Q2.
How can a single-celled organism carry out all its life processes when billions of cells are required to perform similar functions in multicellular organisms like us?
Answer
Because a single cell is small enough that it does not need organs at all — it can do everything by diffusion across its own surface, and by using organelles inside it as miniature organs. Our billions of cells are not needed to do the jobs; they are needed because we are large.
The mechanism — surface area to volume ratio. Every cell exchanges oxygen, food and waste across its surface, but it uses those substances throughout its volume. As a body gets bigger, volume grows much faster than surface area, so the surface becomes too small to supply the inside.
For a sphere of radius r:
surface area = 4πr2 volume = (4/3)πr3
Surface area / volume = 3 / r
So if r is doubled, surface area per unit volume is halved.
An amoeba is a few hundredths of a millimetre across, so its surface-to-volume ratio is enormous. Oxygen diffusing in at the surface reaches every part of it in a fraction of a second. A human is about 1.7 m tall — diffusion alone would take years to carry oxygen from skin to heart, so we must have lungs, blood, a heart and vessels.surface area = 4πr2 volume = (4/3)πr3
Surface area / volume = 3 / r
So if r is doubled, surface area per unit volume is halved.
What does the work inside one cell:
| Life process | In a protist (one cell) | In us (organ systems) |
|---|---|---|
| Taking in food | Pseudopodia engulf it, or cilia sweep it into an oral groove | Mouth, stomach, intestine |
| Digestion | Food vacuole with enzymes | Digestive glands and gut |
| Gas exchange | Straight across the cell membrane | Lungs and blood |
| Removing excess water | Contractile vacuole pumps it out | Kidneys |
| Movement | Pseudopodia, cilia or flagellum | Muscles and skeleton |
| Energy release | Mitochondria | Mitochondria — in every one of our cells too |
The point to take away: multicellularity is not an upgrade in what a cell can do — it is the price of being big. Once an organism grows past the reach of diffusion, it must divide the labour among specialised cells, tissues and organs. That is the same story the animal kingdom tells later in this chapter, from sponges (cellular level) up to organ systems.