Video Transcript
The following is a basic outline of
the structure of a phospholipid. Which of the following options
correctly describes the nature of the structures labeled X and Y? (A) X is hydrophilic and Y is
hydrophobic. (B) X is hydrophobic and Y is
hydrophilic. (C) Both X and Y are
hydrophilic. Or (D) both X and Y are
hydrophobic.
This question is asking about the
properties of different regions of a phospholipid molecule. Let’s first define some of the key
terms in the question.
A phospholipid is a special kind of
lipid that forms the bilayer that our cellular membranes are made up of. Phospholipids form a barrier around
cells and membrane-bound organelles within cells. They arrange themselves as a
bilayer, a double layer with the phosphate heads on the outer surfaces of the
membrane and the fatty acid tails in the center.
Some substances, such as small,
uncharged molecules like water and the respiratory gases oxygen and carbon dioxide,
are able to move freely across the phospholipid bilayer. Large, charged molecules, such as
sodium and chloride ions, cannot move freely across the bilayer membrane. Ions and other large molecules
require transport proteins to cross the membrane. This transport can either be
passive or active, the latter requiring energy from ATP.
So why is it that some molecules
can move easily across the membrane, while others are unable to? This is due to the properties of
phospholipids. Phospholipids have hydrophilic and
hydrophobic regions. Hydro- means water; -philic means
has an affinity for or likes. So hydrophilic means water
liking. -Phobic means fearing, so
hydrophobic means water fearing. The hydrophobic and hydrophilic
regions on the phospholipid molecules are what cause them to arrange themselves into
a bilayer.
The fatty acid chains, or tails,
are hydrophobic so point away from the watery contents of the cell or the tissue
fluid surrounding cells. The best way for them to avoid
water is to point towards one another. As polar molecules, phosphate heads
are attracted to water and therefore hydrophilic. They are exposed to the watery
contents of the cell or tissue fluid in the case of a cell surface membrane.
Let’s apply what we have learnt to
our question. We have seen how the fatty acid
tails of a phospholipid are hydrophobic. These are labeled X in our
diagram. We have also seen how the phosphate
head is hydrophilic. This is labeled Y on our
diagram. So the answer option which
correctly describes the nature of the structures labeled X and Y is (B). X is hydrophobic and Y is
hydrophilic.
