What Is Alive, and What a Cell Is
Biology's one universal claim: every living thing is made of cells, and every cell comes from another cell.
The awkward question first
Biology is the study of life, which would be tidier if anyone could define life. There is no single property that separates the living from the non-living — only a list of things living systems do, and every item on it has an exception. Fire consumes fuel and grows. Crystals self-assemble. A virus carries genes and evolves, but cannot do anything at all without a host cell to work in.
The working list is still worth having. Living things generally:
- are made of cells
- take in energy and use it to stay organised (metabolism)
- hold their internal conditions steady (homeostasis)
- respond to their surroundings
- grow and reproduce, passing on heritable information
- change over generations (evolution)
The last two carry the most weight. A candle flame does the first four in a loose sense; what it cannot do is copy itself with heritable variation. That is the property this whole course circles back to.
Cell theory
The one genuinely universal statement in biology is the cell theory, assembled in the 1830s–50s:
- Every living thing is made of one or more cells.
- The cell is the smallest unit that is itself alive.
- Every cell comes from a pre-existing cell — omnis cellula e cellula.
The third point is the sharpest. It killed spontaneous generation: maggots do not condense out of meat, and broth does not brew its own bacteria — Pasteur showed that a flask sealed against airborne cells stays sterile indefinitely. Every cell alive today is the end of an unbroken chain of divisions stretching back billions of years. Nothing in that lineage has ever once been non-living.
Two kinds of cell
The deepest division in life is not plant versus animal — it is whether a cell keeps its DNA in a separate compartment.
Prokaryotes (bacteria and archaea) do not. Their DNA sits loose in the cytoplasm as a single circular loop. They are small, typically 1–5 µm, and have no membrane-bound organelles at all. They are also, by count, most of life on Earth.
Eukaryotes (everything else — animals, plants, fungi, protists) keep their DNA inside a nucleus, and run their chemistry in specialised compartments called organelles. They are 10–100× larger in diameter, which is the whole point: compartments let a cell run incompatible reactions at once.
The organelles worth knowing now
- Cell membrane — the border, present in every cell. Next lesson.
- Nucleus — the DNA archive. Copies leave; the original never does.
- Ribosomes — protein factories. Universal, in every cell of every organism, which is a strong hint about how old they are.
- Mitochondria — burn glucose with oxygen to make ATP.
- Chloroplasts — capture light to build sugar. Plants only.
- Cell wall and a large vacuole — plants (and, with a different chemistry, bacteria and fungi). The wall stops the cell bursting.
Mitochondria and chloroplasts have their own circular DNA, their own ribosomes, and a double membrane, and they divide on their own schedule. The reason is that they were free-living bacteria, swallowed and kept rather than digested — endosymbiosis. Your cells are a merger, and the mitochondrial DNA you carry came only from your mother.
⚗️ Lab — The Cell Explorer
Three cell types, cut open. Click any part to find out what it does.
- Start with the animal cell and find the nucleus, then switch to bacterial — the nucleus is simply gone.
- Only the plant cell has both a wall and a chloroplast. Both explain something you can see from outside.
- Ribosomes are in all three. That is a clue about what came first.