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AS · Practice questions · The fluid mosaic model

Name the part, then give its role.

Six original Cambridge-style questions on the fluid mosaic model and cell signalling, from labelling to explaining why only some cells respond to a hormone.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Structure explains function.

01
Recall
[3 marks]

Explain what is meant by the term fluid mosaic when describing a cell surface membrane.

  • Fluid: the phospholipid molecules (and many proteins) move about sideways within their own layer. ✓
  • Mosaic: proteins of different sizes and shapes are scattered through the bilayer. ✓
  • So the pattern of proteins in the membrane is constantly changing. ✓
02
Explain
[3 marks]

Explain how hydrophobic and hydrophilic interactions cause phospholipids to form a bilayer in water.

  • The phosphate head is polar (hydrophilic) and is attracted to water. ✓
  • The fatty acid tails are non-polar (hydrophobic) and are repelled by water. ✓
  • So the tails point toward each other in the middle, shielded from water, while the heads face the water on both sides. ✓
03
Recall
[2 marks]

Describe the arrangement of cholesterol and of glycolipids in a cell surface membrane.

  • Cholesterol: between the fatty acid tails of the phospholipids, in both layers of the bilayer. ✓
  • Glycolipids: in the outer layer only, with their carbohydrate chains projecting outward from the cell. ✓
04
Explain
[3 marks]

A membrane was taken from an organism that lives at low temperatures. It contained a high proportion of phospholipids with unsaturated fatty acids. Suggest and explain the advantage of this.

  • Unsaturated fatty acids have kinks (C=C double bonds) in their chains. ✓
  • The kinks stop the tails packing closely together, so the membrane stays fluid at low temperature. ✓
  • A fluid membrane lets proteins move and work, and allows processes such as transport and vesicle formation to continue. ✓
05
Explain
[4 marks]

Glucagon is carried in the blood to all cells in the body, but only liver cells respond to it. Outline the stages of cell signalling in this example and explain why only liver cells respond.

  • Secretion: cells in the pancreas release glucagon (the ligand) into the blood. ✓
  • Transport: the blood carries glucagon to all cells. ✓
  • Binding: glucagon binds to receptors on the cell surface membrane that have a shape complementary to it; this triggers a response inside the cell (for example a second messenger). ✓
  • Only liver cells have glucagon receptors; other cells lack a complementary receptor, so glucagon does not bind to them. ✓
06
Analysis
[3 marks]

Compare channel proteins and carrier proteins.

  • Similarities: both span the membrane (intrinsic proteins) and both are specific to particular ions or molecules. ✓
  • Channel proteins have a water-filled pore that ions pass through, and some are gated. ✓
  • Carrier proteins bind the molecule and change shape to move it across; they can be used in active transport as well as facilitated diffusion, while channel proteins are used only in facilitated diffusion. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.