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Biology Homework (Cell Membrane)

Matthieu-Pierre Currie
October 21, 2019
Explain how hydrophobic and hydrophilic properties contribute to the arrangement of
molecules in a membrane.
Phospholipids form the basis of the cell membrane with the head containing a phosphate
and glycerol group and the tail containing a fatty acid group. Therefore, the head would be polar
or hydrophilic whilst the tail would be non-polar or hydrophobic thus the phospholipid is to be
considered amphipathic due to the dual hydrophobic and hydrophilic properties.
Consequently, due to the amphipathic nature of phospholipids and the attraction between
the hydrophobic hydrocarbon tails in water the phospholipids form a bilayer. Structurally, the
hydrophilic phosphate heads would be facing outwards with the hydrophobic hydrocarbon tails
facing inwards as represented by the fluid mosaic model.
Furthermore, the amphipathic nature of the phospholipids contributes to the positioning
molecules such as proteins, cholesterol and carbohydrates found in membrane structure.
Extrinsic proteins are hydrophilic on their surface and thus are not embedded in the membrane
but attached to the surface of integral proteins or anchored onto the membrane surface. Integral
proteins possess mostly hydrophobic surface thus found embedded within the membrane in the
hydrocarbon chains with the portions of the protein protruding through the regions of the
phosphate heads being hydrophilic at the ends.
Cholesterol is only present in animal cells but nevertheless the amphipathic nature
contributes to its positioning. Cholesterol is primarily hydrophobic but at one end is hydrophilic
due to the presence of a hydroxyl group in its molecular structure. As a result, most of the
cholesterol molecule is found embedded within the membrane in the hydrocarbon chains but the
hydrophilic portion is attracted to the phosphate head and thus the molecule is found in between
the phospholipids in the membrane.
Carbohydrates found in the cell membrane structure are hydrophilic and consequently
found on the outside attached to the hydrophilic phosphate heads to form glycolipids, attached to
the hydrophilic portion of integral proteins to form glycoproteins or attached to the hydrophilic
surface of integral proteins to form glycoproteins.