What counts as alive, and how biologists sort living things
HESI A2 topic: Biology basics
- Living things are made of cells and share eight properties of life.
- The cell is the smallest unit of life; viruses are not considered alive.
- Life is built in levels, from atoms up to the whole biosphere.
- Ranks, broad to specific: domain, kingdom, phylum, class, order, family, genus, species.
- Prokaryotes have no nucleus; eukaryotes do. Both have DNA.
Key facts
- OpenStax lists eight properties of life: order, response to stimuli, reproduction, adaptation, growth and development, regulation (homeostasis), energy processing, and evolution.
- The cell is the smallest unit of structure and function in living things; viruses are not considered living because they are not made of cells.
- Levels of organization run atom, molecule, organelle, cell, tissue, organ, organ system, organism, population, community, ecosystem, biosphere.
Show all 7 key factsShow fewer
- Cell theory: all living things are made of one or more cells, the cell is the basic unit of life, and new cells come only from existing cells.
- Taxonomic ranks from broadest to most specific are domain, kingdom, phylum, class, order, family, genus, species.
- A scientific name has two parts, genus then species, both italicized, with only the genus capitalized (Canis lupus).
- Bacteria and Archaea are prokaryotes (no nucleus, no membrane-bound organelles); the domain Eukarya holds protists, fungi, plants, and animals.
Quick check1 question · 2 more at the end
Which level of biological organization is the smallest unit that can carry out all the functions of life?
Correct answer: C.
The cell is the smallest unit of structure and function in living things, and cell theory states that the cell is the basic unit of life. Levels below it, such as organelles and molecules, are parts of a cell and cannot live on their own. Levels above it, such as tissues, are built from many cells.
A An organelle is a part inside a cell; it cannot carry out all life functions on its own.
B Molecules such as DNA are needed for life but are not alive by themselves; this confuses a building block with a living unit.
C Correct: the cell is the smallest level that is alive on its own; everything below it is only a part of a cell.
D A tissue is a group of similar cells, so it is a higher level built from cells, not the smallest unit.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Living things are made of one or more cells and show order, response to stimuli, reproduction, adaptation, growth and development, homeostasis, energy processing, and evolution. Life is organized from atoms up to the biosphere, with the cell as the smallest unit of life. Biologists classify organisms from domain down to species; prokaryotes lack a nucleus, eukaryotes have one.
1 What makes something alive?Biologists call something alive when it shows a whole set of shared properties, not just one. 1 min
Biologists call something alive when it shows a whole set of shared properties, not just one. OpenStax lists eight, and every living thing is made of one or more cells.
| Property | What it means |
|---|---|
| Order | Highly organized, built from one or more cells |
| Response to stimuli | Reacts to its surroundings, like a plant bending toward light |
| Reproduction | Passes its DNA on to offspring |
| Adaptation | Fits its environment, a result of natural selection |
| Growth and development | Grows and develops following instructions in its genes |
| Regulation (homeostasis) | Keeps its internal conditions stable while the outside changes |
| Energy processing | Takes in energy and uses it to power its chemical reactions (metabolism) |
| Evolution | Populations change over generations as some variants reproduce more |
Viruses are the classic edge case. Scientists do not consider them living, because they are not made of cells.
2 What are the levels of organization?Life is built in layers, and each level is made from the level below it. 1 min
Life is built in layers, and each level is made from the level below it. From smallest to largest:
- Atom
- Molecule (including large ones like DNA)
- Organelle
- Cell, the smallest unit of structure and function in a living thing
- Tissue, a group of similar cells doing a related job
- Organ, several tissues working together
- Organ system, organs with related functions
- Organism
- Population, all members of one species in an area
- Community, all the populations living in that area
- Ecosystem, the community plus its nonliving surroundings
- Biosphere, all of Earth's ecosystems together
3 What does cell theory say?Cell theory sums up three ideas that apply to every living thing. 1 min
Cell theory sums up three ideas that apply to every living thing. Schleiden and Schwann proposed it in the late 1830s, and Virchow added to it later.
- All living things are made of one or more cells.
- The cell is the basic unit of life.
- New cells come only from cells that already exist.
4 How are living things classified?Taxonomy sorts organisms into nested groups, from very broad to very specific. 1 min
Taxonomy sorts organisms into nested groups, from very broad to very specific. The eight main ranks are domain, kingdom, phylum, class, order, family, genus, species. The deeper two organisms match, the more closely related they are.
| Rank | The dog |
|---|---|
| Domain | Eukarya |
| Kingdom | Animalia |
| Phylum | Chordata |
| Class | Mammalia |
| Order | Carnivora |
| Family | Canidae |
| Genus | Canis |
| Species | lupus |
Notice that the groups are nested. Two organisms in the same family must also share the same order, class, phylum, kingdom, and domain. Two organisms that share only a class can still sit in different orders.
An organism's scientific name uses binomial nomenclature: genus plus species, both in italics, with only the genus capitalized. The dog's binomial name is Canis lupus.
5 How do prokaryotes and eukaryotes differ?The biggest split in life is whether a cell keeps its DNA inside a nucleus. 1 min
The biggest split in life is whether a cell keeps its DNA inside a nucleus. All cells share four parts: a plasma membrane, cytoplasm, DNA, and ribosomes.
| Prokaryotes | Eukaryotes | |
|---|---|---|
| Nucleus | No; DNA sits in the nucleoid | Yes |
| Membrane-bound organelles | No | Yes |
| Typical size | About 0.1 to 5.0 micrometers | About 10 to 100 micrometers |
| Domains | Bacteria, Archaea | Eukarya |
| Examples | Bacteria, archaea | Protists, fungi, plants, animals |
For organelles, membranes, and transport in more depth, see our cell structure lesson.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: A population is one species in an area; a community is all the populations in that area, so populations sit inside communities, not the other way around.
Watch out: An ecosystem includes the nonliving parts of the environment, not just the living things.
Watch out: Domain is the broadest rank and species the most specific, not the reverse; organisms in the same genus are more closely related than organisms that only share a class.
Watch out: Prokaryotes lack a nucleus, not DNA; their DNA sits in a region called the nucleoid.
Watch out: In a scientific name the species part is lowercase, not capitalized.
How to remember it
Memory hook: "Dear King Philip Came Over For Good Soup" gives domain, kingdom, phylum, class, order, family, genus, species. For the levels, think "a cell builds a tissue, tissues build an organ."
Self-check:
- Name the smallest level that counts as alive, and explain why a virus does not qualify.
- Put population, ecosystem, and community in order from smallest to largest.
- Two animals share a family. List every rank they must also share.