THE FLUID MOSAIC MODEL

What is the fluid mosaic model

• The fluid mosaic model describes the structure of the plasma membrane.

• This model also applies to the membranes that form the outer boundary of cell organelles, such as the membranes that surround the cell nucleus and other cell organelles.

What does the fluid mosaic model propose

• The plasma membrane and other intracellular membranes should be considered as fluid layers in which proteins are embedded.

What do the terms ‘fluid’ and ‘mosaic’ mean

• The term ‘fluid’ comes from the fact that the fatty chains of the phospholipids are like a thick oily fluid

• The term ‘mosaic’ comes from the fact that the external surface (when viewed from above) has the appearance of a mosaic because of the various embedded proteins set in a uniform background

Phospholipid structure

• The plasma membrane consists of a double layer (bilayer) of phospholipids

• Each phospholipid molecule consists of two fatty acid chains joined to a phosphate-containing group

•  The phosphate-containing group forms the water-loving (hydrophilic or polar) head of the molecule

• The fatty acid chains constitute the water-fearing (hydrophobic or nonpolar) tail of each phospholipid molecule 

How are the phospholipids arranged

• The two layers of phospholipids in a plasma membrane are arranged so that the hydrophilic polar heads are exposed to both the external environment of the cell and the cytosol (the internal environment of the cell)

• The two layers of hydrophobic nonpolar tails face each other in the central region of the plasma membrane

What are the different protein types

• Integral proteins

• Trans-membrane proteins

• Peripheral proteins

Integral proteins

• Embedded in the phospholipid bilayer. 

• Typically, they span the width of the plasma membrane, with part of the protein being exposed on both sides of the membrane

• These proteins are described as being trans-membrane.

Trans-membrane proteins

• Functions as transporters, receptors, channels and carriers

• Integral proteins can be separated from the plasma membrane only by harsh treatments that disrupt the phospholipid bilayer, such as treatment with strong detergents.

Peripheral proteins

• Either anchored to the exterior of the plasma membrane through bonding with lipids, or indirectly associated with the plasma membrane through interactions with integral proteins in the membrane

Where can carbohydrate groups attach and what are they called

• Carbohydrate groups can be attached to the exposed parts of proteins on the outer side of the membrane, creating combinations called glycoproteins - Sugars attached to a protein = glycoprotein

• Other carbohydrate groups, instead, covalently link directly to the lipids in the membrane; these are referred to as glycolipids - Sugars attached to a lipid = glycolipid

• The prefix ‘glyco’ means sugar.

What are carbohydrates function

• cell-to-cell communication

• acting as receptors

• distinguishing cells as ‘self’ (a feature that is vital in the immune system).

How are glycoproteins important

• Vitally important in immune recognition

• Include molecules of the major histocompatibility complex, which is found on the surface of all nucleated cells.

What cells are cholesterol in

• Animal cells only

How does it help the plasma membrane

• An essential component of the plasma membrane, acting in a fluid manner similar to an iceberg

• Makes up about 20% of the membrane by mass

How are cholesterol molecules inserted/where in

• Cholesterol molecules are inserted alongside phospholipid molecules in both leaflets of the membrane.

How does cholesterol act on the plasma membrane at low temperatures

• maintain the fluidity of the membrane by keeping phospholipid molecules separated

• prevent the membrane from becoming too stiff

How does cholesterol act on the plasma membrane at high temperatures

•  stabilise the membrane by raising its melting point and preventing it from becoming excessively fluid