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I was reading Cell Biology by Gerald Karp and came across a section which said-

Membrane fluidity makes it possible for clusters of membrane proteins to assemble at particular sites within the membrane and for specialised structures, such as intercellular junctions, light-capturing photosynthetic complexes and synapses.

I would be obliged to know more about the topic.

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Not necessarily. Sometimes the membrane localized proteins/complexes are anchored to the cytoskeleton or clustered together which limits their movements. See this post.

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Membrane fluidity allows reversible association of membrane compounds, which can be thought of as diluted in a two dimensional fluid; this also implies a crucial property of membrane dynamics, which is the differential association of components into more or less stable molecular groups. Once formed, the more stable will be able to cruise the membrane surface, unchanged. This tells you that certain complexes will, by their stability and membrane fluidity, be in constant motion, and thereby statistically well distributed on the cell surface, a very important feature for many infrastructures (look up "lipids rafts" as a topic for examples). Now, the notion of localization in biology may refer to absolute location (e.g. cell pole), or simply the relative positions of certain compounds. Such is the case for cell junctions, which need to co-localize with certain cytoplasmic compounds, belonging to the cytoskeleton. Relative localization therefore, in this case. Do understand though, that both these infrastructures and photoreceptors need to be well distributed on the cell surface, each for its own reasons (mechanical strength, and optimal photo-capture, respectively). The example of synapses is different, as the exact site of synaptic attachement in between cells plays a role in neural messaging and its regulation. This is, therefore, a different topic, which may involve absolute positioning of infrastructures. However, at these sites, protein movement along membrane surface and ocmplex forming principles remain the same; this is an example of absolute localization.

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    $\begingroup$ Welcome to Bio. Can you add sources or references to allow users to background read on your answer? $\endgroup$ – AliceD Mar 24 '16 at 11:33

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