Membrane structure and the fluid mosaic model
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Question
What is the basic structure of a cell membrane?
Answer
A **phospholipid bilayer** — two rows of phospholipids — with proteins, glycoproteins and cholesterol embedded.
Question
What does 'amphipathic' mean?
Answer
Having **both** a hydrophilic (water-loving) part and a hydrophobic (water-hating) part in the same molecule.
Question
Which part of a phospholipid is hydrophilic, and which is hydrophobic?
Answer
The **phosphate head** is hydrophilic (water-loving); the **two fatty-acid tails** are hydrophobic (water-hating).
Question
Why do the hydrophilic heads face outward?
Answer
They are **attracted to the water** present on both the outer and inner surfaces of the membrane.
Question
Why do the hydrophobic tails point inward?
Answer
They are **repelled by water**, so they are pushed into the centre, away from the water, forming the core.
Question
Why does a bilayer form spontaneously?
Answer
Because phospholipids are amphipathic and there is **water on both sides**: heads go to the water, tails away from it, giving two rows.
Question
What does the hydrophobic core do to permeability?
Answer
It makes the membrane **selectively permeable** — small non-polar molecules pass, but large/polar molecules cannot cross freely.
Question
What does a glycoprotein do?
Answer
Acts in **cell recognition** and cell signalling — its carbohydrate chain on the outer surface is an 'identity tag'.
Question
What does cholesterol do in the membrane?
Answer
Wedges between the phospholipids and **stabilises fluidity**, reducing leakiness to small molecules.
Question
Why is the membrane called a 'fluid mosaic'?
Answer
**Fluid** because phospholipids and many proteins drift sideways; **mosaic** because different molecules are dotted through the bilayer like tiles.
Question
How did the Davson–Danielli model differ from the fluid mosaic model?
Answer
Davson–Danielli put **two continuous protein layers** coating the bilayer; the fluid mosaic model **scatters proteins through** it.
Question
What evidence supported the fluid mosaic model over Davson–Danielli?
Answer
Electron-microscopy images showing **proteins embedded within** the bilayer, not just coating its surfaces.
Question
What is the difference between integral and peripheral proteins?
Answer
**Integral** proteins are embedded right through the bilayer (e.g. channels, carriers); **peripheral** proteins rest on one surface.
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Topic 2.3 hub
Membranes and membrane transport
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