Fluid mosaic model of the cell membrane
In the fluid mosaic model, a cell membrane is a bilayer (a double layer) of phospholipid with proteins embedded in it, like tiles in a mosaic.
Remember
Picture a cell membrane as a selectively permeable phospholipid bilayer that is fluid, with proteins set into it that control which ions and sugars cross, and usually a little carbohydrate on its outer surface.
Membranes are barriers that form the boundaries of the cell and of most of its organelles (the specialised compartments inside the cell, such as the nucleus). These membranes are made mainly of phospholipid and protein, with a smaller amount of carbohydrate.
The framework is the phospholipid bilayer. Each phospholipid is amphipathic: one end, the head, is hydrophilic (water-loving), while the other end, two fatty acid tails, is hydrophobic (water-fearing). In water the molecules line up in two layers, with the tails tucked inwards and the heads facing outwards.
The oily core makes the membrane selectively permeable, which means it lets some substances through and keeps others out. Charged ions and polar molecules (molecules with an uneven spread of charge, such as sugars) are hindered, so they need channel or transporter proteins, which act as controlled routes. Small non-polar molecules such as oxygen, by contrast, slip through.
Proteins are scattered through the bilayer, which is why it is called a mosaic, a patchwork of different pieces. Integral proteins are embedded in the bilayer, and some, called transmembrane proteins, span its whole width. Peripheral proteins attach to the surface. Together they provide transport, signalling and recognition.
The bilayer is also fluid. Phospholipids can flex, rotate and move sideways within their own layer, so the membrane is dynamic rather than rigid. Many proteins can move sideways too, although tethering to the cytoskeleton (the cell's internal supporting framework) restricts some of them.
Key facts
- Composition
- membranes are mainly phospholipid and protein, with a smaller amount of carbohydrate.
- Bilayer arrangement
- the heads face outwards towards the water on each side, and the tails face inwards, hidden from water.
- Barrier
- the oily core lets small non-polar molecules such as oxygen slip through but hinders ions and polar molecules.
- Carbohydrate
- chains are attached to some lipids (forming glycolipids) and some proteins (forming glycoproteins), and sit on the outer surface of the plasma membrane.
- Fluidity
- phospholipids move sideways within their own layer, so the membrane is dynamic rather than rigid.
- Functions
- the membrane acts as a selectively permeable barrier and takes part in communication and signalling through receptors, and in cell recognition.
Exam traps
- Membrane proteins are not confined to the outer surface of the bilayer: integral proteins are embedded in it, and some, such as channels, transporters and receptors, span its full width.
- Ions and other polar molecules do not cross a membrane freely, because the oily core of the bilayer hinders charged and polar substances, so they need channels or transporters.
- A phospholipid bilayer does not have its hydrophilic heads facing inwards: in water the water-loving heads face outwards towards the water and the water-fearing tails tuck inwards, which makes a stable barrier.
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