Question

explain the quasi fluid nature of plasma membrane

Answer 1

Quasi means 'partially' or 'semi'. 

In other words what it's saying is that something that is quasi-fluid acts partly like a fluid and partly like a solid. A good example would be agar jelly which is soft and rubbery and appears solid but yet allows liquids to diffuse through it 

Answer 2

A cell membrane is quasi-fluid because it acts like a solid by enclosing the cell contents (something that a fully fluid liquid could not do) and also like a liquid by allowing molecules to move through it by osmosis and diffusion (something a solid would not do, or at least not at a fast enough rate).
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Hope this will help you