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Question: Keeping in view the fluid mosaic model for the structure of the cell membrane, one of the following ...

Keeping in view the fluid mosaic model for the structure of the cell membrane, one of the following statements is correct with respect to the movement of lipids and proteins from one lipid monolayer to the other (described as flip flop movement)?
A)While protein flip-flop,lipids cannot
B)Neither lipids, nor proteins flip-flop
C)Both proteins and lipids flip-flop
D)While lipids can rarely flip-flop,proteins cannot

Explanation

Solution

In the fluid mosaic model proteins are tightly embedded in the lipid bilayer of the cell membrane by the strong cytoskeletal structures.

Complete answer:
Pituitary gland is a master endocrine gland. The pituitary gland consists of Adenohypophysis (Anterior lobe or Anterior pituitary) and Neurohypophysis (Neural lobe or Posterior lobe or Posterior pituitary).

Adenohypophysis is divided into three parts:

  1. Pars Distalis represents the bulk of anterior lobe.
  2. Pars Intermedia lies between pars distalis and the neural lobe that is Pars intermedia lies between the anterior lobe and posterior lobe of the pituitary. It is avascular and almost non-existent in human beings.
  3. Pars Tuberalis is a structure which encloses secretory cells.
    So, Pars intermedia produces melanocyte-stimulating hormone in foetal life which releases melanin pigment in skin melanocytes that decides the complexion of the baby. However, the pars intermedia starts decreasing production as the baby grows. So it is either very small or entirely absent in adulthood.
    In lower vertebrates like fish, amphibians MSH secreted by pars intermedia is responsible for the darkening of the skin, often in response to the changes of background colours. This change in colour is due to secretion of melanin pigment in dermal melanophore cells of lower vertebrates.
    So, the answer is MSH of pars intermedia of middle pituitary is responsible for the darkening of the skin in lower vertebrates.

Hence option ‘A’ is the correct answer.

Note: Melanin is a skin pigment which brings darkening of skin but in humans colour of the skin is predetermined in the foetal life itself.