Too big to reach
In a big body, many cells are far from the outside.
One cell, no organs
An Amoeba is a single cell. It has no mouth, no lungs and no gut. Yet it feeds and breathes.
How does it manage without any organs?
What this lesson covers
The idea
A single-celled organism's whole surface contacts the environment, so it needs no special organs; in multi-cellular organisms diffusion cannot meet all cells' needs, so specialised tissues take up food and oxygen and a transportation system carries them.
One cell, no organs
An Amoeba is a single cell. It has no mouth, no lungs and no gut. Yet it feeds and breathes.
How does it manage without any organs?
- Its whole surface touches the water around it
- It does not need food or oxygen
- It has a tiny lung and a tiny stomach inside
Add cells, find who is left out
Start with one cell and press Food and oxygen arrive. Then add cells, one row at a time, and try again. See who is left out. Then add organs and transport.
Not every cell touches the outside
A single-celled organism has its whole surface in contact with the environment, so it needs no special organs. In multi-cellular organisms, simple diffusion cannot meet the needs of all the cells. So specialised tissues take up food and oxygen, and a transportation system carries them.
Real bodies are not square blocks, but the idea is the same: the bigger the body, the more cells sit deep inside.
- Cells in a row | All cells | Touch the outside | Inside, left out
- 1 | 1 | 1 | 0
- 2 | 4 | 4 | 0
- 3 | 9 | 8 | 1
- 4 | 16 | 12 | 4
- 5 | 25 | 16 | 9
- 6 | 36 | 20 | 16
Notes
A single-celled organism's whole surface contacts the environment, so it needs no special organs; in multi-cellular organisms diffusion cannot meet all cells' needs, so specialised tissues take up food and oxygen and a transportation system carries them.
Check yourself
Why does an Amoeba need no lungs or gut?
A body is a square block of 7 cells by 7 cells. Only the cells on the outer ring touch the surroundings. How many cells are inside, with no contact?
Answer: 25 cells
The outer ring has 24 cells. 49 − 24 = 25. Or: take the ring away and a 5 by 5 block is left, which is 25.
Food is taken in at one place in a many-celled body, but every cell needs it. What does the body need?
Which sentence is correct?
- It is a single cell, so food and oxygen pass in through its whole surface — correct. Yes! Its whole surface is in contact with the environment.
- It is too small to need oxygen. Every living cell needs oxygen to break food down, however small it is.
- It has a transport system inside. A transport system is for many-celled bodies. A single cell does not need one.
- Simple diffusion alone. Diffusion alone cannot meet the needs of cells deep inside. We saw them left out.
- A transportation system to carry it to all cells — correct. Yes! Specialised tissues take it in and the transport system carries it round.
- Fewer cells. Many-celled bodies keep their cells. They build a transport system instead.
- In a many-celled body, every cell touches the surroundings. Only the cells on the outside do. The ones inside are left out.
- Single cells need specialised tissues to take in food. A single cell takes in food through its whole surface.
- In a many-celled body, cells deep inside cannot touch the surroundings — correct. Yes! That is why organs and transport are needed.