Introduction to Prokaryotic and Eukaryotic Cells: Structure, Function, and Classification

Chapter 3: Introduction to Prokaryotic Cells

Overview of Prokaryotic Cells

Prokaryotic cells are classified into two domains: Archaea and Bacteria. They lack membrane-bound organelles and a true nucleus, making them structurally simpler than eukaryotic cells.

• Domains: Archaea and Bacteria

• Cell Structure: No membrane-bound nucleus; DNA is in a nucleoid region

• Organelles: Lack membrane-bound organelles

Prokaryotic Cell Size, Shape, and Arrangement

• Size: Range from 0.2 to 750 μm; most are 0.5–2.0 μm in diameter

• Shape: Bacilli (rod), Cocci (spherical), Vibrio (comma), Spirillum (spiral), Spirochete (flexible spiral)

• Arrangement: Determined by patterns of cell division (e.g., chains, clusters)

Example: Streptococcus forms chains of cocci; Staphylococcus forms clusters.

Binary Fission in Prokaryotes

Most prokaryotes reproduce asexually by binary fission, producing genetically identical offspring.

1. DNA is copied

2. Cell grows and DNA is segregated

3. Septum forms, dividing the cell

4. Cells separate

Extracellular Structures

• Plasma Membrane: Phospholipid bilayer; selectively permeable barrier

• Membrane Proteins: Transport, signaling, enzymatic activity

• Archaeal Membranes: Ether linkages, branched lipids, monolayer possible

• Cell Wall: Provides rigidity; bacteria have peptidoglycan, archaea have pseudopeptidoglycan or protein

Gram Staining and Cell Wall Types

Gram stain differentiates bacteria based on cell wall structure:

• Gram-positive: Thick peptidoglycan, no outer membrane, stains purple

• Gram-negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), stains pink

Clinical Implications: Gram-negative bacteria are often more resistant to antibiotics due to their outer membrane.

Special Prokaryotic Cell Types

• Mycoplasma: Lack cell wall, have sterol-rich plasma membrane

• L-forms: Bacteria that have lost their cell wall

Transport Across Prokaryotic Membranes

• Passive Transport: Diffusion (simple and facilitated), osmosis

• Active Transport: Requires energy (ATP); includes primary, secondary, and group translocation (phosphotransferase system)

Osmosis: Movement of water across a selectively permeable membrane

External Structures for Movement and Protection

• Flagella: Motility; arrangements include monotrichous, lophotrichous, amphitrichous, peritrichous

• Fimbriae: Short, numerous, for attachment

• Pili: Longer, less numerous, involved in conjugation and attachment

• Glycocalyx: Capsule or slime layer for protection and adherence

Intracellular Structures

• Nucleoid: Region containing circular DNA

• Ribosomes: 70S (50S + 30S subunits), site of protein synthesis

• Cytoskeleton: Protein filaments for shape and division

• Inclusion Bodies: Storage of nutrients and other substances

• Endospores: Dormant, highly resistant structures formed by some bacteria (e.g., Bacillus, Clostridium)

Chapter 4: Introduction to Eukaryotic Cells

Overview of Eukaryotic Cells

Eukaryotic cells are structurally more complex than prokaryotic cells, containing a true nucleus and membrane-bound organelles. They include plants, animals, fungi, and protists.

• Endosymbiotic Theory: Explains origin of mitochondria and chloroplasts from ancestral prokaryotes

Comparison of Prokaryotic and Eukaryotic Cells

Characteristic	Eukaryotes	Prokaryotes
Organisms	Animals, plants, fungi, protists	Bacteria, archaea
Size	Larger	Smaller
Cell Wall	Plants, fungi, some protists	Most have (peptidoglycan or other)
Nucleus	Present	Absent
Genetic Material	Multiple linear chromosomes	Single circular chromosome
Membrane-bound Organelles	Yes	No

Cell Division in Eukaryotes

• Mitosis: Produces genetically identical cells

• Meiosis: Produces gametes with half the chromosome number

Eukaryotic Cell Transport: Endocytosis and Exocytosis

• Endocytosis: Uptake of substances into the cell (includes phagocytosis and pinocytosis)

• Exocytosis: Release of substances from the cell

Classification of Eukaryotes

Kingdom	Examples	Cell Wall	Chloroplasts	Reproduction
Animals	Sponges, worms, insects, vertebrates	No	No	Sexual
Plants	Mosses, ferns, flowering plants	Yes (cellulose)	Yes	Sexual/asexual
Fungi	Yeasts, molds, mushrooms	Yes (chitin)	No	Sexual/asexual
Protists	Amoeba, algae, paramecium	Varies	Varies	Sexual/asexual

Fungi

• Structure: Hyphae (septate or aseptate), mycelium

• Reproduction: Spores (asexual and sexual)

• Diseases: Mycoses (e.g., candidiasis, ringworm)

Protists

• Amoeboid: Move by pseudopodia (e.g., Entamoeba histolytica)

• Flagellated: Move by flagella (e.g., Giardia, Trypanosoma)

• Ciliated: Move by cilia (e.g., Balantidium coli)

• Sporozoans: Non-motile, form spores (e.g., Plasmodium)

Eukaryotic Cell Structures

• Plasma Membrane: Phospholipid bilayer with proteins and sterols (e.g., cholesterol)

• Cell Wall: Present in plants, fungi, some protists (composition varies)

• Flagella and Cilia: Motility structures; cilia are shorter and more numerous

• Ribosomes: 80S (60S + 40S subunits); mitochondria and chloroplasts have 70S ribosomes

• Cytoskeleton: Microtubules, intermediate filaments, microfilaments

• Nucleus: Contains DNA, surrounded by nuclear envelope

• Endoplasmic Reticulum (ER): Rough (with ribosomes) and smooth (lipid synthesis)

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids

• Vesicles and Vacuoles: Storage and transport

• Mitochondria: Site of ATP production; double membrane

• Chloroplasts: Site of photosynthesis in plants and algae; double membrane

Summary Table: Eukaryotic vs. Prokaryotic Structures

Structure	Eukaryotes	Prokaryotes
Flagella	Microtubule-based, whip-like motion	Flagellin-based, rotary motion
Cilia	Present in some	Absent
Cell Wall	Plants (cellulose), fungi (chitin)	Peptidoglycan (bacteria), pseudopeptidoglycan (archaea)
Ribosomes	80S (cytoplasm), 70S (organelles)	70S
Genetic Material	Linear chromosomes in nucleus	Circular chromosome in nucleoid

Mitochondria and Chloroplasts

• Double membrane structures with their own 70S ribosomes and circular DNA

• Mitochondria: Site of ATP production

• Chloroplasts: Site of photosynthesis