or within the plasma membrane.
Functions of Plasma Membrane
- Controls transport of substances.
- Site of cellular respiration.
- Participates in ATP synthesis.
- Supports DNA replication.
- Helps in septum formation during binary fission.
DNA (Nucleoid)
The genetic material of prokaryotic cells is located in a region known as the nucleoid. Unlike eukaryotic cells, this DNA is not enclosed within a nuclear membrane.
Most bacteria possess a single circular, double-stranded DNA molecule. Many species also contain small circular DNA molecules known as plasmids, which carry genes for antibiotic resistance and other advantageous traits.
Characteristics of the Nucleoid
- No nuclear membrane.
- Single circular DNA molecule.
- Double-stranded DNA.
- May contain plasmids.
- Controls all cellular activities.
Polysomes
Polysomes (polyribosomes) are clusters of multiple ribosomes attached to a single messenger RNA (mRNA) molecule. They enable simultaneous synthesis of several copies of the same protein, thereby increasing the efficiency of protein production.
Prokaryotic cells possess 70S ribosomes, which consist of a 50S large subunit and a 30S small subunit.
Functions of Polysomes
- Protein synthesis.
- Increase efficiency of translation.
- Produce multiple protein molecules simultaneously.
- Essential for bacterial growth and metabolism.
Exam Tips
- ✔ Capsule increases bacterial virulence.
- ✔ Cell wall contains peptidoglycan.
- ✔ Gram-positive bacteria stain purple.
- ✔ Gram-negative bacteria possess an outer membrane.
- ✔ Prokaryotes contain 70S ribosomes.
- ✔ DNA is present in the nucleoid region.
Fimbriae or Pili
Fimbriae and pili are short, thin, hair-like appendages present on the surface of many bacteria. They are composed of a protein called pilin. Although both appear similar, they differ in size and function.
Fimbriae are numerous and mainly help bacteria attach to surfaces, whereas pili are comparatively longer and fewer in number. Specialized pili, called sex pili, participate in bacterial conjugation by transferring genetic material from one bacterial cell to another.
Functions of Fimbriae and Pili
- Help bacteria attach to host tissues.
- Assist in biofilm formation.
- Enable bacterial conjugation.
- Transfer plasmid DNA.
- Increase bacterial pathogenicity.
Flagella
Flagella are long, whip-like locomotory organelles that enable bacteria to move in response to environmental stimuli. This directed movement towards favorable conditions or away from harmful substances is known as taxis.
A bacterial flagellum consists of three main parts: the filament, hook and basal body. Movement is produced by the rotation of the basal body powered by the proton motive force.
| Part | Function |
|---|---|
| Filament | Long external whip-like structure responsible for movement. |
| Hook | Connects the filament with the basal body. |
| Basal Body | Acts as the motor that rotates the flagellum. |
Types of Flagellar Arrangement
- Monotrichous – One flagellum.
- Lophotrichous – Tuft of flagella at one end.
- Amphitrichous – Flagella at both ends.
- Peritrichous – Flagella distributed all over the cell.
What is a Eukaryotic Cell?
A eukaryotic cell is a highly organized cell possessing a true nucleus enclosed by a nuclear membrane and numerous membrane-bound organelles. These cells are generally larger and more complex than prokaryotic cells.
Eukaryotic cells are found in plants, animals, fungi and protists. Their compartmentalized organization allows different cellular processes to occur simultaneously with greater efficiency.
Characteristics of Eukaryotic Cells
- Possess a true nucleus.
- Contain membrane-bound organelles.
- Have linear chromosomes associated with histone proteins.
- Usually contain 80S ribosomes.
- Divide by mitosis and meiosis.
- Larger and more complex than prokaryotic cells.
Major Cell Organelles of Eukaryotic Cells
| Organelle | Main Function |
|---|---|
| Nucleus | Controls cellular activities and stores genetic material. |
| Mitochondria | Production of ATP by cellular respiration. |
| Endoplasmic Reticulum | Protein and lipid synthesis. |
| Golgi Apparatus | Modification, packaging and secretion of proteins. |
| Lysosomes | Intracellular digestion. |
| Chloroplast | Photosynthesis (plant cells). |
| Vacuoles | Storage and maintenance of cell turgidity. |
| 80S Ribosomes | Protein synthesis. |
Comparison of Prokaryotic and Eukaryotic Cells
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Cell Size | Small (1–10 µm) | Large (10–100 µm) |
| Nucleus | Absent | Present |
| DNA | Circular | Linear |
| Organelles | Absent | Present |
| Ribosomes | 70S | 80S |
Memory Tricks
- 🦠 Prokaryote = Primitive cell.
- 🏛️ Eukaryote = True nucleus.
- 📦 Capsule → Protection.
- 🧱 Cell Wall → Shape & Support.
- 🧬 Nucleoid → Circular DNA.
- 🏃 Flagella → Movement.
- 🤝 Pili → Conjugation.
- ⚙️ 70S → Prokaryotes | 80S → Eukaryotes.
Differences Between Prokaryotic and Eukaryotic Cells
Although both prokaryotic and eukaryotic cells perform the basic functions necessary for life, they differ significantly in their structure, organization and complexity. The following comparison highlights the major differences between these two cell types.
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Cell Size | 1–10 μm | 10–100 μm |
| Nucleus | Absent | Present |
| Nuclear Membrane | Absent | Present |
| DNA | Circular | Linear |
| Chromosomes | Single | Multiple |
| Histone Proteins | Absent | Present |
| Membrane-bound Organelles | Absent | Present |
| Ribosomes | 70S | 80S |
| Cell Division | Binary Fission | Mitosis / Meiosis |
| Examples | Bacteria, Archaea | Plants, Animals, Fungi, Protists |
Similarities Between Prokaryotic and Eukaryotic Cells
Despite several structural differences, both types of cells possess certain fundamental characteristics essential for life.
- Both contain plasma membranes.
- Both possess cytoplasm.
- Both contain DNA as genetic material.
- Both have ribosomes for protein synthesis.
- Both perform metabolism and respiration.
- Both reproduce and respond to environmental stimuli.
Quick Revision
- ✔ Prokaryotes are primitive cells.
- ✔ Eukaryotes possess a true nucleus.
- ✔ Prokaryotes contain circular DNA.
- ✔ Eukaryotes contain linear chromosomes.
- ✔ Prokaryotic ribosomes = 70S.
- ✔ Eukaryotic ribosomes = 80S.
- ✔ Binary fission occurs in prokaryotes.
- ✔ Mitosis and meiosis occur in eukaryotes.
- ✔ Membrane-bound organelles are absent in prokaryotes.
- ✔ Membrane-bound organelles are present in eukaryotes.
Competitive Examination Points
- Frequently asked in NEET Biology.
- Important for CUET, SSC, Nursing and State PSC examinations.
- Remember: 70S → Prokaryotes, 80S → Eukaryotes.
- Gram-positive bacteria have a thick peptidoglycan layer.
- Gram-negative bacteria possess an outer membrane.
- Binary fission is characteristic of prokaryotes.
- Histone proteins are absent in bacteria.
- Mitochondria and chloroplasts are membrane-bound organelles.
Common Mistakes Students Make
- ❌ Confusing the nucleoid with the nucleus.
- ❌ Assuming all bacteria possess capsules.
- ❌ Mixing up 70S and 80S ribosomes.
- ❌ Forgetting that prokaryotes lack membrane-bound organelles.
- ❌ Confusing binary fission with mitosis.
- ❌ Assuming plasmids are present in all cells.
- ❌ Forgetting that Gram-positive bacteria stain purple.
- ❌ Assuming all prokaryotes have flagella.
One-Minute Revision Table
| Topic | Remember |
|---|---|
| Prokaryotes | Primitive, No nucleus |
| Eukaryotes | True nucleus present |
| DNA | Circular vs Linear |
| Ribosomes | 70S vs 80S |
| Division | Binary Fission vs Mitosis |
| Organelles | Absent vs Present |
Frequently Asked Questions (FAQs)
1. What is a prokaryotic cell?
A prokaryotic cell is a primitive cell that lacks a true nucleus and membrane-bound organelles. Its genetic material is present in a nucleoid region rather than inside a nuclear membrane.
2. What is a eukaryotic cell?
A eukaryotic cell is a highly organized cell possessing a true nucleus enclosed by a nuclear membrane along with membrane-bound organelles such as mitochondria, Golgi apparatus and endoplasmic reticulum.
3. What are examples of prokaryotic organisms?
- Bacteria
- Cyanobacteria
- Archaea
- Mycoplasma
4. Which ribosomes are present in prokaryotic cells?
Prokaryotic cells possess 70S ribosomes, which consist of a 50S large subunit and a 30S small subunit.
5. Which ribosomes are present in eukaryotic cells?
Most eukaryotic cells contain 80S ribosomes, composed of a 60S large subunit and a 40S small subunit.
6. What is the function of bacterial flagella?
Flagella help bacteria move from one place to another and respond to environmental stimuli through a process called taxis.
7. What is the difference between Gram-positive and Gram-negative bacteria?
Gram-positive bacteria possess a thick peptidoglycan layer and stain purple, whereas Gram-negative bacteria have a thin peptidoglycan layer with an outer membrane and stain pink.
8. Why are eukaryotic cells more complex than prokaryotic cells?
Eukaryotic cells contain a true nucleus and specialized membrane-bound organelles that divide cellular functions into different compartments, making them structurally and functionally more advanced.
Conclusion
Prokaryotic and eukaryotic cells represent the two fundamental organizational patterns of living organisms. Prokaryotic cells are simple, primitive and lack membrane-bound organelles, whereas eukaryotic cells are highly organized with a true nucleus and specialized organelles that perform distinct cellular functions.
Understanding the similarities and differences between these two cell types forms the foundation of modern cell biology and is essential for students preparing for Class 11 Biology, NEET, CUET, Nursing Entrance, B.Sc. and other competitive examinations.
Final Revision
- ✔ Prokaryotes are the earliest forms of life.
- ✔ Eukaryotes possess a true nucleus.
- ✔ Prokaryotic DNA is circular.
- ✔ Eukaryotic DNA is linear.
- ✔ 70S ribosomes occur in prokaryotes.
- ✔ 80S ribosomes occur in eukaryotes.
- ✔ Binary fission is the mode of division in prokaryotes.
- ✔ Mitosis and meiosis occur in eukaryotic cells.
- ✔ Membrane-bound organelles are absent in prokaryotes.
- ✔ Membrane-bound organelles are present in eukaryotes.
More Topics
References
- NCERT Biology Class XI
- NCERT Biology Class XII
- Campbell Biology
- Molecular Biology of the Cell – Alberts
- Prescott's Microbiology
- Brock Biology of Microorganisms
- Modern Cell Biology Textbooks