A single cell has to do all seven functions of life by itself, and it can never get very big. Put cells together, let each one specialise, and the group can do things no single cell could manage. That is the trade a multicellular organism makes, and it comes with a cost.
📚 What you need to know
Multicellular organisms are made of many cells that work together and stay attached.
Cells specialise, so there is a division of labour — each cell does one job well.
Levels of organisation: cells → tissues → organs → organ systems → organism.
A tissue is a group of similar cells doing the same job; an organ is several tissues working together.
Multicellular organisms show emergent properties: the whole can do things the individual parts cannot.
Being multicellular allows larger size and more complex behaviour, but needs transport systems and coordination.
Cells stay together through cell adhesion molecules on their surfaces.
Division of labour
An amoeba does everything itself. Nothing it has is specialised, because everything it has must be general purpose. That is the limit of being one cell.
In a multicellular organism the jobs get shared out. A muscle cell only contracts, so it can be packed with the proteins for contracting. A red blood cell only carries oxygen, so it can throw away its nucleus to make room. Neither could survive alone, but together they are far better at their jobs than any single all-purpose cell.
The trade is real. A specialised cell is better at one thing and completely helpless on its own, which is why multicellular organisms need blood, signals and coordination.
Levels of organisation
Once cells specialise, they get grouped. Each level is built from the one before it.
A heart muscle cell can twitch. Heart muscle tissue can contract as a sheet. A heart can pump blood. Same components, and a new ability at every level.
Level
Definition
Example
Cell
The smallest unit that carries out all the functions of life
A muscle cell
Tissue
A group of similar cells doing the same job
Cardiac muscle tissue
Organ
Several tissues working together for one function
The heart
Organ system
A group of organs working together
The circulatory system
Organism
All the organ systems together as one living thing
A human
Emergent properties
This term sounds abstract until you take one example apart. A single heart muscle cell can contract. It cannot pump blood. Put millions of them together with connective tissue, blood vessels and nerves, and the result pumps blood around a whole body.
Nothing new was added. The pumping came from the interaction between the parts, not from any one part. That is an emergent property: the whole is greater than the sum of the parts because of how the parts work together.
Definition worth learning
Emergent properties arise from the interaction of component parts, so the whole is greater than the sum of its parts
🧠
Cells To Organs, Systems, Organism
Cell, Tissue, Organ, System, Organism. Five levels, always in that order, smallest first.
What it costs
Multicellularity is not free, and the better answers say so.
Specialised cells depend on each other. A nerve cell cannot feed itself.
A large body has a small surface area compared with its volume, so diffusion alone is too slow. It needs a transport system.
Cells must be coordinated, which needs nerves or hormones.
Cells must stick together, which they do using cell adhesion molecules on their membranes.
Cell division must be controlled. When it is not, the result is a tumour.
Cell adhesion is the quiet requirement. Before any of this can work, cells have to stay attached in the right arrangement. Proteins on the plasma membrane recognise and bind to matching proteins on neighbouring cells. Without them a multicellular organism would simply fall apart into a pile of separate cells.
Worked examples
WE 1
Explain an emergent property
Explain what is meant by an emergent property, using the heart as an example. (3 marks)
Point 1: the definition
An emergent property arises from the interaction of component parts, so the whole can do more than the parts separately.
Point 2: the parts
A single cardiac muscle cell can contract, but it cannot move blood anywhere.
Point 3: the whole
Millions of these cells, organised into tissues within the heart along with vessels and nerves, together pump blood around the body.
Pumping is a property of the heart, not of any one heart celluse the word “interaction” — it is what separates this from just “working together”
WE 2
Give the levels of organisation
Place the following in order of increasing complexity and define two of them: organ, cell, organism, tissue, organ system. (3 marks)
Step 1: the order
cell → tissue → organ → organ system → organism
Step 2: define a tissue
A tissue is a group of similar cells carrying out the same function together.
Step 3: define an organ
An organ is made of several different tissues that work together to perform one particular function.
Cell, tissue, organ, organ system, organism“similar cells” for a tissue and “several tissues” for an organ are the key phrases
WE 3
Weigh up the advantages
Outline one advantage and one disadvantage of being multicellular rather than unicellular. (2 marks)
Advantage
Cells can specialise, so each does one job very efficiently and the organism can grow larger and behave in more complex ways.
Disadvantage
Specialised cells become dependent on each other, and the organism needs transport and coordination systems to keep them all supplied.
Specialisation buys efficiency and size, at the cost of dependencea disadvantage must be a real biological cost, not “it is more complicated”
💡 Exam tips
Learn the five levels in order and be ready to define any of them.
For emergent properties, use the phrase interaction of component parts.
A tissue is similar cells; an organ is different tissues. That single contrast answers many questions.
Say division of labour when describing why specialisation helps.
Mention transport and coordination when asked for the costs of being large.
Cell adhesion is what holds the whole arrangement together — worth one line.
⚠ Common mistakes
Saying an organ is made of one tissue. It is made of several.
Describing emergent properties as just “working together”. The key word is interaction.
Claiming multicellular organisms have no drawbacks. Dependence and transport are real costs.
Confusing an organ with an organ system. The heart is an organ; the circulatory system is a system.
Saying unicellular organisms are primitive. They are simply organised differently and are extremely successful.
Forgetting that a specialised cell still carries the whole genome.
That completes Cell Structure & Microscopy. The thirteen notes run as one argument: cells are the unit of life, here is how we look at them, here is what is inside, and here is how a pile of identical cells becomes an organism.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
