"Cell" - what is it, definition of the term
The basic biological unit of a living organism is a membrane‑bounded structure that houses cytoplasm, organelles, and genetic material, enabling metabolism, growth, and self‑replication; it constitutes the structural and functional foundation of multicellular entities such as ticks, insects, lice, and fleas.
Detailed information
The basic biological unit of all living organisms, including ticks, insects, lice and fleas, is a microscopic compartment bounded by a selective membrane. Typical dimensions range from 5 µm in small ectoparasites to 30 µm in larger stages. The enclosing barrier consists of a phospholipid bilayer interspersed with transport proteins that regulate ion flux, nutrient uptake and waste removal.
Inside the compartment, a membrane‑bound nucleus houses the organism’s genetic material. Chromatin organizes into chromosomes that replicate during the S phase of the cell cycle, ensuring accurate transmission of hereditary information to daughter compartments. The nucleolus synthesizes ribosomal subunits, which are exported to the cytoplasm for protein assembly.
Organelles perform specialized functions:
- Mitochondria: high-density arrays supply ATP through oxidative phosphorylation, supporting rapid muscle contraction during host attachment.
- Endoplasmic reticulum and Golgi apparatus: coordinate synthesis and modification of secretory proteins, including salivary enzymes that facilitate blood feeding.
- Lysosomes: degrade ingested host proteins and recycle cellular components during periods of starvation.
- Peroxisomes: detoxify reactive oxygen species generated during blood digestion.
Ectoparasitic species exhibit several differentiated cellular types:
- Epidermal compartments form a protective cuticle that resists desiccation and mechanical stress.
- Muscle compartments generate contractile force for locomotion and attachment.
- Digestive compartments line the midgut, possessing microvilli that increase surface area for nutrient absorption.
- Salivary compartments produce anticoagulant and immunomodulatory molecules that suppress host defenses.
- Reproductive compartments produce gametes and support embryonic development within the egg capsule.
Cellular replication follows a tightly regulated mitotic sequence. After each blood meal, mitotic activity increases to expand tissue mass required for engorgement, molting and oviposition. Cytokinesis completes the formation of distinct daughter compartments, which differentiate according to lineage‑specific transcriptional programs.
Metabolic flexibility characterizes these organisms. During fasting, glycolytic pathways dominate, allowing ATP generation without oxygen. Upon blood ingestion, mitochondrial respiration accelerates, reflecting the sudden availability of oxygen‑rich nutrients. This dual capacity enables survival across prolonged off‑host periods and rapid exploitation of host resources.