Question

A student made a pictorial representation of eukaryotic cell membrane and labelled the components as follows:
The student has made errors while labelling the components of membrane. Which of the following hold true regarding the error?
(i) Protein A should be labelled as trans-membrane protein only and not as integral protein
(ii) The polarity of the protein A should be reversed because the cytosolic phase always shows reducing environment.
(iii) Position of cholestrol molecule should be close to polar region as it contains a polar group
(iv) Protein B should be labelled as integral membrane protein and not as peripheral glycoprotein

• A. (i) and (ii)
• B. (iii) and (iv)
• C. (ii) and (iii)
• D. (i) and (v)

Answer 1

Answer: (B)

(iii) and (iv)

The main lipid components of the plasma membrane are phospholipids, cholestrol and galactolipids; their proportion varies in different cell membranes. According to fluid-mosaic model, lipid molecules are present in viscous bilayer and protein moleucles occur at place both inside and on outer side the lipid bilayer. Intrinsic or integral proteins represent more than 70% of the membrane proteins. They are insoluble in water solutions, and are associated with lipids. They attach to oligosaccharides to form glycoproteins. They pass into the lipid bilayer to different depths and establish hydrophobic bonds with lipid molecules. Some of the integral proteins run throughout the lipid bilayer.

The extrinsic or peripheral proteins are superficially on the two surfaces of the membrane, more on the cytosolic face than on the external face. Peripheral proteins are separated by mild treatment with high ionic strength or with metal chelating agents, are soluble in aqueous solution and are usually free of lipids, for example, spectrin.

So, the correct answer is '(iii) and (iv)'.