1.9 Prokaryotic and Eukaryotic Cells

Introduction

Two kinds of structurally different cells have evolved over time. These are prokaryotic cells and eukaryotic cells. Prokaryotic cells include archaea, bacteria, and cyanobacteria, whereas all other forms of life are composed of eukaryotic cells.

Definition

Prokaryotic Cells: Prokaryotic cells are cells that lack a true nucleus and other membrane-bound organelles. The genetic material (DNA) is not enclosed within a nuclear membrane.

Eukaryotic Cells: Eukaryotic cells are cells that have a true nucleus and other membrane-bound organelles. The genetic material (DNA) is enclosed within a nuclear membrane.

• Prokaryotic cells = "before nucleus" — no true nucleus.
• Eukaryotic cells = "true nucleus" — have a true nucleus.

Core Concept

• Prokaryotic cells — include archaea, bacteria, and cyanobacteria.
• Eukaryotic cells — all other forms of life (protists, fungi, plants, animals).

• Prokaryotic cells are simpler and smaller than eukaryotic cells.
• Eukaryotic cells are more complex and larger than prokaryotic cells.

Detailed Explanation

Differences Between Prokaryotic and Eukaryotic Cells

Prokaryotic Cells

• Nucleus: Absent. The genetic material (DNA) is not enclosed in a nuclear membrane. It lies in the cytoplasm in a region called the nucleoid.
• Membrane-bound organelles: Absent. Prokaryotic cells do not have mitochondria, chloroplasts, ER, Golgi, lysosomes, etc.
• Ribosomes: Present (70S). They are smaller than eukaryotic ribosomes.
• Cell wall: Present in most prokaryotes (made of peptidoglycan).
• Cell membrane: Present.
• Cytoplasm: Present.
• DNA: One primary circular chromosome and various plasmids (small circular DNA molecules).
• Locomotion: Some have flagella made of flagellin protein.
• Division: Binary fission (asexual reproduction).
• Size: Generally smaller (1–5 μm).

Eukaryotic Cells

• Nucleus: Present. The genetic material (DNA) is enclosed within a nuclear membrane.
• Membrane-bound organelles: Present. Includes mitochondria, chloroplasts (in plants), ER, Golgi, lysosomes, peroxisomes, vacuoles, etc.
• Ribosomes: Present (80S). They are larger than prokaryotic ribosomes.
• Cell wall: Present in plants (cellulose), fungi (chitin), and some protists. Absent in animal cells.
• Cell membrane: Present.
• Cytoplasm: Present.
• DNA: Linear, double-stranded chromosomes.
• Locomotion: Flagella and cilia made of microtubules (9+2 arrangement).
• Division: Mitosis (for somatic cells) and meiosis (for germ cells).
• Size: Generally larger (10–100 μm).

Intermediate Level

The Two Types of Cells

Why Are Prokaryotic Cells Smaller?

• Prokaryotic cells lack membrane-bound organelles.
• They rely on diffusion for transport of materials.
• Diffusion is efficient only over short distances.
• Therefore, prokaryotic cells remain small.

Why Are Eukaryotic Cells Larger?

• Eukaryotic cells have membrane-bound organelles.
• They have transport systems (ER, Golgi, vesicles) to move materials.
• They have a cytoskeleton for structural support and transport.
• Therefore, eukaryotic cells can be much larger.

Prokaryotic vs Eukaryotic Ribosomes

Note: The "S" stands for Svedberg unit, which is a measure of sedimentation rate during centrifugation. It is not a direct measure of size or mass.

Prokaryotic vs Eukaryotic DNA

Important Points

• 1. Two kinds of structurally different cells have evolved over time:
• - Prokaryotic cells
• - Eukaryotic cells
• 2. Prokaryotic cells include archaea, bacteria, and cyanobacteria.
• 3. Eukaryotic cells include all other forms of life.
• 4. Prokaryotic cells:
• - Nucleus and other membrane-bound organelles are absent.
• - Always unicellular.
• - DNA is stored in the cytoplasm.
• - One primary circular chromosome and various plasmids.
• - Ribosomes are 70S with 30S and 50S subunits.
• - Locomotive structures are composed of repeated flagellin, a hook, and a motor complex.
• - Divide by binary fission.
• 5. Eukaryotic cells:
• - Nucleus and other membrane-bound organelles are present.
• - Unicellular and also multicellular organisms.
• - DNA is stored within the nucleus.
• - DNA is stored in double-stranded chromosomes.
• - Ribosomes are 80S with 40S and 60S subunits.
• - Divide by mitosis and meiosis.

Remember

• - Prokaryotic = before nucleus; no true nucleus
• - Eukaryotic = true nucleus; has a true nucleus
• - Prokaryotic cells: bacteria, archaea, cyanobacteria
• - Eukaryotic cells: protists, fungi, plants, animals
• - Prokaryotic: no nucleus, no membrane-bound organelles, 70S ribosomes, circular DNA, binary fission
• - Eukaryotic: true nucleus, membrane-bound organelles, 80S ribosomes, linear DNA, mitosis and meiosis
• - Prokaryotic: always unicellular
• - Eukaryotic: unicellular and multicellular
• - Prokaryotic: DNA in cytoplasm
• - Eukaryotic: DNA in nucleus
• - Prokaryotic: one circular chromosome + plasmids
• - Eukaryotic: multiple linear chromosomes

Common Mistakes

• 1. Thinking prokaryotic cells have no DNA – Prokaryotic cells have DNA, but it is not enclosed in a nucleus. It lies in the cytoplasm (nucleoid region).
• 2. Thinking prokaryotic cells have no ribosomes – Prokaryotic cells have ribosomes (70S), but they are smaller than eukaryotic ribosomes (80S).
• 3. Confusing prokaryotic and eukaryotic ribosomes – Prokaryotic = 70S (50S + 30S); Eukaryotic = 80S (60S + 40S).
• 4. Thinking all prokaryotes are bacteria – Prokaryotes include both bacteria and archaea.
• 5. Thinking all eukaryotes are multicellular – Some eukaryotes are unicellular (e.g., protists, yeasts).
• 6. Thinking prokaryotic cells divide by mitosis – Prokaryotic cells divide by binary fission, not mitosis.
• 7. Thinking eukaryotic cells have one circular chromosome – Eukaryotic cells have multiple linear chromosomes.
• 8. Confusing plasmids with chromosomes – Plasmids are small circular DNA molecules that are separate from the main chromosome.
• 9. Thinking the cell wall is the same in all cells – Prokaryotic cell wall = peptidoglycan; Plant cell wall = cellulose; Fungal cell wall = chitin; Animal cells have no cell wall.
• 10. Forgetting the Svedberg unit – The "S" in 70S and 80S stands for Svedberg unit, which is a measure of sedimentation rate, not size.
• 11. Thinking prokaryotic cells are always harmful – Most prokaryotes are harmless or beneficial. Only some are pathogenic.
• 12. Forgetting the locomotive structures – Prokaryotic flagella are made of flagellin; eukaryotic flagella are made of microtubules.

Quick Revision

• Two kinds of structurally different cells have evolved over time:
• Prokaryotic cells:
• Nucleus and other membrane-bound organelles are absent.
• Always unicellular.
• DNA is stored in the cytoplasm.
• Prokaryotic cells have one primary circular chromosome and various plasmids.
• Ribosomes in prokaryotic cells are 70S with 30S and 50S subunits.
• Locomotive structures are composed of repeated flagellin, a hook, and a motor complex.
• Prokaryotes divide by binary fission.
• Eukaryotic cells:
• Nucleus and other membrane-bound organelles are present.
• Eukaryotes are unicellular and also multicellular organisms.
• DNA is stored within the nucleus.
• In eukaryotic cells, DNA is stored in double-stranded chromosomes.
• Ribosomes in eukaryotic cells are 80S with 40S and 60S subunits.
• Eukaryotes divide by mitosis and meiosis.

60-Second Revision

• Prokaryotic = before nucleus; no true nucleus
• Eukaryotic = true nucleus; has a true nucleus
• Prokaryotic cells: bacteria, archaea, cyanobacteria
• Eukaryotic cells: protists, fungi, plants, animals
• Prokaryotic: no nucleus, no membrane-bound organelles, 70S ribosomes, circular DNA, binary fission
• Eukaryotic: true nucleus, membrane-bound organelles, 80S ribosomes, linear DNA, mitosis and meiosis
• Prokaryotic: always unicellular
• Eukaryotic: unicellular and multicellular
• Prokaryotic: DNA in cytoplasm
• Eukaryotic: DNA in nucleus
• Prokaryotic: one circular chromosome + plasmids
• Eukaryotic: multiple linear chromosomes
• Prokaryotic ribosomes: 70S (50S + 30S)
• Eukaryotic ribosomes: 80S (60S + 40S)
• Prokaryotic flagella: flagellin
• Eukaryotic flagella: microtubules
• Warning: Prokaryotic cells have DNA and ribosomes, but no nucleus. Prokaryotic cells divide by binary fission, not mitosis. Prokaryotic ribosomes are 70S; eukaryotic ribosomes are 80S. Do not confuse plasmids with chromosomes.