Cells are born, work and die
A cell is not a machine that runs forever. It is made, it does its job, and one day it stops. Many cells in your body die every day, and new cells replace them.
There are two big ideas in this lesson: ageing (slow change over time) and cell death (the end). Death can be planned (apoptosis) or accidental (necrosis).
Features of an ageing cell
An old cell looks and works differently from a young one. Look for these signs:
- Smaller size: the cell loses water and shrinks a little.
- Slower work: enzymes work less well, so the cell makes energy and new material more slowly.
- Waste builds up: brown pigment (such as lipofuscin) collects inside, because old waste is not cleared fast enough.
- Nucleus changes: it can become bigger, darker or oddly shaped.
- Weaker membrane: the cell is worse at letting the right things in and out.
- Stops dividing: many old cells can no longer divide.
Why does this happen? Over time, DNA collects damage, harmful chemicals made during normal life (called free radicals) injure the cell parts, and the protective tips of chromosomes (telomeres) get shorter with every division. When the tips are too short, the cell stops dividing.
Ageing of cells is slow and gradual. Ageing of the whole body is the result of many cells ageing together.
Apoptosis: planned cell death
Apoptosis (say: a-pop-TOE-sis) is a death programme that sits inside every cell. The cell receives a signal, such as 'you are damaged' or 'you are no longer needed', and switches on enzymes called caspases that cut the cell's parts in a controlled way.
- The cell shrinks and pulls away from its neighbours.
- The nucleus and DNA are cut into pieces.
- The cell surface bubbles and the cell breaks into small sealed packets.
- Neighbour cells or special cleaning cells (phagocytes) eat the packets.
Nothing leaks out, so there is no swelling (no inflammation). Apoptosis needs energy (ATP) because the cell works actively to take itself apart.
Necrosis: death by injury
Necrosis happens when a cell is badly hurt by something from outside: a burn, a strong poison, a lack of blood supply, or an infection. The cell cannot control what happens.
- The cell swells because its membrane stops working.
- The membrane bursts.
- The contents spill into the space around.
- The body reacts with inflammation: redness, swelling and pain, and neighbouring cells can be harmed too.
Apoptosis vs necrosis: side-by-side
| Feature | Apoptosis | Necrosis |
|---|---|---|
| Cause | Internal signal; normal or damage found | Outside injury, poison, no blood |
| Planned? | Yes, a programme | No, accidental |
| Cell size | Shrinks | Swells |
| Membrane | Stays whole; makes packets | Bursts |
| Energy (ATP) | Needed | Not needed |
| Neighbours | Not harmed; they eat the packets | Harmed by the spill |
| Inflammation | No | Yes |
Why apoptosis matters
- Shaping the body: removes webbing between fingers and toes, and a tadpole's tail.
- Clearing old cells: old blood cells and gut lining cells are removed and replaced.
- Removing dangerous cells: a cell with badly damaged DNA is told to die so it cannot grow into a cancer. When apoptosis fails, damaged cells can survive and divide again and again. Read more in Cancer biology.
- Balance: the number of cells in a tissue stays steady when cells made equals cells lost.
Too little apoptosis can cause cancer. Too much apoptosis can harm tissues, as in some diseases where useful nerve cells die.
Try it: spot the difference
In the 3D: go to the last step. Press Apoptosis, move the time slider slowly, and write what size the cell becomes. Do the same with Necrosis. Check: apoptosis shrinks, necrosis swells.
At home: take a fresh grape and a dry raisin. The raisin is smaller and wrinkled, like an old, shrunken cell. Now put the raisin in water for an hour and see it swell. Which of these two (shrinking or swelling) looks like apoptosis, and which like necrosis?
Key formulas and definitions
- Apoptosis = planned, shrinks, sealed packets, no inflammation, needs ATP
- Necrosis = by injury, swells, bursts, inflammation, harms neighbours
- Key enzymes of apoptosis: caspases
- Cells in tissue stay steady when: cells made = cells lost
- Ageing signs: smaller, slower, waste (pigment), changed nucleus, shorter telomeres
Worked examples
1. A tissue has 1000 cells. Each day 10 cells die by apoptosis and 10 new cells are made. How many cells after 5 days?
Cells made equal cells lost, so the number does not change. After 5 days there are still 1000 cells. This is the steady balance that apoptosis helps keep.
2. A farmer's hand is burned by hot oil. The skin turns red and swollen. Is the cell death apoptosis or necrosis? Give two reasons.
It is necrosis. Reason 1: the cause is an outside injury (heat), not a planned signal. Reason 2: there is swelling and redness (inflammation), which happens when cells burst and spill.
3. In a frog's life the tadpole tail disappears. Which process does this, and why is it useful?
Apoptosis. The tail cells receive a signal to die on schedule and are eaten without leaking. This removes a part that the frog no longer needs, without harming the rest of the body.
Common mistakes
- Thinking all cell death is bad. Apoptosis is healthy and planned, and your body needs it.
- Mixing up the size change: apoptosis shrinks the cell, necrosis swells it.
- Saying apoptosis causes swelling. It does not, because the contents stay inside sealed packets.
- Thinking necrosis is a programme. It is accidental damage that the cell cannot control.