The fluid mosaic membrane
Relate phospholipid organisation and membrane proteins to selective transport.
What you will learn
- Explain the bilayer's organisation.
- Distinguish integral proteins from peripheral proteins.
Phospholipids organise in water
Phospholipids have hydrophilic heads and hydrophobic tails. In a bilayer, the heads face the aqueous environments and the tails face inward. This creates a hydrophobic interior that impedes the direct movement of many charged substances.
'Fluid' describes the ability of many components to move laterally; 'mosaic' describes the mixture of proteins and other components embedded in or associated with the membrane.
Proteins give specific routes
Integral proteins are embedded in the bilayer; some span it. Channel and carrier proteins allow selected substances to cross. Peripheral proteins are associated with a membrane surface rather than spanning the bilayer.
Facilitated diffusion moves substances down an electrochemical gradient without a direct ATP requirement. Active transport can move substances against a gradient using an energy source. The presence of a protein alone does not prove active transport.
Worked example
A protein spans the bilayer and allows ions to move down their electrochemical gradient. Classify it and the transport process.
Show the worked solution
- Spanning the bilayer makes it an integral membrane protein.
- Providing a selective route makes it a transport protein.
- Movement down the gradient through the protein is facilitated diffusion.
Answer An integral transport protein carrying out facilitated diffusion
Common mistakes
- Not all membrane transport proteins use ATP.
- The membrane interior is hydrophobic, not a watery gap.
What can you explain now?
Which part of a phospholipid faces the watery cytoplasm?
Compare with the explanation
The hydrophilic head
The hydrophobic tails face inward, away from the aqueous environments.
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