Biodiversity, classification and viruses

Choose the scale of the question

Biology describes organisation at several scales. Molecules form cellular structures, cells form tissues in multicellular organisms, and tissues combine into organs and organ systems. An organism is an individual living system. A population comprises individuals of the same species in a defined area; a community includes populations of different species. An ecosystem includes that community together with its non-living environment and their interactions. These levels answer different questions. A statement about all organisms in a pond concerns a community, while one including water chemistry and light concerns an ecosystem.

Not every organism has tissues or organs. A single-celled organism carries out the functions needed for its existence within one cell. Multicellular organisation involves division of labour among cells and communication between them. Counting complexity by body size is unreliable: a large colony and a large multicellular organism need not have identical organisation. Identify the entity and the relationship being described before assigning a level. This also prevents confusing an individual bacterial cell with the whole population growing in a culture.

Use cell structure before visible appearance

The three domains are Bacteria, Archaea and Eukarya. Bacteria and archaea are prokaryotic, lacking a membrane-bound nucleus. They are distinct evolutionary groups, even though both often appear as small single cells. Their molecular features differ; bacterial cell walls typically contain peptidoglycan, whereas archaeal walls do not. It is therefore incorrect to treat every prokaryote as a bacterium. Some archaea inhabit extreme conditions, but extreme habitats do not define all archaea or exclude all bacteria.

Eukarya contains organisms whose cells have a nucleus and other membrane-bound compartments. Animals obtain organic nutrients by ingestion and lack cell walls. Fungi obtain nutrients by absorption, often after secreting digestive enzymes, and their walls characteristically contain chitin. Plants typically have cellulose walls and photosynthetic lineages, although not every plant cell photosynthesises. Diverse organisms commonly grouped as protists include unicellular and multicellular forms with varied nutrition. A broad classroom kingdom scheme is useful for comparisons, but a single visible feature does not establish a complete evolutionary classification.

Classification aims to represent relationships rather than provide a ladder from inferior to superior organisms. Organisms share features because of common ancestry, though similar environments can also favour similar adaptations in different lineages. Compare several lines of evidence, including cellular organisation and molecular sequences. Two organisms living in water are not necessarily close relatives. Likewise, an organism's ability to photosynthesise does not by itself establish that it is a plant: photosynthetic bacteria have a very different cell organisation.

Separate a virus from a cell

A virus consists of a nucleic-acid genome associated with a protein coat, and some viruses also have a lipid envelope. A viral genome may be DNA or RNA. Viruses lack the complete cellular machinery needed for independent metabolism and protein synthesis. They reproduce through host cells, whose machinery produces viral components. Assembly of those components creates new viral particles. This dependence is more precise than merely saying that viruses are small or that they always cause severe disease.

A typical viral infection begins when a particle binds suitable host-cell structures and its genetic material gains access to the cell. Subsequent events vary among viruses. Some viral genomes can persist within host cells rather than immediately producing a large burst of particles. Host specificity depends partly on interactions between viral components and cellular receptors. The existence of bacterial viruses shows that hosts are not limited to animals or humans.

When evaluating a comparison, make a short feature table. Ask whether the entity has ribosomes, a plasma membrane enclosing cytoplasm, a nucleus, and the capacity to reproduce independently of another cell's protein-making machinery. A bacterium has ribosomes and cellular metabolism even though it lacks a nucleus. A virus may possess an envelope but still lacks ribosomes. A membrane-like outer covering alone therefore cannot establish that something is a cell. Keep the evidence attached to the precise claim, and avoid substituting a familiar example for the whole category.