Question
The diagram shows the original drawing of the cell membrane made in 1972 by Singer and Nicolson.
(a) Label with the letter H part of a phospholipid molecule that is hydrophilic.
(b) Distinguish between this model of the cell membrane and the Davson–Danielli model.
(c) Explain the role of molecules such as M in sodium–potassium pumps.
▶️Answer/Explanation
(a)
The part labelled H is the hydrophilic head of the phospholipid molecule. Phospholipids have a polar, hydrophilic (water-attracting) head containing a phosphate group and a non-polar, hydrophobic (water-repelling) tail made of fatty acids. In the fluid mosaic model, the hydrophilic heads face the aqueous environments inside and outside the cell, forming the bilayer structure.
The Singer-Nicolson fluid mosaic model depicts the cell membrane as a bilayer of phospholipids with embedded proteins. The hydrophilic heads interact with water, while the hydrophobic tails are shielded inside the bilayer, creating a stable barrier. The letter H correctly identifies the head region in the diagram.
(b)
The current model of the cell membrane is the fluid mosaic model. It differs from the older Davson–Danielli model in several ways:
- Fluid Mosaic Model:
- Proteins are embedded within the phospholipid bilayer and can move around.
- The membrane is fluid, with flexible movement of components.
- Proteins are not just on the surface—they go through the membrane (integral proteins).
- Davson–Danielli Model:
- Suggested a sandwich-like structure: a phospholipid bilayer with protein layers coating both sides.
- Assumed proteins were only on the outer surfaces, not embedded.
- Didn’t explain how membrane proteins could move or how hydrophobic proteins would interact with water.
Key distinction: The fluid mosaic model has mobile, embedded proteins, while the Davson–Danielli model had static surface proteins.
(c)
Molecules like M act as part of the sodium–potassium pump, a type of active transport protein. These pumps move 3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) in, using ATP as an energy source.
This helps the cell:
- Maintain electrical charge balance (important in nerve and muscle cells)
- Control cell volume
- Keep the proper concentration of ions inside and outside the cell