Being multicellular is not just a matter of having lots of cells. They have to stay attached to one another, in the right arrangement, permanently. The membrane is what makes that possible.
📚 What you need to know
For an organism to be multicellular, its cells must adhere — stick — to one another to form tissues.
The plasma membrane is responsible for cell adhesion, which can be permanent or temporary.
Adhesion is carried out by cell adhesion molecules (CAMs), a type of cell surface protein.
CAMs work by binding cells to other cells or to the extracellular matrix.
The extracellular matrix contains supporting structures such as collagen and provides support for the cells.
Different CAMs are found in different types of cell-cell junction.
The four junction types to know are tight junctions, adherens junctions, desmosomes and gap junctions.
Each junction does a different job, from sealing a gap to letting signals pass directly between cells.
Why adhesion matters
Think about what a tissue actually is. It is not a heap of cells — it is a group of cells of the same type, arranged in a particular way, working together. That arrangement has to be held in place, and it has to survive being stretched, squeezed and moved about.
Your skin gets pulled. Your gut lining gets scraped by food. Your heart muscle contracts hard, thousands of times a day. In every case the cells have to stay attached to each other, or the tissue simply comes apart.
Adhesion is done by cell adhesion molecules, or CAMs. They are cell surface proteins, and they bind in two directions:
The extracellular matrix is the scaffolding outside cells, containing supporting proteins such as collagen. Anchoring to it gives a tissue its strength.
The word “matrix” trips people up because it also appears inside a mitochondrion. They are unrelated. The extracellular matrix is outside cells — a mesh of proteins, including collagen, that cells sit on and hold on to.
The four junctions
Different CAMs form different junctions, and each junction is built for a different job. Learn them by what they are for, not by their names.
Sealing, holding and connecting are three genuinely different jobs, which is why one type of junction was never going to be enough.
Junction
What it does
Where it is useful
Tight junction
Fuses the membranes of neighbouring cells so no substance can leak through the gap between them
The lining of the gut and the bladder, where contents must not escape into the tissue
Adherens junction
Forms a belt around the cell, linked to filaments inside, holding cells in shape as a sheet
Epithelial sheets that need to change shape together
Desmosome
Acts as a strong, localised anchor point, linked to tough filaments inside each cell
Skin and heart muscle, which are under constant mechanical stress
Gap junction
Forms a pore joining the cytoplasms of two cells, allowing ions and small molecules through
Heart muscle, where cells must be signalled to contract together
Permanent or temporary. Most adhesion in a tissue is permanent, but not all of it. A white blood cell rolling along a blood vessel wall makes and breaks adhesive contacts constantly as it hunts for a site of infection. That temporary, controllable stickiness is adhesion too.
🧠
Four junctions, four verbs
Tight junctions seal. Adherens junctions belt. Desmosomesweld. Gap junctions connect. If you can attach the right verb, the explanation writes itself.
Worked examples
WE 1
Explaining the need for adhesion
Explain why cell adhesion is essential for multicellular organisms. (3 marks)
Point 1: forming tissues
Cells must adhere to one another in a fixed arrangement to form a tissue rather than a loose collection of cells.
Point 2: how it is doneCell adhesion molecules in the plasma membrane bind cells to each other and to the extracellular matrix.
Point 3: why it matters
This holds the tissue together under mechanical stress and allows cells to work together as a functioning unit.
without adhesion there are no tissues, and so no organsname CAMs and the extracellular matrix — they are usually where the marks sit
WE 2
Matching a junction to a tissue
Suggest which type of cell junction would be most abundant in the epithelium lining the small intestine, and explain your choice. (3 marks)
Point 1: the choiceTight junctions.
Point 2: what they do
They seal the membranes of neighbouring cells together so that substances cannot pass between the cells.
Point 3: why the tissue needs that
The contents of the gut must not leak straight into the tissue beneath. Everything absorbed has to pass through the cells, which is what lets the body control what it takes in.
tight junctions, because absorption must be controlled and nothing may leak between cells“suggest” questions want your reasoning far more than the name itself
WE 3
Applying junction function to the heart
Cardiac muscle cells are joined by both desmosomes and gap junctions. Explain the advantage of having both types. (4 marks)
Point 1: desmosomes
Desmosomes act as strong anchor points, holding the cells together as the heart contracts repeatedly and forcefully.
Point 2: why that is needed
Without them the mechanical stress of contraction would pull the cells apart.
Point 3: gap junctions
Gap junctions form pores connecting the cytoplasms, so ions can pass directly from one cell to the next.
Point 4: why that is needed
This lets the electrical signal spread quickly, so the muscle cells contract together as one coordinated beat.
desmosomes give strength; gap junctions give coordinationtwo junction types, two functions, two reasons — that structure is exactly four marks
💡 Exam tips
Use the term cell adhesion molecule (CAM), and say it is a cell surface protein.
Mention both binding partners: other cells and the extracellular matrix.
Name collagen as a component of the extracellular matrix when you get the chance.
For junction questions, answer by function first, then name the junction that fits.
Say adhesion can be permanent or temporary — the temporary case is often overlooked.
Different junctions contain different CAMs. That link is explicitly in the syllabus.
⚠ Common mistakes
Confusing the extracellular matrix with the mitochondrial matrix. Completely different things that happen to share a word.
Saying gap junctions “leave a gap” between cells. They are a pore joining two cells, not a space between them.
Treating tight junctions and desmosomes as the same. One seals, the other anchors.
Describing CAMs as lipids. They are proteins.
Forgetting cell-to-matrix binding. Adhesion is not only cell to cell.
Assuming adhesion is passive stickiness. It is a controlled, protein-based process the cell can regulate.
That completes Cell Membranes & Transport. Look back at how the sub-topic hangs together: one amphipathic molecule builds a bilayer, the bilayer becomes a barrier, proteins in it become the doors, sugar chains on it become the name badge, cholesterol keeps it workable, and adhesion molecules on it hold your whole body together. Eight pages, one molecule’s consequences.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
