Levels of Organisation in Multicellular Organisms
Senior 1 · Biology
Levels of Organisation in Multicellular Organisms
Lesson 4 of 5
20 min
At a coffee washing station in Huye or Nyamasheke, the work is shared out. Some workers receive and weigh the red cherries, others sort them, others run the pulping machine, others watch the fermentation tanks, and others turn the beans on the drying tables. Each team becomes very good at one task, and the station produces far more good coffee than the same number of people each doing every step alone. Sharing out work like this is called the division of labour, and a multicellular body runs in exactly the same way.
Definition
Division of labour: the sharing of the jobs needed for life among different specialised cells, tissues and organs, each doing one job well.
Stem cell: an unspecialised cell that can divide and then differentiate into specialised cells. Example: cells in the bone marrow that produce new red and white blood cells.
Look at the Amoeba in the photograph. It is a single cell, yet it must do everything a living thing does: move, feed, digest, get rid of waste, exchange gases, respond and reproduce. It has no organs; each job is done by a part of the one cell.
Figure 1: Doing every job in one cell, and sharing the jobs among specialised cells.
| Advantages | Disadvantages |
|---|---|
| each job is done very well | cells depend on one another: if one system fails, the whole organism suffers |
| the organism can grow large | it needs transport and coordination systems to link its cells |
| it can live in many habitats, including dry land | many specialised cells (nerve cells, red blood cells) cannot divide, so damage may be permanent |
| complex behaviour becomes possible | it needs more food and grows and reproduces more slowly than a single cell |
Warning
"Unicellular" does not mean "unsuccessful". It is tempting to think single cells are primitive failures. In fact bacteria are the most numerous organisms on Earth, and single-celled organisms live in almost every habitat. Specialisation is a different way of living, with its own advantages and costs, not a ladder with humans at the top.
Worked example 1: what fraction of you is red blood cells?
A 2016 study by Ron Sender, Shai Fuchs and Ron Milo estimated that an adult man's body contains about 30 million million () human cells, of which about are red blood cells. What percentage of the cells are red blood cells?
Answer: about 83%, more than four cells in every five. The division of labour is very uneven: one small, simple job, carrying oxygen, is shared by most of the cells in the body.
Worked example 2: evaluating a claim
Keza writes: "Being multicellular is always better than being unicellular." Evaluate her statement.
Conclusion: the statement is too strong. Multicellularity brings clear advantages, but it is not "always better"; it is one successful strategy among others.
Most nerve cells in the brain and spinal cord stop dividing once they are specialised, so they are hard to replace. This is why a serious head or spine injury, for example in a road accident, can cause lasting harm. It is the biology behind Rwanda's rule that every moto rider and passenger must wear a fastened helmet: the helmet protects cells that the body cannot easily rebuild.
Unicellular vs Multicellular, FuseSchool. Listen for the reasons a single cell cannot grow large, and for how the work is divided in a multicellular organism.
Tip
Memory hook: "Specialists need a team." Every advantage of specialisation (skill, size, range, complexity) comes with the same cost: the cells can no longer survive alone.
Practice: the advantages of specialisation
Key points
Sources and credits
Figure 1, the table, the worked examples and the practice questions are written by Ganzaa. The cell counts in worked example 1 are from R. Sender, S. Fuchs and R. Milo, "Revised estimates for the number of human and bacteria cells in the body", PLOS Biology (2016).
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
