Video Transcript
The figure shows the structure of a
cell membrane. Which component is an extrinsic
protein?
To work out which component of the
cell membrane that’s presented in the image is an extrinsic protein, let’s take a
look at some of the structures that have been labeled in the diagram and their
roles. The cell membranes of living
organisms consist mainly of structures called phospholipids. Phospholipids consist of a
phosphate head that is hydrophilic and so attracted to water molecules and fatty
acid tails, which are hydrophobic and so repelled by water molecules. As the extracellular space and the
cytoplasm inside the cell both contain water molecules, the phospholipids arrange
themselves into a bilayer, with their hydrophilic phosphate heads pointing outward
toward these areas and their hydrophobic fatty acid tails pointing inward toward
each other.
The label with the letter V is
pointing to the phosphate head of a phospholipid, oriented outward toward the
extracellular space. The label with the letter W is
pointing to a sugar molecule that is attached to a phospholipid in the bilayer,
which is otherwise known as a glycolipid, as the prefix glyco- means sugar. Glycolipids are useful for cell
recognition to identify whether other cells belong to the same organism or a
different potentially dangerous organism. The structure labeled with a Z is a
different type of lipid called cholesterol, which is useful for regulating the
fluidity of the phospholipid bilayer at a wide range of temperatures.
The two remaining labels, X and Y,
are both pointing to proteins embedded in the phospholipid bilayer. Y is pointing to an intrinsic
protein. These proteins interact with the
inner part of the phospholipid bilayer, spanning the entire width of the cell
membrane. Due to this feature, intrinsic
proteins are very useful for transporting substances across the cell membrane that
would not otherwise be able to cross it due to the hydrophobic nature of the
phospholipid tails.
Finally, label X is an extrinsic
protein. These proteins only interact with
the phosphate heads of the phospholipids and not the fatty acid tails, so lie on the
periphery of the membrane, either pointing toward the cytoplasm, like in this
example, or toward the extracellular space. Extrinsic proteins sometimes act as
receptors and can aid membrane stability. Now we know the correct answer to
our question. The component that is an extrinsic
protein is X.
