Eukaryotes and Prokaryotes (Domains of Life)
The modern phylogenetic tree divides all living organisms into three major domains, differing by their type of cellular organization:
- Domain Bacteria: prokaryotes lacking membrane-bound organelles (mitochondria, Golgi apparatus). Their genetic material is not enclosed by a membrane, and 70S-type ribosomes function in the cytoplasm.
- Domain Archaea: prokaryotes often living in extreme environments (thermophiles, halophiles) or functioning as methanogens. Unlike true bacteria, their cell wall lacks peptidoglycan, and their genetic apparatus features distinct ribosomes and rRNA.
- Domain Eukarya: organisms with a true nucleus (featuring a nuclear envelope and nucleolus) and a complex system of membrane-bound organelles. In medical microbiology, this includes protozoa (Protozoa), true fungi (Eumycota), and chromists (Chromista). The cytoplasm of protozoa is often differentiated into a dense outer ectoplasm and a granular inner endoplasm.
Fungal Cell Structure
Fungi are eukaryotes, but they possess a number of specific cytological features. The body of a fungus is called a thallus.
- Cell wall: rigid and multi-layered. Composed of polysaccharides (chitin, glucans, mannans, cellulose), as well as proteins and lipids. Some species form a capsule.
- Cytoplasmic membrane: contains a unique component — ergosterol. This fundamental difference from mammalian cells (which contain cholesterol) makes ergosterol a crucial target for pharmacological drugs.
- Tinctorial properties: fungi are Gram-positive, and their vegetative cells are not acid-fast.
Ultrastructure of the Mammalian Ovum
A mature ovum (secondary oocyte) has a complex structure. From outside to inside, it is surrounded by:
- Corona radiata (corona radiata) — the outer layer of follicular cells.
- Zona pellucida (zona pellucida) — a glycoprotein barrier.
- Perivitelline space — the cleft before the membrane.
- Plasmalemma — forms microvilli for contact with follicular cells and metabolic exchange.
The cytoplasm contains specific trophic inclusions — yolk granules (containing phosphovitin and lipovitellin), which provide nutrition for the early embryo. Beneath the plasmalemma lie cortical granules containing hydrolytic enzymes, necessary for the cortical reaction (block to polyspermy) upon fertilization.
The main cytological feature of the ovum is the absence of centrioles. The cell is incapable of independent division until the spermatozoon contributes its cell center. The nucleus contains a haploid set of chromosomes and is arrested at metaphase II of meiosis.
Bacterial Cell Wall and Gram Stain
The architecture of the cell wall is the basis for bacterial classification within the domain Bacteria. Differentiation is performed using the Gram stain:
- Gram-positive (G+): have a thick wall consisting almost exclusively of a robust layer of peptidoglycan. Stains dark blue-purple. Examples: Staphylococcus, Bacillus, Lactobacillus.
- Gram-negative (G-): the peptidoglycan layer is thin, but an additional outer membrane composed of phospholipids, lipoproteins, and lipopolysaccharides lies on top of it. Stains red. Examples: Salmonella, Escherichia, Helicobacter.
- Cell-wall-deficient: class Mollicutes (mycoplasmas).
The bacterial cell contains unique substances: N-acetylmuramic acid, D-isomers of amino acids, and diaminopimelic acid. Since humans lack these compounds, the enzymes of their biosynthesis serve as an ideal target for antibiotics. To survive in harsh environments, certain bacteria (e.g., genus Bacillus and Clostridium) form endospores — dormant forms not intended for reproduction.
Mnemonic for ovum layers from outside to inside: Corona Radiata Shines Near the Plasmalemma (Corona radiata, Zona pellucida, Perivitelline space, Plasmalemma).
Class and exam questions
How does the fungal cell membrane differ from the human cell membrane?
Ergosterol is present in the fungal cytoplasmic membrane, whereas cholesterol is found in mammals. This difference is utilized in pharmacology when developing antifungal drugs.
Why is a bacterial cell vulnerable to antibiotics while human cells are not?
Bacteria contain unique cell wall components (peptidoglycan, D-amino acids) that humans lack. Antibiotics selectively block the enzymes that synthesize these substances.
Can a mammalian ovum divide independently?
No, after the first meiotic division, it loses its cell center (centrioles). The capacity for mitosis is restored only after the penetration of the spermatozoon during fertilization.
Are bacterial spores a method of reproduction?
No, endospores are formed inside the mother cell exclusively for survival under extreme conditions (desiccation, UV irradiation). Only a single spore is formed from one cell.
What else to study on this topic
- Structure and functions of organelles (RER, Golgi apparatus, lysosomes, mitochondria)
- Complex intercellular junctions (desmosomes, gap junctions)
- Membrane transport and structure of the plasmalemma
- Cell life cycle: mitosis, necrosis, and apoptosis
- Cytoskeleton, cilia, and microvilli
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