Study Guide: Prokaryotic and Eukaryotic Cells, Microbial Structures, and Classification

Prokaryotic Cells

Definition and Examples

Prokaryotic cells are unicellular organisms that lack a membrane-bound nucleus and other organelles. Their genetic material is found in a single circular DNA molecule located in the nucleoid region. Examples include Escherichia coli and Bacillus subtilis.

• Prokaryotic Domains: Bacteria and Archaea are the two domains. They differ in cell wall composition, membrane lipids, and genetic machinery.

• Distinguishing Characteristics:

  • No membrane-bound nucleus

  • Single, circular chromosome

  • Cell wall (peptidoglycan in bacteria, pseudopeptidoglycan in archaea)

  • Reproduce by binary fission

Shapes and Arrangements

Prokaryotes exhibit various shapes and arrangements:

• Shapes: Cocci (spherical), Bacilli (rod-shaped), Spirilla (spiral), Vibrios (comma-shaped)

• Arrangements: Chains (strepto-), clusters (staphylo-), pairs (diplo-), tetrads, sarcinae

Binary Fission

Binary fission is the process by which prokaryotic cells divide:

• DNA replication

• Chromosome segregation

• Cell elongation

• Septum formation

• Cell separation

If 10 cells divide five times by binary fission, the number of cells produced is $10 \times 2^5 = 320$.

Prokaryotic Plasma Membranes

The plasma membrane is a phospholipid bilayer with embedded proteins. It functions in selective permeability, transport, and energy generation (via the electron transport chain).

Cell Wall Comparison: Bacteria vs. Archaea

• Bacterial Cell Wall: Contains peptidoglycan (murein)

• Archaeal Cell Wall: Lacks peptidoglycan; may contain pseudopeptidoglycan, proteins, or polysaccharides

Gram-Positive vs. Gram-Negative Cell Walls

• Gram-Positive: Thick peptidoglycan layer, teichoic acids, no outer membrane

• Gram-Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), periplasmic space

Prokaryotic Structures for Motility and Adhesion

Flagella and Pili

• Flagella: Long, whip-like structures for motility

• Pili (Fimbriae): Short, hair-like structures for adhesion and conjugation

Flagellar Arrangements

• Monotrichous: Single flagellum

• Lophotrichous: Cluster of flagella at one end

• Amphitrichous: Flagella at both ends

• Peritrichous: Flagella all over the cell surface

Periplasmic flagella (axial filaments) are found in spirochetes and are located between the cell wall and membrane, enabling corkscrew motion.

Endospores

Endospores are dormant, tough, and non-reproductive structures produced by some bacteria (e.g., Bacillus, Clostridium) to survive harsh conditions. They are highly resistant to heat, desiccation, chemicals, and radiation.

Matching Prokaryotic Structures to Functions

Structure	Function
Cytoskeleton	Aids in cellular organization
Pili	Adhesion
Inclusion bodies	Nutrient storage
Fimbriae	Adhesion
Capsule	Protection, exchange genetic information

Shared Properties and Unique Features

• Shared by Bacteria and Archaea: Lack of nucleus, presence of ribosomes, similar cell size

• Comma-shaped cell: Vibrio

• Structures for adhesion: Pili, fimbriae

• Gram-positive to Gram-negative conversion: Damage to the cell wall (e.g., by lysozyme or antibiotics) can result in loss of peptidoglycan, causing Gram-positive bacteria to appear Gram-negative.

Eukaryotic Cells

Endosymbiosis Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells. Evidence includes double membranes, their own DNA, and similarities to bacteria.

Comparison of Prokaryotic and Eukaryotic Cells

Structures/Characteristics	Prokaryotes	Eukaryotes
Uni/multicellular	Unicellular	Uni- or multicellular
Size	Small (0.5–5 μm)	Larger (10–100 μm)
Cell division	Binary fission	Mitosis/meiosis
Plasma membrane	Present	Present
Chromosomes	Single, circular	Multiple, linear
Nucleus	Absent	Present
Ribosomes	70S	80S
Membrane-bound organelles	Absent	Present
Cell wall	Peptidoglycan (bacteria), variable (archaea)	Cellulose (plants), chitin (fungi), absent in animals
Organisms (examples)	Bacteria, Archaea	Animals, plants, fungi, protists

Kingdoms of Eukaryotes

• Protista (e.g., Amoeba, Paramecium)

• Fungi (e.g., Aspergillus, Yeast)

• Plantae (e.g., Moss, Fern)

• Animalia (e.g., Human, Insect)

Parasitic Helminths

Helminths are divided into two main groups: flatworms (Platyhelminthes) and roundworms (Nematoda). They are classified based on body shape, digestive system, and reproductive organs.

Fungi: Spores and Mycoses

• Sexual spores: Zygospores, ascospores, basidiospores

• Asexual spores: Conidiospores, sporangiospores

• Mycosis: Fungal infection in humans (e.g., athlete's foot, candidiasis)

Protists

• Protists are a diverse group of eukaryotic microorganisms, including protozoa and algae.

• Animal-like protists (protozoa) are classified by their mode of locomotion: flagella, cilia, pseudopodia, or non-motile.

Eukaryotic Cell Walls and Glycocalyx

• Cell walls: Present in plants (cellulose), fungi (chitin), absent in animals

• Glycocalyx: A carbohydrate-rich layer outside the plasma membrane; functions in protection, adhesion, and cell recognition

Cilia and Flagella

• Cilia: Short, numerous, used for movement and feeding (e.g., in Paramecium)

• Flagella: Longer, fewer, used for locomotion (e.g., in sperm cells, some protists)

Matching Eukaryotic Organelles to Functions

Structure	Description
Ribosomes	Makes proteins
Golgi apparatus	Modifies and packages proteins
Endoplasmic reticulum	Builds and packages lipids & proteins
Chloroplast	Energy production through photosynthesis
Vacuole	Contains water, nutrients, toxins, or waste products
Mitochondria	Harvest energy & make food

Distinguishing Prokaryotes from Eukaryotes

• Presence of nucleus: Eukaryotes have a true nucleus; prokaryotes do not.

• Cell wall composition: Used to distinguish between groups (e.g., peptidoglycan in bacteria, chitin in fungi)

Eukaryotic Flagella and Pellicle

• Flagella: 9+2 arrangement of microtubules, covered by cell membrane

• Pellicle: Flexible, protein-rich layer beneath the plasma membrane in some protists (e.g., euglenoids)

Algae and Helminth Structures

• Algae: Photosynthetic, aquatic, can be unicellular or multicellular

• Scolex and proglottids: Structures in tapeworms (cestodes); scolex is the head, proglottids are body segments

Vesicles and Vacuoles

• Lysosome: Contains hydrolytic enzymes for digestion

• Peroxisome: Breaks down H2O2 and other toxins

• Secretory vesicle: Delivers proteins to the membrane

• Transport vesicle: Moves substances within the cell

• Vacuole: Stores water, nutrients, toxins, or waste

Additional info: Some content, such as specific examples or detailed mechanisms, was inferred or expanded for academic completeness.