GRE Subject Test: Biology : Understanding Membrane Proteins
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Example Questions
Example Question #1 : Understanding Membrane Proteins
Leucine is a hydrophobic amino acid. In which of the following portions of a transmembrane protein would a leucine residue most likely be located?
Membrane-spanning region
Leucine would not be found in a transmembrane protein
Exoplasmic face
Cytosolic face
Membrane-spanning region
Because leucine is a hydrophobic amino acid, it would make sense that it would be most stable in a hydrophobic environment. The interior of the phospholipid bilayer is a hydrophobic environment; therefore, leucine and other hydrophobic amino acids are more commonly found in the membrane-spanning portions of transmembrane proteins.
Polar and hydrophilic amino acids are most commonly found in the cytosolic and exoplasmic regions of the membrane, as these regions interact with the aqueous environment outside of the membrane.
Example Question #1 : Understanding Membrane Proteins
A researcher finds a large amount of an unusual sugar in the cells of a recently discovered species. Which of the following are potential mechanisms the cell is using to import the sugar?
I. Passive diffusion
II. Receptor-mediated endocytosis
III. Symport
III only
I and II
II and III
I, II, and III
II and III
Because the molecule is a sugar, it is too large to passively diffuse across the plasma membrane and contains polar regions that would make this impossible.
There are several other mechanisms by which the sugar could enter the cell. One of these is receptor-mediated endocytosis. In this process the sugar would bind to receptors on the plasma membrane, which stimulates a budding event and eventually leads to the formation of a vesicle inside the cell.
Symport is another potential mechanism. In symport, the import of a molecule is coupled with the import of another molecule through the same transmembrane protein. For example, glucose has a symport mechanism with sodium ions.
Example Question #1 : Cell Membrane And Organelles
What type membrane protein changes its shape when allowing substances to cross the membrane?
Carrier proteins
Glycoproteins
Channel proteins
Kinases
Carrier proteins
Of the given choices, only channel and carrier proteins allow substances to cross the membrane. While channel proteins create an open pore through which substances can cross, carrier proteins will change their shape in order to allow substances to cross the membrane.
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