Question

According to fluid mosaic model (proposed by singer and Nicolson), plasma membrane is composed of:
a. Cellulose and hemicelluloses
b. Phospholipid and integral protein
c. Phospholipid, extrinsic protein, intrinsic protein
d. Phospholipids and hemicellulose

Answer 1

An improved model of cell wall structure was proposed by Singer and Nicolson in 1972 which is widely accepted as fluid mosaic model. According to this model a plasma membrane consists of phospholipid bilayer and extrinsic and intrinsic proteins.

Complete answer:
The cell membrane is composed of lipids that are arranged in a bilayer. The lipid component is made of phosphoglycerides arranged in a bilayer such that the lipids arranged within the membrane have their polar head (hydrophilic part) towards the outer side and hydrophobic tail towards the inner side. This ensures the nonpolar tail of saturated hydrocarbons is protected from aqueous environment.
The cell membrane contains proteins and carbohydrates as well. The ratio of protein and lipid varies in different cell types. Also, membrane proteins can be classified as integral or peripheral proteins.
Intrinsic proteins or integral proteins are partially or totally buried in the membrane. These proteins are permanently attached to the membrane by interaction between its hydrophobic units and phospholipids of the membrane.
Extrinsic proteins or peripheral proteins are the membrane proteins that are partially attached to the membrane by weak intermolecular forces, either outside or inside the phospholipid membrane. These basically serve the function of receptors or used as transporting molecules across the bilayer.

Additional information:
Cellulose is the main component of the cell wall of a plant cell. The cellulose is a homopolymer known to be hard and stiff which provides the strength to the plants to stand straight. Cellulose is not present in animal cells, as it restricts the movement of the cell. Furthermore, hemicellulose is also a polymer like cellulose which is present in plant cells, but it is a heteropolymer. It is present along with the cellulose but has less strength and possesses random and irregular structure.

Note: The fluid nature of membranes is important for functions like cell growth, formation of intercellular junctions, secretions, endocytosis, cell division etc. out of which, the most important function is the transport of molecules across it, which is achieved by the help of proteins.