Biology · Life, Evidence & Biological Scale
Living systems, their shared features and their diversity
Put a viable dry bean seed beside a salt crystal. Neither walks across the table. A crystal can become larger when more material joins its surface. The seed can produce a growing plant under suitable conditions. Motion and increasing size, taken alone, are poor tests of life.
No earlier biology course is needed. The data lesson explains its own arithmetic. Chemistry and mathematics offer useful next steps, but you can read this chapter before completing their maps.
Back to the Biology mapBy the end, you can
Explain why a living organism is more than something that moves or gets larger.
Look for a connected living system
Known cellular organisms consist of one or more cells. A cell is a bounded, organised system in which interacting molecules sustain biological activity. Metabolism is the network of chemical transformations involved in using materials and energy. Regulation helps maintain conditions compatible with that activity; response connects an organism to its surroundings.
Inheritance and reproduction connect generations, while evolution concerns changes in inherited characteristics of populations across generations. An individual need not reproduce to be alive. A sterile animal is still an organism, and a seed's temporary lack of visible growth does not by itself establish that it is dead.
Homeostasis means regulated conditions within workable limits, not perfect stillness. Think of adjusting a room's heating as the outside weather changes: keeping a condition within bounds can require continuing activity. The comparison helps explain regulation, but a thermostat is not an organism.
A boundary allows exchange
Follow a seedling over time rather than judging one photograph. Ask what enters it, what leaves it, and what changes inside. The diagram groups those exchanges so that the plant appears as a continuing process. It does not claim that every input enters by the same route, or that every organism uses sunlight.
Follow what crosses the plant's boundary
Read the diagram in words
A schematic plant receives light energy and takes in carbon dioxide, water and minerals. It releases matter such as oxygen and water vapour and transfers energy to the surroundings as heat. These arrows group different exchanges; the plant also respires. The drawing is not a quantitative balance or a complete account of every pathway.
- Light energy → Plant processes
- Carbon dioxide, water and minerals → Plant processes
- Plant processes → Matter exchanged with surroundings
- Plant processes → Energy transferred as heat
- Carbon dioxide, water and minerals
- Carbon dioxide · Water and minerals
- Matter exchanged with surroundings
- Oxygen, water vapour and other exchanges
Shared principles allow different ways of living
A bacterium, a fungus and a bean plant differ greatly in structure and in how they obtain resources. Start by asking the same questions of each: where is its living organisation, how does it obtain materials and energy, and how does it relate to its environment and to other generations? Similar questions need not have identical answers.
Viruses expose the limits of a simple checklist. They contain hereditary material and evolve, but lack cells and depend on host cells to make more viruses. Whether they are called living depends partly on the definition being used. That boundary question belongs within biology; it does not make the study of viruses less biological.
Worked example
A claim that asks too little
Claim: 'The crystal grows, so it is alive.' The observation concerns size. The conclusion concerns an organised biological system. The missing step is evidence about cells, metabolism, regulation and inheritance. One shared feature cannot carry the whole conclusion. We should ask for the relevant evidence rather than add up superficial similarities.
Check your understanding
A viable dormant seed shows no visible change today. Is that enough to call it dead?
Show reasoning
No. Visible growth is only one observation. The seed's capacity to resume development under suitable conditions matters; a photograph alone does not determine viability.
Why does a sterile animal challenge the rule 'anything alive must reproduce'?
Show reasoning
It is a living individual even though it cannot produce offspring. Reproduction describes the continuity of a lineage; it is not a requirement that every individual fulfil.
What to carry forward
Recognise life through connected biological organisation and its history, with care at boundary cases.
References
Resources supporting this lesson’s explanations and examples.