Explain the fluid mosaic model of membranes
A phospholipid bilayer forms through hydrophilic and hydrophobic interactions, with proteins arranged within and around it.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain membrane transport and investigation reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Relate phospholipid and protein structure to their roles, and distinguish a concentration gradient from the direction of net movement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which part of a phospholipid faces the watery cytoplasm?
What you will learn
- Describe the phospholipid bilayer using hydrophilic and hydrophobic interactions
- Explain why the membrane is called fluid and mosaic and relate protein arrangement to the model
A bilayer forms in water
Phospholipids have a hydrophilic phosphate head and hydrophobic fatty-acid tails. In an aqueous environment, the heads face the cytoplasm and extracellular fluid while the tails face inward, away from water, producing a bilayer.
The hydrophobic interior restricts the direct passage of many polar or charged particles. The bilayer is not a solid wall: many phospholipids and some proteins can move laterally within it.
The model is fluid and mosaic
Fluid describes lateral movement of many membrane components within the plane of the bilayer. Mosaic describes the mixture of phospholipids, proteins, cholesterol and carbohydrate-containing molecules.
Proteins may be embedded in one layer or span the bilayer. Their hydrophobic regions interact with the bilayer interior, while hydrophilic regions can face aqueous environments or line channels. The model represents an organised, dynamic structure, not a uniform sheet.
Pause and explain
What does “fluid” describe in the fluid mosaic model?
Worked example
A drawing places phosphate heads against the watery cytoplasm and tails toward the centre, with a protein spanning both layers. Explain two features of the model.
Show the worked solution
- Hydrophilic heads face an aqueous environment.
- Hydrophobic tails face inward, and a spanning protein is embedded across the bilayer.
Answer The hydrophilic phosphate heads face the aqueous cytoplasm and extracellular fluid, while hydrophobic tails face inward to form the bilayer interior. The protein spans the bilayer, illustrating the mosaic arrangement of proteins among phospholipids.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the arrangement of phospholipids in a cell-surface membrane.
Give me a hint
State which parts face water and which face inward.
Compare my reasoning
- Hydrophilic heads face aqueous surroundings.
- Hydrophobic tails face inward toward each other.
Phospholipids form a bilayer with hydrophilic phosphate heads facing the aqueous cytoplasm and extracellular fluid and hydrophobic fatty-acid tails facing inward toward one another.
Look for these in your work
- Bilayer stated
- Both orientations correct
Explain why an integral membrane protein can remain associated with the phospholipid bilayer.
Give me a hint
Relate protein regions to the bilayer environment.
Compare my reasoning
- Hydrophobic protein regions interact with the bilayer interior.
- Hydrophilic regions may face water or form a pore.
Hydrophobic regions of an integral protein interact with the bilayer’s hydrophobic interior. Hydrophilic parts can face the aqueous surroundings or line a channel through the membrane.
Look for these in your work
- Interior interaction explained
- Protein regions related to water
A student describes the membrane as a rigid phospholipid sheet with proteins attached only outside. Correct the model.
Give me a hint
Include component movement and embedded proteins.
Compare my reasoning
- Many membrane components can move laterally.
- Some proteins are embedded in or span the bilayer.
The membrane is a fluid mosaic: many phospholipids and some proteins move laterally, and proteins can be embedded in one or both layers or span the bilayer.
Look for these in your work
- Fluidity corrected
- Mosaic and protein arrangement corrected
Common mistakes
- Pointing hydrophobic tails toward water
- Describing fluid as the whole membrane flowing like a liquid
- Drawing every protein as a channel spanning both layers
What can you explain now?
Which part of a phospholipid faces the watery cytoplasm?
Compare with the explanation
The hydrophilic phosphate head
The head interacts with the aqueous environment.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How do phospholipid heads and tails arrange in water, and what do “fluid” and “mosaic” mean?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.