Describe membrane components and their roles
Cholesterol, glycoproteins, glycolipids, channels, carriers and receptors contribute to membrane properties and cell interactions.
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
Where are cholesterol molecules found in the membrane?
What you will learn
- Describe the positions of cholesterol, glycolipids and glycoproteins in membranes
- Relate membrane components to stability, fluidity, permeability, transport and cell recognition
Locate membrane molecules
Cholesterol lies among phospholipids in the bilayer, with its hydrophobic region among fatty-acid tails and its small hydrophilic region near the head groups. Its presence affects membrane stability and fluidity and reduces permeability to some small polar molecules.
Glycoproteins are proteins with carbohydrate chains attached; glycolipids are lipids with carbohydrate chains. Their carbohydrate chains project from the outer surface and can act as cell-surface markers or antigens.
Components provide distinct functions
Phospholipids form the basic selective barrier. Cholesterol helps stabilise membrane properties. Channel proteins provide hydrophilic pathways for selected particles; carrier proteins bind selected substances and change shape to move them across.
Cell-surface receptors bind specific signalling molecules. Glycoproteins and glycolipids can support cell recognition. A molecule’s presence alone does not prove its function: relate structure and location to the stated role.
Pause and explain
What is a glycoprotein?
Worked example
A membrane diagram labels a carbohydrate chain attached to a lipid and a protein forming a pore. Name each structure and one role.
Show the worked solution
- Carbohydrate attached to a lipid is a glycolipid.
- A pore-forming transmembrane protein can provide a selective channel.
Answer The carbohydrate-bearing lipid is a glycolipid, whose outer carbohydrate chain can contribute to cell recognition. The pore-forming integral protein can act as a selective channel for particular particles.
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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.
Distinguish a glycolipid from a glycoprotein and state where their carbohydrate chains project.
Give me a hint
Identify the molecule carrying the carbohydrate.
Compare my reasoning
- A glycolipid is a lipid with carbohydrate attached.
- A glycoprotein is a protein with carbohydrate attached; chains project outward.
A glycolipid has a carbohydrate chain attached to a lipid; a glycoprotein has one attached to a protein. Their carbohydrate chains project from the outer surface of the cell-surface membrane.
Look for these in your work
- Both structures distinguished
- Outer orientation stated
Relate cholesterol’s location to one role in a membrane.
Give me a hint
Locate it among phospholipids and name a membrane property.
Compare my reasoning
- Cholesterol lies between phospholipids.
- It helps stabilise membrane properties such as fluidity and permeability.
Cholesterol lies among the phospholipids, with its hydrophobic region among their tails. It helps stabilise membrane properties, including fluidity, and reduces permeability to some small polar molecules.
Look for these in your work
- Location accurate
- Role accurately bounded
A small charged ion crosses a membrane through a protein pore. Identify the likely membrane component and explain why direct passage through the bilayer is limited.
Give me a hint
Compare a hydrophilic route with the bilayer interior.
Compare my reasoning
- A channel protein provides a selective hydrophilic pathway.
- The bilayer interior is hydrophobic and impedes charged ions.
A channel protein can provide a selective hydrophilic pathway for the ion. The hydrophobic interior of the phospholipid bilayer impedes the direct passage of charged particles.
Look for these in your work
- Channel role identified
- Barrier linked to hydrophobic interior
Common mistakes
- Placing membrane carbohydrate chains on the cytoplasmic face in the syllabus model
- Treating cholesterol as a protein channel
- Saying all membrane proteins have the same transport or recognition role
What can you explain now?
Where are cholesterol molecules found in the membrane?
Compare with the explanation
Among phospholipids within the bilayer
Cholesterol is interspersed among phospholipids.
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Make it stick
Where are cholesterol and carbohydrate chains located, and how do channels, carriers and receptors differ?
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