According to the 'unit membrane model, the thickness of the cell membrane is about
a. 200 nm
b. 7.5 nm
c. 150 nm
d. 10 nm

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Hint: The unit membrane model was discovered by Robertson, where he explained that the plasma membrane usually comprises three layers, it is said to be trilaminate, which means two lipid layers that sandwich the protein layer.

Complete answer:
- The cell membrane is also called a plasma membrane, present in all cells, the main function is to separate the inner part of the cell with the outer environment.
- If the cell wall is present, it is attached to the plasma membrane.
- The plasma membrane is again composed of a lipid layer mainly involved selectively transporting and regulating the movement of some substances in and out of the cell.
- The lipid layer present in this is phospholipids.
- Along with this, there are protein layers that are present in between the lipid layers, and the proteins present here are membrane or integral proteins.
- Plasma membrane is the selective permeable membrane which allows the movement of ions in and out of the cell.
- So many models are proposed to explain the structure of the plasma membrane.
- Initially the unit membrane model was proposed by Robertson, where the another model by the name called fluid mosaic model was proposed by Nicholson and singer.
- The bilipid layer is nearly 3.5 to 5 nm, whereas the dense protein layer is up to 2.5 nm so the total is 7.5nm.

Hence, The correct answer is option (B).

Note: In the case of plasma membrane the carbohydrates are also attached to the proteins and lipids of the membrane, when the carbohydrates attached to lipids form the glycolipids and which are attached to the proteins are said to be glycoproteins.