Class 9 BiologyChapter 2NCERT 2.5 and 2.5.1
Cell theory and the life of a cell
Three short sentences that tie all of biology together.
Think it over
- A bacterium, a banyan tree and you look nothing alike. What single rule could possibly be true of all three?
- Can a brand new cell ever appear from non-living stuff, like mould from bread? How would you test it?
- Skin cells keep dividing for your whole life. Why do they not pile up into a lump?
In one minute
- Schleiden (1838): all plants are made of cells. Schwann (1839): all animals are too. Virchow (1855): new cells come only from existing cells.
- Cell theory: all living things are made of one or more cells; the cell is the basic unit of structure and function; all cells come from pre-existing cells.
- Every cell has a definite life span; cells that die too late or too early cause problems.
- Contact inhibition stops many animal cells dividing when they touch neighbours. Cancer cells lose it and form tumours.
- Haberlandt (1902): any living plant cell can grow into a whole plant, an ability called totipotency.
Cell theory, the unifying idea of biology
One observation runs through all of biology: every organism is made of cells. It took nearly two centuries, and better microscopes, for scientists to be sure of it. Scroll through the story.
- 1665 Robert Hooke
Sees box-like compartments in a thin slice of cork through his own microscope (about 200 to 300X) and calls them cells.
- 1838 Matthias Schleiden
German botanist: all plants are made of cells.
- 1839 Theodor Schwann
German zoologist: all animals are made of cells too.
- 1855 Rudolf Virchow
German scientist: new cells form only from cells that already exist. The cell theory is complete.
- 1898 Camillo Golgi
Italian scientist: a special stain shows a thread-like network in nerve cells of a barn owl. Electron microscopes confirm it decades later: the Golgi apparatus.
- 1902 Gottlieb Haberlandt
Austrian botanist: a living plant cell can grow into a whole plant. Totipotency, the root of plant tissue culture.
- 2010 J. Craig Venter and team
A laboratory-made copy of a bacterium's DNA runs a living cell. Only the DNA was made; the rest of the cell came from a living one.
Three German scientists built the cell theory between them. Matthias Schleiden, a botanist, studied plants and in 1838 concluded that every plant is built of cells. A year later, in 1839, the zoologist Theodor Schwann showed that the same holds for animals. Then in 1855 Rudolf Virchow answered where cells come from: only from cells that already exist.
- 1
Every living thing is made of one or more cells.
- 2
The cell is the basic unit of structure and function of living things.
- 3
All cells arise from cells that already exist.
Together these unite all of biology, from bacteria to humans, and explain how life carries on through cell division.
Quick check
Who added the idea that new cells come only from pre-existing cells?
Show the answer
D. Rudolf Virchow Rudolf Virchow, in 1855, expanded the cell theory with this idea.
Do cells live and divide for ever?
No. Every cell has a definite life span, and a healthy one keeps to a simple plan: grow and divide only under control, stay in its own place, do its work, then die once it is no longer needed. A new cell steps in to carry on the same work. Timing matters both ways: a cell that outlives its time causes trouble, and so does one that dies before it should.
What tells a cell to stop dividing? In many animal cells it is touch: once a cell is pressed against its neighbours, it stops. This brake is contact inhibition. Cancer cells ignore it, so they go on dividing without control and build up into tumours.
Both dishes start with a couple of cells that grow and divide.
- 1A few cells in a dish
Both dishes start with a couple of cells that grow and divide.
- 2Dividing until they touch
Both kinds divide and spread out across the dish.
- 3Normal cells stop, cancer cells do not
Once normal cells touch their neighbours, they stop dividing: contact inhibition. Cancer cells have lost it; they keep dividing and pile up into a lump, a tumour.
Plant cells play by other rules. Boxed in by rigid walls, they have no contact inhibition and grow in a pattern of their own.
Quick check
What is contact inhibition?
Show the answer
B. Cells stopping division when they touch neighbouring cells In many animal cells, contact with neighbours switches division off. Cancer cells lose this control.
Totipotency: one plant cell, a whole new plant
Could a single plant cell rebuild the whole plant? In 1902 the Austrian botanist Gottlieb Haberlandt proposed that it could. Feed any living plant cell well and keep it in the right conditions, he said, and it can give rise to a complete plant, even a fully mature cell taken from a permanent tissue. So plant cells can produce, and turn into, many different kinds of cell. The name for this power is totipotency.
A whole branch of biology, plant tissue culture, grew out of this idea. Step through it below.
Take a single living cell, even a mature one, and give it nutrients and the right conditions.
- 1One living plant cell
Take a single living cell, even a mature one, and give it nutrients and the right conditions.
- 2It divides
The cell divides again and again into a small mass of cells.
- 3Shoots and roots
The cells of the mass change into different kinds of cells: some form a shoot, others roots.
- 4A complete plant
A whole new plant grows from that one cell. This ability is totipotency, the basis of plant tissue culture.
So look again at what one tiny cell manages. It frees energy from food. It builds substances and ships them out. It splits to make new cells. And with other cells, it keeps a whole body working. A bacterium, a tree, you: every living thing is built from cells.
Quick check
Totipotency means a plant cell can:
Show the answer
C. Grow into a complete plant under the right conditions Haberlandt proposed that a living plant cell, given suitable nutrients and conditions, can develop into a whole plant.