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Singer and Nicholson’s (1972) fluid mosaic model , which has been only very slightly modified despite an additional 35 years of discovery, is the now the accepted model for plasma membrane structure.
1. On a piece of paper, describe your understanding of the biochemical structure and organization of the plasma membrane. A superior response will include the molecular interactions responsible for its physical organization.
2. Recall that the biochemical composition and structural organization of the plasma membrane render it ‘semi-permeable’ as Overton observed over 100 years ago. That is, some molecules can diffuse through it while others cannot. Review the list of molecules below and for each one propose and explain a reason that references the structure of the membrane for the indicated permeability property.
Hydrogen ion (H+): Impermeable.
Ions are charged and are hydrophobic. They cannot dissolve in the hydrophobic interior of the plasma membrane and so cannot diffuse directly through the membrane.
Oxygen (O2): Permeable.
Oxygen is a non-polar molecule because electrons are shared equally by both atoms. Therefore, it can diffuse directly through the hydrophobic interior of the plasma membrane.
Carbon dioxide (CO2): Permeable.
Although the chemical bonds of this molecular are polar because the oxygen atoms pull electrons away from the carbon, the oxygen atoms are arranged on either side of the carbon in such a way that it distributes the charge more or less evenly across the surface of the molecule. Thus, the molecule as a whole is non-polar. As such, the molecule is able to diffuse directly through the hydrophobic interior of the plasma membrane.
Glucose: Impermeable.
The hydroxyl groups on the molecule are charged and therefore non-polar. These charges prevent glucose from diffusing directly through the hydrophobic interior of the plasma membrane.
Calcium ion (Ca2+): Impermeable.
The calcium ion, like the hydrogen ion, is charged, Consequently it cannot diffuse directly through the hydrophobic interior of the membrane.
Water (H2O): Permeable.
Oddly water can diffuse directly through the membrane even though it is a highly polar molecule due to the unequal charge distribution that arises from the oxygen atom pulling electrons away from the two hydrogen atoms. Its very small size is believed to be important to its ability to diffuse directly through the membrane.
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