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2.3.8Inquiry Process

Cell Adhesion — IB Biology HL Revision Notes & Topical Questions

5 min read 6 parts Lesson 54 of 160

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Part 1 of 6Cell Adhesion Molecules

2.3.8 Cell Adhesion

Cells in a tissue must stick together and communicate.

CAMs
  • Cell adhesion molecules are cell surface proteins that bind cells to other cells or to the extracellular matrix.
Extracellular matrix
  • A network of molecules such as collagen outside cells that supports them.
Selectivity
  • Cells bind to others carrying the same type of CAM, which helps tissues organise.
Importance
  • Adhesion is essential for multicellular organisms, as it lets cells form tissues.

Part 2 of 6Types of Junction

Types of Junction
Tight junctions
  • Proteins seal neighbouring cells so that fluids cannot leak between them, as in the intestinal lining.
Adherens junctions
  • Link the cytoskeleton of adjacent cells through CAMs and give mechanical strength.
Desmosomes
  • Strong spot-like attachments that resist stretching, common in skin and heart muscle.
Gap junctions
  • Channels linking the cytoplasm of two cells so small molecules and ions can pass between them.
JunctionMain functionExample tissue
TightSealGut lining
AdherensAttachmentEpithelia
DesmosomeStrengthSkin, heart
GapCommunicationHeart muscle

Part 3 of 6Temporary and Permanent Adhesion

1
Cells in tissues such as skin form long-lasting junctions.
2
White blood cells bind temporarily to blood vessel walls using CAMs.
3
They can then squeeze between cells and move into infected tissue.
4
Temporary adhesion allows cell movement, while permanent adhesion holds tissue structure.
5
Loss of adhesion is a feature of cancers that spread to other parts of the body.

Part 4 of 6Why It Matters

Tissue integrity
  • Strong junctions stop tissues from tearing under stress.
Barrier function
  • Tight junctions control what passes between cells and force substances through the cells instead.
Coordination
  • Gap junctions let cells in heart muscle contract together.
Development
  • Cells rely on adhesion to arrange into the correct tissues in the embryo.

Part 5 of 6Worked Example

Question Explain why glucose in the intestine must pass through the cells of the epithelium and cannot leak between them.

  1. Neighbouring epithelial cells are joined by tight junctions at their upper edges.
  2. These junctions are made of proteins that seal the gaps between cells.
  3. Substances therefore have to cross the plasma membranes by channels or carriers, so the cell controls what enters the body.

Answer Tight junctions block the route between cells, forcing glucose to pass through membrane transport proteins.

Part 6 of 6Exam Checklist & Tips

2.3.8 – Key Points for the Exam

  • Cell adhesion molecule A cell surface protein that binds cells to other cells or to the extracellular matrix.
  • Tight junction A junction that seals the gap between adjacent cells.
  • Desmosome A junction that anchors cells together and resists mechanical stress.
  • Gap junction A channel between adjacent cells allowing small molecules to pass.
  • Extracellular matrix The network of molecules outside cells that supports them.

Common Mistakes

  • Mixing up tight junctions (sealing) and gap junctions (communication).
  • Saying CAMs are lipids.
  • Assuming adhesion is always permanent.
  • Forgetting that the extracellular matrix contains collagen.

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Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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