Draw a phospholipid and explain its polarity
A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails.
AS Level; Cambridge International Biology 9700, 2028–2030. Original explanatory diagrams and examples support syllabus learning; written answers are self-checked and teacher review remains pending.
Before you begin
- Recognise that biological molecules are built from smaller molecules and that structure is related to function.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which part of a phospholipid is hydrophilic?
What you will learn
- Describe the molecular structure of a phospholipid
- Distinguish its hydrophilic polar head from its hydrophobic non-polar tails
One glycerol, two tails and a phosphate head
A phospholipid contains a glycerol backbone, two fatty-acid tails and a phosphate-containing head group. The phosphate head is polar and hydrophilic. The fatty-acid tails are non-polar and hydrophobic.
When drawing a simplified phospholipid, show one head and two tails. A phospholipid differs from a triglyceride, which has three fatty acids and no phosphate head.
Polarity supports membrane arrangement
Because the head interacts with water and the tails avoid water, phospholipids can form a bilayer in aqueous environments. The heads face the watery surroundings and the tails point inward towards each other. This arrangement is important in cell membranes; detailed membrane structure is covered in Topic 4.
“Hydrophilic” means attracted to or interacting with water; “hydrophobic” means avoiding water. These terms describe how regions behave in water, not whether a molecule can move.
Pause and explain
How many fatty-acid tails does a phospholipid have?
Worked example
A molecule has a polar phosphate group, glycerol and two non-polar fatty-acid tails. Identify it and predict its orientation in a bilayer.
Show the worked solution
- Two tails and a phosphate head identify a phospholipid.
- Heads face water; tails face away from water.
Answer It is a phospholipid. In a bilayer, its polar hydrophilic phosphate head faces the watery surroundings, while its two non-polar hydrophobic fatty-acid tails point inward away from water.
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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 a phospholipid’s components and identify its polar region.
Give me a hint
Count the tails and head group.
Compare my reasoning
- It has glycerol, two fatty-acid tails and a phosphate group.
- The phosphate-containing head is polar and hydrophilic.
A phospholipid contains glycerol, two fatty-acid tails and a phosphate-containing head group. The head is polar and hydrophilic; the tails are non-polar and hydrophobic.
Look for these in your work
- Components correct
- Polarity correctly identified
Explain how phospholipid polarity leads to a bilayer in water.
Give me a hint
State which regions face water.
Compare my reasoning
- Hydrophilic heads interact with water.
- Hydrophobic tails point inward away from water.
In water, hydrophilic phosphate heads face the aqueous environments, while hydrophobic fatty-acid tails point inward away from water. Opposing layers form a phospholipid bilayer.
Look for these in your work
- Head orientation
- Tail orientation
A diagram labels a phospholipid’s tails “polar and water-attracting.” Correct both labels.
Give me a hint
Use the head-tail polarity distinction.
Compare my reasoning
- The tails are non-polar and hydrophobic.
- The phosphate-containing head is polar and hydrophilic.
The fatty-acid tails are non-polar and hydrophobic. The phosphate-containing head is polar and hydrophilic and faces water in a bilayer.
Look for these in your work
- Tail labels corrected
- Head labels and orientation stated
Common mistakes
- Drawing a phospholipid with three fatty-acid tails
- Calling the phosphate head hydrophobic
- Pointing the tails towards the aqueous environments
What can you explain now?
Which part of a phospholipid is hydrophilic?
Compare with the explanation
Phosphate-containing head
The polar head interacts with water.
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Make it stick
How does a phospholipid differ from a triglyceride, and how does polarity affect bilayer arrangement?
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.